Engineering Chemistry (revised edition)

Engineering Chemistry (revised edition)
ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Electrical Conductance
Substances that allow the passage of current through them are called conductors and the phenomenon is called electrical conductance. Metallic: the conduction is only due to the presence of free mobile electrons. Electrolytic: The substances which conduct electricity both in the fused state and in the aqueous solution are called electrolytic conductors or electrolytes. Types of Electrolytes Based on the extent of dissociation, electrolytes are classified into two types: Strong electrolytes: For example, HCl, HNO3, NaOH, KOH, NaCl and KCl. Weak electrolytes: For example, CH3COOH, HCOOH and NH4OH. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Mechanism involved in typical electrolytic conduction.
Electrolytic Conductance Mechanism involved in typical electrolytic conduction. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Specific Conductance The reciprocal of specific resistance is called the specific conductance (k). The ratio of length to the cross-sectional area is called the cell constant. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Measurement of Specific Conductance
For measurement of unknown resistance for ionic solutions AC current is used and a specially designed conductivity cell is used t o contain the solution of the electrolyte. This conductivity cell then forms one arm of the circuit which permits the measurement of resistance using the Wheatstone bridge. The conductivity cell is available in many designs but the essential components are two platinum electrodes of known dimensions, coated with platinum black . The ionic solution for which the resistance is to be measured is confined between the two electrodes of area of cross-section (a) kept at a distance (l ) from each other and specific conductance is computed. Different types of conductivity cells Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Wheatstone Bridge The Wheatstone bridge setup for measurement of resistance consists of two resistances R2 and R3, a variable resistance R1 and the conductivity cell with unknown resistance Rx. The setup is fed with an AC power and a suitable headphone detector is used. When the bridge is balanced, no current passes through the bridge and therefore, we have Using this relation we get, or Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Equivalent Conductance
Molar Conductance Ionic Conductance Conductance of a solution depends on the number of ions. It is observed that molar conductance increases upon dilution. This may be due to the increase in the degree of dissociation of the electrolyte or due to decreased ion–ion force of attraction. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Applications of Kohlrausch Law of Independent Ionic Migration
To calculate the Λo of weak electrolytes To calculate the ionic conductance To calculate the solubility of the sparingly soluble salt Engineering Chemistry (revised edition) ISBN: Copyright©2013 Wiley India Pvt. Ltd. All Rights Reserved

Plot of molar conductance vs. (concentration)1/2
Factors Affecting Conductance Temperature Concentration Plot of molar conductance vs. (concentration)1/2 Engineering Chemistry 2nd Edition ISBN: Copyright©2013 Wiley India Pvt. Ltd. All Rights Reserved

Transport (Transference) Number
Interionic Attraction Theory of Conductance It is a general observation that molar conductance of a solution increases on dilution. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Electrophoretic effect
Hydration of Ions The molar conductance at infinite dilution of an electrolyte is the maximum value that an electrolyte can offer. Electrophoretic effect Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Electrochemical Cells
The devices which convert electrical energy into chemical energy or vice versa are called electrochemical cells. Based on the activity taking place in them, these devices are classified into two major categories: Electrolytic cells. Galvanic (voltaic cells): These are further divided into chemical cells; concentration cells. The devices in which chemical changes occur in the presence of applied electrical energy are referred to as electrolytic cells. The devices in which electrical energy is generated on account of the chemical reactions occurring in them are known as galvanic cells. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved ved

Redox Reactions Such reactions in which oxidation and reduction take place simultaneously are known as redox reactions. For example, in the reaction: Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Electrode Potential Origin of Electrode Potential
Origin of electrode potential (reduction) Origin of electrode potential (oxidation) Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Standard Electrode Potential (E0)
Oxidation Potential If oxidation occurs at the electrode, at equilibrium the potential of the electrode can be termed as oxidation potential. For the zinc electrode, the reaction can be represented as: Reduction Potential If reduction occurs at the electrode, at equilibrium the potential of the electrode is referred to as reduction potential. For the zinc electrode, the reaction can be represented as: Standard Electrode Potential (E0) If is defined as the potential that exists between the metal or the gas and its aqueous solution of unit concentration at 298 K when the sum of all partial pressures of the gaseous reactants and products, if any, is equal to 1 atm pressure. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Galvanic Cells The cell consists of two half-cells. Oxidation takes place in one half-cell and reduction in the other as indicated by the half-reactions Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Salt Bridge Engineering Chemistry (revised edition)
ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved ed

