2 The reverse process of adsorption is called desorption . Surface Chemistry Deals with phenomena that occur at the interfaces or surfaces.Adsorption – It refers to the accumulation of a substance at the surface than in the bulk . It differs from absorption which involves accumulation of a substance in the bulk than at the surface .Sometimes both adsorption & absorption occur simultaneously . This is called sorption.The reverse process of adsorption is called desorption .
3 The substance which gets adsorbed is referred to as adsorbate & the surface over which adsorption occurs is called adsorbent .Adsorption is a surface phenomenon.Solids as adsorbent should be finely divided.Should have large surface areaTherefore, charcoal, silica gel, alumina gel, clay,colloids, metal in finely divided state act asgood adsorbents.
4 Examples of Adsorption Adsorption of a gas by charcoal.Adsorption of a dye by charcoal.Purification of raw sugarRemoval of moisture from air.
5 Difference between Adsorption & Absorption It is the phenomenon in which particles of gas or liquid get uniformly distributed throughout the body of the solid.It is the phenomenon of higher concentration of particles of a gas or liquid on the surface than in the bulk.Occurs at uniform rate.Rapid in the beginning and rate slowly decreases.e.g. Anhydrous calcium chloride absorbs water.e.g. Silica gel adsorbs water vapour
6 Mechanism of Adsorption Adsorption is due to the fact that the surface particles of the adsorbent are not in the same environment as the particles inside the bulk.Inside the adsorbent, all the forces acting between the particles are mutually balanced but on the surface the particles are not surrounded by atoms or molecules on all sides.They possess unbalanced force or residual attractive force.
7 These forces of the adsorbent are responsible for attracting the adsorbate particles on its surface. The extent of adsorption increases with the increase of surface area per unit mass of the adsorbent at a given temperature and pressure.
8 Thermodynamic AspectEntropy decreases during adsorption, ∆S is negative.For the process to be spontaneous, ∆G must be negative.∆H should have sufficiently high negative value as –T∆S is positive.Hence, adsorption is an exothermic process.
9 Types of AdsorptionDepending upon the nature of forces which hold the molecules of the adsorbate(GAS) on the surface of the adsorbent(SOLID), adsorption is classified into two types:1. Physical adsorption (Physisorption)2. Chemical adsorption (Chemisorption)
10 Difference between Physisorption & Chemisorption Forces between the adsorbate molecules & adsorbent are weak vanderwaal’s forces.Forces between the adsorbate molecules & adsorbent are strong chemical forces Chemical Bonds).Low enthalpy of adsorption, 20 to 40 KJ/mol.High enthalpy of adsorption, 80 to 240 KJ/mol.Not specific in natureHighly specific in nature.ReversibleIrreversibleDepends on the nature of gas. More liquefiable gases are adsorbed easilyDepends on the nature of gas. Gases which can react with the adsorbent show chemisorption.Low temperature is favourable for adsorption. Decreases with increase of temperature.High temperature is favourable for adsorption. Increases with increase of temperature.
11 No appreciable activation energy is needed. High activation energy is sometimes needed.It depends on surface area. It increases with an increase of surface area.It also depends on surface area. It increases with an increase of surface area.It results into multimolecular layers on adsorbent surface under high pressure.It results into unimolecular layer.
12 Factors affecting adsorption Nature of adsorbate – A gas which has greater intermolecular attraction can be more readily adsorbed than a gas which has less intermolecular attraction .Nature of adsorbent – Since adsorption is a surface phenomenon hence an adsorbent having larger surface area acts as a better adsorbent .Temperature – Adsorption is exothermic . This is because ∆S for the process is –ve since gas getting adsorbed is becoming less random . To make ∆G=-ve the value of ∆H should be sufficiently negative . Hence adsorption generally decreases with increase in temperature . However chemisorptions first increases and then decreases with increase in temperature. This is because it requires energy of activation . But after it is achieved further increase in temperature decreases adsorption
13 Pressure – With increase in pressure adsorption generally increases however the increase is non-uniform . The behavior can be explained by Freundlich isotherms which is based on the relation – x/m = kp1/nx/m = mass of adsorbate per unit mass of adsorbent, p= pressure and 1/n = 0 to 1At very low pressure 1/n=1 hence x/m = kpAt high pressure, 1/n=0 it means x/m= k.Thus at very low pressure x/m is directly proportional to p while at very high pressure x/m is independent of p .
15 Applications of Adsorption In gas masks –Coal minesTo control humidityRemoval of colouring matter from solutionsBlue Lake testFroth Floatation processChromatographyManufacture of AmmoniaHydrogenation of vegetable oilProduction of high vacuumCuring diseases: Drugs are used to kill germs by getting adsorbed on them.
