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ENGINEERING CHEMISTRY FRESHMAN ENGINEERING INSTITUTE OF AERONAUTICAL ENGINEERING Dundigal, Hyderabad
Electrochemistry and Batteries Introduction Electrochemistry is the branch of chemistry which deals with the transformation of electrical energy into chemical energy and vice versa. It is the study of phenomena at electrode solution interfaces. Electrochemistry deals with the relationship between electrical, chemical phenomena and the laws of interaction of these phenomena. The branch of electrochemistry is of major technical importance. The laws of electrochemistry form the basis of electrolysis and electrosynthesis. The knowledge of electrochemistry is of immense importance to study about the causes of destruction of materials due to corrosion. In electro-chemistry, there are two processes: electrolysis and electromotive process. Both these processes are interrelated. 1
The two processes above can be summarized as follows: 1. Electrical energy causing chemical reactions → Electrolysis (Electrolytic cell) 2. Chemical reactions producing electrical energy → Electromotive (Galvanic cell) 3. Electromotive → Electron + Motion
Conductance: Thesubstances which allow the passage of electric current are called conductors. Metals such as copper and silver are good conductors of electricity. Substances which allow the electricity to pass through them in their molten states or in the form of their aqueous solutions are called electrolytic conductors or electrolytes and this property is known as electrolytic conductance. Electrolytic conductors conduct electricity due to the migration of ions. The examples of electrolytic conductors are sodium chloride, potassium chloride, etc. Electrical conductors are of two types: 1. Metallic conductors and 2. Electrolytic conductors.
Electrolytic Conductance: Electrolysis is the process of decomposition of electrolyte when electric current is passed through the solution. Electrolysis is carried out in an apparatus called electrolytic cell. It contains a +ve electrode called anode and a –ve electrode called cathode. The current enters at anode and leaves at cathode. Ohm’s law states that the current (I) flowing through a conductor is directly proportional to the potential difference (E) applied across the conductor and is inversely proportional to the resistance of the conductor. Thus, where I is the current in amperes and Vis the potential difference applied across the conductor in volts. Thus, where R is the proportionality constant and is known as the resistance of conductor in ohms. Thus, the re-sistance of a conductor is directly proportional to the potential difference applied across the conductor and inversely proportional to the current carried by the conductor.
Specific conductance: Specific resistance (r) is defined as the resistance of an electrolyte solution of 1 cm in length and 1 cm 2 cross-section, i.e. resistance of 1 cm3 of the electrolytic solution. The unit of specific resistance is ohm centimetre. The reciprocal of specific resistance (r) is called specific conductance. This may be defined as the conductance of 1 cm3 of a material and is denoted by K
Equivalent Conductance: Equivalent conductance is of greater importance in case of electrolytic solution. It is defined as the conductance of an electrolyte solution containing 1 g equivalent of the electrolyte. It is usually denoted by L. the relation between equivalent conductance () and specific conductance is given as- =V x K where V is the volume of the electrolyte solution in millilitres containing 1 g equivalent of the electrolyte. If the concentration of the electrolyte solution is C gL–1, then volume containing 1 g equivalent of the electrolyte will be 1/C litres or 1000 C ml. Thus, by using the above equation, The unit of equivalent conductance is ohm–1 cm2eq–1.
Molar Conductance: Molar conductance is defined as the conductance of an electrolytic solution containing 1 mol of electrolyte. It is denoted by λ m. If Vm ml is the volume of the solution containing 1 g-mol of the electrolyte, then λ m = V m X K If m is the concentration of the solution in moles per litre, then The unit of λ m is S m 2 mol –1 or ohm –1 cm 2 mol –1
Cell Constant: It is a constant characteristic of the cell in which the electrolyte is taken and its value depends on the distance between the electrodes and the area of cross- section of the electrodes. If the area of cross-section is in cm 2 and distance between the electrodes is in cm, the unit of cell constant is cm –1.
Variation of Conductance with Dilution: On dilution, the volume of an electrolyte solution increases. Thus, the number of ions furnished by the electrolyte per unit volume is decreased. Hence, the specific conductance decreases on progressive dilution. Electrolytes are of two types: (i) strong electrolytes (ii) weak electrolytes. Strong Electrolytes: These electrolytes are completely ionized at ordinary concentration. They show an increase in conductance at lesser dilutions. For example, salts such as KCl, NaCl, BaCl2, CaCl2, etc.; acids such as HCl, H2SO4, etc.;and strong bases such as NaOH, KOH, Ca(OH)2, Mg(OH)2, etc.; have very high equivalent conductance at ordinary concentration. Since the conductance of a solution of a given electrolyte depends on the number and speed of ions, these two factors will be considered in the determination of conductance. 1. Strong electrolytes are completely ionized at ordinary concentration; hence, with increase in dilution,the number of ions will remain the same as before. 2. In the case of concentrated electrolyte, the speed of ions is very low; but on dilution, the speed of ions will be more. This is because the interionic forces of attraction of the oppositely charged ions decrease on dilution. Thus, the equivalent conductance at infinite dilution reaches its maximum value due to increase in the speed or mobility of ions but not due to the number of ions in case of strong electrolytes.
Weak Electrolytes: These electrolytes dissociate into a very small extent at ordinary concentration. 1. In dilute solution, the interionic attraction forces between the ions are not significant since the ions move freely. Thus, on dilution, there is a little or no effect on the speed of ions in the electrolytic solution. 2. On dilution, the degree of dissociation increases so that the number of ions increases. Thus, the equivalent conductance increases on dilution because of the increase in the number of ions in the electrolytic solution.
Effect of Concentration on Equivalent Conductance Let us consider a weak electrolyte AB, which is ionised as When the electrolyte solution is diluted, more electrolyte molecules break up and more ions are formed,i.e., the degree of dissociation is increased. As a result of which the solution exhibits enhanced conductivity. If the process of dilution is continued, more and more electrolyte molecules undergo dissociation. But at some stage of dilution all the electrolyte molecules are ionised, i.e. ionisation is completed and at that point on further dilution, no more ions are produced and equivalent conductance does not increase. This limiting value is known as equivalent conductance at infinite dilution ( 0 or ∞ ). Strong electrolytes are completely ionised in solution at all concentrations and hence one would not expect any variation of equivalent conductance with concentration (dilution). But it is found that for strong electrolytes, equivalent conductance () increases with dilution, and the cause of this variation isfundamentally different from weak electrolytes. The conductivity of an electrolyte depends on the speed and concentration of ion. The motion of an ionic species in an electric field is retarded by the oppositely charged ions due to inter-ionic attraction. On dilution, the concentration of electrolyte decreases and the retarding influence of oppositely charged ions decreases. Consequently, the speed of ions increases, and the equivalent conductance of electrolyte is increased
Kohlrausch’s law of independent migration of ions Kohlrausch’s law states that at infinite dilution, each ion of an electrolyte makes a definite contribution to the total equivalent conductance of the electrolyte, and this contribution is independent of the presence of other ionic species. Kohlrausch’s law is applicable to calculate: 1. Equivalent conductance of weak electrolytes at infinite dilution. 2. Absolute ionic mobility. 3. Solubility of sparingly soluble salt. 4. Degree of ionization. 5. Ionic product of water.
Calculation of equivalent conductance of weak electrolytes at infinite dilution ‘The equivalent conductance of an electrolyte at infinite dilution is the sum of the equivalent conductance of the constituent ions at infinite dilution’ i.e.
Calculation of absolute ionic mobility (absolute velocity): The speed of an ion produced by an electrolyte varies with the applied potential difference. Ionic mo-bility is defined as the distance travelled by an ion per second under the influence of a unit potential gradient, i.e. 1 V cm –1. Thus, if an ion moves a distance equal to x cm in t seconds, when a potential gradient of v Vcm –1 is applied, its mobility U may be expressed as
Galvanic Cell A galvanic cell is made of two half cells. One is oxidation or anodic half cell and the other one is reduction or cathodic half cell. Daniel cell is an example of galvanic cell having zinc and copper electrodes. The first half cell consists of zinc electrode dipped in ZnSO4 solution and the second half cell is made of copper electrode dipped in copper sulphate solution. Both half cells are connected externally by metallic conductor (connecting wire) and internally by a bent glass tube having saturated solution of a strong electrolyte (KCl) called salt bridge. It acts as a bridge between the two half cells.
A zinc or a copper galvanic cell can be represented as Zn/ZnSO4 || CuSO4/Cu The double bar shows a salt bridge, i.e. electrolyte–electrolyte junction. The chemical reactions taking place at both electrodes may be written as follows: At anode: Oxidation takes place with the liberation of two electrons. Zn → Zn e– (oxidation) At cathode : Reduction occurs and cuprous ion is reduced to metallic copper. Cu e– → Cu (reduction) The overall reaction is Zn + Cu +2 Zn +2 + Cu The electrode showing oxidation reaction is anode and the other electrode where reduction occurs is cathode. As per IUPAC convention, the anode is always represented on the left and the cathode always represented on the right side of the cell. As the connection is complete, the flow of electrons will be externally from anode to cathode and internally from cathode to anode through the salt bridge. The flow of current is due to the difference in electrode potentials of both the electrodes. The potential difference in the cell is called the EMF and is measured in volts. It can be measured by the potentiometer. The flow of current becomes slow after using the electrodes for a long time because of the polarization of the electrodes.
Salt bridge: Salt bridge is a U-shaped tube containing concentrated solution of an inert electrolyte such as KCl, KNO3, and K2SO4 or a paste of inert electrolyte (whose ions do not take part in redox reaction and do not react with the electrolyte) in agar–agar medium or gelatin. Functions of salt bridge: Salt bridge helps to complete the circuit by allowing the ions to flow from one solution to the other without mixing the two solutions. It helps to maintain electrical neutrality of the solution in the two half cells.
Reference Electrodes The potential of half cell or single electrode potential is the potential difference between the metal and its salt solution in which it is dipped. The determination of the potential of a single electrode is not possible. Since any circuit necessarily contains two electrodes, a reference electrode is an electrode which has a stable and well known potential. It is used as half cell to build an electrochemical cell and to determine the potential of other electrode. A fixed potential difference is applied between the working electrode and the reference electrode; this potential drives the electrochemical reaction at the working electrode surface. The current produced from the electrochemical reaction at the working electrode is balanced by a current flowing in the opposite direction at the counter electrode. The examples of reference electrodes are standard hydrogen electrode, calomel electrode, silversilver chloride electrode, etc.
Standard Hydrogen Electrode (Normal Hydrogen Electrode: It is a redox electrode which is widely used as reference electrode. It can be used as either anode or cathode depending upon the nature of the half cell for which it is used. The SHE consists of a platinum electrode immersed in a solution with a hydrogen ion concentration of 1.0 M. The platinum electrode is made of a small square of platinum foil which is platinised and known as platinum black. [Platinum black provides a surface on which the hydrogen gas can be in contact with the hydrogen ions (aq.).]
Nernst equation for a cell reaction:
Quinhydrone Electrode: It is a type of redox electrode which can be used to measure the H + ion concentration of a solution. The electrode consists of an inert metal electrode (a platinum wire) in contact with quinhydrone crystals and a water-based solution. Quinhydrone is slightly soluble in water, formed by equi–molar mixture of hydroqui-none and quinone.
Ion Selective Electrode: An ion selective electrode is a sensor which converts the activity of a specific ion dissolved in a solution into an electrical potential which can be measured by a voltameter or pH-meter, e.g. glass electrode. Glass Electrode: A glass electrode is a type of ion-selective electrode and consists of a thin-walled glass bulb attached to a glass tube. A very low melting point and high electrical conductivity glass are used for the construction of this bulb. The glass tube contains a dilute solution of constant pH of HCl (0.1 N) solution. A silver– silver chloride electrode or platinum wire is immersed as reference electrode in the HCl solution. A Glass Electrode
ELECTRODE POTENTIAL When a metal rod is dipped in its salt solution (electrolyte), the metal atom tends either to lose electrons (oxidation) or to accept electrons (reduction). The process of oxidation or reduction depends on the nature of metal. In this process, there develops a potential between the metal atom and its corresponding ion called the electrode potential. Oxidation: M → Mn + + ne– Reduction: Mn + + ne– → M The tendency of oxidation results in the dissolution of metal and in the release of electron density on the surface of metal with respect to electrolyte solution. Similarly in reduction, the positive charge density is more on the surface of metal with respect to the solution. Consequently, they attract oppositely charged particles and form a double layer of charge called the Helmholtz electrical double layer There is a dynamic equilibrium between metal and metal ion, and the potential difference between the two is called the electrode potential or the equilibrium potential. It is measured in volts. The potential difference for oxidation reaction is called the oxidation potential and that of reduction is called the reduction potential.
Batteries or Commercial Cells Definition- a) A battery is a storage device used for the storage of chemical energy and for the transformation of chemical energy into electrical energy. b) Battery consists of group of two or more electrical cells connected together electrically in series. c) Battery acts as a portable source of electrical energy. Batteries are of three types, namely, 1. Primary batteries (or) primary cells 2. Secondary batteries (or) secondary cells 3. Fuel cells (or) flow batteries
Primary Cell: In a primary cell, a chemical reaction proceeds spontaneously and its free energy is converted into electrical energy. The production of electrical energy at the expense of the free energy of the cell is called discharging of the cell. But in a secondary cell, electrical energy is passed into the cell when a chemical reaction is induced, and the products remain on the electrodes. These products react in the backward direction at our choice and liberate free energy in the form of electrical energy. These cells accumulate electrical energy in the form of chemical reaction and later on the reaction is reversed to release electrical energy. This process is called charging of the cell. Examples of primary cells: Voltaic cells, Daniel cell, Leclanche cell, Weston cadmium cell, and lithium cell. Dry Cell (leclanche Cell)
Secondary Cells (or) Accumulator Batteries: In secondary cells, the cell reaction can be made to proceed in the reverse direction by passing direct current through it from an external source. Therefore, it can operate in both the ways, i.e., it receives electrical en- ergy and also supplies it. When it supplies electrical energy, it operates like an electrochemical cell and gets rundown. The cell needs to be recharged. During recharging, it operates like an electrolytic cell. Example: Lead storage battery Nickel–cadmium battery Lithium-ion cell battery
Lead–acid battery: If a number of cells are connected in series, the arrangement is called a battery. The lead storage battery is one of the most common batteries that are used in the automobiles. A 12 V lead storage battery is generally used, which consists of six cells, each providing 2 V. Each cell consists of a lead anode and a grid of lead packed with lead oxide as the cathode. These electrodes are arranged alternately, separated by a thin wooden piece and suspended in dil. H2SO4(38%), which acts as an electrolyte. Hence, it is called lead-acid battery. Lead–acid battery
Lead Storage Cells: To increase the current output of each cell, the cathode and the anode plates are joined together, keeping them in alternate positions. The cells are connected parallel to each other (anode to anode and cathode to cathode). The cell is represented as Pb|PbSO4(s), H2SO4(aq.)|PbSO4(s), Pb
Fuel Cells: A fuel cell is an electrochemical cell which converts chemical energy contained in readily avail-able fuel oxidant system into electrical energy. Principle: The basic principle of the fuel cell is same as that of an electrochemical cell. The fuel cell operates like a galvanic cell. The only difference is that the fuel and the oxidant are stored outside the cell. Fuel and oxidant are sup-plied continuously and separately to the electrodes at which they undergo redox reactions. Fuel cells are capable of supplying current as long as reactants are replenished. Fuel+Oxidant OxidationProducts+Electricity Examples: (1) H2– O2fuel cell (2) Propane – O2fuel cell (3) CH3OH – O2fuel cell Schematic diagram of hydrogen– oxygen fuel cell
THEORETICAL QUESTIONS 1. What are concentration cells? Explain an electrolyte concentration cell and its application. 2. Describe the construction of a glass electrode. How is it significant in its applications? 3. Derive Nernst’s equation. How is it useful? 4. Discuss the working principle of the primary batteries. 5. What is a galvanic cell? Also describe a Daniel cell. 6. Differentiate between an electrochemical cell and an electrolytic cell. 7. What is electrochemical series? Discuss its three important applications. 8. Describe the construction and working of a calomal electrode. 9. Describe the construction of a galvanic cell. Write down the electrode reactions and the formula for its EMF. 10. What are fuel cells? Explain the working of the hydrogen– oxygen fuel cell?
Introduction Corrosion is a process of formation of the compound of pure metal by the chemical reaction between metallic surface and its environment. It is an oxidation process. It causes loss of metal. Hence, disintegration of a metal by its surrounding chemicals through a chemical reaction on the surface of the metal is called corrosion. Thus, corrosion is defined as destruction or deterioration of metals by chemical reaction. Example: Formation of rust on the surface of iron, formation of green film on the surface of copper. The responsible factors for the corrosion of a metal are the metal itself, the environmental chemicals,temperature and the design CORROSION AND ITS CONTROL
Causes of Corrosion In nature, metals are not found in free state due to their reactivity. It is the ore from which the metals are extracted by metallurgical processes. Metallurgy requires a large amount of heat energy. Why metals are not found in their free state? The answer to this question is that the metals are thermodynamically unstable in their free state. Since they are stable in the form of certain compounds, the extracted metal has higher energy. This is given as:
Effect of Corrosion Corrosion is a slow process, taking place mainly on the surface of the metals, but the damage caused by it is enormous. The corrosion in metal objects such as equipments, instruments, chemical plants, structures, etc., makes them inefficient. In some types of corrosion it is invisible and seen only when accident occurs. Consequences of corrosion cause a great loss of economy and life. The following harmful effects are specific. 1.Corrosion reduces the thickness of the metal, resulting in the loss of mechanical strength and failure of the structure. 2. Because of the deterioration of appearance, the cost of machine is reduced. 3. Efficiency of a machine is reduced due to corrosion. 4. Because of corrosion, pipes are blocked and pumps are difficult to operate. 5 Boilers are damaged because of corrosion. 6. Buildings and historic monuments are damaged due to corrosion (e.g., Taj Mahal).
Theories of Corrosion: There are three theories of corrosion: (i) acid theory, (ii) dry or chemical corrosion and (iii) galvanic or electrochemical or wet corrossion. Acid Theory of Corrosion This theory suggests that corrosion of a metal (iron) is due to the presence of acids surrounding it. According to this theory, iron is corroded by atmospheric carbon dioxide, moisture and oxygen. The corrosion products are the mixture of Fe(HCO3)2, Fe(OH)CO3 and Fe(OH)3. The chemical reactions suggested are given below: This theory is supported by the analysis of rust that gives the test for CO3 2– ion. Further, the process of rusting is reduced by the presence of lime and caustic soda (these two can absorb CO2, thus reducing corrosion).
Dry or Chemical Theory of Corrosion: According to this theory, corrosion on the surface of a metal is due to direct reaction of atmospheric gases such as oxygen, halogens, oxides of sulphur, oxides of nitrogen, hydrogen sulphide and fumes of chemicals with metal. The extent of corrosion of a particular metal depends on the chemical affinity of the metal towards reactive gas. Oxygen is mainly responsible for the corrosion of most metallic substances when compared to other gases and chemicals. Oxidation corrosion (Reaction with oxygen): Some of the metals react directly with oxygen in the absence of moisture. Alkali and alkaline earth metals react with oxygen at room temperature and form corresponding oxides, while some metals react with oxygen at higher temperature. Metals such as Ag, Au and Pt are not oxidised as they are noble metals.
Wet or Electrochemical Theory of Corrosion: It is a common type of corrosion of metal in aqueous corrosive environment. This type of corrosion occurs when the metal comes in contact with a conducting liquid or when two dissimilar metals are immersed or dipped partly in a solution. According to this theory, there is the formation of a galvanic cell on the surface of metals. Some parts of the metal surface act as anode and rest act as cathode. The chemical in the environment and humidity acts as an electrolyte. Oxidation of anodic part takes place and it results in corrosion at anode, while reduction takes place at cathode. The corrosion product is formed on the surface of the metal between anode and cathode. To understand the wet theory, let us take the example of corrosion of iron. Oxidation of metal takes place at anode while the reduction process takes place at cathode. By taking rusting of iron as an example, the reaction can be explained as that it may occur in two ways: (i) evolution of hydrogen and (ii) absorption of oxygen
Types of Corrosion Based on the reactions and physical states, there are different types of corrosions. They are (a) Galvanic corrossion (b) Pitting corrosion (c) Stress corrosion (d) Crevice corrosion (e) Erosion corrosion (f) Soil corrosion (g) Micro-biological corrosion (h) Water-line corrosion (i) Differential aeration corrosion (j) Intergranular corrosion
(a) Galvanic corrosion: This type of electrochemical corrosion is also called bimetallic corrosion. When two dissimilar metals are connected and exposed to an electrolyte, they will form a galvanic cell. The anodic metal will be oxidised and it will undergo corrosion. Zinc and copper metals connected with each other in an electrolyte medium form a galvanic cell. Zinc acts as anode and undergoes corrosion while cathode Prevention of galvanic corrosion: Galvanic corrosion can be avoided by (i) Coupling metals close to the electrochemical series. (ii) Fixing insulating material between two metals. (iii) Using larger anodic metal and smaller cathodic metal. Examples of galvanic corrosion: steel screws in brass marine hardware; steel pipe connected to copper plumbing; steel propeller shaft in bronze bearing; zinc coating on mild steel; and lead–tin solder around copper wires. Galvanic corrosion
(b) Pitting corrosion: Due to the formation of cracks on the surface of a metal, local straining of metal, sliding under load, chemical attack, there is formation of a local galvanic cell. The crack portion acts as anode and rest of the metal surface acts as cathode. It is the anodic area which will be corroded and the formation of a pit is observed. This type of corrosion is thus called pitting corrosion,Metals owing to their corrosion resistance to their passive state show pitting and ultimately result in the formation of passivity. Presence of external impurities such as sand, dust, scale embedded on the surface of metals lead to pitting. For example, stainless steel and aluminium show pitting in chloride solution. Pitting corrosion at the surface of iron
(c ) Stress corrosion: In a metallic structure if there is a portion under stress, it will act as anode and rest part of the structure will act as cathode. It is now a galvanic system and hence anodic part which is small in area will corrode more. Stress corrosions are observed in the following system. Caustic embrittlement is a type of stress corrosion occurring in steel tank (boiler) at high temperature and in alkaline medium. Boiler water has Na2CO3; it will be hydrolysed at high temperature to give NaOH. It flows into hair cracks and crevices. There it reacts with iron and forms Na2FeO2(sodium ferroate) which decomposes to give Fe3O4 (ferroferric oxide) and NaOH. The reaction is as follows: 3Na2FeO2 + 3H2O Fe3O4 + H2 + 6NaOH
(d) Crevice corrosion: If surface of painted metal is scratched, it will undergo corrosion. The scratched portion acts as small anode and the rest part will act as cathode forming a local cell. Crevice corrosion is formed near joints, rivets and bolts. Changes in the concentration of oxygen/acidic medium causes crevice corrosion. (e) Erosion corrosion: Due to mechanical wear and tear, corrosion occurs on the surface of a metal and is called erosion corrosion. (f) Soil corrosion: underground pipes, cables, etc. corrode due to soil corrosion. It is caused due to moisture, pH of soil and micro-organisms. (g) Microbiological corrosion: Some types of bacteria consume oxygen and cause differential aeration type of system which results in corrosion. The corrosion occurs at the portion poor in oxygen concentration. For example, the bacillus and algae diatoms.
Inter-granular corrosion: This corrosion is observed in case of alloys. The corrosion product is observed at the boundaries of grains. Externally, it is not seen. There is a sudden failure of material due to this corrosion. For example, during the welding of stainless steel (an alloy of Fe, C, Cr), chromium carbide is precipitated at the grain boundaries and the region adjacent to grain boundaries becomes depleted of chromium composition and is made anodic, and is more susceptible to corrosion. It occurs on microscopic scale and follows the path of grain boundary until the affected grain is completely dislodged. Thus, it leads to sudden failure without any external indication of the attack. It can be minimized by preventing heterogeneous due to slow cooling of alloy. Rapid quenching after heat treatment of a metal is the remedy of inter-granular corrosion
Factors Influencing Corrosion: Since corrosion is a process of destruction of metal surface by its environment, the two factors that govern the corrosion process are: (i) Metallic and (ii) Environmental (i) Nature of metal: Different properties of a metal are responsible for corrosion. These properties are as follows: a) Position of metal in galvanic series: b) Hydrogen over voltage: c) Purity of metal: d) Relative areas of anode and cathode: e) Physical state of the metal: f) Nature of oxide film: g) Volatility and solubility of corrosion product:
(ii) Nature of environment: The role of environment in the corrosion of a metal is very important. Environmental parameters like temperature, humidity, pH, etc. play important role. The effect is discussed here. (a) Temperature: The rate of diffusion increases by rise in temperature, hence the rate of corrosion is also increased. At higher temperature, passive metals also become active and undergo corrosion. But higher temperature reduces the concentration of oxygen and hence corrosion is reduced homogeneous solid solution, hence no local action and no corrosion. (b) Humidity of air: In humidity, gases like CO2, SO2, NOx are dissolved which form electrolytes. It will cause galvanic corrosion. Some oxides are water soluble, humidity washes away the corrosion products and metal surface is further corroded. Other soluble corrosion products can also be washed away by humidity, causing further corrosion. (c )Impurity of atmosphere: Pollutants like H2S, SO2, CO2 and acid vapours cause more pollution where they dissolve. In sea water (salty in nature which acts as an electrolyte) corrosion rate increases. Some suspended particles are dissolved in humidity and form electrolyte which helps in corrosion. (d) pH value of the medium: pH value means concentration of H+ (acidic nature). In acidic medium (pH less than 7), corrosion is faster. Also, in basic medium pH > 7, some metals such as Pb, Zn, Al, etc. form complexes and hence they corrode.
Measurement of Corrosion: The process of corrosion is a slow chemical reaction. During the corrosion process, there is loss of weight of the metal. The loss in mg/dm2 per day or inches per year measures the extent of corrosion. Rate of corrosion (R) =kw/atd where w = loss of weight of metal in milligrams a = area of metal surface in sq. cm t = exposure time for corrosion in 100th part of an hour or a day d = density in g/cm2 k = constant of proportionality The condition is that the surface of the metal must be uniform. There are several units for the measurement of the rate of corrosion (R), i.e. mg/dm 2 /day, oz/ft 2 /day, inch/year or millimetre/year. A cleaned metal is taken, weighed and exposed for corrosion. Time taken in the process is recorded and then the piece of the metal is taken out; corrosion product is removed and is weighed again. The loss in the weight of the metal is noted which helps to calculate corrosion rate.