EMF of the Cell and Free Energy Change
A salt bridge helps to bring about internal contact between the electrodes; to minimize liquid junction potential; to minimize polarization. EMF of the Cell and Free Energy Change Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Electrochemical Conventions and Notations
There are two electrodes in a galvanic cell. Each of these electrodes is referred to as a half cell. If reduction occurs at an electrode, it is called a reduction electrode or a positive electrode. If oxidation occurs at an electrode, it is called an oxidation electrode or a negative electrode. Oxidation and reduction potential values of an electrode are numerically the same, with opposite signs. As per IUPAC, the potential of an electrode is always expressed as reduction potential only, irrespective of the reactions occurring at the electrode. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

The EMF of a cell is represented as:
The interphase across which a potential develops is denoted by either a single vertical line (|) or a semicolon (;). Representation of a cell will have the oxidation electrode (anode) on the LHS and the reduction electrode (cathode) along the RHS. The EMF of a cell is represented as: Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Nernst Equation Nernst equation for an electrode
The potential of an electrode depends on concentration and temperature. A quantitative equation relating the electrode potential with these parameters was deduced by Walter Herman Nernst. Nernst equation for an electrode Nernst equation for a galvanic cell Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Thermodynamics of an Electrochemical Process
The relation between EMF of the cell and the thermodynamic parameters H and S can be derived based on this fundamental relation between G and EMF: Relating free energy change to enthalpy change by Gibbs-Helmholtz equation, we get Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Application of Nernst equation
The potential of an electrode and EMF of a cell can be calculated at any temperature and concentration. Knowing potential of an electrode, the concentration of the reactant can be calculated. The concentration of a solution in the galvanic cell can be determined. The pH of a solution can be calculated by measuring the EMF. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Measurement of EMF of the Cell
EMF and Potential Difference The terms EMF and potential difference are used indiscriminately, but there is a clear distinction between them. Differences between EMF and potential difference Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Potentiometric Measurement
Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Reference Electrodes The criteria for an electrode to act as a reference electrode are: The potential of such an electrode should be known, under the conditions of utility. The potential of the electrode should show minimum variation with temperature. In other words, the potential gradient with respect to temperature should be minimum. The most commonly used reference electrode is the hydrogen electrode. The potential of all other electrodes are measured with respect to the hydrogen electrode. Hence, it is called a primary reference electrode. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Standard Hydrogen Electrode (Normal Hydrogen Electrode)
The half-reaction is Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

However, SHE/NHE has its own inherent limitations:
Construction of SHE is a difficult activity. It is difficult to maintain unit molar concentration of hydrogen throughout and to pass hydrogen always at exactly 1 atm pressure. Presence of arsenic compounds would easily get absorbed on platinum foil, thereby poisoning the surface. It would affect equilibrium of the reaction. In addition, SHE cannot be used in presence of strong oxidizing and reducing agents. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Calomel Electrode Engineering Chemistry (revised edition)

Silver–Silver Chloride Electrode

Single Electrode Potential
Measurement of Single Electrode Potential Case 1: If the experimental electrode is an oxidation electrode, the cell so constructed shall be Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Case 2: If the experimental electrode is a reduction electrode, then the cell formed is represented as: Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Characteristics of Electrochemical Series
When the electrodes are arranged in the increasing order of their standard reduction potential values, the series so formed is termed as electrochemical series. Characteristics of Electrochemical Series Lithium is the first member of the series. Highly reactive metal systems are at the top of the series. In other words, good reducing agents are at the top of the series. All good oxidizing agents are at the bottom of the series. Hydrogen system is at the middle of the series. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Applications of Electrochemical Series
Higher the reduction potential, greater is the tendency of the element to get reduced. Lower the reduction potential, greater is the reducing ability. A metal placed higher in the series is anodic to other metals which lie below it. Knowledge of electrochemical series helps in selection of electrode assemblies, to construct the galvanic cells of the desired EMF. The polarity of the electrode system and the electrode reaction can be easily predicted. Spontaneity and feasibility of the cell under construction can be easily predicted. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Types of Electrodes Metal–metal ion electrode: It consists of a metal in contact with its ions or metallic salt in equilibrium. Metal–insoluble salt electrode: It consists of a metal and its sparingly soluble salt, which are in equilibrium with a solution containing the same anion as that of the salt. Amalgam electrode: Amalgam is the alloy of any metal with mercury. An amalgam electrode is a modification of metal–metal ion electrode. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Gas electrode: It consists of a noble metal like platinum which is simultaneously in contact with a gas and a solution of ions of the gas. Redox electrode: In this electrode, the electrode potential results due to the presence of ions of the same substance present in different oxidation states. Ion-selective electrode (ISE): In this electrode, a suitable non-porous membrane separates two solutions, containing similar ions of different concentrations, and acts as an electrochemical membrane. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