16 Catalysis – It refers to the process of altering the rate of reaction by using a catalyst . The catalyst refers to a substance which alters the rate of reaction without itself getting consumed in the reaction . There are two aspects a catalystActivity – It is the ability of catalyst to alter the rate of reaction . It can be increased by increasing the surface area of a catalyst .Selectivity – It is the ability of catalyst to guide the reaction towards particular product formation .Depending on the phase of catalyst and reactants it is of two types –
17 Homogeneous catalysis – It is a catalysis in which the catalyst is in the same phase as reactants and products .e.g. C12H22O11 + H2O H+→ C6H12O6 (+) + C6H12O6 (-)Heterogeneous catalysis – It is a catalysis in which the catalyst is in a different phase from reactants and products .e.g. N2 + 3H2 Fe/Mo→ 2NH3
18 Mechanism of heterogeneous catalysis – It includes the following steps –Diffusion of reactants on the surface of catalyst .Adsorption of reactants on the surface of catalyst .Reaction between the reactants while remaining adsorbed on the surface of catalyst .Formation of product on the surface of catalyst .Desorption of product from the catalyst.Diffusion of the product away from the catalyst .
19 Enzyme Catalysis Enzymes are complex nitrogenous compounds which are called bio-catalysts i.e they catalyse the various biochemical reactions occurring inside the bodies of living organisms .Enzymes are highly specific i.e a particular enzyme can only act on a given substrate only.Enzymes are highly efficient i.e. a single enzyme can catalyse millions of molecules in a minute . It is because the rate of regeneration of an enzyme is very high .Enzymes are basically special proteins . Hence when subjected to extremely high temperature they get denatured i.e they lose their biological activity due to change in their tertiary and secondary structure .Enzymes are most activite between C and a pH range of 5-8 .
20 In presence of activators and co-enzymes, enzyme activity increases. Activators are generally metal ions: Na+, Mg2+,Co2+,Cu2+Inhibitors or poisons destroy the catalytic activity of enzymes.
21 Mechanism of enzyme catalysis – The most accepted mechanism is lock and key mechanism which can be described by the following diagram It should be noted that the shape of enzyme is complimentary to the shape of susbstrate . The enzyme usesIt’s functional groups present at it’s active site to carry out the reaction on the substrate.
23 Shape selective catalysis – The catalytic reaction which depends on the pore structure of catalyst and the size of the reactant and product molecule is called shape selective catalysis . Zeolites are example of shape selective catalysts Sodium alumino silicates in which some Si atoms are replaced by Al atoms . Chemically Zeolites are They It finds application in cracking of petroleum . ZSM-5 is an example of zeolite which is used for this purpose .
24 COLLOIDSCollloids are heterogeneous mixtures which contains fine particles of dispersed phase spread throughout a solvent like medium called dispersion medium . The size of particles of dispersed phase varies from 1nm to 1000 nm . Colloids may be classified as follows –
26 b) On the basis of size of colloidal particles – Multimolecular colloids – Those colloids in which the particles of dispersed phase are small but when they aggregate together they form particles of colloidal dimensions . e.g. Sulphur Sol Macromolecular colloids – In these colloids the particles of dispersed phase themselves are of colloidal dimensions . They are generally formed by macromolecules e.g.DNA , proteins etc .Associated Colloids – These colloids show dual nature . They act as electrolytes at low concentration but behave as colloids at high concentration . Soaps are examples of these type of colloids . CMC(Critical Micelle Concentration) is the minimum concentration above which soap molecules aggregate together to form soap micelles. Kraft temperature is the temperature above which micelle formation takes place .
27 c) On the basis of affinity between D/P & D/M Lyophilic sols – Those sols in which the particles of D/P and D/M strongly attract each other are called lyophillic sols . It literally means liquid loving or solvent loving sols . Lyophillic sols are quite stable hence it is not easy to separate them. Also they are reversible sols because even if they are separated they can be formed back again by simply adding the D/P to D/M . e.g. Gum, gelatinLyophobic sols – Those sols in which the particles of D/P & D/M have very weak attraction . It literally means liquid hating or solvent hating sols .They are comparatively unstable . Also they are called irreversible sols because once separated it cannot be formed back again by mixing D/P & D/M.e.g Fe(OH)3 sol.