Protection from Corrosion Metals can be protected from corrosion by various methods. Some of them are as follows:. Proper designing. Using pure metals. Using corrosion inhibitors. Modifying the environment. Cathodic protection—sacrificial anode, impressed current, cathode. Heat treatment. Applying surface coatings
Sacrificial Anodic Protection: Advantages: It is a simple method. It has low maintenance and installation cost. It does not require external power; cathodic interferences are minimum. Disadvantages: More than one anode is required. Current output is less. It does not work properly in high corrosive environment. The sacrificial anode must be replaced periodically as used when it is consumed. Due to corrosion, there is a great loss of material and money. Therefore, it is essential to protect metals from corrosion. Since there are two components involved in corrosion—the metal and environment—both are considered in corrosion protection. Following methods have been adopted for the protection of metal from corrosion: (i) If it is unavoidable, the anodic area should be very large as compared to the cathodic area. (ii) Proper designing of the equipment is the best way of controlling corrosion. The design of the metal should be such that even if corrosion occurs, it is uniform and does not result in intense and localised corrosion.
(iii) Two different metals used in the structure should be such that they are occupying near positions in the galvanic series. (iv) Putting an insulator between two metals resists corrosion. (v) As far as possible, metal used in a structure should be extremely pure. Small amount of impurity causes corrosion. (vi) While using an alloy, it should be completely homogeneous. (vii) Design or fabricate equipment or metal parts in such a manner that they have minimised sharp edges and corners and also avoid, as far as possible, the crevices in joints, etc. (viii) The equipment must be supported on legs to allow the passage of air which prevents differential
Modifying the Environment It can be done either by removing the corrosive agents from the environment or by neutralising the corrosive effect by adding certain substances. For example, by deaeration or by deactivation of substance by the addition of chemicals; to deactivate sodium sulphate, oxygen is added to sodium sulphate. To neutralise the acidic character of the corrosive environment due to HCl, CO2 and SO2, bases like lime, NH3 and NaOH are used. The modification of environment also helps in protection from corrosion. (i) Deaeration removes oxygen by adjusting temperature and mechanical ageing. (ii) Deactivation involves addition of chemicals such as Na2SO3, NH2 – NH2.H2O which absorb oxygen. (iii) Dehumidification of environment is done by adding alumina or silica gel. These chemicals absorb humidity from metallic surface. In humidity, gases such as CO2, H2S, SO2 and HCl give acidic medium responsible for corrosion. They are neutralised by NH3 or NaOH or lime.
Cathodic Protection: There are some chemicals which reduce the rate of corrosion. These chemicals are called corrosion inhibi-tors. They are of two types, anodicand cathodic. Phosphate, chromate and tungstate protect anode. They form sparingly soluble products which are adsorbed on the surface of metal and hence check corrosion. Cathodic protection is done by organic amines, mercaptans, thiourea and substituted urea. The above chemi-cals retard reduction reaction taking place at cathode. Also, by the use of salts of Mg, Zn or Ni, the insoluble hydroxides of Mg, Zn or Ni are deposited preventing corrosion. For protecting corrosion–electrochemically, there are two methods for the protection from corrosion: a) Sacrificial anodic protection: b) Impressed current cathodic protection:
a) Sacrificial anodic protection: Underground steel pipes are protected from corrosion by this method. A magnesium rod is fixed near the metal under protection (Fe) and both are connected with a conducting wire. Magnesium is more positive than iron and, hence, in electrochemical cell it acts as anode and the iron acts as cathode. According to the principle of galvanic cell, it is anode that undergoes oxidation, and, hence, corrosion occurs at anode saving cathode (iron) from corrosion. Thus, magnesium sacrifies itself for saving the iron. Usually, buried pipes and storage tanks made of steel are connected to Mg blocks. Mg protects steel sacrificially; Zn or Mg sheets are hung around the ship hull to protect the steel or iron base of ship by sacrificial protection.
b) Impressed current cathodic protection: This is another method for the cathodic protection of metals. The object or the metal to be protected (the metal is exposed to soil or other electrolyte) is made anode by connecting to an external battery. The battery contains an inert graphite anode which is buried in a back fill to maintain electrical contact. Sacrificial anodic protectionImpressed current cathodic protection
Applying Surface Coatings: The surface of pure metal is homogeneous and it improves corrosion resistance. Corrosion resistance de-pends on the nature of the environment. For example, the corrosion resistance of aluminum is reduced in alkaline environment. Preparation of Surface for Metallic Coatings The surface of the metal is covered with rust, oxide scales, oil, grease, dust, etc. These substances produce porous, discontinuous or non-adherent coatings in order to get uniform, smooth, cohesive and adhering coat-ing. These unwanted substances covering the metal should be removed perfectly before giving protective coating. The methods are as follows: Sandblasting: This method is used for removing oxide scales. It is suitable for large steel surfaces and also useful whenever maximum protection from the coating is required. The process consists in introducing sand (an abrasive) into an air steam under pressure of 25 to 100 atmospheres. The blast is imparted on scales and also causes a certain degree of hardening of the cleaned metal surface. Solvent cleaning: This method is used to remove oils, greases and fatty substances. The organic solvents like naphtha, chlorinated hydrocarbons, toluene, xylene and acetone are used in this method. This is followed by cleaning with steam and hot water containing wetting agents and alkalies. This treatment provides a metal surface, readily wetted by aqueous which is particularly required for electroplating.
Mechanical cleaning: It removes loose rust and other impurities and is done by hand cleaning with bristle brush plus some abrasive (like sand) and detergent. Tools such as knife scapers, wire, brushes, grinding wheels and cutters are also used for removing scales. The remaining dust and loose particles of dirt are then removed by solvent cleaning followed by steam or hot water treatment. Flame cleaning: It involves heating the metal surface with a hot flame to remove moisture and loosely adhering scales. This is followed by wire brushing. Alkali cleaning: This method is used particularly to remove old paint coating from metal surfaces. Triso-dium phosphate along with soap and wetting agents such as caustic soda are used as cleaning agents. This method is followed by rinsing with water and then immersion in an acidic solution of 0.1% chromic acid to remove traces of alkalies.Alkali cleaning method can be made more effective by the application of electric current and making the metal cathodic in alkali medium. Pickling and etching: It is a convenient method to remove scales. It is usually accomplished by immers-ing the metal (except Al) in an acid pickling solution. Aluminum is picked in alkaline solution. Acid pickling of steel is done with dilute sulphuric acid or in cold HCl solution to which some inhibitor is added. For the cleaning of copper, brass or nickel, dilute HNO3 or a mixture of dilute HNO3 and dilute H2SO4is used. If the cleaning operation is carried out in an efficient manner, it provides a clean, smooth surface for elec-trode deposition. Moreover, the deposit obtained is adherent, tough, smooth and bright in appearance
Methods of application of metal coatings: Metallic dipping causes a coat of metal over the base metal. It is of two types, anodic coating and cathodic coating. Functioning of anodic coating. In galvanised steel, Zn serves as anode; while iron of steel serves as the cathode. Therefore, the iron is protected, even if it is exposed, when a part of the zinc coating is scrapped off
PASSIVITY The process in which a metal exhibits higher corrosion resistance is called passivity. When a very thin, invisible and highly protective film is formed on the surface of a metal or an alloy, it is called passivity. This film is insoluble and non-passive. A metal is passive in a certain environment if its corrosion rate is very low. By the change of the environment, the passivity of a metal may change and may become active towards corrosion. The formation of a passive film on the metal surface in determined by the Pourbaix diagram, which depends on the electrode potential and pH of the medium. Low carbon steel does not corrode in conc. HNO3 due to protection effect of passive film. However, in dil. HNO3 it does not form a stable passive film and therefore dissolves steel. Passive film is formed on the surface of aluminium, Cr, Si, Ti in air, water and dilute acids. There is a good corrosion and oxidation resistance of stainless oxide passive layer. A damage of passive film may cause intensive localised corrosion (pitting corrosion). Passive oxide layers are dissolved in electrolytes containing SO4 2– and chloride. Phosphate and chromate ions stabilise passive films, promoting the regain of its defects.
Paints: A paint is a mechanical dispersion of one or more pigments in medium (liquid, non- volatile) drying oil and thinner. When a metallic surface is painted, the thinner evaporates while the drying oil forms a dry pigmented film after oxidising itself. A paint has following qualities: (i) it should be spread easily on the surface, (ii) it should form a tough, uniform and adherent film, (iii) the coating of paint should not crack after drying, (iv) it should have high covering power, (v) it should neither be oxidised nor reduced in environment and (vi) the colour due to point should be shining and stable Constituents of a paint: Pigment: It is a solid substance which is of different colours depending upon the composition, e.g. zinc oxide, white lead, lithophone, titanium oxide (all are white in colour). Red lead, ferric oxide, chrome red are of red colour. Chromium oxide is green, while carbon black is black. Pigments provided (i) strength to point, (ii) protection to the film by reflecting harmful ultraviolet light, (iii) resistance against abrasion/wear, (iv) impermeability to moisture and (v) aesthetic appeal to the paint film. A good pigment should be (i) opaque, (ii) chemically inert, (iii) non-toxic so that there is no bad effect on the health of a painter as well as an inhabitant, (iv) cheap and (v) freely mixable with film forming constituent oils. Drying oil: It is also called vehicle. It is a film-forming constituent. They are glycerides of higher fatty acids (saturated or unsaturated), e.g.Drying oil may be vegetable oil. Drying oil absorbs oxygen and forms peroxides, hydroperoxides, etc. and forms tough, coherent, insoluble and highly cross-linked structure on the surface. They provide toughness, durability, adhesion and water proofness to the film. When paint is applied in the form of a film, the drying oil absorbs oxygen from air and forms peroxides and hydroperoxides at double bonds. These peroxides isomerise, polymerise condense to form tough, elastic, insoluble, infusible polymer film.
Thinner: It is a highly volatile liquid, it reduces viscosity of the paint and increases its elasticity. It increases the penetration power of drying oil. The common thinner is turpentine oil. Other thinners are benzene, mineral spirit, xylol, kerosene, etc. Some other constituents of paint are— driers(oxygen carrier catalyst) helpful in drying, e.g. tungstate, linoleates of Co, Mn, Pb and Zn, turpentine, mineral spirit, naphtha, methylated naphthalene etc. Extenders or fillers: They reduce the cost and increase durability, e.g. barytes (BaSO4), talc, asbestos, gypsum (CaSO4)etc. They serve to fill voids in the film and act as carriers for pigment colour. Plasticisers: They are used to increase elasticity and to minimise cracking, e.g. dibutyl phosphate, tri-cresyl phosphate, dibutyl tartarate etc. Anti-skinning agents prevent skinning and gelling of paint film, e.g. polyhydroxyphenols.
THEORETICAL QUESTIONS 1. What is corrosion? Why do metals corrode? 2. Why most of the metals are found in the ore form and not in the pure form? Explain. 3. Describe the electrochemical theory of corrosion. 4. Show the reactions involved in the hydrogen evolution and oxygen absorption types of corrosion. 5. Describe the factors on which corrosion depends. 6. Discuss various methods of protection corrosion. 7. Discuss the wet theory of corrosion. 8. Explain the rusting of iron with the help of electrochemical theory of corrosion. 9. Explain two methods to prevent from corrosion. 10. Define passivity with an example. 11. Explain the acid theory of corrosion. 12. Why rusting of iron is fast in saline water than in ordinary water?
13. What is differential aeration corrosion? 14. What is meant by passivity? 15. What is cementation? 16. Give the functions of pigments. 17. What is electrochemical series? 18. Explain sacrificial anodic and impressed current cathodic protection. 19. Give the characteristic features of paints and their functions. 20. Differentiate anodic and cathodic protection methods. 21. Differentiate galvanising and tinning. 22. Rusting of iron is quicker in saline water than in ordinary water. Why? 23. What is cathodic protection? 24. State the two conditions for wet corrosion
Multiple-Choice Questions 1. Corrosion is a process of (a) reduction (b) oxidation(c) ozonolysis (d) electrolysis 2. In the wet theory of corrosion (a) dry cell is formed (b) galvanic cell is formed (c) concentration cell is formed(d) none 3. Which one of the following causes corrosion of iron? (a) oxygen (b) hydrogen(c) strong base (d) moisture and oxygen 4. The method in which the base metal is heated with another powdered metal to prevent corrosion is known as (a) electroplating (b) metal spraying (c) pack cementation (d) metal cladding 5. The method to prevent corrosion of iron by zinc coating is called (a) galvanisation (b) electrolysis (c) cathode protection (d) anode protection
6. The rusting of iron is catalysed by (a) Zn (b) Fe (c) Al (d) H2O (H+) 7. A process in which metal is protected from corrosion by dipping it in molten zinc is known as (a) tinning (b) galvanisation (c) cladding (d) electroplating 8. During wet corrosion (a) the anodic part undergoes reduction (b) the cathodic part undergoes oxidation (c) the anodic part undergoes corrosion (d) none 9. The chemical composition of rust is (a) Fe2O3 ⋅ H2O (b) Fe3O4 ⋅ xH2O (c) Fe2O3 ⋅ xH2O (d) Fe3O4 ⋅ H2O 10. When the ratio of anodic to cathodic area decreases, the rate of corrosion (a) decreases (b) increases (c) has no effec
POLYMER Polymers are important engineering materials which have diversified uses in day-to-day life Natural polymers such as polysaccharides, cellulose, starch and proteins have been parts of every living being since the beginning of life. In addition, there are numerous man-made polymers, which have a wide range of applications. The term ‘Polymers’ is derived from two Greek words: ‘poly’ means many and ‘mers’ means unit or part. A polymer is a high molecular weight compound formed by the joining of a large number of small units.
CLASSIFICATIONS OF POLYMER Polymers can be classified in several ways. Based on Structure of Polymers Based on Tacticity (Configuration) Based on Synthesis
Based on Structure of Polymers On the basis of the structures, polymers can be classified into three groups. Linear chain polymer: In these polymers, monomers are joined together in a chain. They have high density and high melting point due to a well-packed structure, e.g. polyethylene and nylon. Branched chain polymer: In these polymers, the chain of the polymer contains branches of monomers,which hinders the close packing of polymeric chains, and, hence, these are less tightly packed in comparison to the linear chain polymers. They have low melting points and less density in comparison to the linear chain. Cross-linked polymers: In the cross-linked polymers, monomers are cross-linked together in all the three dimensions, e.g. bakelite.
Based on Tacticity (Configuration) There are three different types of polymers depending upon the relative geometric arrangement of the functional(side) groups. Isotactic polymer: All functional groups are on the same side of the polymer chain. Syndiotactic polymer: All the functional groups are arranged in regular on alternate sides of the polymeric chain. Atactic polymer: All the functional groups are arranged randomly on both sides of the polymeric chain
Based on Synthesis On the basis of synthesis, polymers are of two types: Addition polymers: These are formed by the polymerisation of monomers without the elimination of atoms or groups. Condensation polymers: These are formed by the polymerisation of monomers with the elimination of small molecules such as NH3, H2O, CH3 and OH.
On the basis of Thermal processing behaviour, polymers may be classified further into two groups- Thermoplastic polymers: Thermoplastics are polymers that soften on heating and become hard on cooling. These are the polymers in which inter-molecular forces of attraction are moderate and there are no cross-links between the chains, e.g. polyethylene, polypropylene, PVC and nylon-66. Thermosetting polymer: On heating, extensive cross-link is formed in these polymers between the polymeric chains and, thus, they become hard, e.g. Bakelite, urea– formaldehyde resin, terylene.
The important Thermoplastics are discussed here. Polyethylene (polyethene): Polyethylene (PE) is one of the most common polymers. It is used for a variety of products, such as polyethylene bottles, polyethylene bags, sheets, dishes and coatings on milk cartons. The widespread use of this polymer is due to the low cost of starting materials. Properties: 1. It is a soft flexible polymers. 2. PE is a good electrical insulator. 3. It is resistant to moisture, O2, CO and UV light. 4. It is resistant to acids, alkalis, salt solution at room temperature. Polyethylene is of two types: (a) Low-density polyethylene (LDPE) (b) High-density polyethylene (HDPE) :
(a) LDPE: It is polymerised at a high pressure of 5000 atmospheres The presence of branches in the polyethylene molecules does not allow it to pack close together, and hence, its density is low (0.91–0.94 g cm–3 and is known as a low density polyethylene. Properties: (1) LDPE has a linear structure with extrusive branching. (2) It has low degree of cystallinity, low tensile strength, stiffness, hardness. (3) It has poor chemical resistance. LDPE is chemically inert and a poor electrical conductor. It is used in films for packing, toys, insulation wires, pipes, etc. (b) HDPE: It is polymerised under 6–7 atmospheric pressure at 60–70°C in the presence of Ziegler–Natta catalyst (AlEt3 + TiCl4) dispersed in an inert solvent. The polyethylene molecules produced by these methods are linear. The ratio of the side chain to the main chain is less than 1/200. Since, linear molecules can pack themselves, the density of these polyethylene molecules is high (0.95– 0.97 g cm–3). HDPE is also chemically inert but it is more stiffer and harder than LDPE. It is not attacked by chemicals. They are used in toys, pipes, bottles and in bags for packing. Because of its excellent insulation properties, it is used for wire and cable coating as well.
Polypropylene:It is prepared by passing propylene through Ziegler–Natta catalyst (AlR3–TiCl4) at 100°C at 10 atmospheres.
Polytetrafluoroethylene (Teflon) (PTFE): It is prepared by the polymerisation of tetrafluoroethylene under pressure in the presence of free radical initiator (benzoyl peroxide). Teflon is also known as fluon. Properties: Because of the presence of highly electronegative fluorine atoms, there are strong attractive forces between different chains in the teflon molecules. This strong attractive force is responsible for high toughness and high chemical resistance towards all chemicals except hot alkali metal and hot fluorine. Engineering applications: It is used in making seals and gaskets, stopcock for burettes and impregnating glass fibre, asbestos fibre, etc. which have to withstand high temperature. It is also used for insulation of electrical items and for making non-sticky surface coating, particularly for cooking utensils. It is used for storing corrosive chemicals.
Polyvinyl chloride (PVC): It is obtained by heating vinyl chloride in an inert solvent in the presence of benzoyl peroxide. PVC is also known as Koroseal. Properties: 1. It is due to the presence of electronegative chlorine atom on alternative carbon atoms, and attraction between different strands takes place due to difference in polarity. Hence,the polymer will have high softening temperature and is very hard. 2. It is colourless, odourless and non-inflammable. 3. It is resistant to weather, chemicals and oil. 4. Its plasticity can be increased by the addition of a plasticizer (ester of phthalic acid). Uses: It is used in making sheets, pipes, raincoats, handbags, table clothes, plastic dolls, floor covering,electrical insulation and coating on electrical cables. It is used in preparing bottles to pack food stuff, water and cosmetics. PVC is of two types: Rigid PVC and Placticized PVC.
RUBBER (ELASTOMER) Rubbers are high polymers capable of returning to their original length, shape or size after being stretched or deformed. Rubber is also known as elastomer. The rubber obtained from natural sources is called natural rubber, and the polymers prepared in the laboratory, which are similar to natural rubber, are known as synthetic rubber. About 200 plants and shrubs have been found to produce latex on tapping. Sapotaceae latex gives trans-isomer of isoprene; it is called Gutta percha. Natural Rubber Natural rubber is obtained from nearly 500 different plants, but the main source from which the commercial natural rubber is obtained is the tree Hovea braziliansis. It is in cis form known as natural rubber. The rubber is obtained from latex (milky sap) collected from the cuts made in the bark of the tree. The latex is diluted to 15–20% and filtered to remove impurities such as leaves, pieces of bark, etc. The obtained latex contains hydrocarbon with impurities such as fatty acids, proteins and resins in an emulsified form. The latex is then coagulated with acetic acid or formic acid. The crude or raw rubber is composed of 95% hydrocarbons, 4% protein and 1% of resins. The latex is diluted to 15–20% and filtered to remove suspended impurities such as bark, leaves, dirt, etc. It is treated with acetic acid or formic acid. The rubber coagulates as white coagulum. It is washed with water and passed between two rollers where rubber comes out as a sheet resembling crepe paper. It is called crepe rubber, which can be dried and used. In the second type of process, the rubber sheets are hung in smoked chamber for 3–4 days at 40–50°C. This rubber is called smoked rubber. Destructive distillation of natural rubber gives isoprene as the main product.
Vulcanisation of Rubber The raw or crude rubber is of very little use because it has very undesirable properties such as low tensile strength, possesses elasticity only over a limited range of temperature and becomes softer, more plastic and sticky on heating and brittle on cooling. Its solubility in organic solvents (such as CHCl3,benzene and petrol) is of advantage for preparing rubber derivatives and adhesive solutions. It swells in water and attacked by acids, bases O2. It reacts with O2 to give epoxide with bad smell. In order to give more strength and more elasticity, natural rubber is heated with sulphur or sulphur compounds at 150°C temperature for a few hours.
The sulphur combines chemically at double bonds of different rubber springs and a cross-linked network is formed. This process is known as vulcanisation of rubber. The vulcanisation process was invented by Charles Goodyear in The vulcanisation can also be accomplished with certain peroxides, gamma radiation and several other organic compounds. The vulcanisation process can be enhanced in the presence of certain organic substances known as accelerators. The common accelerators contain nitrogen, sulphur or both.
Advantages of vulcanised rubber 1. Vulcanised rubber has good tensile strength. 2. The working temperature of vulcanised rubber is enhanced up to 100°C. The temperature range of raw rubber is 10–60°C. 3. The elasticity of vulcanised rubber is very low. 4. It has good resistance to organic solvents. 5. It acts as good electrical insulator. 6. It has low water absorption tendency. 7. It has good impact resistance. 8. It possesses good resistance, i.e., articles made from it return to original shape when load is removed. Application of natural rubber 1. It is used in tank lining in chemical plant for storing corrosive chemical. 2. It is used in the manufacture of tyres, toys, sport items etc. 3. In used as sandwitching material between metal surface to prevent liberation in machine parts.
Thiokol (polysulphide rubber or GR-P): Polysulphide rubbers are the condensation products of ethylene dichloride and sodium tetrasulphide. Polymers | 109 Thiokol (yellow solid) Properties The properties of polymers depend on the length of the aliphatic groups and the number of sulphur atoms present. The polymer behaves like elastomer when four sulphur atoms are present per monomer and it does not behave as elastomer if only two sulphur atoms are present per monomer. Thus, in case of thiokol, four methylene groups should be present in the dihalide to induce elastic properties. Some of the important properties are: (i) Thiokol is resistant to the action of oxygen and ozone. (ii) It is also resistant to the action of petrol lubricants and organic solvents. (iii) Thiokols show outstanding resistance to swelling by organic solvents, but benzene and its derivatives cause some swelling. (iv) Thiokol films are impermeable to gases to a large extent. (v) Thiokols are vulcanised with metal oxides such as zinc oxide. (vi) Thiokol has poor heat resistance and low tensile strength. The odour of thiokol is very bad. It tends to lose its shape under continuous pressure. Uses Thiokol mixed with oxidising agents is used as a fuel in rocket engine. It is used in engine gaskets and other such products that come into contact with oil. Thiokols are used for hoses and tank lining for the handling and storage of oils and solvents
Fibres-: Fibres are thin,long,thread like materials which are extremely long compared to its width, atleast hundred times longer than its width. Fibres can be classified into two groups Natural Fibre = Natural Fibre naturally occurs in plants and animals e.g. wood,silk,cotton Manufactured Fibre = It is further classified in to two groups Regenerated Fibre = These are made from natural fibres.e.g. rayon ( made from cellulose ), cellulose acetate (made from cellulose ) Synthetic Fibre = These are made from chemicals e.g. nylons (polyamide ),terylene, polyester etc
COMPOUNDING OF PLASTICS The pure material obtained from the process of polymerisation is known as polymer. The usefulness of long chain pure polymer is very little. After adding some materials, polymer becomes useful and termed as plastic. These additives improve the workability of plastics and change the properties of plastics. The following types of additives are found to be present in plastic: Fillers: These are substances that increase hardness, tensile strength and the mechanical properties of plastics. Some important fillers are carbon-black (C-black), chalk, china clay, cellulose fillers, metallic oxide (ZnO, PbO, etc.), metal powders (Al, Cu, Pb, etc.), carborundum, quartz, etc. Fillers also reduce the cost of final compounds. Binder (resin): In plastics, binder holds different constituents together. They are generally low-molecularweight materials and withstand high temperature. Plasticisers: Plasticisers are generally small liquid molecules that are chemically similar to the polymer and create gaps between polymer chains for greater mobility and reduced inter-chain interactions. Commonly used plasticisers are camphor, some phosphate esters (dioctyl phthalate), etc.
Stabilisers: Stabilisers are used to improve the thermal stability of plastics, e.g. polyvinyl chloride. At moulding temperature, PVC undergoes decomposition and decolourisation. So during their moulding, stabilisers are used. The commonly used stabilisers are white lead, lead chromate, red lead, etc. Colouring materials: Organic dyestuffs and inorganic pigments are used as colouring materials. They give bright transparent colours. For example, carbon black—black, anthraquinone—yellow and phthalocyanine
Liquid Crystal Polymers Liquid crystal polymers (LCP) are a class of aromatic polyester polymers. They are also known as super polymers. Liquid crystallinity in polymers occurs either by dissolving a polymer in a solvent (lyotropic liquid crystal polymer) or by heating a polymer above its glass transition temperature (thermotropic liquid- crystal polymers). These show the characteristic of ordinary liquid crystals and also retain the properties of polymers
Fabrication of plastic The fabrication of plastic is the manufacturing process of plastic articles. The com-monly used methods for fabrication of plastic articles are welding, extrusion, moulding, etc. Welding: Similar to metal welding, plastic welding also involves the use of heat to melt two or more pieces together. In this method, thermoplastics are used which are unsuitable for adhesive binding. Extrusion: It is a manufacturing process in which plastic material is melted and formed into a continuous profile. This method of fabrication is generally used for the manufacture of pipes, tubes, window frames, rods, etc.
Moulding Process: The commonly used methods are Injection moulding Compression moulding Transfer moulding Blow moulding Injection moulding Compression moulding Blow moulding Transfer moulding
Fibre-Reinforced Plastic (FRP) Fibre-reinforced plastics are a category of plastics that specifically use fibrous material to mechanically enhance the strength and elasticity of plastics. The plastic material without fiber reinforcement is known as matrix. Enhanced strength and elasticity in a fiber- reinforced plastic depends on the mechanical properties of both the fibre and the matrix, as well as the fibre length and orientation within the matrix. Mainly fibre-reinforced plastics are classified into two groups: Carbon fibre reinforced plastics (CFRP) Glass-fibre reinforced plastics (GFRP)
There are various methods for producing fibre- reinforced plastic such as continuous lamination, pultru-sion, rotational moulding, injection moulding, hand lay up, etc. Application of FRP: They find extensive use in space crafts, aeroplanes, acid storage tanks, motorcars, and building materials. Melamine FRP is used for insulation and in making baskets. Advantages of FRP: 1. They are non-inflammable and resistant to corrosion, chemicals, etc. 2. The cost of production of FRPs is low. 3. They possess low coefficient of thermal expansion
Biodegradable polymers Biodegradable polymers are defined as the degradable polymers in which degradation is caused by the action of naturally occurring microorganisms such as algae, fungi, and bacteria. They are easily compostable. During compositing, they yield CO2, H2O, inorganic compounds and biomass at a rate consistent with other compostable materials without leaving toxic residues. They are classified into following groups: Naturally occurring biodegradable polymers Biosynthetic polymers Synthetic biodegradable polymers Some important biodegradable polymers of synthetic polymers are poly alkene esters, polyamide esters, polyvinyl esters, polyvinyl alcohols, polylactic acid and its copolymers and polyanhydrides.
