Saturday, January 19, 2008

IIT JEE Ch.8 ELECTROCHEMISTRY - Core Point for Revision

JEE Syllabus


Electrochemical cells and cell reactions;
Electrode potentials;
Nernst equation and its relation to ΔG;
Electrochemical series,
emf of galvanic cells;


Faraday's laws of electrolysis;
Electrolytic conductance, specific, equivalent and molar conductance,
Kohlrausch's law;
Concentration cells.

electrochemical cell

or Voltaic or Galvanic Cells

In this cell, a chemical reaction produces electrical energy.

In this cell, the electrons being transferred from the reducing agent to the oxidizing agent travel through a wire and thus provide an elctric current.

a electrochemical cell is represented as

Ө Zn|ZnSO-4║CuSO-4|Cu In this symbol additionally On zinc side as it is a cathode a - sign is placed in O (as shown) and on Cu side as it is anode a + sign is placed in O.

cell reactions;

Reaction at the electrodes are called half cell reactions as both the elctrodes are kept seperate from physical contact and ion movement only is permitted through salt bridge.

At zinc electrode Zn → Zn^2+ +2e¯ (oxidation)

At Cu electrode: Cu^2+ +2e¯ → Cu (reduction)

Electrode potentials

When an electrode in put in solution of ions, a charge is developed between the solution and the electrode. This charge is termed a electrode potential. The electrode potential cannot be measured individually and it is measured in reference to a standard hydrogen electrode.

Standard Hydrogen Electrode

An electrode in which pure dry hydrogen gas is bubbled at 1 atm and 298K about a platinized platinum plate through a solution containing H^+ ions ( for example - HCl solution)

The emf produced is taken as zero volts. All other potential are expressed with SHE potential as zero.

Nernst Equation: The cell potential of a half cell (as well as that of a complete cell) depends upon the concentrations of involved ions, pressure of the gaseous species (if involved) and the temperature. The relation connecting them is given by the Nernst equation.

It is expressed as

E = E° - (RT/nF)ln Q°

Q° = Product of concentration (or pressure) of products each raised to the corresponding stochiometric number/Product of concentration (or pressure) of reactants each raised to the corresponding stochiometric number

n = number of electrons involved in the hall cell reaction

Electrochemical series is the series in which various elements are arranged in the order of their reduction or oxidation potentials.

Emf of galvanic cells

E(Cell) = E(R) - E(L)


In electrolytic cell, electric energy is used to cause a chemical reaction to take place.

Faraday's laws of electrolysis:
Quantitative Relationships in Electrolytic Cells

Determining the amount of electrical energy necessary for accumulating a given amount material from the electrolytic cell.

First law: It states that the amount of any substance that is liberated at an electrode during electrolysis is directly proportional to the quantity of electricity passed through the electrolyte.

W α Q (w = weight of substance deposited and Q is charge = ampere * time)

Second law: It states tht when the same quantity of electricity is passed through different electrolytes amount of different substances liberated or deposited at the different electrodes are directly proportional to the chemical equivalents9i.e., equivalent weight) of substances.

One faraday (F) is the amount of electrical energy required for flow of 1 mole of electrons.

To three significant digits, 1 faraday equals 96,500 coulombs(coul).

Current flow is measured in amperes (A)which is coulombs/seconds or coul/s,

Electrolytic conductance

The flow of electric current through an electrolytic solution is known as electrolytic conduction.

Electrolytic conduction also follows Ohm's law.

The equivalent conductivity of an electrolyte may be defined as the conductance of a volume of solution containing one equivalent mass of a dissolved substance when placed between two parallel electrodes which are at a unit distance apart, and large enough to contain between them the whole solution.

The molar conductivity of a solution gives the conducting power of ions produced by one molar mass of an electrolyte at any particular concentration.

Kohlrausch's Law on the independence of migrating ions: The molar conductivity of an electrolyte equals the sum of the molar conductivities of the cations and the anions; n = number of anions or cations.

Λ = v+Λ+ + vˉΛˉ

Concentration cells.

Concentration Cells are electrochemical cells that have two equivalent half-cells of the same material differing only in concentrations. One can calculate the potential developed by such cells using the Nernst Equation. A concentration cell produces a voltage in the process of reaching equilibrium, which will occur when the concentration in both cells are equal.

Concentration cell methods of chemical analysis compare a solution of known concentration with an unknown, determining the concentration of the unknown via the Nernst Equation.

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