28 Preparation of colloids Chemical method – Sols can be prepared by carrying out chemical reactions e.g.. Double displacement, hydrolysis, oxidation, reduction etc.As2O3 + H2S → As2S3 + H2OFeCl3 + H2O → Fe(OH)3 + HCl2 AuCl3 + 3HCHO + 3H2O 2Au + 3HCOOH + 6HClSO2 + 2H2S 3S(sol) + 2H2O
29 Bredigs arc method – This method is generally used to prepare metal sols.In this method the metalwhose is sol is to be preparedis taken in the form of anelectrode and is immersedin the dispersion medium .When an electric arc is struckbetween the metal electrodes,The intense heat is producedwhich vaporizes the metalwhich then condenses to formparticles of colloidal size.
30 Peptization – It is the process of converting a precipitate into a colloid by shaking the precipitate with the D/M and a small amount of an electrolye which is called peptizing agent . The colloidal particles adsorbs one of the ions of the electrolye causing development of charge stabilizing the sol.This causes the development of the positive or negative charge on precipitates which ultimately break up into smaller particles of the size of a colloid.
31 Purification of Colloids DialysisProcess of removing a dissolved substance from a colloidal solution by means of diffusion through a semi-permeable membrane.The apparatus used in this method is called dialyser. It consists of a bag made of parchment or cellophane. The bag is filled with the impure sol to be purified and is suspended in a tank through which pure water is circulated. The impurities of electrolytes present in the sol diffuse out of the bag leaving behind pure sol in the bag.
32 ElectrodialysisDialysis is a slow process. However, it can be expedited by applying an electric field.Under the influence of electric field, the impurity ions move faster to the oppositely charged electrodes and the process gets quickened.This process is referred to as electrodialysis.
33 Ultrafiltration – Colloids can also be purified by passing it through a special filter paper in which the size of pores has been reduced by passing a solution of colloidion (4% nitro cellulose) through it . The pores trap D/P particles but not D/M. the D/P can then be added to pure D/M to obtain a pure sol.
34 Properties of Colloids Colligative properties – Since colloids have a large molar mass, the value of their colligative properties is small . Colour – The colour of a colloid results due to scattering of light by the colloidal particles.The exact colour for a given wavelength of light depends on two factors – The size of the colloidal particles The nature of light perceived (reflected or transmitted ) Tyndall effect – It is defined as the scattering of light by colloidal particles . It is responsible for making the path of light visible. The scattering of light depends onThe wavelength of light usedThe size of colloidal particlesIt is observed when two conditions are satisfied:The diameter of the dispersed particles is not much smaller than the wavelength of the light used.b) The refractive indices of dispersed phase and dispersion medium differ greatly in magnitude.
35 Brownian motion – It refers to the random zig-zag motion of particles of D/P as seen through a microscope. It takes place due to continuous collision of particles of D/P with adsorbs other particles of D/P & D/M.a) This motion is independent of the nature of the colloid but depends on the size of the particles & viscosity of the solution. Smaller the size & lesser the viscosity and faster is the motion.b)It does not allow the particles to settle down and is responsible for the stability of the sols.
36 One of the most important properties of colloidal solutions is that colloidal particles posses a definite type of electrical charge. In a particular colloidal solution, all the colloidal particles carry the same type of charge, while the dispersion medium has an equal but opposite charge. Thus, the charge on colloidal particles is balanced by that of the dispersion medium and the colloidal solution as a whole is electrically neutral. For example, in a ferric hydroxide sol, the colloidal ferric hydroxide particles are positively charged, while the dispersion medium carries an equal and opposite negative charge. The stability of a colloidal solution is mainly due to the presence a particular type of charge on all the colloidal present in it. Due to the presence of similar and equal charges, the colloidal particles repel one another and are thus unable to combine together to form larger particles. This keeps them dispersed in the medium and the colloidal remains stable. This is why sol particles do not settle down even on standing for a long time.
37 Based on the nature of charge, the colloidal sols may be classified as positively charged and negatively charged sols.
38 Positively charged sols: Metallic hydroxide sols e. g Positively charged sols: Metallic hydroxide sols e.g., Fe(OH)3, Al(OH)3, Cr(OH)3, etc., TiO2 sol, haemoglobin, sols of basic dyes such as methylene blue etc. Negatively charged sols: Metal sols e.g., Au, Ag, Cu, Pt etc. sols, metal sulphide sols e.g., As2S3, CdS etc. sols; starch sol, sols of acid dyes such as Congo red etc.
39 The colloidal particles have a tendency to preferentially adsorb a particular type of ions from the solution. A colloidal particle usually adsorbs those ions which are in excess and are common to its own lattice.This preferential adsorption of a particular type of ions imparts a particular type of charge to colloidal particles. For example, when a ferric hydroxide sol is prepared by the hydrolysis of ferric chloride in warm water, the colloidal particles of Fe(OH)3 formed have a tendency to adsorb preferentially the Fe3+ ions present in the solution. This is because Fe3+ ions are common to the lattice of Fe(OH)3 particle. The Fe3+ ions thus adsorbed impart positive charge to the colloidal particles present in the sol.