Applications of biodegradable polymers : Packaging:Compostable bags help in the disposal of vegetable matter which gets converted to carbon diox-ide and methane. Agriculture:Biodegradable polymers are synthesized from the materials derived from the processing of crops or from petrochemical feedstock with conventional processing methods, thereby minimizing the cost of raw materials. The problem of landfill solid wastes can be reduced. Medical applications: Biodegradable polymers are used as surgical implants in blood vessels for a long-term drug release and as absorbable surgical sutures for the treatment of eye. Most commonly used biocompatible sutures are made of PGA and PLA and their copolymers are used as a surgical thread. PGA gets absorbed in 15 days. Bone-fixation devices: Biodegradable polymers such as PGA, PLA and polydioxanone are used as bone-fixation devices since they have high compatibility with bone and ligament compared to metal. When metal implants are used, the removal of metal may cause weakness of bone and refractures may occur. But biode-gradable polymers when used for bone fixation allow free movement of ligaments and bones
THEORETICAL QUESTIONS 1. What is corrosion? Why do metals corrode? 2. Why most of the metals are found in the ore form and not in the pure form? Explain. 3. Describe the electrochemical theory of corrosion. 4. Show the reactions involved in the hydrogen evolution and oxygen absorption types of corrosion. 5. Describe the factors on which corrosion depends. 6. Discuss various methods of protection corrosion. 7. Discuss the wet theory of corrosion. 8. Explain the rusting of iron with the help of electrochemical theory of corrosion.
Introduction The term engineering materials is used to include a wide variety of materials used in construction and fabrication. Engineering materials include cementing materials or binding materials such as lime, cement, gypsum and ceramics. Ceramics includes a variety of materials such as glass, refractories, clay products, lubricants, rocket propellants, etc. MATERIALS CHEMISTRY
Cement Cement is a construction material which possesses adhesive and cohesive properties and is used for binding building blocks, bricks, stones, etc. Cements are classified into four types based on the chemical composition. The essential constituents of cement used for construction are the compounds of calcium (calcareous and argillaceous; calcium + silicon). Classification of Cements-Cements are classified into various types as follows: 1. Natural cement 2. Puzzalona cement 3. Slag cement 4. Portland cement
Natural Cement-Natural cement is made by subjecting the argillaceous limestone to calcinations at high temperature and then pulverizing the calcinated mass. Natural cement possesses low strength and hydraulic qualities. It is used in the preparation of mortars and in the construction of dams and foundations Puzzalona Cement-It has been the oldest cement used by Romans for the construction of domes and walls. Natural puzzalona is a deposit of volcanic ash produced by rapid cooling of lava mixed with slaked lime. It is a mixture of aluminium silicate, calcium silicate and silicates of iron. They form hydraulic cementing materials and possess hydraulic properties. Puzzalona cements are not used directly but can be used by mixing with Portland cement for various applications. Slag Cement-Slag cement is obtained by mixing blast furnace slag (aluminium silicate) and calcium, then the mixture is poured into cold water. The granular cement produced is dried and mixed with lime. The mixture is then pulverised to fine powder. Slag cements are slow-setting cements. They can be hardened by adding accelerators such as clay, salt or caustic soda. The strength of slag cement is less. It is mainly used for making concrete for construction in waterlogged area where the tensile strength is less important. Portland Cement -Portland cement is made by the calcination of calculated amount of clay and lime, followed by gypsum for retarding calcination. The setting and hardening properties resemble Portland rock, so it is named as Portland cement. It is a mixture of calcium silicates and calcium aluminates with small amount of gypsum. All Portland cements are hydraulic in nature, which are capable of setting and hardening under water by the interaction of water with the constituents of cement.
Chemical Composition of Cement: Cement contains silica, lime and alumina. The proportion of these constituents in cements should be main-tained to get good quality cements. (1) MgO should be less than 6%. (2) Sulfur content should always be less than 2.75%. (3) The ratio of alumina to iron oxide should be (4) Total loss on ignition should be less than 4%.
Setting and Hardening of Portland Cement The hydration of cement followed by gelation is called setting and the subsequent crystallisation is called hardening. The strength of cement depends on the amount of gelation and the extent of crystallisation. Cement is mixed with water to produce a plastic cement paste. The paste is subjected to hydration and gelation and finally crystalline products are formed. 1. Initial setting of cement involves hydration of tricalcium aluminate. 2. Second step of the reaction involves gelation in which tobermonite gel is formed. It also produces calcium hydroxide and hydrated tricalcium aluminate.
3. Crystallisation of tricalcium aluminate takes place. Even though initial reaction involves the formation of tetracalcium alumiate, hardening of tricalcium aluminate takes place finally through crystallisation. Final setting and hardening of cement paste may be due to the formation of tobermonite gel with the crystallisation of calcium hydroxide and hydrated calcium aluminate. Mentioned below is the flow chart representing setting and hardening of cement.
Complete setting and hardening of cement takes place in 20 days. 1. Hydration of cement takes place in one day 2. Next hydration of C3S is completed in seven days The gel of C3A crystallises and dicalcium silicate (C2S) begins to hydrate in 7–28 days. The strength of cement is increased in 7–28 days due to the gelation of both dicalcium silicate and tricalcium silicate. Initial setting and final setting are recognised based on the ability of the penetration of vicat needle into the mass. Initial set takes place when the needle does not penetrate the paste beyond certain depth; final set takes place when the needle does not penetrate the mass at all. For ordinary cements, initial setting should be less than 45 minutes to allow sufficient time for mixing, transportation and placing between the aggregates. Final setting should not be more than 10 hours. Quick setting cement requires 5 minutes for initial setting and less than 30 minutes for final setting. Hardening involves increasing strength of cement. It depends on the chemical combination of cement and water and occurs faster in the beginning and goes on decreasing gradually. It is a relatively slow process. The heat of hydration of cement constituents is useful in the formulation of cements for different purposes. Information regarding heat of hydration helps in preventing freezing water in the cement paste in winter. It is used in controlling the speed of setting and hardening of cement. The liberated heat should be quickly dis-sipated, otherwise serious stress crackings occur in concrete constructions.
Special Cements 1. Water-proof cement: This cement makes the concrete impervious to water and resist absorption of water. This type of cement is prepared by adding water proofing agents like calcium stearate, aluminum stearate, gypsum and tannic acid to Portland cement during grinding and powdering process. 2. High alumina cement: High alumina cements are made by fusing bauxite and limestone mixture at 1500–1600ºC in rotary kiln and the resulting mass is subjected to grinding.Setting time is similar to portland cement but hardening is very rapid. It also has superior resistance to chemicals. 3. Acid-resistant cements: They are produced by mixing finely ground quartzite with silicon in suitable proportions. Sodium silicon fluoride, or ethyl acetate are used as setting agents; silica gel gets precipitated. On drying, it becomes very porous and permeable to liquids. Hence, it should be always kept wet to prevent shrinking of gel. 4. Barium strontium cements: The calcium in Portland cement is replaced by barium or strontium.Tribarium and dibarium silicates offer more resistance to radioactive rays. Hence, they are used in concrete shields for atomic piles.
Concrete Cement alone cannot be used for construction work, because it is sensitive to moisture and liable to get internal stresses. In order to overcome these, concrete is used. It is a building and structural materials obtained by mixing binding materials (cement or lime), mineral aggregates (sand, crushed stone, gravel, broken bricks, slag, etc.) and water in suitable proportion. It is compact, rigid, strong and durable. If concrete is to be used in beams, pillars, etc., where tensile stresses occur, concrete is reinforced with steel rods, glass fibres, etc. to improve the strength. The resulting cement is called RCC (reinforced concrete construction). RCC possesses greater rigidity, moisture and fire resistance. Its maintenance cost is practically negligible
Decay of Cement (Effect of CO2)-The constituents of cements are likely to be attacked by acids and salts present in water. CO2 in acidic water causes leaching out of pure lime. The above cycle of reaction continues to occur till all the CO2 is consumed. Hydrolysis of silicates and aluminates in the cement also results in the decay of cement. Prevention of Cement Decay The decay of cement can be minimized by coating the surface with epoxy resin paint or using drying oils. As a result, acidic water fails to enter into cement. When alkaline water is carried by sewage pipes, the SO2 component of cement is attacked. To prevent it, the inner surface of concrete pipe is treated with SiF4 which produces insoluble CaF2.
Lubricants When one solid surface is sliding past over another solid surface, friction and wear is developed due to the relative motion of two contacting surfaces which results in the loss of energy as heat. As the equipment gets heated up, it is damaged and sometimes result in welding or seizure. Any substance introduced between the two moving and sliding surfaces with a view to reduce frictional resistance is known as ‘lubricant’. ‘Lubrication’ may be defined as the reduction of friction between two relatively moving surfaces by the interposition of some other substance (lubricant) between them.
Functions of Lubricants 1. Lubricants help in reducing frictional forces between two sliding surfaces. Even highly polished and extremely smooth surfaces show many irregularities and peaks called asperities when viewed under a microscope; when pressed together they exhibit high friction and wear. Even highly polished surfaces can be welded together by the application of pressure. But when a lubricant is placed, the separation of two surfaces takes place and interlocking of peaks and valleys does not occur, and sliding takes place conveniently. 2. It reduces wear and tear and surface deformation. When a lubricant is placed between the two sliding surfaces, direct contact between them is avoided. Without lubricant, small peaks would be sheared which results in wear and tear and surface deformation; excessive wear in machinery or any part result in malfunctioning of entire unit.
3. It prevents the loss of heat energy produced by frictional forces between two sliding surfaces and acts as a coolant. In a machine, frictional heat is produced at the point of contact between the rubbing parts. Cool oil which is flowing on a heated surface carries away the heat. 4. It reduces running cost and maintenance cost of machines and tools. A lubricant prevents corrosion and rusting of machine parts. The rust inhibitor in lubricant protects the machine parts. 5.It acts as a seal. In internal combustion engines, lubricant is used as a seal between the wall of the cylinder and piston and prevents the leakage of gases under high pressure from the cylinder. 6. It prevents the accumulation of dirt and foreign matter entering the bearing.
Classification of Lubricants Lubricants are classified according to the state of aggregation as liquid, semi-solid and solid lubricants. Their use in machinery depends on its working conditions. They contain 90% base (usually petroleum fractions called mineral oils) and less than 10% additives. 1. Liquid lubricants: Lubricating oils are widely used for the lubrication of machine tools. They act as sealing agents, corrosion inhibitor and cooling medium. Majority of them are of petroleum origin; some of them are of vegetable or animal origin. Examples: Animal oils: whale oil, lard oil, tallow oil, seal oil, etc. Vegetable oils: mustard oil, sunflower oil, cotton seed oil, etc.
2. Semi-solid lubricants: When a liquid lube oil cannot be maintained, lubrication is done by a semi-solid lube. Greases are very good semi-solid lubricant. An automotive wheel bearing is best lubricated by grease. A semi-solid lubricant is obtained by combining lubricant oil with thickening agent. They are special soaps of sodium, calcium, baricum, aluminium, etc. Non-soap thicknes are carbon-black, silica gel, poly urea, etc. They have high resistance to friction than oils and can support heavier loads at low speeds. Semi-solid lubricant are used (a) where oil is not suitable for machines, (b) when machines are working with heavy load at low speed and (c) where sealing is required without dust particles and moisture and contamination is not acceptable.
3. Solid Lubricants: In certain aero-space devices and some other environments, liquid and semi-solid lubes cannot be used. Solid lubricates such as MoS2, mica, chalk, wax, soap, graphite, which can with stand heavy load and low speed can be used. They consist of a number of layers of atoms held together by weak van der Waals forces which makes them soft and smooth to act as lubricant. Because of its slippery touch, non- inflammability and resistance to oxidation, graphite is a widely used solid lubricant. It can be used as powder or as colloidal dispersion in water (aqua dag) oil (oil dag). Graphite is used in IC. engines, lathes, and air compression engines, but is ineffective in vacuum conditions and above 370ºC. MoS2 can be used upto 800ºC. It possess very low coefficient of friction. Hence, it can be used in air frame lubrication and wire drawing. Other organic solid lubes are copper phthalocyanine and teflon. Gases: In recent times, gases have also been used as lubes in precious spindles, fans, compressors, etc., because of their low viscosity which is independent of temperature. Hence, the viscous resistance is minimum and there is no effect of variation in temperature on lubrication. There is no risk of contamination of gas if enclosed system is used as lubricant.
Mechanisms of Lubrication Three mechanisms have been proposed to explain the action of lubricants. They are: 1. Thin film or boundary lubrication 2. Fluid film or hydrodynamic lubrication 3. Extreme pressure lubrication 1. Thin film or boundary lubrication: In this type of lubrication, a thin film of lubricant is adsorbed on the surface and held by van der Waals forces. When the lubricant is not viscous enough to generate a film of sufficient thickness for the separation of surfaces under heavy loads, friction is reduced by thin film lubrication. Thin film lubrication is applied when (a) the speed in very low, (b) the shaft moves from rest position to operation, (c) the load in heavy and (d) the oil has low viscosity. Some peaks or asperities may have higher thickness than the film of lubricant which results in wearing and tearing. Hence, the chemical or physical forces on some metal surfaces would avoid the direct contact of metals and absorb a thin layer of lubricating oil. In such conditions, lubricant forms a thin layer and gets adsorbed. Boundary film lubrication
2. Fluid film or hydrodynamic lubrication This type of lubrication is also known as thick film lubri-cation. It is carried out with the help of liquid lubricants. In fluid film lubrication, the two sliding sur-faces are separated by a thick film of about 1000 Å which is applied to prevent direct surface-to-surface contact. Wearing and tearing of metals is minimised. Fluid film lubrication
3. Extreme pressure lubrication It involves chemical action on the part of lubricant Under heavy load and high speed conditions, high local temperature is generated. The liquid film may not stick, it may decompose and vapourise. Hence, special additives called extreme pressure additive are blended with lubricating oils to form more durable film to withstand high temperature and pressure. Chlorinated esters, sulfurised oils and tricresyl phosphates are used as extreme pressure additives.
Flash and Fire Point Flash point is defined as the minimum temperature at which the lubricating oil gives off its vapours that ignite for a moment when a flame is brought near it. Fire point is the lowest temperature at which the vapours of the oil burn continuously for at least 5 seconds, when a flame is brought near it. The flash point of the lubricating oil is above the operating temperature, because a good lubricating oil should not volatalise under the working conditions usually. The fire points are 5 to 40°F higher than flash points. But the flash and fire points do not have any relation with the lubricating property of oil. Flash and fire points indicate the occurrence of fire accident. Before fire occurs flash appears. Flash point apparatus
Application of Lubricants Selection of lubricants for few typical applications is given under. Lubricants for IC engines: In internal combustion (IC) engines, the lubricants are generally exposed to high temperatures. Therefore, they should possess high viscosity index and should be thermally stable. Hence, petroleum oils containing additives which produce high VI and oxidation stability are used in IC engines. Lubricants for refrigerators: For refrigeration, the selected lubricant must possess low pour point, low viscosity and low cloud point. Maximum limit of pour point is –40ºF for lighter grade and –13ºF for heaviest grade oils, and viscosity should be 85–325 say bolt universal seconds at (545) 100ºF. Naphthalene base oils satisfy these conditions and hence are suitable for refrigeration.
Lubricants for very high pressure and low speed: Concrete mixers, lathes, railway joints, tractor rollers, etc. are exposed to very high temperature and move at low speed. A film of oil or grease is difficult lathes, railway joints, tractor rollers, etc. are exposed to very high temperature and move at low speed. A film of oil or grease is difficult to maintain. Hence, a dry power of solid lubricants such as graphite, mica is used. Lubricants for gears: The lubricants used in gears are exposed to extreme pressures, hence thick mineral lubricating oils are used so that they should stick to the gear teeth. The oil also should possess high resistance to oxidation and high tensile strength. Lubricants should possess good lubricating properties, non-corrosive nature, and chemical stability.
Selection of a Lubricant The selection of a lubricant depends on the operating conditions. Many conventional lubricants do not work at lower temperatures, and at higher temperatures, they may break down. The type of lubricating oil suitable for different conditions is given in Table. Table : Types of lubricating oil suitable for different conditions Operating condition Types of lubricant oil High speed, heavy load Extreme pressure lubricants Low speed, heavy load High oiliness boundary film lubricants Less load, high speed Oil with low viscosity Low temperature High fluidity High temperature Oxidation resistance and less volatility
Refractories Refractories are inorganic materials which can withstand very high temperatures without softening or suffering deformation. Therefore, they are used for the construction of kilns, ovens, crucibles, retarts, furnaces, etc. The main function of refractories varies depending on the purpose to which they are subjected like confining heat within the furnace, transmitting or storing heat in regenerators. The selection of a refractory for a particular purpose depends on the service conditions, like working temperature to which it is exposed, the nature of the materials which come into contact with it, temperature fluctuations, load applied and the nature of chemical reactions which occur. Characteristics of a Refractory A good refractory materials should possess (1) high-temperature resistance under working conditions, (2) good abrasion resistance by dusty gases and molten metals, (3) low permeability or ability to contain heat, (4) high mechanical strength, structural strength and crack resistance to withstand overlying load, (5) thermal strength to withstand thermal shock due to rapid and repeated temperature fluctuations and (6) high resistance to change in physical, chemical and mechanical properties. If a given refractory, materials does not have the above-mentioned characteristic properties, it will fail in service.
Factors Affecting refractory materials 1. Chemical reaction with the environment in which it is working. It should be in a chemically similar environment. For example, acidic refractories should not be used in furnace using basic fluxes or slags and vice-versa. 2. Porous refractory materials causes penetration of slag to greater depths and consequently destruction of refractory. It also affects the reactions with gases. 3. Deposition of carbon from carbon monoxide in fireclay refractories in blast furnace is an important cause of its failure. It results in high internal stresses resulting in the destruction of bricks. 4. Rise in temperature beyond safe limit quickly brings about the destruction of refractory. 5. Agitation or movement of the materials also causes failure of refractory. If agitation is on the surface, new surface is quickly exposed to chemical attack and is destroyed. 6. Another important cause of failure of a refractory is spalling. It is the breaking or cracking or peeling of fragments or spalls from the surface of refractory brick. Spalling may be thermal or mechanical or structural. Thermal spalling is due to unequal expansion or contraction caused by difference in temperature at different parts of furnace or by rapid changes in temperature. Mechanical spalling: It is due to carelessness in loading the furnace or due to removal of materials from the furnace thereby damaging the refractory. Structural spalling: It is due to change in composition of the refractory due to reactions with slag. As a result, the coefficient of expansion changes. Spalling of a refractory can be reduced by proper design, construction and operation of the furnace.
Classification of Refractories Refractories are broadly classified into three categories. 1. On the basis of their chemical nature a. Acidic refractories: They are made from acidic materials such as alumina (Al2O3) and silica (SiO2). They are resistant to acid slags, but attacked by basic materials such as CaO and MgO. Examples: Silica, alumina and fire clay refractories (fire clay refractories contain silicate mineral kaolinite. Silicate refractories which rank next to fireclay refractories are produced from quartzite and quartz pebbles). b. Basic refractories: Basic refractories are those which consist of basic materials. They are not attacked by basic materials, but attacked by acidic materials. They find extensive use in steel- making open hearth furnaces. Example: Magnesite, dolomite, chrome magnesite refractories. c. Neutral refractories: They are not completely neutral in chemical sense. They consist of weakly basic/ acidic materials such as carbon, zirconia (ZrO2), chromite (FeOCrO2), graphite and silicon carbide. 2. On the basis of fusion temperature a. Normal refractory: Fusion temperature is 1580–1780oC. Example: Fire clay refractory b. High refractory: Fusion temperature is 1780–2000oC. Example: Chromite refractory c. Super refractory: Fusion temperature is above 2000oC. Example: Zirconia 3. On the basis of oxide content a. Single oxide refractory Examples: Alumina, magnesia and zirconia refractory b. Mixed oxide refractory Example: Spinel and mullite c. Non-oxide refractory Example: Borides, carbides and silicates
Properties of Refractories Chemical inertness: The refractory materials used as lining for furnace should be chemically inert. It should not react with slags, reagents, furnace gases, fuel ashes and products produced inside the furnace. Acidic refractories should not be in contact with alkaline product, vice-versa. Example: Silica bricks being acidic cannot be used in a basic furnace and magnesite bricks being basic can-not be used in acidic furnace. Refractoriness: It indicates the ability of a refractory to bear its own weight at high temperature. The refrac-tory materials should not undergo deformation with increase in temperature. Higher the softening tempera-ture, more valuable is the refractory. Refractoriness is expressed in pyrometric cone equivalents (PCE). The materials whose refractoriness is to be tested is taken in the form of cone and a similar sized standard cone is also taken. Then both the cones are heated uniformly at 10°C per minute. The standard cones possess definite softening temperature. Standard cones are given the number 22 to 42 with increasing softening temperature. When the test cone gets softened and loses its shape, one of the standard cones whose refractoriness is close to that of the test cone will also soften. The serial number of that standard cone is noted and this number is the PCE of test cone. The soften-ing temperature range of this pair is also noted. For example, silica brick has PCE of 33 with a softening temperature range of 1700–1750°C. This indicates that particular refractory can be used in the temperature range of 1700°C.
Thermal expansion and contraction: The expansion and contraction of a good refractory should be negli- gible with change in temperature. Repeated contractions and expansions of a refractory materials will lead to the breakdown of refractory materials. The lower the thermal expansion and contraction, the better the quality of the refractory. Porosity: Combustible materials like sawdust when used as raw materials for making bricks make them po-rous. Certain foaming agents are added to make the refractory porous. The porosity of a refractory is the ratio of its pore volume to the total or bulk volume. Porosity influences the strength of a refractory. If it is highly porous, molten reactants, gases, slags, etc., penetrate and damage the brick. As a result, its abrasion resistance and mechanical strength decreases. Contrary to it, in a porous brick the pores act as an insulator for the furnace. Porosity decreases thermal spalling. Thermal spalling: The breaking, cracking, peeling of a refractory can be reduced by (1) using materials with low porosity and low coefficient of expansion, (2) avoiding sudden changes in temperatures and (3) avoiding overfiring during construction and finishing of internal lining of refractories
NANO MATERIALS: Nano materials are the materials of nano-metre (10–9 m) dimension. The properties of nano materials significantly differ from bulk materials. For example, the electronic structure of metals and semi-conductor. crystals greatly differs from those of isolated atoms and bulk materials. Some of the nano materials like gold particles of 1–2 nm size exhibit unusual catalytic properties. Large pieces of gold and silver show inert (noble) behaviour. Silver is widely used in jewellery because of its inert nature, but it shows anti-bacterial activity at nano scale. Now-a- days nano particles of silver are used in wound dressing. The uniqueness of nano particles is due to two factors: 1. Smaller particles have a relatively large surface area than their volume. 2. Below 100 nm size, quantum effects can change the magnetic and electronic properties in unpredictable manner. Nano-particle-reinforced polymeric materials could replace structural metallic components in the auto-mobile industry. Widespread use of the nano composites could reduce the consumption of 1.5 billion litres of gasoline by the fleet of vehicles in 1 year thereby reducing CO2 emission by about 5 billion kg annually.
Synthesis of Nano Particles: A number of methods have been developed for the synthesis of nano particles. Some of them are: Precipitation: In this method, metal salts are dissolved in appropriate solvent and reducing agents such as alcohols, glycols, metal borobydrides, hydroxy methyl phosphonium chloride are added. For example, gold hydro sols. of 10–640 A size can be prepared by reducing chloroaurate ions in sodium borohydride solution. Semiconductor nano crystals of CdSe, AgI, TiO2, Cus, ZnS, ZnO, etc., are prepared by kinetic control of precipitation. Solvothermal synthesis: In this method, the material is heated in an autoclave at very high temperature i.e. above the boiling temperature of solvent, and a pressure above atmospheric pressure are employed. Nano crystals of CdSe have been prepared by heating a mixture of cadmium stearate and selenium powder in tolu- ene and in the presence of catalyst tetra hydronaphthalene. Microwave solvothermal method can be employed for the synthesis of nano particles of MFe2O4 (M = Mn, Lo, Ni, Zn) by heating ammonical solutions of metal nitrates in the microwave oven for 4 minutes. Thermolysis: In this method, high boiling organic solvents are used to prepare nano crystals. For example, nano crystals of CdSe have been prepared by reacting dimethyl cadmium dissolved in tri-n-octyl phosphine with tri-n-octyl phosphine selenide in hot tri-n-octyl phosphine oxide in 120–300°C range of temperature. The size of the particle depends on the reaction temperature. Larger particles are produced at higher temperatures. Other organo-metallic nano crystals like CdSe, PbSe, InP, ZnSe, GaSn, InSb, etc., have also been prepared by thermolysis. By the thermal decomposition of selected precursors, alloys of controlled composition like Cd 1 – x MnSe have been prepared.
Sonochemical reduction: Ultrasound radiation of 20 kHz to 10 MHz is used for sonication. When a liquid is exposed to ultrasound radiation, bubble formation, collapse and breaking of chemical bond occurs. As a result, nano-sized particles of crystalline or amorphous nature are produced. For example, nano particles of copper were obtained by the sonochemical reduction of copper hydrazine carboxylate in aqueous solution. If argon gas is used for sonication, metallic copper and cuprous oxide nano particles are formed. Vapour-phase reductions: In this method, evaporation of pre-formed semiconductor powders or coevapo-ration of two components like zinc metal and sulphur to produce zinc sulfide nano particles takes place.
Techniques for Characterizing Nano Materials There are some optical and electrical techniques to characterize the nano materials. They are used to study the influence of the optical structural and surface morphological properties of nano materials. A commonly used technique for optical characterization is UV-visible spectroscopy. Optical properties are important for the films. The measurement of transmission or reflection of a sample provides to determine absorption edge. The absorption edge and energy band gap can be determined from the transmission measurement which shifts with the size of the nano particle. The absorption of photons takes place and electrons can be raised when an incident radiation passes through thin film material with energy equal or greater than that of the band gap. The material absorbs little and can transmit the photons with less energy. The ability of a material to absorb photons of a given wavelength is measured quantitatively by the optical absorption coefficient measured in units of reciprocal distance.