It should have sufficient mechanical strength.
The membrane is the vital component of ISEs and the criteria for the choice of the membrane depends on many aspects: It should not be affected by the action of oxidizing and reducing agents. It should have sufficient mechanical strength. It should have low electrical resistance. It should not attacked by organic solvents. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Based on the nature of the membrane, the ISEs are classi ed into different categories:
Glass electrode: These electrodes are responsive selectively towards hydrogen ions (H+). Crystalline ISEs: A thin crystalline membrane is used in these ISEs. Fluoride ion-selective electrode is the best example of crystalline ISEs Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Ion exchange ISEs: Enzyme ISE: Engineering Chemistry (revised edition)

Glass Electrode Engineering Chemistry (revised edition)

The glass surface gets hydrated both on the outside and inside.
This glass surface acts as an electrochemical membrane and is non-conducting. The exact mechanism is not clearly known but the following steps are suggested: The glass surface gets hydrated both on the outside and inside. H+ ions get absorbed on the hydrated layer. The exchange occurs between alkali metal ions in the glass surface and the H+ ions present in the solution. In the dry layer sandwiched between the hydrated layers, electrical conduction occurs by Frankel’s mechanism. In other words, Li+ or Na+ ions move from one interstitial position to another. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Advantages of Glass Electrode
It is portable and compact. Unlike other systems, the equilibrium is easily attained. It gives precise and accurate determination of pH, even in the case of very dilute solutions. Unlike the potentiometric method, a few drops of the solution are enough for measurement. It is a stable electrode system and can be used in presence of strong oxidizing and reducing agents. It can be used in the presence of viscous biological fluids. It can detect and estimate H+ ions in presence of other types of ions. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Limitations of Glass Electrode
The glass membrane is very thin and fragile, and so it has to be handled with at most care. Even though care is taken to select the glass of relatively high electrical conductance, it offers fairly high electrical resistance. Fluoride ions in the sample attack the glass surface and alter the composition of the membrane. Glass electrodes with Corning-015 cannot be used for accurate measurement of pH > 9. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Engineering Chemistry (revised edition) ISBN: 978-81-265-4475-2

Cell for pH determination

Concentration Cells Electrode Concentration Cells
A concentration cell is a galvanic cell which generates electrical energy at the expense of chemical energy. Electrode Concentration Cells Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Electrolyte Concentration Cells

Liquid Junction Potential
In the electrolyte concentration cell, the two electrodes are separated by a diffusion membrane. The concentrated solution will contain an excess of Cl− ions which are less mobile and acquires negative charge. This is the scenario on either side of the diffusion membrane in contact with the two solutions. As a result, potential difference is developed at the junction of the solutions, and this potential difference is called liquid junction potential. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Electrolyte Concentration Cell without Transference
In these cells, the two same electrodes materials are dipped in the two solutions containing the same electrolyte of different concentrations. The electrodes are connected through a salt bridge Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved ved

The electrodes of the cell can be represented as:

Applications of Concentration Cells
Concentration cells are not used as a source of electrical energy, as the EMF drastically decreases with time. It appears to be a theoretical concept, but has indirect applications. Determination of Valency of Ions Determination of the Solubility of a Sparingly Soluble Salt Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Batteries A battery is a combination of cells either in series or parallel or both, in order to get the required amount of electrical energy. It is a portable source of electrical energy. The galvanic cells used to form batteries can be classified as: Primary cells: In these cells, the chemical energy is converted into electrical energy as long as the chemical components are active. Secondary cells: In these cells, the redox reaction that converts chemical energy into electrical energy can be reversed by passage of current. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Components of a Battery
Anode It contains active materials which spontaneously take part in the oxidation reactions. Cathode It contains active materials which spontaneously take part in the reduction reactions. Electrolyte It helps in the migration of the ions leading to the generation of electrical energy. Separator It is a thin polymeric membrane which prevents the mixing of products formed at the electrodes. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Discharging and Charging of a Battery
The process in which spontaneous redox reaction occurs is called discharging. The process of conversion of an inactive material back into active materials in a cell is called charging. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Characteristics of a Battery
Voltage or EMF: It depends on the change in free energy of the reaction. According to Nernst equation, A good battery is one which has a flat discharge rate (constant EMF) Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Current: It is a measure of the rate of discharge reactions of the cell. It depends on the amount of active materials. Higher the amount of active materials, longer will be the time for which EMF is generated. Current capacity: It is defined as the amount of current generated in unit time. It is measured in terms of ampere-hour (A h) and depends on the discharge conditions. Power density: Energy density: Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Energy efficiency: Cycle life: It is applicable only to rechargeable (secondary) cells. The EMF of cell decreases during discharging. If it is followed by charging process, the amount of active materials in the cell increase and hence the EMF. Shelf life: The duration of storage under specified conditions at the end of which the cell still has the ability to deliver specified performance. A good battery should possess a long shelf life. Self-discharge: It is defined as the loss of active materials of the cell due to localized action on the electrode even when the cell is not in discharge mode. Longer the self-discharge, lower would be the shelf life. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Alkaline Dry Cell This cell is same in construction as the dry cell but modified to replace NH4Cl as electrolyte by sodium hydroxide or potassium hydroxide, hence the name alkaline dry cell. Replacing NH4Cl with NaOH/KOH prevents corrosion of zinc cathode since it does not dissolve in basic medium. The cell reactions are: Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

This cell, also known as Ruben–Mallory cell
Mercury Cell This cell, also known as Ruben–Mallory cell Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Silver Button Cell Engineering Chemistry (revised edition)

This cell, also known as alkaline storage battery
Nickel–Cadmium Cell This cell, also known as alkaline storage battery Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Modern Batteries Metal–Air Batteries The cell is represented as:

The characteristics of zinc–air batteries are as follows:
These are light and have high energy density. These relatively eco-friendly and have unlimited capacity. These suffer from low shelf life. Still, they are known for low energy cost. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

The characteristics of nickel–metal hydride batteries are as follows:
They have long shelf life and cycle life. They have high capacity and rapid recharge capability. They have poor charge detention capacity. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Lithium Batteries Lithium is a light metal with low electrode potential and good conductivity. It is, therefore, a good material for batteries, and can be expected to have high potential and high energy density. The group of batteries where lithium is used as an anode are known as lithium batteries and they were commercialized in 1990. These batteries have the following characteristics: The batteries are light in weight and compact. They are known for low maintenance and have high energy density. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Fuel Cells Comparison with Conventional Galvanic Cells
The conversion into electrical energy involves a number of steps and there is loss of energy at every step. The efficiency of the process is around 40%. There is also a viable way of converting the chemical energy of fuel directly into electrical energy through catalytically activated redox reactions. Such devices are called fuel cells. Comparison with Conventional Galvanic Cells They consist of two catalytic electrodes. The reagents used are fuel and oxidant. The fuel and oxidant are not stored in the cell. No pollutants and hence fuel cells are environmentally friendly. No toxic species are formed in a fuel cell. They do not need charging. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Fuel Cells Advantages Limitations
They offer high energy conversions (almost 75%). These cells have high energy density. These cells use inexpensive fuels. Limitations The electrodes used are either Pt, Ag or the alloys of noble metal which are prohibitively costly. The power generated is moderate. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Hydrogen–Oxygen Fuel Cell

Advantages of Hydrogen–Oxygen Fuel Cell
It just emits water vapor and no other harmful chemicals to the environment. Hydrogen is the lightest element and provides a lot of energy, and also it can be transported easily to places where required. Hydrogen fuel can replace the use of batteries and causes less noise pollution. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Limitations of Hydrogen–Oxygen Fuel Cell
Hydrogen is explosive. For very good efficiency, it turns out to be very expensive, and even the initial research which is to be carried out costs a lot of money. Hydrogen is actually gas, so a large investment of cost and energy is required to compress it into liquefied form. While using these fuel cells in an automobile, a high pressure is created inside the engine, which is quite risky. Another great disadvantage would be that since hydrogen is never present as it is, but always in combined form with either oxygen or some other element, it needs to be separated first. Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved ed

Types of Fuel Cells Alkaline fuel cells Methanol–oxygen fuel cell
Phosphoric acid fuel cells (PAFCs) Molten carbonate fuel cells (MCFCs) Solid oxide fuel cells (SOFC) Solid polymer electrolyte fuel cells (SPEFCS) Microbial fuel cells (MFCs) Engineering Chemistry (revised edition) ISBN: Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

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