40 Similarly, during the preparation of AgCl sol using excess of KCl solution, the Cl– ions are preferentially adsorbed and the colloidal particles acquire negative charge. However, if an excess of AgNO3 is used, Ag+ ions get preferentially adsorbed and the colloidal particles acquire positive charge.
41 ElectrophoresisDue to the presence of a particular type of electrical charge, the colloidal particles present in a colloidal dispersion move towards a particular electrode under the influence of an electric field. The direction of movement of the colloidal particles is decided by the nature of charge present on them. If the colloidal particles carry positive charge, they move towards cathode when subjected to an electric field and vice versa. The phenomenon is called electrophoresis and may be defined as the movement of colloidal particles towards a particular electrode under the influence of an electric field.
42 The phenomenon of electrophoresis clearly indicates that the colloidal particles carry a particular type of charge. The property can be used to find the nature of charge carried by colloidal particles in a colloidal dispersion. Electrophoresis is an important phenomenon and finds several applications in industry.
43 Coagulation or Flocculation The phenomenon involving the precipitation of a colloidal solution on addition of an electrolyte. A small amount of electrolyte is necessary for the stability of a sol because the ions of the electrolyte get adsorbed on colloidal particles and impart them some charge. However, when an electrolyte is added in substantial amount the positively charged ions of the electrolyte neutralize the charge on colloidal particles and compel the sol to get coagulated.
44 Hardy-Schulze rule: The coagulation capacity of an electrolyte depends upon the valence of ion responsible for causing coagulation. As we have seen above, the ion responsible for causing coagulation is the one which carries charge opposite to that present on colloidal particles. For example, a positively charged sol gets coagulated by the negatively charged ions of the added electrolyte. From a study of the coagulation behavior of various electrolytes towards a particular sol, Hardy and Schulze suggested a general rule known as Hardy-Schulze rule.The rule can be stated as the greater is the valence of the oppositely charged ion of the electrolyte added to a colloidal solution, the faster is the coagulation of the colloidal solution.
45 Thus, higher the charge on oppositely charged ion greater is its coagulating power. For example, the coagulation power of different cations for coagulating a negatively charged sol of As2S3 follows the order. Al3+ > Ba2+ > Na+ Similarly, for the coagulation of a positively charged sol such as Fe(OH)3, the coagulating power of different anions follows the order. [Fe(CN)6]4- > PO43- > SO42- > Cl-
46 The coagulation of colloidal solution can also be achieved by any of the following methods. By electrophoresis: In electrophoresis, the charged colloidal particles migrate to the oppositely charged electrode and get discharged. This results in the coagulation of the colloidal solution. By mixing two oppositely sols: When two sols carrying opposite charges are mixed together in suitable proportions, the colloidal particles of one sol neutralize the charge present on the particles of the other sol and both get coagulated. By persistent dialysis: We have already seen that a small amount of electrolyte is essential to make a sol stable. When a sol is subjected to persistent dialysis, the traces of electrolyte also pass out through the membrane.
47 Protective ColloidsLyophobic sols such as those of metals (e.g. Au, Ag, Pt etc.) are not very stable in the sense that they get easily coagulated (precipitated) in the presence of an electrolyte. This poses a big problem in their storage and usage. Contrary to this, lyophilic sols are much more stable and do not get coagulated easily under similar conditions. It has been observed that in the presence of certain lyophilic colloids such as gum Arabic, gelatin, starch etc. the hydrophobic sols acquire greater stability towards coagulation, i.e. they get protected and do not get coagulated easily when an electrolyte is added. The process of protecting a lyophobic sol from being coagulated (precipitated) on addition of an electrolyte by the use of a lyophilic colloids is called protection and the lyophilic colloid used for purpose is called a protective colloid. For example, the addition of gelatin (a lyophilic colloid) to a gold sol (lyophobic sol) protects the latter from being coagulated on addition of sodium chloride solution.
48 The exact mechanism of protection is not very clearly understood The exact mechanism of protection is not very clearly understood. However, it is believed that the lyophilic colloid particles get adsorbed on the surface of the colloid particles present in the lyophobic sol. The adsorbed lyophilic particles thus form an envelope around the lyophobic sol particles and protect them from the action of electrolytes.
49 EmulsionsApplications of colloids(refer to NCERT)Practice NCERT intext and exercise questions)