Applications of nano materials 1. The potential application of nano particles is in the design of new supercomputers. It includes zero dimensional quantum dots¤, one-dimensional quantum dot, planar arrays of ordered structures, nano scale circuits, etc. 2. Nano particles are potential components in the generation of bimetallic nano structures and nano mechanical device based on DNA. 3. The magnetic nano metal particles are widely used in magnetic separation, magnetic drug transport and magneto-optical data storage. 4. In communication technology, nano wires 20 times thinner and longer than conventional wires are used. 5. Nylon nano composites containing small amount of clay are capable of withstanding high temperature environments and used in automobile air intake covers. 6. Catalysts that are stable at high temperatures and can be used in smaller possible amount have been discovered. For example, Rhodium hydrosols are the effective catalysts for the hydrogenation of olefins in organic phase. The complex oxide barium hexa aluminate BaO3Al2O3 retains its catalytic activity at high temperature. Conventional methods to synthesise the catalyst tend to reduce the catalytic activity by reducing its surface area, Coordinating polymers (polyvinyl pyrrolidone) seem to protect nano metal particles in their catalytic activity towards hydrogenation of olefins and hydration of nitriles. Nano chemical routes catalyse the chemical reactions at much lower temperature, pressure and in a very short period of time
7. A combination of nano materials with enzymes improves the durability of enzymes, create localised high concentration of proteins and reduces cost by minimising losses. 8. In the field of medicine and surgery, nano technology possesses several potential applications. Mutations in DNA could be repaired and cancer cells, toxic chemicals, viruses could be destroyed with the help of nano devices. Sensor systems which detect the emerging diseases in the body would shift the focus from the treatment of disease to early detection and prevention. They can perform tasks inside the body that would otherwise not be possible. For example, nano-scale devices smaller than 50 nm can easily enter most cells, and those smaller than 20 nm can move through the walls of the blood vessels. As a result, a nano-scale device can interact with molecules on both the cell surface and within the cell. 9. Nano technology significantly improves energy efficiency, storage and production of solar cells. Solar cells are expensive and nanometre- sized solar cells provide more energy at a cheaper price. This would reduce the usage of fossil and nuclear fuels. further, the solar fuel is cleaner, safer and environment friendly or ecofriendly.
10. Nano technology can provide a platform for integrating research in proteomics (study of structure and properties of proteins) including the way they work and interact with each other inside cells with other scientific investigations into the molecular nature of cancer. 11. Nano technology also helps in reducing chemical pollution. By providing thorough control of matter, nano technology will enable us to prevent chemical pollution. Any waste atoms could be recycled, since they could be kept under control. Even immense tonnage of excess carbon dioxide in the atmosphere (a green house gas) could be swiftly and economically removed.The smallness of their size coupled with wireless technology facilitates the development of sensors and systems of real-time occupational safety and health managenment. 12. Nano technology based fuel cells, lab-on-chip analysers and opto- electronic devices have potential to be useful in the safe, healthy and efficient design of work itself. In fuel cells: Hydrogen fuel cells being non-polluting are the great source of power. Nano materials are used as catalysts in hydrogen storage systems. Nano particles of platinum are used in hydrogenation. Nano technology is used to create more efficient ultra-thin hydrocarbon membranes which allow to build light-weight fuel cells. These membranes allow only hydrogen ions to pass through, but not those of oxygen.
Carbon nano tubes Carbon nano tubes are capable of storing hydrogen in fuel cell-powered cars. They absorb hydrogen just like sponge absorbs water and hold until it is needed. They can hold about 8% by weight of hydrogen. Conventionally, hydrogen is stored as compressed gas or as a liquid. To power a car, a large storage tank to store hydrogen is required. Carbon nano tubes are efficiently used for the purpose. Carbon nano tubes are large molecules of pure carbon that are long, thin and thousands of nanometres long. Nano tubes are 100-times stronger than steel. Single-walled carbon nano tube is 1 of nm diameter and multi-walled carbon nano tube is of 10 nm in diameter. Carbon nano tubes are prepared by laser evaporation method. In this method, graphite is mixed with iron and nickel catalyst and heated to about 1200°C and then the material is irradiated with laser. Carbon Nano tubes are allotrope of carbon made of graphite carbon with unique electrical, electronic and mechanical properties. They found wide application in the field of electronics, information technology and medical fields. Carbon nano tubes are large molecule of pure carbon that are long and have tube-like structure. The diameter ranges from 1–2 nm and 100–1000 nm long. They are hundred times stronger than steel.
There are two types of nano tubes: 1. Single-walled CNT with diameter 1 nm 2. Multi-walled CNT with diameter 10 nm Carbon nano tubes exhibit interesting chemical and physical properties with a wide range of potential applications. The CNTs are ameanable for opening the closed end and various substances can be filled. They can be derivatised through various chemical methods. For example, a multi-walled CNT can be opened by boiling them in concentrated HNO3 and it can be filled with Ni or Pt or with gases like H2. Halogens, amines when filled in CNT make it soluble in organic solvents. Electrical properties: They vary between semi-conducting to metallic depending on the diameter and chirality of the tube. Metallic nano tubes have very high electrical conductivity (about billion amps/cm2) and thermal conductivity since they have negligible amounts of defects which scatter electrons and they have very low resistance. Mechanical properties: These are quite unique. CNTs are stiffest and strongest materials in terms of elastic modulus and tensile strength which is about 20 times that of steel. But because of their hollow structure they undergo buckling under compression, bending stress. Because of their high electrical conductivity, they are poor transmitters of electromagnetic radiation and applied as light weight shielding materials for protecting electronic equipment against electromagnetic radiation. They act as storage devices for storing H2 gas. A CNT can hold 6.5% H2 by weight. For every six carbon atoms of CNT, one lithium atom can be stored. Hence, CNTs can be used to store charged carrier lithium in lithium batteries. By using nano tubes filters, it is possible to filter out bacteria and viruses from water. Only few potential applications are there for inorganic nano tubes. The nano tubes of MoS2 and WS2 act as semiconductors while those of NbS2 and NbSe2 are metallic in nature with NbSe2 a super conductor at lower temperatures. Nano tubes are resistant to oxidation, chemical attack. Thermal conductivity of CNTs is two times higher than diamond, whereas electric conductivity is 100 times higher than copper.
Fullerenes Fullerenes are the classic three-dimensional carbon nano materials. The most common and most stable fullerene is buckminster fullerene, a spheroidal molecule which resembles soccer ball consisting of 60 carbon atoms. (Buckminster fullerene is the most abundant cluster of carbon atoms found in carbon soot.) Other fullerenes that contain 70, 76, 84, 90, 96 carbon atoms and those that contain 180, 190, 240 and 540 carbon atoms are also available. The soccer ball-like molecules are prepared in helium by passing about 150 amps through a carbon rod and extracting the soot with benzene. The resulting solution contains C60 and C70, i.e., the systems with appropriate material inside the fullerene ball are conducting and are of particular interest. For example, the hollow structure can accommodate a drug molecule, while outside the ball is resistant to interaction with other molecules in the body. Thus, they act as safe containers of drug. They can enter cancer cell without reacting with them. A common method of producing fullerene is to send a large current between two nearby graphite electrodes in an inert atmosphere. The resulting carbon plasma arc between the electrodes cools into a residue is soot from which many fullerenes have been isolated.
Use of Fullerenes 1. They find wide application in the field of drug delivery system, optical devises. 2. Fullerenes have been used to produce tiny diamonds and thin diamond films. 3. They find use in the preparation of super-conductors, electronic devices, micro-electric devices, soft ferro- magnet. 4. They inhibit the activity of HIV and used in medicine for the treatment of AIDS. 5. It leads to the discovery of new coating, lubricants, catalysts, electronic and optical devices. 6. They find wide applications in preparing conducting films, alkali, metal-doped fullerenes and super conductor.
Quantum Dots They are also known as artificial atoms. Quantum dots are semi-conductor with quantum confinement properties in all three dimensions. They measure from 1–10 nm. They display any chosen colour in UV region which helps to develop multi colour lasers. Quantum dots may be metallic such as gold, chalcogenised compounds, e.g., CdSe2, PbSe, CdS, etc. Quantum dots find potential application in photo voltaic cell. They absorb photon from solar radiation and release electrons to generate electricity. They find use in cellular imaging and light emitting diodes.
Insulators Insulators are the substances which are capable of retarding the flow of heat or electricity through them. Insulators are broadly classified into two types: o 1. Thermal insulators o 2. Electrical insulators or dielectrics Thermal Insulators The substances having extremely low thermal conductivity are known as thermal insulators. Thermal insulators are mainly used to prevent the loss of heat which takes place by conduction, convection and radiation. Thermal insulators are normally employed when the flow of heat has to be stopped from outside environment to the equipment which has to be operated at low temperature and when flow of heat is to be stopped from furnace to the outer environment. Examples: include, refrigerators, cold storages, furnaces, ovens, boilers, steam carrying pipes, etc. A good thermal insulator should have large number of fine pores instead of few large pores (which causes heat transfer by convection). Because of the low thermal conductivity, air and gases entrapped in pores act as good thermal insulators. The surface of a thermal insulator should be waterproof and pores should be closer, because if thermally insulating air in pores is replaced by thermally conducting moisture, the thermal conductivity increases enormously. Thermal insulators are classified into two types on the basis of suitability of temperature and range of insulator. They are organic (suitability upto 150°C) and inorganic (suitable for high temperature, i.e., above 150°C). Examples: Wool, leather, silk, wood pulp, saw-dust, cotton wool, cattle hair, charcoal powder, etc. : Asbestos, glass wool, hydrous calcium silicate, mineral wool. They can withstand 400–1100°C temperature
Characteristics of a Good Thermal Insulator 1. Its thermal conductivity is extremely low. 2. It should be waterproof and fireproof. 3. It should be chemically stable to the surrounding conditions of high temperatures. 4. It should have low density. 5. It should be mechanically stable and capable of bearing the load applied on it during working. 6. It should be odourless during use. 7. Cost of the thermal insulator should be low.
Electrical Insulators Electrical insulators are those materials through which electrical charges cannot pass through. All the electrons are bound to the parent molecule in an insulator, and there are no free electrons for conduction. They are non-metallic materials of high specific resistance. Dielectrics are the materials whose main function is to store the electrical charges, i.e., they prevent the loss of electricity through certain parts in an electrical system, whereas the insulators are those materials through which electrical charges cannot pass through. Characteristics of Electrical Insulator 1. The electrical charges cannot pass through the insulator. 2. They possess low conductivity, i.e. high resistivity (109–1020 ohm-cm at room temperature). 3. They should have low thermal contraction and expansion. 4. They should be resistant to chemicals, solvents, acids, alkalis, oils and other organic solvents. etc. 5. Dielectrical losses should be minimum for an ideal insulator. The dielectric losses are causes by (a) the absorption of electrical energy. (b) the leakage of current through the material. 6. The absorption of electrical energy results in dissipation of electrical charge in the insulator. 7. At high temperature, the leakage of current takes place through the insulating materials due to conduction. 8. The electrical properties of the materials should not be altered by change in chemical composition. 9. They should have low dielectric constant. Dielectric constant is the ratio of the capacitance of a capacitor containing dielectric materials to the capacitance of the capacitor with vacuum as the dielectric. 10. A good dielectric materials should have low porosity. With increase in porosity, the moisture-holding capacity increases which adversely affects the electrical properties.
Superconductors Superconductivity was invented by kamerlingh Onnes in 1911 during his experiments on electrical resistivity of metals and alloys at sufficiently low temperatures. With decrease in temperature, the electrical resistivity of many metals decreases. The phenomenon in which the electrical resistively of the materials suddenly falls to nearly zero when it is cooled to a very low temperature is known as ‘superconductivity’ and the materials under this condition is called ‘superconductor’. In order to study the phenomenon of superconductivity, mercury was chosen and it was purified by repeated distillation. With decrease in temperature, the resistivity is lowered, and at 4.2 K it becomes zero and the materials become superconductor. The temperature at which the materials undergoes a phase transition from a state of normal conductor to superconductor is known as critical temperature or transition temperature. Above critical temperature, it exhibits normal conductivity. Such materials in their superconducting state are diamagnetic and repel the magnetic field. The phenomenon of exclusion of magnetic field is known as Meissner effect, which is useful in determining the critical temperature of superconductors. When a magnet approaches a superconductor, it induces a current in the superconductor. Since there is no resistance to current, it induces its own magnetic field.
Application of Superconductors 1. Some superconductors act as excellent catalysts for industrial processes. For example, YBa2Cu3O7 – x and related cuprates act as catalysts in oxidation or dehydrogenation reactions. 2. La2 – x SrxCuO4 is a good sensor for alcohol vapours. On contact with certain gases, the electrical resistivity of superconductor changes sharply. 3. Because of their speed of operation and efficiency, they replace the conventional doped metallic conductors and alloy conductors in electronic circuits. Superchips made of superconductors for computers. These can function 1000 times faster than silicon chips. 4. Superconducting magnets capable of generating high fields with low power consumption are being employed in scientific tests and research equipments. 5. Because of their small size and less energy consumption, superconductors are used in magneto-hydrodynamic power generators to maintain plasma.
6. Superconductors are used in magnetic resonance imaging (MRI) in the field of medicine as a diagnostic tool. Based on the production of cross-sectional images, any abnormalities in body tissues and organs can be detected. Magnetic resonance spectroscopy is used in the chemical analysis of body tissues. 7. Superconducting cables can be used to transmit electric power over long distances without resistive losses. 8. High efficiency separating machines are built using superconducting magnets, which are also used to separate tumour cells from healthy cell by high-gradient magnetic separation method. 9. They can be used as a memory or storage device in computers, since the current can flow without any change in its value with time in a superconductor. 10. In Japan., superconducting magnets have been used to levitate an experimental train above its track and can drive it at a great speed of 500 km/h with minimum expenditure of energy. A similar magnetic propulsion system may be used to launch satellites into orbits directly from the earth without the use of rockets. 11. Using superconducting elements, one can built up an extremely fast and large scale computer in compact size. It consumes less than 0.5 W power.
Theoretical Questions 1. What is the average composition of cement. What are the raw materials used in the manufacture of Portland cement. 2. How Portland cement is manufactured? Give a detailed flow diagram. 3. Explain setting and hardening of cement. 4. Give an account on the classification of cement and the composition of each type. 5. Give physical and chemical reactions involved in setting and hardening of Portland cement. 6. (a) Explain the manufacture of cement in detail. (b) Write notes on setting and hardening of cement. 7. Write chemical reactions that take place during setting of cement and explain. 8. What is meant by lubricant? What are the functions of a lubricant? and prime requisites of lubricant 9. Define and explain the cloud point and pour point of a lubricant with a neat diagram. 10. State and explain different mechanisms of lubrication with examples. 11. What are extreme pressure additives? Why these additives are used for lubrication? 12. What is the significance of flash and fire points of a lubricant? How are they determined experimentally?
13. What is the importance of viscosity and viscosity index of a lubricant? 14. How are refractory materials classified? Give examples of each. 15. What are basic refractories and how these differ from acidic refractiories? 16. What are the characteristics of a good refractory? 17. (a) What are the causes of failure of refractory? (b) Which of the following is neutral refractory: (i) fireclay bricks (ii) silica bricks 18. What is spalling? Write different types of spalling of refractory. 19. What are the insulators? How are they classified? 20. Mention the characteristics of thermal and electrical insulators? 21. What are super conductors? how YBa2Cu3O7 – x is prepared? 22. Write the engineering applications of thermal and electrical insulators? 23. What is super conductivity? Explain the phenomenon of super conductivity. 24. Discuss the important applications of super conductors. 25. Differentiate dry and wet process of Portland cement. 26. Discuss the basic principle of lubrication. 27. Describe the following properties of refractories: (1) RUL test and (2) refractoriness. 28. What is meant by lubrication? Describe the theories of lubrication. 29. Distinguish between hydrodynamic lubrication and boundary lubrication. 30. What are nano materials? How are they synthesized? 31. Write about the applications of nano materials.
Multiple-Choice Questions 1. The raw materials for Portland cement are (a) Lime stone and gypsum (b) Lime stone and alumina (c) Lime stone, clay, gypsum and coal powder (d) clay and lime stone 2. Lubricant used in machines working at low temperature should have (a) high pour point (b) high flash point (c) low flash point (d) low pour point 3. Lubricants are used to (a) reduce corrosion (b) reduce wearing (c) reduce seizure (d) all the above 4. For liquid lubricants, (a) flash point > pour point (b) flash point > fire point (c) flash point = fire point (d) flash point < fire point 5. Machine working at high temperature, and loads are lubricated by (a) liquid lubricant (b) mineral oil (c) solid lubricant (d) grease 6. With increase in temperature, the viscosity of a lubricating oil (a) decreases (b) increases (c) remains un altered (d) first increases and then decreases 7. A good example for fluid film lubrication is (a) Hydrocarbon oil (b) Coconut oil + Kerosene(c) Graphite (d) MoS2
8. The lubricant used in boundary lubrication is (a) Graphite (b) MoS2 (c) Oil suspension of both (d) All the above 9. Dolomite is an example of ____________ refractory. (a) aidic (b) basic (c) neutral (d) amphoteric 10. Thermal spalling is caused due to (a) unequal expansion and contraction of furnace due to difference of temperature (b) the difference in design of furnace (c) carelessness in loading of furnace (d) none of the above 11. Silica and alumina are the examples of (a) basic refractories (b) acidic refratories (c) neutral refractories (d) none of the above 12. Which of the following is a neutral refractory? (a) Alumina (b) Magnesite (c) Silica (d) Silicon carbide 13. A good refractory materials should have (a) thermal strength to withstand high temperature (b) high mechanical strength (c) crack resistance (d) all the above 14. The porosity of insulators should be (a) high (b) low (c) moderate (d) cannot be predicted
40. Which of the following is a neutral refractory? (a) Graphite (b) Carborundum (c) Both (a) and (b) (d) Diamond 41. The gas used in electric transformers in insulation is (a) N2 (b) CO2 (c) SF6 (d) Air 42. Major component of Portland cement is (a) CaO (b) MgO (c) 3CaO◊SiO2 (d) SiO2 43. Which of the following constituents of cement contain least heat of hydration? (a) C2S (b) C4Af (c) C3A (d) C2A 44. In the final stage of setting of cement, clinkers are mixed with gypsum (a) to increase the rate of setting (b) to reduce the rate of setting (c) to make the cement soft and porous (d) to make the cement more brittle 45. Tobermorite gel is chemically (a) hydrated tricalcium aluminate (b) hydrated tricalcium silicate (c) hydrated tricalcium silicate (d) slaked lime 46. Which of the following constituent of cement has least setting time? (a) Dicalcium silicate (b) Tricalcium silicate (c) Tricalcium aluminate (d) Tetracalcium aluminate 47. The high percentage constituent in cement is (a) tricalcium aluminate (b) dicalcium aluminate (c) dicalcium silicate (d) tricalcium silicate 48. The chemical formula for lime stone is (a) MgCO3 (b) CaCO3 (c) Na2CO3 (d) Li2CO3 49. The Chemical formula for gypsum is (a) MgSO4◊H2O (b) CaSO4◊2H2O (c) CaCl2 (d) CaCO3 50. An inorganic material that can withstand high temperatures without softening or suffering any deformation in shape is called (a) cement (b) refractory (c) glass (d) chalk
WATER INTRODUCTION Water is a natural gift on the earth. It is essential for humans, animals and plants. Human beings use water for drinking, cooking, bathing, cleaning and washing. Chemically, water consists of two atoms of hydrogen and one atom of oxygen and hence have the molecular formula H2O. In several chemical reactions, water is formed along with the main product, e.g., acid reacts with a base to give salt and water. Alcohol and organic acids react to give ester and water. Water is found in three physical states: liquid (water), solid (ice) and gas (vapour). The freezing point of water is 0°C and the boiling point is 100°C. Several special behaviours found in water are due to the hydrogen bond present in it. In nature, water is present in abundance. About 96% of water forms the oceans, 2.2% forms the ice and snow, 1.2% forms the groundwater and only 0.003% of water is useful for human consumption. It is therefore essential to use available water carefully and economically, the rest forms the surface water and the water present in the atmosphere. The various sources of water are rivers, tanks, reservoirs, seas, oceans, w ells, tubewells, rain, glaciers, etc.
Hardness of Water Water is a very good solvent and even called as the universal solvent. Most of the inorganic salts are soluble in water. The water that has calcium and magnesium salts dissolved in it causes hardness of water. Water passes through rocks and flows on the ground. The calcium and magnesium salts that are present in the rocks are dissolved in water and make it hard. Hard water does not give lather with soap, while soft water easily gives lather with soap. Hence, the hardness of water is its characteristic which resists the lathering with soap. Chemically, it can be understood as follows: A soap is a sodium salt of higher fatty acid such as stearic acid (C17H35COOH). Soap in the absence of Ca2+ and Mg2+ gives lather with water easily, but in the presence of Ca2+ and Mg2+ reacts with them and forms insoluble soap that appears as precipitate (formation of scum). Reaction
Disadvantages of Hard Water In domestic use (a) Washing: When hard water is used for washing purposes, it does not produce lather freely with soap; rather it produces sticky precipitates of calcium and magnesium soaps. The precipitation continues to take place till all calcium and magnesium salts in water are precipitated. After that the soap gives lather with water. As a result, wastage of soap takes place and in the presence of iron salts it may cause staining of cloth. (b) Bathing: Hard water forms sticky precipitate with soap on bath tub and body. Thus, cleaning quality of soap is reduced and a lot of it is wasted. (c) Cooking: Due to the presence of dissolved hardness producing salts, the boiling point of water is elevated. Hence, more fuel and time are required for cooking. The dissolved salts are deposited as carbonates on the inner walls of the utensils. (d) Drinking: Hard water causes bad effect on our health. Calcium forms oxalate crystals in urinary tract.
In industrial use (a) Textile industry: Hard water produces precipitates of calcium and magnesium with soap. The fabrics when dyed later on do not produce exact shades of colour. Iron and magnesium salts containing water produces coloured spots on the fabric. (b) Sugar industry: Water containing sulphates, nitrates, alkali carbonates if used in sugar refining causes difficulties in the crystallization of sugar. Moreover, the sugar so produced may be deliquescent. (c) Dying industry: The dissolved calcium, magnesium and iron salts in hard water react with costly dyes forming undesirable precipitates, which yield impure shades and give spots on the fabric being dyed. (d) Laundry: Hard water causes wastage of soap. Iron salts cause colouration of clothes. (e) Pharmaceutical industry: Hard water if used for the preparation of pharmaceutical products such as drugs injections, ointments may produce undesirable products in them. f) Paper industry: Iron salts affect the colour of the paper.
Types of Hardness Hardness in water is of two types: (i) Temporary hardness and (ii) Permanent hardness. Temporary hardness is due to the presence of bicarbonates of calcium and magnesium that can be removed by boiling. On boiling, soluble (Ca +2 /Mg +2 ) (HCO3)2 is converted into insoluble carbonate that can be removed by filtration. Permanent Hardness Permanent hardness cannot be removed by boiling. It is due to the dissolution of CaCl2, CaSO4, MgCl2 and MgSO4 in water. These salts cannot be removed by boiling. They are removed by different other methods. Fe +3, Al +3 and Mn +7 also cause hardness in water but they are rarely found in hard water.
Meaurement of Hardness To estimate hardness, it is essential to know the various units to measure it. The hardness of water is measured in terms of CaCO3 because it is highly insoluble in water and also its molecular weight is 100 that makes the calculation easier. Units of Hardness Parts per million (ppm): It is the number of parts of equivalent CaCO3 per 106 part of water. For example, 50 ppm hardness means 106 parts of water has 50 parts of equivalent CaCO3. Milligram per litre (mg/L): It is the number of milligrams of equivalent CaCO3 per litre of water. One milligram per litre hardness means 1 mg of equivalent CaCO3 per litre of hard water. 1 kg of water = 1000 × 1000 mg = 106 parts 1 ppm = 1 mg/L Degree Clarke: It is the number of grams of CaCO3 per gallon of water, i.e., the number of parts of calcium carbonate equivalent hardness per 70,000 part of water. Hardness 1°Clarke (1°Cl) means 1 gram equivalent CaCO3 present in 1 gallon of hard water. Degree French: It is the number of parts of CaCO3 per 105 parts of hard water. Hardness 1°French (1°Fr) means 1 part of equivalent CaCO3 per 105 parts of hard water. Relationship among units of hardness: 1 ppm = 1mg/L = 0.1°Fr = 0.07°Cl Any water sample with hardness less than 150 ppm is good and potable, while beyond 350 ppm is not suitable for consumption.
Methods to Determine Hardness: Hardness of water is determined by the EDTA method, which involves the complexometric titration. EDTA is a strong complexing agent. EDTA (ethylene diamine theoretic acid) is a strong complexing agent whose structure is given as As such it is not very soluble in water; hence, disodium salt of EDTA is used in complexometry. Principle: The EDTA solution is standardized with a standard solution of calcium carbonate, prepared by dis-solving a known weight of calcium carbonate in dil. HCl and then making up the solution to a known volume with distilled water. The permanent hardness of water can be determined by titrating the water after boiling well to remove the temporary hardness as carbonates of calcium and magnesium. When hard water is treated with EBT at pH 10, Ca2+ /Mg 2+ ions form unstable wine red coloured complex. When it is titrated with EDTA under similar conditions, EDTA extracts metal ions from the complex and forms a stable colourless complex and releases EBT into the solution. Hence, the colour of the solution changes to blue at the end point
EDTA forms complexes with different metal ions at different pH values (Fig below). Calcium and magne- sium ions present in water form complexes with EDTA in a buffer solution of ammonium chloride and ammonium hydroxide at pH = 10 Metal–EDTA complex
Alkalinity of Water: The presence of anions, such as CO3 –2, HCO3– and OH–, results in alkalinity of water. The estimation of alkalinity in water is done by titrating water sample against standard acid using phenolphthalein and methyl orange as indicators. In this titration, two indicators are used as the different anions give end points at different pH values. Out of the three anions CO3 –2, OH – and HCO3 –, any two of them can exist in water together.
Experimental Procedure: 100 ml of water sample is taken in a flask and two drops of phenolphthalein (pH > 10) is added to it. The colour will become pink due to pH > 10. This water sample of pink colour is titrated against N/50 HCl solu-tion. At the endpoint, pink colour disappears. This endpoint is termed as P-endpoint. Now add two drops of methyl orange indicator to the same water (pH falls below 7). At the endpoint, pink colour reappears. This endpoint is M-endpoint (methyl orange endpoint). REACTIONS: Calculation At the phenolphthalein endpoint (at P ml), the alkalinity thus calculated is phenolphthalein alkalinity in terms of CaCO3 and it is given by The alkalinity of methyl orange (at M ml) is given by
Conclusions A summary of the conclusions is given in Table Table: Alkalinity of water in terms of phenolphthaleine (P) and methyl orange (M) indicators
Boiler Feed water In several industries, boilers are used to produce steam. The water used for making steam must have the following Characteristics: 1. The pH value should be nearly 7. 2. It should have very low hardness (less than 0.2 ppm). 3. Caustic and soda alkalinity should not exceed 0.45 ppm and 1.00 ppm, respectively. Boiler feedwater is treated to avoid (i)carry over (priming and foaming), (ii) caustic embrittlement, (iii) boiler corrosion, and (iv) scale and sludges formation.
Carry Over The phenomenon of carrying of water along with impurities by steam is called carry over. This is due to priming and foaming. Carry over is undesirable because it decreases the efficiency and results in some practical difficulties in the successful operation of the boilers. Foaming of water makes it difficult to know the exact level of water in the boiler. Water takes grit along with it at high velocity and damages the walls of the cylinder. A considerable amount of heat is wasted by the removal of hot boiler water. Priming and Foaming During functioning of the boilers, some water droplets pass with steam. This process of wet steam formation is called priming. Priming in boilers is due to (i) the presence of the suspended and dissolved solids, (ii) high of water in boilers, (iii) faulty design of boilers and (iv) sudden steam demands. Formation of priming reduces the efficiency of the boilers and causes damage to the system. Priming can be controlled by 1. proper evaporation and using adequate heating surfaces. 2. controlling rapid change in steam velocities. 3. proper design of boilers by maintaining low water levels in the boilers. 4. filtering water before feeding to boilers. 5. by blow down.
Formation of stable, persistent foam or bubbles at the surface tension of water in the boiler is called foaming. Foaming is due to the presence of oil or alkali in boiler feed water. The oil and alkali react with water to form soap, as a result the surface tension of water decreases. More foaming will cause more priming. Oil is introduced into the boiler through lubricants. It results with the formation of wet steam that harms the boiler cylinder and turbine blades, etc. Foaming is due to the presence of oil drops, grease and some suspended solids. Silicic acid and aluminium hydroxide are used as clarifying agents to minimise foaming. Oil can be removed by sodium aluminate or alum. It can also be removed by cataphoresis. Polyamide and castor oil also act as anti-foaming agents. Thus, foaming can be prevented by 1. adding anti-foaming agents which reduces the surface tension, e.g., castor oil, prevents foaming. 2. blow down, removing the concentrated boiler water and replace the water by fresh feed water.
Caustic Embrittlement It is the brittlement of the boiler due to the increased concentration of caustic alkali in boiler water. It is a type of boiler corrosion caused when excess of caustic alkali is present in water. In high-pressure boilers, sodium carbonate is hydrolysed to yield NaOH. The NaOH thus formed concentrates after long use. It causes inter-granular cracks on the boiler walls, especially at the stress points. The concentrated alkali is dissolved iron as sodium ferroate (NaFeO2) which enter through minute cracks and causes brittlement of the boiler parts, especially at the bends, joints and rivets, even causing failure of boiler. The formation of cracks in the boilers due to increased concentration of NaOH is called caustic embrittlement. Caustic cracking can be explained by the following concentration cell. i.e., iron at bends and joints is in contact with concentrated alkali and causes corrosion. The iron at plane surface is in contact with dilute alkali and acts as cathode.
Caustic embrittlement can be prevented by 1. using Na3PO4 as softening reagent instead of Na2CO3 in external treatment of boiler water. 2. adding tannin, lignin to boiler water, which blocks the hair cracks in the boiler walls. 3. adding Na2SO4 to boiler water to prevent caustic cracking in boiler. At different pressures, the proportions of sodium sulphate to NaOH as follows: 4. neutralizing alkali with very small quantity of acid
Sludge and Scale Formation In industries, boilers are continuously used for stream generation. As a result of continuous evaporation of water, the concentration of dissolved salts increases. When the ionic product of the salts exceeds the solubility product, the salts are thrown out of boiler. These salts may get deposited on the inner walls of the boiler to form scales or they may float on boiler water as loose and slimy precipitates called sludges. Scales are hard and sticky deposits formed on the inner walls of the boiler. They are very difficult to remove. Scales are formed due to the presence of MgCl2, CaSO4, Mg(OH)2, Ca(OH)2 and silica in water. Formation of Scales Decomposition of Ca(HCO3)2: Ca(HCO3)2 decomposes to an insoluble calcium carbonate salt. In low-pressure boilers, calcium carbonate causes scale formation. In high pressure boilers, it becomes soluble. Hydrolysis of magnesium salts: Magnesium salts dissolved in water undergoes hydrolysis and form a precipitate of Mg(OH)2. Presence of silica: Silica present in water gets deposited as CaSiO3. This is sparingly soluble in water. Decomposition of CaSO4: It is soluble in cold water but solubility decreases with rise in temperature. At high temperature in boilers, CaSO4 gets deposited as hard scale which is very difficult to remove. The chemicals found in the sludges and scales are CaSO4, Ca(OH)2, Mg(OH)2, carbonates of Ca2+ and Mg2+, phosphates and silicates.
Disadvantages of scale formation: 1. Wastage of fuel: Scales have poor thermal conductivity so the rate of heat transfer is reduced. In order to provide a steady supply of heat to water, overheating is done and this causes increase in fuel consumption. 2. Lowering of boiler safety: Due to overheating, to maintain steady supply of heat, the boiler material becomes soft and distortion of boiler tubes takes place. 3. Decrease in efficiency: Scales may get deposited in the valves and condensers of the boiler and choke them there by decreasing the efficiency. 4. Danger of explosion: When thick scales crack due to uneven expansion, water suddenly comes in contact with overheated walls and a large amount of steam is formed instantaneously. Sudden high pressure is developed and causes explosion of the boiler. Removal of scales: Scales can be removed by mechanical and chemical methods. 1. Mechanical methods: If scales are loosely adhering, they can be removed with a scraper or a piece of wood or wire brush. They can also be removed by blow down. If the scales are brittle, thermal shocks are to be given. 2. Chemical methods: If scales are hard and adhering, they can be removed by dissolving in chemicals. Calcium carbonate scales are dissolved in 5−10% HCl. Calcium sulphate complex is highly soluble by adding EDTA. Ca–EDTA complex is highly soluble in water.
Formation of Sludges Sludges are loose and slimy precipitates. They do not stick to the walls of the boiler. They are formed by substances having more solubility in hot water. Examples, magnesium carbonate, magnesium sulphate, magnesium chloride, calcium chloride form sludges. The solubility of these salts increases with increase in temperature. Sludges are formed at comparatively colder portions of the boiler such as bends, joints, rivets, etc. Disadvantages of sludges: 1. Sludges are poor conductors of heat. They tend to waste a portion of heat generated and decreases the efficiency of boiler. 2. Sludges settle in regions of poor water circulation such as pipe connections and plug openings, thereby causing chocking of the pipes.
Sludges and scales in boiler
Softening of Water The process of removing the hardness causing salts from water is called softening of water. Soft water is essential for many industries such as textiles, laundries, paper, ice, brewing, canning, etc. Water used for steam generation should be perfectly soft in boilers. 1. To minimize boiler troubles, the water used must be perfectly soft otherwise loss of efficiency and of boiler tube takes place. 2. Hardness causing salts are removed by external and internal treatment methods. There are two treatment methods for preventing scale formation. They are Internal treatment: In this method, raw water is treated inside the boiler. Internal treatment means addition of suitable chemicals to reduce scale and sludge formation. It is mainly based on the solubility product. If the product of the concentration of ions exceeds the solubility product, it precipitates.This is a corrective method to remove slight residual hardness and to remove corrosive nature of water. External treatment: This treatment is given outside the boiler before the feed water enters in. External treatment methods include lime–soda process, permutit process, ion-exchange process. Pre-heating: Feed water is heated before it enters the boiler. Hot flue gas leaving the boiler is also used for pre- heating. Water may be heated in a heat exchanger. Advantages of pre-heating is as follows: 1. Fuel will be saved. Instead of feeding boiler with water at atmospheric temperature, it is fed with hot water. 2. A portion of temporary hardness may be removed Ca(HCO3)2 CaCO3 + CO2 + H2O 3. Dissolved O2, CO2 are removed when water is heated to 65oC. As these gases have corrosive effect, their removal is advantageous. The main principle involved in internal condition is that scales formed are converted into sludges.
Internal Treatment Methods After feeding water to the boiler for steam generation, scale formation can be prevented by internal treatment method. The precipitation process applied includes carbonate conditioning, phosphate conditioning, colloidal conditioning, calgon process and treatment with sodium alluminate. These reduce the scale and sludge formation in the boilers. Carbonate conditioning: Scale formation in low pressure boilers can be avoided by adding sodium carbonate to boiler water, where CaSO4 is converted into calcium carbonate, a sludge which can be removed by blow down operation. CaSO4 + Na2CO3 CaCO3 + Na4SO4 By adding Na2CO3, the concentration ratio of carbonate ion to sulphate ion is made greater than the solubility product ratio of CaCO3 to CaSO4. CaCO3 is a loose sludge which can be scraped off by blow down operation. CO3 2– /SO4 2– > CaCO3/CaSO4 i.e., the concentration of carbonate ion is greater than concentration of sulphate. Precipitation of calcium carbonate occurs in preference to calcium sulphate. In high pressure boilers, excess of sodium carbonate may get hydrolysed to NaOH which causes caustic embrittlement.
Phosphate conditioning: Phosphate conditioning is generally applied to high pressure boilers. When sodium phosphate in added to boiler water, it reacts with magnesium and calcium salt forming soft sludges of Mg3(PO4)2 and Ca3(PO)2. Trisodium phosphate is used when the alkalinity of boiler water in 9.5–10.5 and calcium gets precipitated at pH of 9.5. If alkalinity of boiler water is too high, NaH2PO4 (acidic) is used, and if the boiler water is slightly alkaline, Na2HPO4 is used. Colloidal conditioning: Colloidal substances like kerosene, tannin, agar-agar, etc. are added to low-pressure boilers. These substances gets adsorbed over the scale forming precipitates and yield non-sticky, loose deposits which can be easily removed by blow down. Calgon conditioning process: The word calgon means calcium gone, i.e. the removal of Ca2+. Sodium hexametaphosphate is called calgon. It reacts with calcium ion and forms a water-soluble compound. Calcium sulphate present in hard water forms sludge which on drying in boiler changes to scale and, hence, reduces the efficiency of the boiler.
Treatment with sodium aluminate: When boiler water is heated with sodium aluminate, it gets hydrolysed to give sodium hydroxide and a gelatinous precipitate of The NaOH formed above reacts with MgCl2 to form another flocculant precipitate of Mg(OH)2. These two precipitate entrap colloidal impurities such as oil drops, sand and make them settle down at the bottom that can be easily removed.
External Treatmemt Methods Lime–Soda process (L–S Process) This is the most important method of chemical water softening. The principle involved in this process is to convert all soluble hardness causing impurities into insoluble precipitates which can be removed by settling and filtration. In lime–soda process, a calculated amount of lime and soda is added to the hard water (generally some ex-cess quantity is added). Lime removes the temporary hardness caused by Ca +2 and Mg +2 and also permanent hardness caused by Mg +2. It also removes dissolved CO2 and acidity, if any. Soda removes the permanent hardness caused due to Ca +2. Reactions involving soda (Na2CO3):
Cold lime–soda process: In this process, water is treated with lime and soda at room temperature. For this purpose, two types of water softeners are known, intermittent type and continuous type. Intermittent type: In this type of softener, raw water is mixed with a calculated amount of lime and soda at room temperature. Inside the chamber, there is mechanical stirrer. During mixing, stirring is started and a small amount of precipitate from previous treatment is also mixed. It accelerates precipitation. After some time stirring is stopped and chemical reaction completes. Precipitate is collected at the bottom as sludge which is taken out. Clear water is taken out with the help of floating pipe Intermittent type of cold lime–soda softener
Continuous type: This process provides soft water continuously. There are two chambers shown in. Raw water and soda + lime are continuously allowed in to the chamber through inlet pipes. The circular vertical chamber is provided with a shaft containing number of paddles. Mechanical stirrer is used for mixing. A small amount of coagulant like alum/aluminium sulphate is mixed to get heavy precipitate. Sludge settles at the bottom and softened water from outlet Continuous cold lime–soda softener
Hot lime–soda process: In this process, raw water is treated with lime and soda at about 80°C with raw water. The equipment has a heating coil for maintaining the temperature. At higher temperature precipitation takes place and coagulant is not required Process: It consists of a big steel tank having an inner circular vertical chamber containing open bottom. The upper end of which is in the form of a funnel and the lower end is open. Raw water and chemicals fall in to this funnel and get mixed. The steam is allowed to get in through the steam inlet. By the time the mixture goes down, the reaction is complete. When the water rises up in the tank, the sludge separates and settles down. The sludge can be removed from the bottom. Continuous hot lime–soda softener
Advantages of hot L–S process are as follows: 1. Precipitation is rapid and completes in 15 min. 2. The residual hardness (15–30 ppm) is far less than the cold process. 3. Precipitate and sludge settle down rapidly. Hence, no coagulant is needed. 4. Lime and soda are required in small quantity. 5. Little or no coagulant is added. 6. Pathogenic bacteria are killed in hot L–S process. 7. Fe and Mn salts are removed. They cause unpleasant odour. Fe causes redness to water. 8. Dissolved gases like CO2 are driven out. 9. The pH value of treated water increases, thereby preventing corrosion of distribution pipes. 10. The process is economical. Disadvantages of L–S process are as follows: 1. A large amount of sludge is produced which poses disposal problem. 2. Softened water contains sodium which is not suitable for boiler feed. 3. It is not suitable for house-hold purposes because it is difficult to know how much lime and soda should be added and how to filter off precipitate. 4. It is also not suitable for many industries because it leaves water supersaturated with CaCO3
Zeolite Process Zeolite is a three-dimensional silicate. In zeolite, Si (IV) atom is replaced by Al (III) atom and, hence, changes it into anion. Common zeolite is naturally occurring aluminosilicate with the general composition Na2O. Al2O3.xSiO2.yH2O (x= 2 to 10 and y= 2 to 6). Zeolite has a cage-like structure. It is derived from SiO4 tetrahedron. Zeolites are of two types: Natural zeolite: They occur naturally and are porous. Example: natrolite Na2O ⋅ Al2O3 ⋅ 4SiO2 ⋅ 2H2O. Synthetic zeolite: They can be prepared from feldspar china clay and soda ash. They are porous and possess gel structure. They possess high exchange capacity than natural zeolites.
Besides natural zeolite, synthetic zeolite is also prepared with the help of soda ash, feldspar and china clay on heating. The apparatus is made of cylindrical vessel inside it where zeolite salt is kept. Raw water bed percolated inside the apparatus through beds and, thus, ion-exchange reactions take place where Na – ions is replaced by Ca +2 /Mg +2 ions. After the use of this process for a certain time, zeolite is exhausted, i.e. all Na + of the zeolites are removed by Ca 2+ /Mg 2+, and hard water will not be further softened.Exhausted zeolite can be regenerated or reactivated by heating it with brine solution (10% NaCl solution). Zeolite softener
Advantages of zeolite process: 1. Hardness of water is removed with residual hardness of about 10 ppm. 2. It is easy to operate. 3. It occupies less space. 4. The process can be made automatic and continuous. 5. This process is very cheap since regenerated permutit is used again. Disadvantages of zeolite process: 1. Since the process is an ion-exchange process in which 2Na+ is replaced by Ca2+/Mg2+ ion, the soft water obtained by this process has excess of Na+. 2. Hard water containing acid destroys the zeolite. 3. If suspended particles (turbidity) are present, the pores of the zeolite are blocked and softening is not possible. 4. Bicarbonate and carbonate ions, if present in water, are not removed and are present as sodium salt resulting in the alkalinity of water. 5. The colours present in hard water cannot be removed by this process
Ion-exchange Process or Demineralisation Process Ion exchangers are of two types: anionic and cationic. They are copolymers of styrene and divinyl benzene. The polymers have two types of functional groups. The first group has (SO3H) or COOH group in which H+ ion is replaced by cations such as Mg 2+ /Ca 2+. The second group contains substituted amino groups, such as –NH + 2 OH or >NH + OH – or N + OH –, in which OH – is replaced by anions in water. General representation of cation and anion exchangers are H–R and R–OH, respectively. Structures of some cations and anions are shown in Fig. Structures of (a) cation-exchange resins and (b) anion-exchange resins
Desalination The process of removal of dissolved salts (NaCl, KCl) present in water is known as desalination. Water is divided into three categories on the basis of salinity: (a) Sea water: The salinity is greater than mg/L. (b) Brakish water: The salinity is in the range of 1000−35000 mg/L. It has peculiar salty taste. (c) Fresh water: The salinity of water is less than 1000 mg/L. Brakish water and sea water are not fit for drinking as well as for industrial purposes. They can be subjected to desalination to make them suitable for drinking. Important desalination methods are: (i) reverse osmosis, (ii) electrodialysis and (iii) distillation.
Reverse Osmosis Process Osmosis is the process in which the flow of solvent takes place from dilute to concentrated solution through a semi-permeable membrane. In this process, only solvent can flow but not the solute, which produces a pressure called osmotic pressure on the side of more concentrated solution. When the flow of solvent under pressure from more concentrated solution to solvent or to the less concentrated solution through a semi-permeable membrane takes place is called reverse osmosis. Reverse osmosis cell
This method is applicable mainly for the desalination of sea water. Sea water and pure water are separated by a semi-permeable membrane made up of cellulose acetate fitted on both sides of a perforated tube. Inventions are in progress to search for better membrane. Polymethacrylate and polyamides have been proved to be better membranes. The process is very easy. It is used to make pure water. It removes the ionic and non-ionic substances in the water. It can also remove suspended colloidal particles. The life of a membrane is nearly two years, and it should be replaced after this period. By this process, sea water is made fit for drinking. Water obtained after being treated by this process is used in boilers.
Advantages of the process 1. The process removes ionic as well as non-ionic dissolved salts easily. 2. It is effective in removing colloidal impurities in water. 3. The process is economical and convenient. The process can be carried out at a room temperature. 4. It is suitable for converting sea water into drinking water
Electrodialysis Dialysis is a process in which diffusion of smaller particles takes place through a semi-permeable membrane. By this process, crystalloids are removed from colloids. The process has been successfully applied for the purification of sea water. Sea water is called brackish water (salty water). It has 3.5% salt. Dialysis removes salt from sea water (brine) through a membrane. Principle: In electro dialysis, ion-selective membrane is used, which permits the passage of only one kind of ions having specific charge, i.e., cation selective membrane allows the passage of cations only but not anions and vice versa. When electric current is passed through saline water enclosed between ion-selective membranes, cations of the dissolved salt move towards cathode through a cation-selective membrane. Similarly, anions of the salt move towards anode. As a result, the concentration of ions in saline water becomes free from dissolved salts and turn into fresh water. In electro dialysis, two electrodes (anode and cathode) are dipped in brine, separated by a semi-permeable membrane Cathode is placed near the cation-exchange membrane, and anode is placed near the anion exchange membrane. When current is passed through electrodes, oppositely charged ions present in sea water pass through membrane towards respective electrodes. Chloride ions pass towards anode, while sodium ions pass towards cathode. After some time, the water in the middle chamber becomes pure and is taken out. Cation membranes are permeable to cations due to the presence of negatively charged functional groups inside the membrance.
When outer chambers become more concentrated with brine, they should be replaced by fresh water. Now, different types of semi- permeable membrane have been developed as per the size of cation and anion. In electro dialysis cell, several pairs of membrane are used. These membranes are made of synthetic materials (plastics). Electrodes attract oppositely charged particles shown by arrows. Pure water is obtained in alternate chambers Line diagram of electrodialysis
Potable/Domestic/Municipal Water The water supplied by municipality should be fit for human consumption. It should satisfy the following requirements: 1. It should be colourless, odourless, and pleasant to taste. 2. Turbidity should not exceed 10 ppm. TDS should not exceed 500 ppm. It should not be very alkaline (pH 8.0). 3. It should be free from dissolved gases. 4. It should be free from objectionable minerals such as Pb, As, Cs, Mn and dissolved gases such as H2Sand CO2. 5. It should be free from pathogenic micro-organisms (coliform bacteria are used as indicator organisms,whose presence suggests water is contaminated.)
Treatment of municipal water involves the following: 1. Screening: Water is allowed to pass through the mesh screens whereby large floating matters are removed 2. Sedimentation: Water is allowed to stand undisturbed for 2−5 h in big setting tanks. Suspended particles settle down due to gravity and clear water raises which can be drawn out with the help of the pumps. Disadvantages It requires long big tanks. It takes a long time. It removes 70−75% suspended matter. If water contains clay and colloidal impurities, coagulants are added before sedimentation. 3. Coagulation: Colloidal particles from the water are removed by adding coagulants such as alum, Al2(SO4)3, NaAlO2, etc., which produces flocs. Smaller particles gather together to form bigger flocs. They can be easily removed by filtration. Some bacteria and colour are also removed. Al2 (SO4)3 + 3 Ca (HCO3)2 → 2 Al(OH)3 ↓ + 3aSO4 + 6 CO2 NaAlO2 + 2 H2O → Al(OH)3 ↓ + NaOH FeSO4 + Mg(HCO3)2 → Fe(OH)2 + MgCO3 + CO2 + H2O 4Fe(OH)2 + O2 + 2H2O → 4 Fe (OH)3 4. Filtration: Colloidal matter, bacteria, micro-organism are removed. Water is passed through a large area sand bed. The filter may be pressure filter or gravity filter. 5. Sterilization/disinfection: Water after passing through sedimentation, coagulation, and filtration still contains a small percentage of pathogenic micro-organisms such as bacteria. Its removal can be achieved by sterilization
The chemicals used for killing bacteria are called disinfectants. Water can be sterilized by the following methods: (a)Boiling: Water is boiled for 10−15 min, where most of the pathogenic bacteria are killed. (b)By adding bleaching powder. Bleaching powder in calculated amount is added to water and allowedto stand for several hours. CaOCl2 + H2O → Ca(OH)2 + Cl2 Cl2 + H2O → HCl + HOCl HOCl + Germs → germs are killed Disadvantages: Bleaching powder introduces Ca2+ hardness in water and adds lime residue. Excess of it gives bad smell and bad taste. Excess chlorine is irritating to mucous membrane. (c)By chlorination Cl2 + H2O → HCl + HOCl Bacteria + HOCl → killed bacteria The quantity of chlorine to be added is important. The disinfection will not complete if chlorine is insufficient. If excess chlorine is added, it causes irritation, bad taste, and odour.
De-chlorination Over chlolrinated water can be de chlorinatede by passing it through a bed of activated carbon. Excess chlorine can also be removed by adding SO2 (or) NaSO3. SO2 + Cl2 + 2H 2O → H2SO4 + 2HCl Na2SO3 + Cl2 + H2O → Na2SO4 + 2 HCl (d) By using chloramine (NH2Cl): It is obtained by mixing chlorine and NH3 in 2:1 ratio. It has better bactericidal action than chlorine. It is more stable and not producing any irritating odour. NH3 + Cl2 → HCl + NH2Cl NH2Cl+ H2O → NH3 + HOCl (e) Disinfection by ozone: O3 is prepared by passing silent electric discharge through cold, dry O2. It is highly unstable. O3 → O2 + (o) O3 is an excellent, harmless disinfectant. It is highly unstable and decomposes to give nacent oxygen (o). (o) is a powerful oxidizing agent. It oxidizes organic matter in water and also kills bacterias.
Solved Examples Numerical problems based on this chapter can be solved using the table that gives the equivalence of different salts, ions and compounds in terms of CaCO3 which is also called the multiplication factor.
Theoretical Questions 1. A water sample contains 408 mg of CaSO4 per litre. Calculate the hardness of water in terms of CaCO3 equivalent. 2. Why do we express hardness of water in terms of CaCO3 equivalent? 3. Write various units of hardness and the relationship between them. 4. What are zeolites? How do they function in removing the hardness of water? What are the limitationsof this process? 5. What is the basic principle applied to remove the hardness of water by lime–soda process? 6. What are ion-exchange resins? 7. What do you understand by the hardness of water? What are its causes? 8. Distinguish between temporary and permanent hardness of water. 9. How is water softened by lime–soda process? Describe its types and suitable chemical reactions. 10. What is calgon? What is its application in water treatment?
11. What is meant by the exhaustion of cation and anion exchangers? How can they be regenerated? 12. Why is demineralisation process preferred over zeolite process for the softening of water for use in boilers? 13. How is water analysed for alkalinity? How the alkalinity due to various ions can be determined? ml of a sample of water on EDTA titration with EBT as indicator consumed 13 ml of M EDTA till the endpoint is reached. Calculate the hardness of water in terms of ppm. 15. The hardness of 1000 L of a water sample was completely removed by passing it through a zeolite softener. The softener then required 30 L of NaCl solution containing 1.5 g/L of NaCl for regeneration. Calculate the hardness of the sample of water.
Multiple-choice Questions 1. Hardness of water is caused by (a) CaCl2 (b) NaCl (c) Na2CO3 (d) K2SO4 2. Hard water contains (a) Na ⊕ (b) Mg2+ (c) Ca2+ (d) both (b) and (c) 3. The chemical formula of zeolite is (a) FeSO4.7H2O (b) Al2(SO4)3.18H2O(c) Na2Al2O4 (d) Na2O. Al2O3. xSiO2. yH2O 4. Permanent hardness of water is due to (a) HCO3– (b) CO3– (c) Cl– (d) Na ⊕ 5. Temporary hardness can be removed by (a) zeolite process (b) ion exchange (c) boiling (d) none 6. The demineralisation of water is called (a) zeolite process (b) ion-exchange process (c) lime–soda process (d) none 7. Which is not the unit of hardness of water? (a) ppm (b) epm (c) degree Clark (d) none of these 8. The relation between mg/L and ppm is (a) 1 mg/L = 1 ppm (b) 10 mg/L = 1 ppm (c) 1 mg/L = 10 ppm (d) 1 mg
9. In EDTA titration, the colour of the end point is (a) red (b) blue (c) yellow (d) no change 10. The colour of phenolphthalein in acidic medium is (a) colourless (b) pink (c) yellow (d) blue 11. Tannin, lignin are used for (a) phosphate conditioning (b) carbonate conditioning (c) colloidal conditioning (d) calgon conditioning 12. Blow down operation causes the removal of (a) sludges (b) scales (c) NaCl (d) acidity 13. Temporary hardness of water can be removed by a) filtration (b) screening (c) boiling (d) sedimentation 14. Purest form of natural water is (a) sea water (b) river water (c) rain water (d) lake water 15. Calgon is a trade name given to (a) sodium hexametaphosphate (b) magnesium phosphate (c) calcium silicate (d) sodium sulphate 16. The phenomenon of carrying of water along with impurities by steam is (a) priming (b) carry over (c) foaming (d) embrittlement 17. Brakish water mostly contains dissolved (a) KCl (b) MgCl2 (c) CaCl2 (d) NaCl 18. Water can be sterilized by using (a) Cl2 (b) CCl4 (c) CaCO3 (d) NaOH 19. pH of alkaline water is (a) 7 (b) more than 7 (c) less than 7 (d) Brakish water can be purified by using (a) lime–soda process (b) permutit process (c) filtration (d) reverse osmosis method
21. Hard water contains (a) Ca+2 and Mg+2 (b) K+ and Na+ (c) CO2 and O2 (d) NO3– and PO43– 22. Water containing calcium chloride and magnesium sulphate causes (a) temporary hardness (b) permanent hardness (c) both (d) softness 23. Best method of removing hardness of water is (a) ion exchange (b) permutit (c) lime–soda (d) boiling 24. Hardness of water is expressed in terms of equivalents of (a) MgCO3 (b) CaCO3 (c) Na2CO3 (d) K2CO3 25. Caustic embrittlement is caused due to the presence of (a) NaCl (b) NaOH (c) MgCO3 (d) KNO3 26. Priming and foaming in boilers produce (a) wet steam (b) dry steam (c) soft steam (d) hard steam 27. The exhausted cation exchange resin can be regenerated by treating with (a) dil. NaOH (b) dil. HCl (c) distilled water (d) dil. NaCl 28. A hard sticky precipitate formed on the inner surface of the boiler is called (a) sludge (b) embrittlement (c) coating (d) scale 29. Which of the following is responsible for temporary hardness? (a) MgCl2 (b) NaHCO3 (c) MgSO4 (d) Mg (HCO3)2 30. The water which is fit for drinking is called (a) hard water (b) brakish water (c) potable water (d) mineral water 31. Which of the following is a curdy precipitate? (a) sodium stearate (b) calcium stearate (c) potassium stearate (d) sodium carbonate 32. Which indicator is used for the determination of hardness by EDTA method? (a) methyl orange (b) methyl red (c) EBT (d) FSB-F 33. pH of purest water is (a) 7 (b) 14 (c) 10 (d) Water shows hardness, when it contains (a) alkalinity (b) acidity (c) dissolved sodium salts (d) dissolved Ca and Mg salts 35. Loose and slimy precipitate formed within the boiler is called (a) scale (b) sludge (c) priming (d) corrosion
ENERGY SOURCES Introduction A fuel is a combustible substance which on proper burning produces a large amount of heat energy. The heat evolved during combustion can be used economically for industrial and other uses. For example, coal is used in locomotives and as reducing agent in blast furnace. Petrol is mainly used in internal combustion engines and for doing mechanical work. There are solid, liquid, and gaseous fuels that are available for firing in boilers, furnaces, and other combustion equipment. Right type of fuel can be selected depending on various factors such as storage, availability, handling, pollution, and landed cost of fuel. Combustion is the process of chemical reaction between fuel and oxygen. During combustion heat and products of combustion are released. The combustion process is an exothermic chemical reaction, i.e., a reaction that releases energy as it occurs. Symbolically, combustion can be represented as Fuel + Oxidizer (Oxygen) → Products of combustion + Energy Heating value of a fuel is the amount of energy or heat released per unit mass during combustion of that fuel. The main elements of combustion are carbon, hydrogen, sulphur, oxygen and nitrogen. With the advent of nuclear fuels, which generate heat by nuclear reaction, the common fuels may be termed as chemical fuels.
Classification of Fuels Classification of fuels is based on two factors: (i) based on occurrence (natural or primary and artificial or secondary) (ii) physical state of the fuel (solid, liquid, gas) or state of aggregation. Nuclear fuels are nowadays used for power generation. It includes 92U235 and 94Pu239. Natural Fuels (Primary Fuels) Some fuels are found in nature and are used in the same form. These are called natural fuels, e.g. wood, coal, natural gas and petroleum. Artificial Fuels (Secondary Fuels) The fuels that are derived from natural fuels (primary) are called artificial or secondary fuels, e.g. petrol, producer gas and charcoal.
Comparison of solid, liquid and gaseous fuels: advantages and disadvantages
Characteristics of a Good Fuel A good fuel has the following features: 1. It should be cheap and easily available. 2. It should be dry and should have less moisture content. Dry fuel increases its calorific value. 3. It should be easily transportable, otherwise cost of fuel will increase. 4. It must have high calorific value. 5. It must leave less ash after combustion. In case of more ash, the fuel gives less heat. 6. The combustion speed of a good fuel should be moderate, otherwise it will not solve the problem of heating. 7. It must have moderate ignition temperature. Low (burning)/ignition temperature can cause fire accident. 8. It should not burn spontaneously to avoid fire hazards. 9. It should not give harmful gases after combustion. 10. Its handling should be easy. 11. The combustion of a good fuel should not be explosive. 12. The combustion of a good fuel should not result in the release of toxic gases such as CO, CO2, CH4,etc
Calorific Value of a Fuel The calorific value of any fuel is a very important property. It measures the heat produced by the fuel. The higher the calorific value, the better will be the quality of fuel. Calorific value is defined as “the amount of heat produced by the combustion of unit mass or unit volume of a fuel”. It is characteristic of every substance and is important for thermodynamic design and calculation of combustion system. The calorific value is measured in several units of heat; they are calorie, kilocalorie, British thermal unit and Centigrade thermal unit (Centigrade heat unit). Calorie: The amount of heat which increase the temperature of one gram of water by 1°C in known as 1 calorie. Kilocalorie: The amount of heat that increases the temperature of 1 kg of water by 1°C (i.e., 15°C to 16°C) is called kilocalorie.
British thermal unit (BTU): The amount of heat required to raise the temperature of one pound of water by 1°F (60°F to 61°F) is BTU. 1 BTU = 252 cal = kcal 1 kcal = BTU Centigrade heat unit (CHU): It is the amount of heat required to raise the temperature of one pound of water through 1°C. 1 kcal = BTU = 2.2 CHU Relationship among all the above units of heat is given as follows: 1 kcal = 1000 cal = BTU = 2.2 CHU Joule is also a unit of energy. 1 cal = 4.18 J
There are two types of calorific values of a fue High Calorific Value (HCV) or Gross Calorific Value (GCV) It is defined as the amount of heat energy produced by the combustion of unit mass (unit volume) of a fuel when the combustion products are allowed to cool at the room temperature. Generally, fuels contain hydrogen. When the calorific value of hydrogen-containing fuels is determined experimentally, hydrogen is oxidized to steam; when the products are cooled to room temperature, steam undergoes condensation to produce water and releases the latent heat of condensation of steam which must be included in the measured heat which is called GCV. Actually, during combustion, the products are not allowed to escape into the atmosphere and they are cooled to the room temperature. Hence, the net heat energy recovered on the combustion of fuel is lower. Low Calorific Value (LCV) or Net Calorific Value (NCV) It is defined as the amount of heat energy produced by the combustion of unit mass (unit volume) of fuel when the combustion products are allowed to escape out into the atmosphere. LCV does not include the latent heat of steam or water vapour formed.
Relationship Between HCV and LCV If hydrogen is present in a fuel, the above-mentioned chemical reaction will take place and 2 g of hydrogen will produce 18 g of H2O or 1 g of hydrogen will produce 9 g of H2O. If x gram hydrogen is present in a fuel, it will produce 9x g of water and 9x g of water vapour that will release 9x × L cal heat on cooling ( L cal/g is the latent heat of water vapour). So, LCV = HCV – latent heat of water vapour LCV = HCV – (mass of hydrogen × 9 × latent heat of steam) = HCV – (9 × H /100× 587) = HCV – (0.09 H × 587) where H is the percentage of hydrogen in the fuel and latent heat of steam is 587 kcal/kg.
Determination of Calorific Value The calorific value of a fuel can be determined by an equipment called calorimeter. The Bomb and Boy’s calorimeters are the two main methods. Here, only the Bomb calorimeter has been discussed. Bomb Calorimeter The bomb calorimeter is a device that is used for the determination of calorific values of the solid and liquid fuels. Since the combustion of fuel is explosive, it is called the bomb calorimeter. A bomb calorimeter is made of a cylindrical stainless steel vessel called bomb. It contains a crucible having known mass or volume of a fuel fixed on a screw. A magnesium fuse wire is connected to the electrodes and the electrodes are connected to a 6 V battery. A copper calorimeter envelops the bomb. The calorimeter has a known amount of water having stirrer and Beckman’s thermometers. There are two jackets around the calorimeter, i.e. air and water, as shown in Fig. 6.3 to prevent heat radiation. As battery is connected to the electrodes, current flows through magnesium wire, which ignites the fuel (cotton or cotton thread is also sometimes used for the ignition of fuel). The initial and final temperatures of water in the calorimeter are measured with Beckman’s thermometer. Calculation of calorific value Let mass of fuel be m gram. Thus, Mass of water in calorimeter = W gram Water equivalent of calorimeter = w gram Initial temperature of water = T1°C Final temperature of water = T2°C Since the system is closed, the calorific value obtained will be an HCV.
Solid Fuels Because of environmental hazards, trees were buried inside the earth. By the action of temperature, pressure and bacterial actions over a period of thousands of years, they converted into a brown-black solid named coal. Since wood contains higher carbon percentage in the form of cellulose, lignocelluloses, they are transformed into the form of coal. The process of conversion of wood into coal is called coalification. Depending upon the percentage of carbon, hydrogen, moisture and calorific value, four different types of coals exist. It is called the ranking of coal. Dry wood has the following composition: carbon (48–50%), oxygen (42–44%), hydrogen (5–6%) and traces of minerals. The calorific value of wood is 4000–4500 kcal/kg and, thus, is used as a domestic fuel. Wood can be converted into charcoal by a process called carbonization (destructive distillation of wood). Charcoal is used as absorbent of gases and for de colourisation of sugar.
Coal It is a carbonaceous matter produced by the decomposition of vegetable and animal matter buried inside the earth’s crust or under oceans. It is a non-renewable energy source formed by the decomposition of accumulated vegetation over a period of millions of years. Coal is one of the major source of energy in many industries such as steel, cement and paper, because of its easy availability and least risk of fire hazards. Ranking of Coal During coalification of wood, the first stage of coal is peat. It is ranked the lowest among coals. Other coals are lignite, bituminous and anthracite. It has been reported that graphite is also the final stage of coalification. Wood → Peat → Lignite → Bituminous → Anthracite
Analysis of Coal The analysis of coal is helpful in its ranking. The assessment of the quality of coal is carried out by these two types of analyses: (i) proximate analysis and (ii) ultimate analysis. Proximate Analysis Proximate analysis gives information regarding the practical utility of coal. In this analysis, the percentage of carbon is indirectly determined. This analysis includes percentage of moisture, volatile substance, ash content and carbon. Moisture: A known mass of finely powdered air-dried coal is taken in a crucible. It is heated up to 110°C for an hour and cooled to room temperature in a desiccator. The moisture is removed as water vapour and the process is repeated. The weight of coal is reported on moisture% basis till the constant weight is obtained. Percentage of moisture=Loss of weight of coal/Weight of coal taken×100
Volatile matter: Dried sample of coal left in the crucible is covered with a lid and placed in a muffle furnace maintained at 950°C exactly for 7 minutes. The crucible is then taken out, cooled in the air and then in desiccator and weighed. The loss in weight is reported as the percentage of volatile matter Percentage of volatile matter = Loss in weight due to removal of volatile matter/Weight of coal taken×100 Ash content: Coal, free from moisture and volatile matter, is heated in a crucible at about 700°C in a mufflef urnance in the presence of air. It undergoes combustion and results in the formation of ash. Crucible is cooled to room temperature and weighed. Heating, cooling, weighing is repeated to get constant weight of the residue. The residue is reported as as h. The mass of ash is then determined. Percentage of ash=Mass of ash × 100/Mass of coal Carbon: Since the main component of coal is carbon, it can be determined by subtracting the sum of the percentage of moisture, volatile substance and ash content from 100. Carbon % = 100 – (% of moisture + % of volatile matter + % of ash)
Significance of analysis: Proximate analysis gives quick and valuable information regarding commercial classification and suitability of coal for industrial use. It consists of a complex mixture of gaseous and liquid products resulting from the thermal decomposition of coal. The amount of decomposition and yield of V.M. depends on the conditions of heating, particularly temperature. Moisture: A high moisture content in the fuel takes some heat liberated in the form of latent heat, reduces the calorific value, increases the cost of transportation and causes wastage of heat. Hence, the lesser the moisture content, the better is the quality of a fuel. But moisture up to 10% produces a uniform fuel bed and less of fly ash. Volatile matter: It is due to combustible and non-combustible gases. A coal containing high volatile matter burns with long flame, high smoke and low calorific value. volatile matter also influences the design of the furnace since the higher the volatile matter, the larger is the combustion space required. Ash: Ash is a residual, incombustible matter produced by burning of coal. It creates cleaning and disposal problem. Ash adds impurities in metallurgical operations. It obstructs the flow of air and heat.
1. Ash reduces heating value of coal. 2. Ash content increases the cost of transportation, handling, storage and disposal. 3. It determines the quality of coal. hence, the lesser the percentage of ash, the better is the quality of coal. Fixed carbon: The higher the fixed carbon in a coal, the greater is its calorific value and better is the quality of coal. It helps in designing the furnace and shape of fire box. It increases from low ranking coals such as lignite to high ranking coals such as anthracite. It helps in designing furnace and fire box.
Ultimate Analysis It is the elemental analysis of coal. This analysis includes percentages of C, H, O, S, N and ash content in coal and better quality of CO. The two components can be determined in a single experiment.
Liquid Fuels (Petroleum) Petroleum is one of the best primary liquid fuel. It is also known as crude oil. Petrol, diesel, kerosene are main liquid fuels. They are secondary liquid fuels derived from petroleum. These fuels are used for domestic works, auto vehicles and power generation. The word meaning of petroleum is ‘rock oil’ (petra = rock, oleum= oil). Petroleum is dark-brown viscous liquid. Petroleum is a mineral found deep in earth’s crust. It is a mixture of number of hydrocarbons (paraffins, olefins, aromatics and naphthalene), nitrogen, sulphur, oxygen containing optically active compounds along with traces of compounds of heavy metals such as Fe, Co, Ni and V. The unpleasant odour of petroleum is due to the presence of some foul smelling sulphur compounds.
Petroleum does not have definite composition. Its composition varies with the place of origin. It is a complex mixture of various hydrocarbons and a small quantity of optically active compounds of S, N, O and traces of Fe, Cu, V, etc. Composition of petroleum is given as: C = 80 to 87.1 %, H = 11.1 to 15.0 %, S = 0.1 to 3.5 %, O = 0.1 to 0.9 %, N = 0.4 to 0.9 %. Origin of petroleum There are two theories to explain the origin of petroleum. Carbide theory: This theory is also called inorganic theory or Mendeleev’s theory. Metals inside the earth react with carbon and form metal carbides. These carbides are converted into hydrocarbons in the presence of moisture or steam which on further hydrogenation polymerize to give a complex mixture of paraffin's, olefins, and aromatic hydrocarbons. Drawbacks: This theory was unable to explain the presence of nitrogen, sulphur and optically active compounds found in petroleum. Engler’s theory or organic theory: According to this theory, organic matters, animals, vegetation and marine animals died and accumulated in sea. There, they were decomposed under high temperature and pressure by anaerobic bacteria to give a dark viscous liquid called petroleum. This theory is better accepted. The presence of optically active compounds in petroleum favours Engler’s theory.
Classification of crude oil There are three types of petroleum. Paraffinic base type crude oil: It has main saturated hydrocarbons CnH2n + 2(n = 1 to 35) alongwith naphthalene and aromatics hydrocarbons from C18H38 to C35H72 are solids, called waxes. Asphaltic base type crude oil: It has mainly naphthalene and cycloparaffins with a small quantity of aromatic and saturated hydrocarbons. These oils on distillation leave asphalt as residue. Mixed base type: This type of crude oil is a mixture of above two, paraffinic base type and asphaltic type. It has high percentage of semisolid waxes.
Synthetic Petrol (Gasoline) Petrol is mainly obtained from crude oil from oil wells but can also be obtained by synthetic processes. Two synthetic processes are given here, viz. (i) Fisher–Tropsch process and (ii) Bergius process. Fischer-Tropsch process: The Fischer–Tropsch process is a collection of chemical reactions that convert a mixture of carbon monoxide and hydrogen into liquid hydrocarbons. The process, a key component of gas to liquids technology, produces a synthetic lubrication oil, synthetic fuel from coal, natural gas or biomass. When solid feedstock—coal—is used, the solid feedstock is converted into gaseous reactants CO, H2 and alkenes. In the Fisher–Tropsch plant that uses CH4 as feedstock, steam reforming occurs which converts methane into CO and H2 A variety of catalysts such as cobalt, iron, ruthenium and Ni can be used for the Fischer–Tropsch process. Nickel favours the formation of methane. Cobalt catalysts are more active when feedstock is natural gas. Iron catalysts are preferred for lower quality feed stocks such as coal or biomass. This process was developed in Germany by F. Fisher and H. Tropsch in In this process coal is converted into coke. Coke is then treated with steam to obtain water gas.
Bergius process: This process is the oldest one. In this process, low ash content coal is taken and powdered. It is now mixed with heavy oil to make a paste alongwith a catalyst (Ni or Sn oleate). This paste is heated with hydrogen at 450°C and 200–250 atmospheric pressure in a converter. The process is called hydrogenation. The hydrocarbons formed in this process are converted into low boiling liquid hydrocarbon. The issuing gases are led to condenser; crude oil is obtained which is fractionated. The middle oil is hydrogenated to get more gasoline and heavy oil is mixed with coal for making paste. The yield of gasoline by this process is 60%. A flow diagram of this process is given in Fig. Nowadays improved processes are introduced which include Exxon donner solvent (EDS) process. This process is improved by using suitable catalyst. Bergius process of hydrogenation of coal to gasoline
Refining of Gasoline Ideal gasoline must be (i) cheap and readily available, (ii) it must burn without ash or soot on combustion, (iii) it should mix easily with air and it should be knock resistant. The quantity of petrol available from distillation alone was not sufficient to meet our requirements. Hence, two processes have been developed: (i) rebuilding hydrocarbon molecules called reforming and (ii) breaking heavier hydrocarbons to lighter molecules called cracking. Reforming: Reforming of gasoline is mainly aimed at reducing the knocking of gasoline. The straight run gasoline produced by fractional distillation of crude oil is subjected to structural modification. Reforming is carried out thermally or catalytically. These modifications are essentially a result of rearrangement of molecules without greatly disturbing the average molecular weight.
1. Thermal reforming: It is carried out in a reactor maintained at 500–600°C at 80 atm pressure. The straight run gasoline is subjected to heating in the reactor to give branched alkanes. The products are cooled by spraying cold oil and then fractionated to remove residual gasoline. During reforming, cracking occurs to yield alkenes and alkanes. They may also undergo de hydrocyclization or dehydrogenation to yield naphthalene. 2. Catalytic reforming: Catalytic reforming is carried out in order to improve the yield and to produce better quality of gasoline. In this method, platinum metal supported on alumina in used as catalyst and reforming is done at 460–530°C and at 35–50 atm pressure.
3. Chemical reactions during reforming: Gasoline obtained from fractionation of crude oil or by synthesis contains certain undesirable chemicals (unsaturated straight chain hydrocarbons and sulphur compounds). Sulphur compounds lead to the corrosion of internal combution engines whereas remaining ones are oxidised to form gum and sludge. Refining is thus needed to obtain high class gasoline.
Cracking The quality and yield of petrol produced by the fractionation of petroleum is low. Hence, the middle oil and heavy oil fractions are cracked to give petrol. Cracking is the process of conversion of bigger hydrocarbon molecules into smaller hydrocarbons of lower molecular weights.
The process of cracking brings about (i) the conversion of high boiling fractions into low boiling fractions suitable for automobile and (ii) the production of raw materials for petrochemical industries.
Knocking In internal combustion engines, diesel or gasoline mixed with air is used as fuel and ignited in the cylinder. The ignition is brought about by an electric spark (in petrol engines) and compressing air in diesel engines. After the ignition is initiated by a spark, the fuel air mixture must burn smoothly and rapidly and the flame should spread uniformly throughout the gaseous mixture. The expanding gas drives the piston down the cylinder due to high pressure and provides power stroke. The four strokes in petrol engine are: 1. Suction stroke: Here the fuel–air mixture is drawn into the cylinder during induction. 2. Compression stroke: Fuel–air mixture is compressed in the cylinder. 3. Power stroke: Fuel–air mixture is ignited by electric spark. The hot gases produced due to combustion increase the pressure and push the piston down the cylinder. 4. Exhaust stroke: The piston ascends and expels exhaust gas from the cylinder and the next cycle starts again. The efficiency of an engine is directly related to compression ration (CR)
CR is the ratio of the volume of gas in the cylinder at the end of suction stroke to the volume of gas at the end of compression stroke. Compression ratio directly governs the efficiency of an engine. More the CR value, better will be efficiency of the engine. CR depends upon the type of constituents in gasoline. But in certain circumstances, the smooth burning of fuel is interrupted due to the presence of certain impurities. The last portion of the fuel ignites instantly and produces shock waves. it results in rattling sound in the engine called knocking. it results in rattling sound in the engine called knocking. Knocking decreases the efficiency of engine. The tendency of knocking is based on chemical structure of hydrocarbons. Branched chain alkenes burn more easily than straight chain alkenes. Lower alkenes (e.g., C4H10) burn easily than higher alkenes, e.g. C7H16. Also alkenes are better than alkanes and aromatic hydrocarbons; they burn more easily than cycloalkenes. Alternatively, the order of knocking tendencies is straight chain alkenes > cycloalkene > olefins > aromatics. For internal combustion engine, n-alkanes are not fit for modification. Knocking is the measure of octane number. Hence, for n- heptane, octane number is 0, and that of isooctane is 100.
Octane number or Octane rating The anti-knocking value of fuel can be increased by adding tetraethyl lead (TEL). The oxides of lead formed as combustion products inhibit freeradical chain reaction responsible for knocking. Additives like ethylene dibromide and ethylene dichloride are also added to petrol to avoid the contamination of atmosphere by vapours of lead and PbO2. They form lead halides which escape into the atmosphere. They are not eco-friendly. Other additives tricresyl phosphate, 2,4-ditertiarybutyle-4- methyl phenols act as lead scavengers to prevent the oxidation of lead. The small quantity of methyl cyclopentadienyl manganese tercarbonyl (MMT) is also used nowadays in Canada and European countries in place of TEL, but results in Mn pollution in air and soil. Nowadays leaded petrol is used as aviation fuel known as avgas.
Prevention of Knocking -Knocking can be prevented by using: (i) Good quality fuel with higher octane number. (ii)By adding anti-knocking agents like tetraethyllead, methyl cyclopentadienyl manganese tricarbonyl (MMT) isooctane, diethyl telluride, etc. (iii) By retarding spark plug ignition. It has been observed that maximum knocking is from a straight chain alkane n- heptane and hence its octane number (anti-knocking values) is assigned as zero while isooctane knocks minimum (say zero), its octane number has been given hundred. Hence, the octane number of gasoline is the percentage of isooctane in the mixture of isooctane and n-heptane which has the same knocking as the gasoline itself. Therefore, octane number 60 of a gasoline sample means, the mixture of 60% isooctane and 40% n-heptane which passees same knocking as the gasoline. The higher the octane number, the lower it is knocking. There are certain compounds which are used as anti-knock compounds. Tetraoctyl lead (C7H5)4Pb is better anti-knock but lead is dangerous for health. Methyl cyclopentadicyl manganese is also used as anti-knock but manganese is also harmful. Organic compounds like methanol, ethanol, methyl t-butylether, etc. are also inserted as anti-knocking agent blended with gasoline. Aviation gasoline has octane number even more than 100. The fuel marked means a mixture of isooctane (100 ml) + TEL (2 ml).
Cetane Number or Rating The knocking characteristics of diesel oils are usually expressed in terms of cetane number. It is a measure of ignition lag or delay of the fuel. It is the time period between the start of ignition and start of combustion of the fuel. For the determination of cetane number, n- hexadecane and a-methyl naphthalene are taken as standard. (C16H34) n-hexadecane is a saturated hydrocarbon having a short ignition lag as compared to any commercial diesel fuel. Its cetane number is 100. It ignites below compression temperature. α-Methyl naphthalene has a very long ignition lag as compared to any commercial diesel oil. Its cetane number is taken as zero.
In diesel engine, fuel is exploded not by spark but by temperature and pressure. The suitablity of diesel fuel is determined by its cetane number. Cetane number is the percentage of n-hexadecane in n- hexadecane and α-methyl naphthalene which has the same ignition characteristics as that of the sample under test. Cetane number 60 means, it has the same ignition characteristics as that of 60 parts of cetane and 40 parts of α-methyl naphthalene. The order of cetane number for the following is given as n-alkanes > napthalenes > alkenes > branched alkanes > aromatics Thus, it is inferred that the hydrocarbons which are poor gasoline fuels are good diesel fuels. Alkyl and nitrites and di-tertiary peroxides are used as additives to increase the cetane number of fuel. They are called pre-ignition dopes.
Power alcohol: It is an excellent alternative motor fuel for gasoline IC engines. It can also be used in combination with gasoline. Blends of alcohol with gasoline or gasoline with benzol or benzene are used as fuels. When alcohol is blended to gasoline, then the anti-knock property of gasoline increases. Alcohol- blended fuels have lesser starting difficulties. They burn cleaner than gasoline and do not produce any toxic emissions. They are biodegradable. The raw materials for alcohol are derived from sugarcane and corn which are readily available.
Gaseous Fuels Natural gas is the primary gaseous fuel. A variety of secondary fuels are obtained from coal or petroleum. They include coal gas, producer gas, water gas derived from coal, and LPG, CNG derived from natural gas and oil gas formed by cracking of kerosene oil. Natural Gas Natural gas is primarily methane gas. It is a fossil fuel. It is formed in coal beds (coal bed methane). Other sources are town gas and biogas. The main source of natural gas is oil fields, and the gas is called associated natural gas. (The gas formed in natural gas field is called associated gas.) Also, natural gas formed with petroleum and diesels is called wet gas and that formed with crude ore is called dry gas. The calorific value of wet gas is higher than the dry gas because of higher percentage of heavy unsaturated molecules. Composition of natural gas: Natural gas contains ethane, propane, butane and pentane alongwith main component methane Before the use of natural gas, other heavier hydrocarbon, CO 2, N2, He, and H2O are removed. Commercially natural gas is produced from oil fields and natural gas fields. The gas obtained from oil wells is also called casing head gas. Town gas is a mixture of city and other gases mainly CO. It is used similar to the natural gas. The gas is used for cooking and lighting purposes. The waste product coaltar is used for road making. Biogas is also methane-rich and widely used as domestic fuel. The approximate composition of natural gas is CH4= 70–90%, C2H6= 5–10%, H2= 3%, CO + CO2= 0.7%. The calorific value of natural gas is to kcal/m 3. Sometimes harmful H2S gas flows in traces which can be removed by 2-amino ethanol
Natural gas has several applications: 1. It is used as a very good domestic fuel. 2. It is used in the preparation of ammonia (used for urea manufacturing). 3. It is used to prepare carbon-black which is used as filler for rubber industry. 4. It is used to prepare synthetic proteins for animal feed. (Methane on fermentation gives synthetic proteins used as animal feed.) Natural gas is being sent to thousands of kilometres through pipes.
Biomass In rural areas, the major fuel sources are wood, agricultural wastes, cattle dung, dry plants, etc. These fuels cater about 75% of our need. They are called biomasses. In villages, traditional chullas are used which have poor efficiency (about 10%). It produces a lot of pollution due to smoke. The biomasses are primary fuels and can be converted into more efficient secondary fuels. Wood is converted into charcoal and cattle dung can be converted into biogas. The remaining part of the sugarcane is called bagasse that is used as fuel and as raw material for making paper. Rice husk is also one of the important biomass from which liquid fuel can be extracted. Biogas Biomasses undergo bacterial fermentation and result into biogases. Alternatively, biogas is obtained by the anaerobic fermentation of the dung, plant wastes and household wastes (vegetable waste) in the absence of oxygen and in the presence of water. The main components of biogas are methane (55%), CO2 (35%), H2 (7.4%), N2 (2.6%) and H2S (present in traces). The calorific value of common biogas (gobar gas) is 5300 kcal/m3. The natural gas may also be considered as a biogas. It is formed by animals and plants and other vegetations buried inside the earth. Bacterial actions, temperature, high pressure and water convert them into natural gas.
Biogas Plant: Biogas is prepared in an apparatus called biogas plant. There are two types of biogas plants: (i) floating gas holder type and (ii) fixed dome type. Floating Gas Holder Type Floating gas holder type plants were made of metallic sheet (iron sheet) that rusted and caused gas leakage. Fixed Dome Type Biogas Plant This type of plant is made of concrete, bricks and cement. It has underground digester tank. One side of the tank is connected with inlet chamber while the other side is connected with the overflow tank. There is a dome connected with the gas pipe having a valve (Fig.6.12)
LPG (liquified Petroleum Gas) Nowadays LPG has been a common fuel for domestic work and also in most of the industries. The main components of LPG or cooking gas are n-butane, isobutane, butylene and propane (traces of propene and ethane). The hydrocarbon are in gaseous state at room temperature and at atmospheric pressure but can be liquified under higher pressures (Fig. 6.13A). The gas can be compressed under pressure in containers and sold under trade names like Indane, Bharat Petroleum gas, HP gas, etc. LPG is kept in metallic cylinder attached with burner through pipe. It has two stoppers, one at the cylinder and other at burner. LPG has special odour due to the presence of organic sulphides which are added specially for safety measure. The gas can be compressed under pressure in containers and sold under trade names like Indane, Bharat Petroleum gas, H. P. gas etc. The gas is obtained from natural gas or as a byproduct in refineries during cracking of heavy petroleum products.
Characteristics of LPG 1. It has high calorific value: kcal/m3. 2. It gives less CO and least unburnt hydrocarbons. So it causes least pollution. 3. It gives moderate heat which is very good for cooking. 4. Its storage is simple. It is colourless. 5. It has the tendency to mix with air easily. 6. Its burning gives no toxic gases though it is highly toxic. 7. It neither gives smoke nor ash content. 8. It is cheaper than gasoline. It burns with little air pollution and leaves no solid residue. Hence, it is used as fuel in auto vehicles also. 9. It is dangerous when leakage is there. It is highly knock resistant. 10. LPG can be extracted from natural gases and also from refining of crude oil. Cryogenic process is best for the extraction for natural gas. Advantages of LPG 1. LPG is used as domestic fuel and as a fuel for internal combustion engines. 2. It is used as feedstock for the manufacture of various chemicals and olefins by pyrolysis. 3. LPG in used in industries as portable blow lamps, welding, annealing, hardening, steel cutting, etc. Disadvantages 1. It is difficult to handle as fuel. 2. Engines working at low compression ratio cannot use LPG as fuel.
CNG (Compressed Natural Gas) Natural gas contains mainly CH4. When natural gas is compressed at high pressure (1000 atm) or cooled to –160°C, it is converted into CNG. It is stored in cylinder made of steel (Fig. 6.13B). It is now replacing gasoline as it releases less pollutants during its combustion. It is environmentally clean alternative to those fuels which produce toxic pollutants. In some of the metro cities, CNG-vehicles are used to reduce pollution. LNG (liquified natural gas) is different from CNG. LNG is costlier than CNG. Advantages of CNG 1. Due to higher temperature of ignition, CNG is better fuel than petrol and diesel. 2. Operating cost of CNG is less. Cost of production is less. It can be easily stored. 3. It releases least pollutants like CO and unburnt hydrocarbons. 4. Spark plug of CNG-engines are not carbonified. 5. It undergoes regular combustion.
Disadvantages 1. Response to blending is poor. 2. Faint odour; leakage cannot be detected easily. 3. CNG tanks require a large tank space. 4. Refueling network for CNG is very expensive. Combustion Problems
Theoretical Questions 1. Define fuel. What are the characteristics of a good fuel? 2. What is meant by calorific value of fuel? Explain the method of determination of calorific value of liquid and gaseous fuels. 3. How do you analyse coal by (a) proximate and (b) ultimate analysis? Give their significance. 4. What is octane number? 5. Explain Fischer–Tropsch’s and Bergius methods for the synthesis of petrol. 6. What are the advantages of gaseous fuels? 7. How do you analyse flue gas by Orsat’s methods? 8. Why tetraethyl lead is mixed with gasoline? 9. Write notes on LPG, CNG and biogas. 10. Give an account of the advantages and disadvantages of solid fuels over gaseous fuels. 11. What is LCV and HCV of fuel? Describe Junker’s method for the determination of calorific value of gaseous fuels. 12. Give an account of the classification of the fuels with suitable examples. 13. Define octone number of gasoline. What is its significance and how is it measured? Why is ethylene dibromide added when TEL is used as an antiknock reagent? 14. What is biodiesel? How is it obtained? What are its advantages? 15. What is petroleum? How is it refined? What are the fractions obtained and their uses? 16. What is cracking? How gasoline is obtained from fixed bed cetelytic cracking
Multiple-Choice Questions 1. A good fuel should possess (a) high ignition temperature (b) moderate ignition temperature (c) high calorific value (d) both (b) and (c) 2. Ignition temperature of a fuel is the (a) temperature at which the fuel ignites for a moment but does not burn after heat. (b) temperature attained when the fuel is burnt. (c) lowest temperature at which the fuel must be pre-heated so that it starts burning smoothly. (d) temperature at which the fuel can be stored safely. 3. An example of a secondary fuel is (a) wood (b) coal (c) natural gas (d) gobar gas 4. Purest form of coal is (a) lignite (b) bituminous (c) peat (d) anthracite 5. Gobar gas contains (a) CO2 (b) H2 (c) CH4 (d) all of these 6. Biogas contains (a) CO2 (b) CH4 (c) C2H4 (d) C2H2 7. Power alcohol is an example of (a) liquid fuel (b) artificial fuel (c) secondary fuel (d) all of these 8. Gross calorific value is also known as (a) high calorific value (b) low calorific value (c) net calorific value (d) none 9. The correct relationship between HCV and LCV is (a) LCV = HCV HL (b) LCV = HCV − 0.09 HL (c) HCV = LCV − 0.09 HL (d) HCV = LCV HL 10. The most impure form of coal is (a) anthracite (b) peat (c) wood (d) lignite
11. One kilogram of cattle dung produces biogas which gives (a) 1000 kJ of heat (b) 10 kJ of heat (c) 800 kJ of heat (d) 500 kJ of heat 12. Bomb calorimeter is used for the determination of calorific value of (a) solid fuel (b) liquid fuel (c) gaseous fuel (d) both (a) and (b) (e) both (b) and (c) 13. The fuel which has the highest calorific value is (a) wood (b) petrol (c) methane (d) hydrogen 14. The optimum temperature for fermentation in biogas plant is (a) 10–20°C (b) 25–35°C (c) 35–50°C (d) 55–75°C 15. The calorific value of biogas is (a) 1500 kcal/m3 (b) 2500 kcal/m3 (c) 5300 kcal/m3 (d) 43 kcal/m3
16. The other name of biogas is (a) producer gas (b) gobar gas (c) natural gas (d) none 17. The highest ranking coal is (a) lignite (b) peat (c) anthracite (d) bituminous 18. Natural fuel among the following is (a) oil gas (b) coke (c) petrol (d) coal 19. Major constituent of LPG is (a) methane (b) ethane (c) benzene (d) butane 20. The calorific value of a fuel is expressed as (a) kcal/m (b) kcal/kg (c) cal/cm3 (d) kcal/g 21. The calorific value of a fuel can be theoretically calculated by _________ formula. (a) Dulong’s (b) Newton’s (c) Avagadro’s (d) Raman’s 22. Compressed natural gas mainly contains (a) CO (b) N2 (c) CH4 (d) SO2 23. The jet engine fuel is ____________. (a) kerosene (b) petrol (c) diesel (d) petroleum ether 24. Calorific value of diesel is (a) kcal/kg (b) kcal/kg (c) kcal/kg (d) kcal/kg 25. The greases used for lubrication can be obtained by fractionation of (a) diesel oil (b) kerosene oil (c) vegetable oil (d) heavy oil
PHASE RULE Introduction Phase diagrams are of considerable significance both industrially and commercially, particularly for steels, alloys, ceramics and semiconductors. These are the basis of separation procedures in petroleum industry, food formulations and in the preparation of cosmetics. Phase diagrams are an important tool to deal with the behavior of heterogeneous systems. Phase rule helps in predicting the effect of temperature, pressure and concentration on a heterogeneous system in equilibrium. The equilibrium condition for any system is that the chemical potential of each constituent throughout the system must be the same. If there are several phases of each constituent, then the chemical potential of each constituent in every phase must have the same value. For example, if at any temperature and pressure liquid water is in equilibrium with water vapour and solid ice, then Phase diagrams can be discussed in terms of relationships, the phase rule, derived by Gibbs. We shall derive the rule first, and then apply it to a wide variety of systems. The phase rule requires a careful use of terms, so we begin by presenting some definitions.
Phase Phase is a homogeneous system which is separated from other phases of the system by means of a boundary. It is a physically distinct and mechanically separable part of a system. The phase has to fulfill the following conditions: 1. It should be physically homogeneous. It must have the same composition and physical properties in all its parts. 2. It should be separated from other phases of the system at equilibrium by surfaces of contact. 3. The phases of a system are in dynamic equilibrium with one another through the exchange of chemical species. Common examples of some phases 1. In a system of water and ice all the ice pieces are in one phase and water in another phase, so there are two phases. 2. Two liquids which are completely miscible with each other, e.g. glucose in water, sodium chloride in water, ethanol and water possess one phase. 3. Two liquids immisible in all proportions., e.g., benzene and water and oil and water contain two phases.
Component It is an element or a compound present in a system. The concentration of the component can undergo independent variation. It does not stand for the total number of constituents of the system. The number of components of a system at equilibrium is defined as the minimum number of molecular species in terms of which the composition of each phase may be expressed quantitatively. Examples: 1. In the water system, the phases that occur are ice, liquid water, and water vapour: It is a one-component system, as each phase is in a different physical form and the composition of each of these phases can be expressed in terms of the single constituent represented by H2O. 2. Decomposition of CaCO3: There are three molecular species–CaCO3, CaO and CO2, but the composition of each phase can be expressed in terms of any two of the species. If any two of the constituents are chosen, the composition of all the phases can be expressed. For example, if CaO and CO2 are the two components, then CaO = CaO + OCO2 CO2 = OCaO + CO2 CaCO3 = CaO + CO2 Thus, the composition of phases present at equilibrium can be fixed by two variables. Hence it is a two component system.
Degree of Freedom The degree of freedom or variance (F) is defined as the smallest number of independent variables such as pressure, temperature, and concentration that must be specified in order to describe the state of the system completely. The degrees of freedom of a system may be one, two, three or zero. Accordingly, the systems are known as uni-variant (or mono-variant), bi-variant, tri-variant and non-variant systems, respectively. For example: 1. Consider a gaseous system containing CO2 and nitrogen. The system is completely defined when three variables of temperature, pressure and concentration are specified. Then the system is said to have three degrees of freedom, i.e., F = A saturated solution of sodium chloride in equilibrium with solid NaCl and water vapour. The system can be completely defined if temperature is specified. At a fixed temperature, the other two variables (the composition of NaCl solution and vapour pressure) have a definite value. So the system has one degree of freedom, i.e., F = 1.
Uses of Phase Diagrams-The following are the uses of phase diagrams: 1. Phase diagrams are useful in understanding the properties of materials in a heterogeneous equilibrium system. 2. The study of low-melting eutectic alloys which are used in soldering can be carried out using phase diagrams. 3. From the phase diagram, it is possible to predict whether an eutectic alloy or a solid solution is formed on cooling a homogeneous mixture of two metals.
Phase rule Willard Gibbs in 1876 deduced the phase rule from thermodynamic considerations. This rule gives a relationship between the number of degrees of freedom (F), the number of components (C) and the number of phases (P) in a heterogeneous system at equilibrium. The relationship is as follows: F = C– P + 2
Uses of Phase Rule 1. Phase rule provides a convenient basis for the classification of systems with the help of phases, components and degrees of freedom. 2. The phase rule indicates that different systems having the same degrees of freedom behave in a similar fashion. 3. It is applied to macroscopic systems, hence information about the molecular structure is not required. 4. The rule considers only the number of phases but not their amounts.
Limitations of Phase Rule 1. The phase rule can be applied only for systems in equilibrium. It is not applicable to systems which attain equilibrium at a later stage. 2. All the phases must be present under the same conditions of temperature and pressure. 3. Only three degrees of freedom are allowed to influence the equilibrium system. 4. The rule considers only the number of phases but not their amounts.
One-Component System: The Water System Under normal conditions the system ‘water’ is a three-phase, one-component system. In this system, water vapour water ice exist in equilibrium. All the three phases can be represented by one chemical formula H2O. Hence, it is a one-component system. If three phases exist together, we have P = 3, C = 1 and the phase rule F = C – P + 2 = 1 – = 0 The system therefore has no degrees of freedom. It is called a non-variant or invariant system, i.e. none of the variables, i.e., pressure, temperature, concentration, can be varied without one of the phases disappearing from the system. Thus, the three phases can only exist together under one condition of temperature and pressure. If only two phases exist together, say ice and water or ice and vapour or water and vapour, the system now has one degree of freedom and is said to be univariant.
This means that one of the degrees of freedom, say temperature, may be varied without causing any alteration in the number of phases: F = C – P + 2 C = 1, P = 2 F = 1 – = 1 When pressure and temperature are required to define the system completely, the system represented by areas has two degrees of freedom and is bivariant: F = C – P + 2 C = 1 P = 1 F = 1 – = 2
Curves OA: This curve is known as vapourisation curve as it represents equilibrium between liquid water and vapour at different temperatures: Liquid water vapour The curve starts from ‘0’, the freezing point of water and ends at A, the critical temperature of water, 374°C. Beyond point A, the two phases, liquid water and vapour, merge into each other, i.e. the number of phases it takes is two because liquid water and vapour exist along the curve OA. For any given temperature, there exists a fixed value of pressure and for each vapour pressure, temperature, has also a fixed value. Thus, the degree of freedom on any point of this curve is 1 or it is univariant, F = 1. Therefore, F = C – P + 2 = 1 – = 1 Curve OB: OB is the sublimation curve or the vapour pressure curve of ordinary ice. Along this curve, the two phases of solid ice are in equilibrium with its vapour. This curve shows vapour pressure of ice at various temperatures. The curve starts at O and ends at B, i.e. absolute zero (–273°C). At this phase, no vapour can exist and, therefore, only ice is present. On the other side of OB, ice is in equilibrium with vapour. Therefore, two phases exist and the degrees of freedom is 1 each. F = C – P + 2 = 1 – = 1 Vapour pressure can be maintained only at a fixed temperature.
Curve OC: OC is the melting point curve or fusion curve of ice. It indicates the effect of pressure on the melting point of ice. The fact that the curve OC is inclined towards the pressure axis indicates that the melting point of ice is lowered by an increase in pressure. At any point on curve OC, ice and liquid water exist in equilibrium. It has one degree of freedom and the system is univariant. The melting point must have a definite value. Triple Point O- The three curves OA, OB, and OC meet at point O, the triple point. Here all the three phases, namely, ice, water and vapour co- exist. Further, P = 3, since there are three phases and one component. So F = C – P + 2 = 1 – = 0 In case of ice at the triple point, the system is non-variant. This occurs at a temperature of °C and pressure 4.5 mm of Hg. Phase would not exist and one of the phases would disappear if temperature and pressure are varied from this value. In the area AOB, only vapour exists and in the area AOC, only water exists. In the area BOC, three areas are bivariant and in order to locate any point in the area, temperature and pressure must be defined. F = C – P + 2 = 1 – = 2
Metastable Equilibrium Sometimes, water can be cooled below its freezing temperature without separating solid ice. The vapour pressure curve AO can be extended to the dashed line A by cooling below its freezing point without separating ice, i.e. water can be super-cooled by carefully eliminating solid particles. Along the curve OA, the liquid is in metastable equilibrium with vapour. As soon as a small particle of ice is kept in contact with supercooled liquid, it changes into ice and the curve merges in ‘OB which lies below OA’. Metastable systems possess a higher vapour pressure than stable systems at the same temperature. The supercooled water/vapour system is metastable or unstable. It soon reverts to the stable system ice/vapour, when there is slight disturbance or when ice crystal is introduced. Areas AOC, AOB, BOC The areas between the curves show the conditions of temperature and pressure under a single phase–ice water or vapour is capable of stable existence. Area AOC represents stable conditions for one-phase system, i.e. water. Area AOB represents conditions for the one-phase system water vapour. The area BOC represents the condition for one-phase system, i.e. ice. In all the three areas there being one phase and one component, we have F = C – P + 2 = 1 – = 2 Thus, each system, water, water vapour, or ice, has two degrees of freedom, i.e. the system is bivariant
Two-Component System: When a single phase is present in a two-component system, the number of degrees of freedom is F= 2 – = 3, i.e. the three variables temperature, pressure and concentration of one of the components must be specified. Construction of the phase diagram of a two-component system will require besides temperature, pressure coordinates, the use of one concentration coordinate, which is possible in the space model or solid model.
A two-component system can be simplified by considering various types of equilibria separately. These are (1) liquid– gas (2) solid–gas (3) liquid–liquid and (4) solid–liquid equilibria. Solid–liquid systems are of great importance because they are connected with all crystallisation problems. These systems are characterised by the absence of a gas phase and are little affected by small changes in pres-sure. Systems where the gas phase is absent are called condensed systems. The measurement on solid–liquid equilibrium in condensed systems is carried out at atmospheric pres-sure. The phase rule takes the form F= C– P+ 1. The reduced phase rule is more convenient to apply to a solid–liquid two-component condensed system. The variables are temperature and conentration of one of the constitutents. Solid–liquid equilibrium is represented on temperature composition diagrams.
Simple eutectic system Eutectic means easy to melt. A binary system in which two components react chemically and each component reduces the freezing point of the other is called eutectic system. In such a system, the two components are miscible in all proportions, i.e., eutectic system is a binary system consisting of two different, totally immis-cible components in solid and miscible in liquid state without any chemical interaction. The solid solution of a two-component system having lowest freezing point is called eutectic mixture. The lowest melting point that can be obtained corresponding to eutectic mixture is known as eutectic point. The eutectic mixture possesses sharp melting point and definite composition. But it cannot be regarded as a com-pound, because (1) the compound does not possess components in stoichiometric proportion and (2) when the components are examined under a microscope, they appear as separate crystals of the compound. Consider a two-component system having two components Aand B. In liquid state, these components are completely miscible and these solutions on cooling yield pure Aor pure Bas solid phases. Figure 7.3 consists of Curve AC, the freezing point curve of A. On addition of Bto A, the freezing point of A falls along curve AC. Along this curve, solid Ais in equilibrium with the liquid solution of Bin A. Curve BC,the freezing point curve of B. The curve exhibits decrease in freezing point by the addition of Ato B. Along this curve, solid Bis in equilibrium with the liquid solution of Ain B. When reduced phase rule equation is applied, F= C– P+ 1 = 2 – = 1 The degree of freedom is 1, i.e., both equilibria are monovariant.
Eutectic point C: The curves ACand BCintersect at the eutectic point C, and the three phases—solid Ag, solid Pb and solution— are in equilibrium. Applying reduced phase rule to the system, F= C– P+ 1 = 2 – = 0, i.e., At the eutectic point C, the system Ag/Pb/solution is non- variant. Both the variables, temperature (303°C) and composition (97.5% Pb and 2.5% Ag), are fixed. If the tem-perature is raised above eutectic temperature, the solid phases Ag and Pb disappear and if the system is cooled below the eutectic point, only solid Ag/Pb exists where solution phase is non-existent. Area above AOC:This region is a single-phase system. The solution of molten Ag and Pb exists. When condensed/reduced phase rule is applied, F= C– P+ 1 = 2 – = 2. Thus, Ag/Pb system is bivariant. The area below ACrepresents the phase Ag+ solution and that below BC, the phase Pb+ solution. The area below 303°C represents solid Ag+ and solid Pb. All these areas have two phases and one degree of freedom. F= C– P+ 1 = 2 – = 1
Solid Solutions - These are the solid phases containing more than one component. The phase rule makes no distinction between the kind of phase (solid, liquid, gas) that occurs, being concerned with how many phases are present. Two general classes of solid solutions can be found on structural grounds: (1) substitutional solid solution and (2) interstitial solid solution. A substitutional solid solution is one in which solute atoms or molecules are substituted for solvent atoms or molecules in the crystal structure. For example, nickel has a face-centered cubic structure; if some nickel atoms are replaced at random by copper atoms, solid solution is obtained. The substitution is possible only when they do not differ in size. An interstitial solid solution is one in which the solute atoms or groups occupy interstices in the crystal structure of solvent. For example, carbon atoms may occupy some of the interstices in the nickel structure. Interstitial solid solutions can occur only when solute atoms are small compared to the solvent atoms.
Iron–carbon diagram: The discussions on phase diagrams would be incomplete without the iron–carbon system, which is the prin-cipal theoritical basis for iron and steel industry. The phase diagram shown in Fig. 7.5 extends from pure iron to the compound iron carbide Fe3C.Pure iron is ductile. Molten iron in pure form freezes into δ-ferrite at 1537°C.
Difference Among Melting Point, Triple Point and Eutectic Point- At the melting point, a solid and a liquid having the same composition are in equilibrium. At the triple point, three phases are in equilibrium. At the eutectic point, two solids and liquid are in equilibrium. By definition,all eutectic points are melting points, but all melting points need not be eutectic points. Similarly, all eutectic points are triple points, but all triple points need not be eutectic points.
Heat Treatment Heat treatment is the combined operations of heating and cooling of a metal or an alloy in solid state in order to get desired properties. Heat treatment is mainly aimed at (1) increasing strength, toughness, hardness, ductility to steel, (2) relieving internal stresses and strains and (3) normalising steel which has been already subjected to mechanical (or) heat treatment. After heat treatment, the properties of high-carbon steels are altered to a much greater extent than lowcarbon steels. The heat treatment methods include the following processes:
(1) Hardening: Many desirable properties of alloys, ceramics and structural materials depend on the components being in the form of solid solutions. The hardening and tempering of steel involve the existence of solid solutions of carbon in different iron carbon compounds. The solid solution stable at high temperature is hard, and in order to retain hardness, proper compositions and temperatures are obtained as indicated by phase diagrams. The steel is immediately quenched in oil or water so that it does not have time to form the solid solution which is stable at lower temperature. By heating up again at little lower temperature, partial conversion to softer solid solution occurs which is stable at lower temperature. In this way, the steel may be given different degrees of hardening.
(2) Transformation hardening: Plain carbon steel on heating to a temperature above 723°C for longer periods of time allows the formation of austenite phase and the dissolution of more carbon in fcc structure. The fcc structure is changed to bcc and excess carbon forms cementite on slow cooling. If the steel is quenched by plunging into water to 204°C, the carbon atoms do not have sufficient time to form cementite, but remain trapped in the bcc lattice. Excess carbon gets precipitated in the hot metal and prevents slipping of planes. Thus, quenched steel is quite hard and strong and has lower ductility. This heat treatment is called transformation hardening, i.e., it involves transformation of austenite to cementite which is a hard steel.
(3) Tempering: The carbon steel in austenite state is quenched by plunging in water (or) brine, the unstable g-iron in fcc and held carbon in solid soln. is changed to stable a- iron at that temp. Quenched steel is not useful for construction purpose because of its brittleness. The quenched steel is then tempered by reheating to below a-iron to g-iron transition temperature. At elevated temperature, there is greater atomic mobility. The stresses and strains are relieved and excess carbon is rejected as Fe2.4C. The sample is then cooled in air, water, oil, etc., to get resistance to abrasion and the steel is capable of withstanding shock loads. (4) Annealing: The steel is cooled very slowly so that equilibrium is established and stress-free steel is obtained. It involves dissolution of carbon in g-iron followed by slow and controlled cooling. The steel becomes soft, ductile and machineable. Annealing decreases the hardness and strength of steel. Therefore proper heating must be done. It depends on the chemical composition of steel and on the structure and properties of the desired product. If steel is heated to a higher temperature the grain size increases and the material becomes weak. This is required only for drawing the metal into a wire. Mild and medium carbon steel (Hypo-eutectoid is < 0.81% C) are heated to just above upper critical range. Hard steel (hyper-eutectoid, i.e., 70.8% C) are heated above upper critical range. Annealing consists of three stages, heating to the right temperature, soaking (or) keeping at that temperature and slow cooling in a furnace (or) buried in sand, ash (or) lime.
Advantages (a) Annealing removes internal stresses. (b) It changes ductility, toughness, electrical and magnetic properties. (c) It removes gases entrapped within the mass. (d) It improves machinability.
5) Normalising: Normalsing means to bring back the structure and properties that are normal for a sample. By normalising, fine, stronger pearlite is produced. It is a better method for producing the steel with high tensile strength, yield strength, impact resistance. Both normalising and annealing treatments are given for making the steel soft and homogeneous. Normalising is the corrective treatment for grain refinement in steel after it has been subjected to rolling, forging, etc. Hyper-eutectoid steels are rarely normalised because higher temperatures. are used and there is a tendency to form coarser grains which reduce the strength. Normalising differs from annealing in two ways: (a) In normalising, cooling is done in still air and not in a furnace. Therefore, the rate of cooling is faster which prevents the formation of coarse grains at high temperature. (b) In annealing, hyper-eutectoid steels are heated above lower critical range, and in normalising, both hyper- and hypo-eutectoid steels are heated above upper critical range.
Theoretical Questions 1. State the phase rule and explain the terms involved with suitable examples. 2. Discuss the phase diagram of Ag–Pb system and its use in obtaining pure metals. 3. What are interstitial solid solutions? 4. Define phase, component and degree of freedom with two examples each. 5. Describe, in detail, the phase diagram of one-component water system. 6. Illustrate the number of components in the equilibrium given below: 7. Why are the number of phases in the following two sets different? (a) Oil + Water (b) Alcohol + Water 8. E xplain the concept of metastable state with the help of a phase diagram of water. 9. What are the differences between melting point, triple boint and eutectic points? 10. What is meta-stable equilibrium? Explain with an example. 11. What is eutectic point? 12. Discuss the application of phase rule to Ag–Pb system. 13. Describe the salient features of Fe-C system and its relevance to heat treatment of steel. 14. E xplain the terms case hardening, normalizing, nitriding and cyaniding? 15. What is the significance of Gibbs’ phase rule? Discuss its applications and limitations.
Multiple Choice questions 1. For one-component system, the phase rule is (a) F = 3 – P (b) F = 2 – P (c) F = 1 – P (d) none of these 2. A saturated solution of NaCl in water has degrees of freedom equal to (a) 1 (b) 2 (c) 3 (d) 4 3. At a triple point, (a) both temperature and pressure are fixed (b) only temperature is fixed (c) only pressure is fixed (d) sometimes temperature and sometimes pressure is fixed 4. For one-phase and one-component system, the degrees of freedom are equal to (a) 1 (b) 2 (c) 3 (d) 4 5. A system with zero degree of freedom is known as (a) mono-variant (b) bivariant (c) invariant (d) none of these 6. The reduced phase rule for condensed system is (a) F = C – P + 2 (b) F = C – P + 1 (c) F = C – P (d) F = C – P The phase rule is applicable to (a) homogeneous system (b) reversible system (c) irreversible system (d) heterogeneous system 8. For a one-component system in a single phase, the degree of freedom is (a) 0 (b) 1 (c) 2 (d) 3 9. A one-component system has four phases. Can the four phases co-exist in equilibrium? (a) No (b) Yes (c) Sometimes (d) None of these 10. Heat treatment is aimed at (a) increasing strength, toughness and
11. Annealing of steel helps to (a) decrease the hardness (b) decrease machinability (c) increase internal stresses (d) bring back the original structure 12. For water system, the maximum number of degrees of freedom is (a) 0 (b) 3 (c) 2 (c) Low carbon steels are hardened by (a) quenching (b) case hardening (c) cyaniding (d) nitriding 14. A system consists of water in contact with its vapour; the number of degrees of freedom is (a) unpredictable (b) one (c) two (d) zero 15. Phase rule for one-component system is (a) F = 3-P (b) F = 2 – P (c) F = C – P (d) none of the above 16. Making hard steel resistant to abrasion is called (a) annealing (b) tempering (c) case hardening (d) hardening
17. Low-carbon steels are hardened by (a) quenching (b) case hardening (c) nitriding (d) cyaniding 18. When lead in added to silver progressively, the melting point of the resulting alloy is (a) increased (b) decreased (c) unchanged (d) cannot be predicted 19. Strength and hardness of steel is increased by (a) normalizing (b) annealing (c) quenching (d) all of the above 20. A system consists of water in contact with its vapour. The degree of freedom is (a) 0 (b) 1 (c) 2 (d) The hardest structure that appears on the iron–carbon equilibrium is (a) ledeburite (b) cementite (c) astenite (d) pearlite 22. A system containing liquid water and water vapour has the number of phases (a) 0 (b) 1 (c) 2 (d) A saturated solution of NaCl is a _____________ system. ( b ) (a) one-phase (b) two-phase (c) three-phase (d) no-phase 24. The number of components of water system having three phases—ice, water, and water vapour—is (a) one (b) two (c) three (d) four 25. The decomposition of CaCO3 in closed vessel is represented by CaCO3(s) → CaO(s) + CO2(g). It has _____________ number of phases and components, respectively. (a) 2 and 3 (b) 3 and 2 (c) 2 and 2 (d) 3 and The appearance of same substance in more than one crystalline form is known as (a) polymorphism (b) metamorphism (c) racemisation (d) isomerism 27. For one-phase and one-component system, the degree of freedom is equal to (a) 1 (b) 2 (c) 3 (d) _____________ is the temperature at which a polymer substance changes from one form to another. (a) Triple point (b) Transition temperature (c) Ignition temperature (d) Equilibrium temperature
SURFACE CHEMISTRY Introduction -Adsorption is the phenomenon of concentration or assimilation of a gas or liquid on the surface of a solid or liquid with which it is in contact. Adsorbent is the material which provides the surface on which adsorption occurs and the substance adsorbed or attached is called adsorbate. Examples of adsorbents are charcoal, silica gel, clay, fullers earth, alumina gel, etc. Adsorbent molecule (adsorbate) Fig. 8.1 The adsorption of gas on solid surface
Differences between absorption and Adsorption Adsorption is a surface phenomenon and it is a fast process. In absorption, diffusion into the interior of matter takes place, hence it is a slow process. For example, when a blotting paper is kept in contact with ink, ink is absorbed into the paper. on the other hand, if a dilute solution of litmus is shaken with animal charcoal the surface of charcoal takes away some of the litmus and all the litmus is concentrated on charcoal. In adsorption, equilibrium is easily attained but in absorption it takes some time to reach equilibrium. Absorption is a bulk phenomenon but adsorption is a surface phenomenon. Adsorption depends on the surface area of adsorbent. Consequently, it is more rapid on finely divided and on a rough surface. Such effect is not observed in absorption. Fig. 8.2 Differences between absorption and adsorption
Types of Adsorption Adsorption is not necessarily a physical phenomenon. It may be a chemical process involving chemical reaction between the surface atoms of adsorbent and the atoms of adsorbate. This type of adsorption is known as chemisorption. It results in surface complex. For example, O2 is chemisorbed by carbon and H2 by nickel under suitable conditions. Chemisorption differs from physical adsorption in the following respects: 1. Physical adsorption occurs appreciably at very low temperature, i.e, below boiling point of adsorbate. chemisorption occurs at all temperatures. 2. The magnitude of physical adsorption decreases with rise in temperature. The magnitude of chemisorption increases with temperature. 3. The heat evolved during physical adsorption is very low, i.e., 4–40 kj mol–1. It is very high (40–400 kj mol–1) in chemisorption.
4. Chemisorption is irreversible as the gas adsorbed cannot be recovered from adsorbent as such on lowering the pressure of the system at the same temperature. Physical adsorption is reversible as the gas adsorbed can be recovered by simply lowering the pressure of the system. 5. Physisorption may extend beyond a monolayer. Chemisorption operates within short distance only. It does not extend beyond the monolayer of gas molecules. 6. In physisorption, the adsorbate molecules are held together by weak van der Waals forces. Hence, activation energy for disorption in very low. In chemisorption, adsorbate molecules are held by strong valence forces and its activation energy for disorption is high. Adsorption isotherms-The adsorption of a gas on a solid adsorbent in a closed vessel is a reversible process. The amount of gas adsorbed depends on equilibrium pressure (p) and temperature. Adsorption isotherm is a graph plotted between a given magnitude of adsorption and pressure at a given temperature. It may be given on a graphical curve which is known as Freundlich adsorption isotherm
Freundlich Adsorption Isotherm The relation between the magnitude of adsorption and pressure can be expressed mathematically by an empirical equation known as Freundlich adsorption isotherm. where x/m is amount of gas adsorbed per unit mass of the adsorbent at pressure P. k and n are variables which depend on the nature of the solid, gas and the nature of adsorbent. The extent of adsorption x/m increases with increase in pressure (P) and becomes maximum at saturation pressure P0. At P0, the rate of adsorption becomes equal to the rate of desorption. Further increase of pressure has no effect on adsorption.
From the adsorption isotherm (Fig. 8.4), the following observations can be made:
Limitations of Freundlich adsorption isotherm 1. It fails when pressure is high and when the concentration of adsorbent is very high. It is valid only within a limited range of pressure. 2. The constants k and n change with temperature. 3. It has no theoretical foundation. It is only an empirical formula.
Langmuir’s theory of adsorption (MonoLayer adsorption) Langmuir proposed a better equation to explain adsorption isotherms on the basis of theoretical consideration. Langmuir proposed that the surface of solid possesses fixed number of adsorption sites per unit area and each site could adsorb only one molecule of gas. Therefore, the surface of solid is covered by mono-molecular gaseous layer. Since the solid surface is assumed to be homogeneous, the molecular adsorption at each site is independent of other adjacent sites occupied or vacant. The adsorbed gas molecules remain localised without any interaction with neighbouring molecules. One site adsorbs one molecule. When a uni-molecular layer is formed by the adsorption of gas, no further adsorption occurs, i.e., saturation is obtained. In Langmuir’s adsorption, there is a dynamic equilibrium between adsorbed gas molecules on the surface of solid and evaporation of adsorbate from the surface of adsorbent.
Before adsorption, the surface remains vacant and the rate of condensation stays maximum. As the surface becomes covered, the rate of desorption (evaporation) is less in the beginning and it increases when the surface becomes covered. At equilibrium, the number of molecules evaporating/unit time from the same surface, i.e., rate of adsorption and rate of disorption, is in equilibrium. Based on the above postulates, the rate of adsorption depends on the pressure P and the number of vacant sites on the surface (1 – θ), where q is the fraction of surface occupied by gas molecules. Now, since the rate of adsorption is proportional to the pressure (P) of the gas as well as uncovered surface (1 – q) of the adsorbent available for adsorption, thus Rate of adsorption α P (1 – θ) = k1P(1 – θ)
Adsorption of Gases by Solids Several methods for determining the adsorption of gases on solid adsorbent have been derived. In one such method, the gas is contained in a vessel of known volume at a given temperature. The pressure of the gas is measured by a monometer attached to the vessel. The adsorbent is slowly introduced into the vessel. Adsorption takes place quickly and the pressure of the gas falls which is noted on the monometer. Knowing the fall of pressure, the quantity of gas adsorbed by solid can be calculated, assuming Boyle’s law to hold good. As a result of adsorption, the residual forces on the surface of adsorbent decrease. Hence, there is a decreaseof surface energy which appears as heat. Adsorption is invariably accompanied by decrease in the enthalpy of the system.
Factors influencing Adsorption The magnitude of gaseous adsorption depends on the following factors: (1) temperature, (2) pressure, (3)nature of the gas and (4) nature of adsorbent. Effect of temperature and pressure: Since adsorption is accompanied by evolution of heat, according to Le Chatelaine's principle, the magnitude of adsorption should increase with fall in temperature and increase with increase in pressure. Nature of gas and nature of adsorbent: The more readily soluble and easily liquefiable gases such as ammonia, hydrochloric acid, chlorine, sulfur dioxide are adsorbed more than permanent gases such as hydrogen, nitrogen and oxygen. This is because the van der Waal’s forces involved in adsorption are more predominant in the former category than the latter category of gases. Since adsorption is a surface phenomenon, it is evident that the greater the surface area per unit mass of the adsorbent, the greater is its capacity for adsorption under the given conditions of temperature and pressure.
COLLOIDS In Greek, kolla means glue and eidos means like. Depending on their ability to diffuse in liquid medium,substances are classified into (1) crystalloids and (2) colloids. 1. Crystalloids: Crystalloids diffuse rapidly in solution and can pass very easily through animal and vegetable membranes. Examples: urea, sugar and other crystalline substances. 2. Colloids: Colloids diffuse very slowly in solution and cannot pass through vegetable and animal membranes. Examples: Gelatin, starch and proteins. But according to Graham, every substance irrespective of its nature can be a colloid or crystalloid under suitable conditions, e.g., soaps show crystalloid character in alcohol in which they are freely soluble and show colloidal character in water in which they are sparingly soluble. Thus, it is not feasible to classify the substances into crystalloids and colloids. Hence, the term colloidal substance has been replaced by colloidal state.
True Solution, Colloidal Solution and Suspension A true solution is one in which the particles are invisible and do not settle on standing, e.g., molecules of sugar in water. When the particles settle down on standing and can be separated very easily from the solvent, such mixtures are called suspensions. They are large enough to be visible to the naked eye or under a microscope. They are hetergeneous in nature. In between these two limits, there are particles which are bigger than molecules but smaller than suspension particles. They cannot be seen under a microscope. Such particles are said to be in ‘colloidal state’ and when suspended in liquid are referred to as colloidal solutions. A colloidal system is made of two phases. The substance distributed as colloidal particles is called dispersed phase. The continuous phase in which the colloidal particles are dispersed is called dispersion medium, e.g., in a colloidal solution of copper in water, copper particles constitute dispersed phase and water the dispersion medium.
Classification of Colloids or Sols Colloids are of two types: (1) lyophilic or reversible and (2) lyophobic or irreversible. If water is the dispersion medium, the terms hydrophilic and hydrophobic are used. Lyophilic Colloids (Lyo = Liquid, Philic = Love) A colloidal system obtained by warming or shaking the substance with a suitable solvent is known as lyophilic colloid, e.g., gelatin, starch, protein, gum, rubber. These colloids are reversible in nature. On evaporating the dispersion medium, the residue can be reconverted into colloidal state by the addition of liquid. They are stable and cannot be easily precipitated. The affinity of colloidal particles for the medium is due to hydrogen bonding with water. If a protein (as in egg) is considered as dispersed phase, hydrogen bonding would occur between water molecules and the amino group of protein.
Lyophobic Colloids (Lyo = Liquid, Phobic = Hate) These colloids are not formed easily. Lyophobic colloids are formed by substances such as As2S3, Fe(OH)3, gold and other metals which are sparingly soluble and thus their molecules do not readily pass through colloidal state. There is no hydrogen bonding when these colloids are dispersed in water. These colloids are known as irreversible colloids since the residue obtained by evaporating dispersion medium cannot be readily reconverted into colloid by ordinary means. These sols or colloids are not stable and can be readily precipitated. Examples: Dispersion of gold, iron (III) hydroxide and sulphur in water.
Characteristics of Lyophilic and Lyophobic Colloids The important properties of lyophilic and lyophobic colloids are described below. Heterogeneous character: Unlike true solutions, colloidal systems are heterogeneous in nature. They consist of two phases, viz., dispersed phase and dispersion medium. Ease of preparation: Lyophilic colloids are prepared by mixing the material (starch, protein) with a suitable solvent. Lyophobic sols cannot be prepared by simply mixing the solid material with the solvent.
Charge on particles: Lyophilic colloidal solutions have little or no charge at all, whereas lyophobic colloids carry positive and negative charges which give them stability. Viscosity: Lyophilic sols are viscous in nature. The particle size increases due to solvation. Viscosity of lyophobic colloid is almost similar to that of dispersion medium. Colligative properties: The colligative properties such as osmotic pressure, lowering of vapour pressure, elevation of boiling point and depression of freezing point depend on the number of solute particles in a given weight of solvent. Since the colloidal particles are aggregation of molecules, the number of particles will be very small as compared to the number of particles in true solution for a given mass of colloids. Hence, unlike true solution, colloidal system gives very low osmotic pressure and shows very small elevation of boiling point and depression of freezing point.
Optical properties: (1) Tyndall effect: A colloidal solution exhibits Tyndall effect. When a strong beam of light is passed through a colloidal solution and observed at right angles, the path of light shows up as a hazy beam or cone. This is due to the fact that colloidal particles absorb light energy and emit it in all directions in space. This phenomenon is known as Tyndall effect and the illuminated path is known as Tyndall cone. Tyndall effect is due to the scattering of light from the colloidal surface. The appearance of dust particles in a semi-darkened room when a sun beam enters or when a light is thrown from a projector in cinema hall is the well- known example of Tyndall effect. The dust particles are large enough to scatter light which makes the path of light visible.
Tyndall effect is used to distinguish between a true solution and a colloidal solution, since the ions of solute molecules are very small to scatter light and the beam of light passing through true solution is not visible when viewed from the side. Conditions for the system to show tyndall effect are as follows: (a) The diameter of particles of the dispersed phase must not be much smaller than the wavelength of light used. (b) The difference between the refractive indices of the dispersed phase and dispersion medium must be appreciably high. Lyophilic colloids have very small difference of refractive indices and hence the Tyndall effect is very small. On the other hand, lyophobic colloids shows Tyndall effect because they satisfy the above condition.
(2) Colour: The colloidal sols are often coloured. The colour of a colloidal solution is not always the same as that of the dispersed phase in bulk. It depends on the size and shape of the dispersed particles. For example, a gold sol is red when the particles are extremely fine and in spherical shape, but when the dispersed particles are bigger and flat it appears blue in colour. (3) Visibility: Even under a powerful microscope, colloidal particles connot be seen. It is not possible to see a particle whose diameter is less than half the wavelength of light used. Thus, the particles of diameter less than 200 mμ cannot be seen, since the shortest wavelength of visible light is 400 mμ, e.g., gold sol cannot be seen by a microscope.
Kinetic Property of Colloidal Solutions Brownian movement When a colloidal sol is observed under an ultra microscope, we can find a continuous, zigzag random motion of particles (Fig. 8.14). This kinetic activity of particles of colloid is known as Brownian movement. It depends on the size of the dispersed phase and viscosity of the dispersion medium. Brownian movement is due to molecular impacts from the medium on all sides of dispersed particles. The movement of colloidal particles is much slower than that of molecules of the medium, because colloidal particles are heavier than molecules of dispersion medium.
Brownian movement proves the existence of molecules and thermal motion of molecules, since the colloidal particles acquire almost same energy possessed by the molecules of the dispersion medium. Fig. 8.14
Coagulation or precipitation The stability of lyophobic solution is due to the adsorption of positive or negative ions by dispersed particles. The repulsive forces between the charged particles do not allow the particles to settle down. If by any means the charge is removed, then the particles get precipitated and settle down under gravity. The settling down or flocculation of discharged solution particles is called coagulation or precipitation of sol. Coagulation can be brought about by (1) the addition of ectrolytes, (2) electrophoresis, (3) mixing two oppositely charged sols and (4) boiling.
Addition of electrolytes: When excess of electrolyte is added to the sol, the dispersed particles get precipitated. Sol particles adsorb oppositely charged ions and get discharged. Electrically neutral particles then aggregate and settle down as precipitate. It depends on the valency of the effective ion. the higher the valency of the effective ion, greater is its power of precipitation. Example: For precipitating As2S3 sol (negative), the precipitating power of Al+3, Ba+2, Na+ ions is in the order Al+3 > Ba2+ > Na+ For precipitating Fe(OH)3 sol (positive), [Fe(CN)6]3–, SO4 2–, Cl– have been used. Then the order of precipitating power is [Fe(CN)6]3– > SO4 2– > Cl– The precipitating power of an electrolyte is expressed by its flocculation value. It is the minimum concentration in millimoles per litre required to cause the precipitation of a sol in 2 hours. the smaller the flocculation value, the higher is the precipitating power of the ion.
By electrophoresis: The charged particles of colloids migrate to oppositely charged electrodes. They get discharged and precipitated soon after they come into contact with electrode. By mixing oppositely changed sol: Positive particles of one sol are attracted by negative particles of second sol. Mutual adsorption and precipitation of both the sols occur. By boiling: On boiling, the collision between sol particles and water molecules removes adsorbed layer, charges from particles are taken away and precipitation occurs.
Protective action of colloids Lyophilic colloids are stable and reversible. The stability depends on the degree of hydration. Lyophobic sols are easily precipitated when a small amount of the electrolyte is added. These sols are also stabilised if a small amount of lyophilic sol is added. The property of lyophilic sols to prevent the precipitation of a lyophobic sol is called protection. The lyophilic sol used to protect a lyophobic sol. from precipitation is known as protective colloid. Example: When a small amount of gelatin (lyophilic colloid) is added to a gold sol (lyophobic sol), lyophobic sol is protected which no longer gets precipitated. It is because the particles of lyophobic sol adsorb the particles of lyophilic sol, and the lyophilic colloid forms a protective cover around lyophobic sol particles.
The protective power of lyophilic sol is expressed in terms of gold number. Gold number is defined as the number of milligrams of a lyophilic colloid that will just prevent the precipitation of 10 ml of gold sol on addition of 1 ml of 10% solution of NaCl. The start of precipitation of gold sol is indicated by a colour change from red to blue with increase in particle size. The smaller the gold number, the greater is the protective action of the lyophilic colloid.
Micelles The molecules of colloids at low concentration act as strong electrolytes. At higher concentration, they form thermodynamically stable particles of colloidal dimensions called association colloids or ‘micelles’. Micelles have lyophobic tails which get congregated and lyophilic heads which provide protection. Example: Colloidal aggregate of soap (sodium oleate, sodium stearate) or detergent molecules formed in the solvent. Sphere represents lyophilic groups. Stalks represent lyophilic groups. The zigzag hydrocarbon tail is shown by a wavy line and the polar head by a hollow circle (Fig. 8.15).
Cleansing Action of Soaps and Detergents The cleansing action of soap is due to (a) solubilisation of grease into the micelle and (b) emulsification of grease. Solubilisation: When soap solution is added to a fabric, the tails of the soap anions penetrate into the grease stain. The polar heads protrude from the grease surface and form charged layer around it. The hydrocarbon tails are in the interior of the micelle and the COO– ions on the surface. The grease droplets are suspended by mutual repulsions. The emulsified stains of grease are washed away with soap solution. Emulsification: When soap or detergent molecules are ionised in water, the anions are made of oil-soluble hydrocarbon tails and water- soluble polar heads. The polar heads protrude from grease surface and form charged layer. By mutual repulsion, the grease droplets are suspended in water. The emulsified grease stains are washed away.
Applications of colloids 1. Electroplating of rubber: Rubber is a colloidal suspension of negatively charged particles in water. By electrophoresis, it can be made to deposit on various tools. These negative particles migrate towards anode and get deposited on it during electrolysis. 2. Leather tanning: The coagulating ability of colloids is used in leather tanning. Leather is a positively charged colloid when mixed with wood, mutual coagulation takes place and leather surface gets hardened. 3. Chrome tanning: Leather can be subjected to chrome tanning when hydrated chromic oxide penetrates into leather under the influence of electric field. 4. Medicine: The ease of adsorption and assimilation of colloids makes their use in a number of medicinal and pharmaceutical preparations where colloidal gold, iron, calcium, etc., are administered (orally or injected) to raise the vitality of human system. Trivalent Al+3 and Fe+3 colloids are used in the coagulation of blood. Many skin ointments consist of physiologically active components dissolved in oil and made emulsion with water. Penicillin and streptomycin antibiotics are produced in colloidal form suitable for injection. and scatter the light of blue colour. This is an application of the Tyndall effect. 5. Food: Many food materials are colloidal in nature. For example, butter-milk (emulsion of fat in water), cheese, fruit jelly, eggs, whipped cream, protoplasm, blood, etc. Ice cream is a dispersion of ice in cream. Bread is a dispersion of air in baked dough. 6. Artificial rain is due to the aggregation of minute colloidal particles. Clouds are charged particles of water dispersed in air. Electrified sand is thrown into clouds to get rain. 7. Blue colour of the sky: The outer atmosphere contains colloidal dust particles dispersed in air. As the rays of the sun strike the colloidal particles, they absorb sunlight
EXPLOSIVES AND ROCKET PROPELLANTS Explosives and rocket fuels are closely related materials, since in both the cases exothermic chemical reaction resulting in large amount of energy takes place. The materials which were earlier used as explosives are now used as rocket fuels. An explosive is a substance which when subjected to thermal or mechanical shock gets oxidized exothermically into the product of increased volume with sudden release of large amount of potential energy. In explosive reactions, being exothermic, the products get heated up to very high temperatures and exert high pressure on the surroundings which can be exploited for constructive or destructive purposes. When an explosion occurs in a confined space, the high-pressure conditions developed within the system shatter the confining walls or if developed under relatively slower and controlled rate, the energy may be used to propel projectiles. The quantity of power realizable from a given weight (or volume) of an explosive is called powerto- weight (or volume) ratio which is small in case of gases. In order to achieve a better ratio, solids and/or liquids are used as explosives.
Classification of Explosives Explosives are broadly classified into three groups: (1) primary explosives or detonators, (2) low explosives or propellants and (3) high explosives. Primary Explosives or Initiating High Explosives or Detonators These are highly sensitive explosives which explode under a slight shock or blow by ignition. They have to be very carefully handled. They are used in very small quantities to initiate explosion of less sensitive explosives such as TNT. Hence, they are called initiating high explosives. Primary explosives are used in blasting caps and catridges. Examples of primary explosives include:
Leade azide (PbN6): Beacause of its low cost, excellent initiating action, stability in storage lead azide reacts with brass, hence caps loaded with lead azide are made of aluminium. It cannot initiate explosion of TNT Tetracene (C2H7N7O): Tetracene ignites easily and has high heat of explosion and produces large volume of gases. It has low initiating property hence it is not used by military. It is only used as a detonator. Mercury fulminate [Hg(CNO)2]: It is more sensitive and expensive and more toxic than lead azide. Hence, its usage is limited. Diazonitrophenol (DDNP): It is quite sensitive and can initiate explosion in less-sensitive high explosives. It is widely used in commercial blasting caps.
Low Explosives or Propellants They do not explode suddenly but burn. The chemical reactions takes place comparatively slowly and burning proceeds from the surface inwards in layers at approximately 20 cm/s and the evolved gases disperse readily without building up pressure. Examples: Cellulose nitrate, gun powder. Low explosives are used as propellants (to propel missiles) and in fire works (i.e., pyrotechniques). Low explosives are divided into two categories depending on their applications. Millitary explosives: RDX, HMX, TNT, nitrocellulose, tetryl, picric acid, PETN, DDNP, Lead azide, etc. Industrial explosives: GTN, dynamite, etc., are used for industrial purposes.
High Explosives These explosives are insensitive to mechanical shock and to the flame i.e.; they do not explode on ignition. They explode with great violence only when initiated with the help of detonators. They are stable and possess high energy content than primary explosives. The rate of explosion is about 1500 to 10,000 m per second. Under the influence of high temperature large volume of gases are evolved which produces shattering effect. High explosives are sub-divided into 1. Military high explosives, 2. Blasting or industrial purpose explosives. Examples: GTN: Glyceryl trinitrate RDY: Cyclo trimethylene trinitroamine PETN: Pentaerythetol tetranitrate TNT: Trinitrotoluene
Ammonium nitrate: It is half as powerful as TNT (2,4,6 trinitrotoluene) and employed in making binary explosives. It is dangerous to store near any inflammable material. It cannot be used in contact with brass, since it produces a detonator – tetrammino cupric nitrate. TNT: It is used in shell-firing and under-water explosives. It can be loaded in containers because of its low melting point (81°C). Because of its (1) non-hygroscopic nature and (2) inertness to metals, TNT is used as safe explosive in the manufacture, storage and transportation. RDX or cyclonite (cyclotrimethylene trinitroamine): It is a powerful high explosive. It is more sensitive and less toxic than TNT. It is used both in military and industrial purpose explosive. Picric acid (or trinitrophenol): It is replaced largely by TNT since it forms shock-sensitive picrates with metals. Explosives, based on their state of aggregation are classified as solid (e.g.,TNT), liquid (e.g., nitroglycerine) and gaseous (e.g., mixture of oxygen and acetylene).