Chapter 1:- Concept of Electrode Potential

B.sc 2nd year Book
Advance Inorganic Chemistry
(Page 2)

Faraday’s Laws of Electrolysis

In 1834, Michael Faraday, on the basis of his investigations found that chemical changes at the electrodes during electrolysis depend on the quantity of electricity passed through the electrolyte solution and stated exact relationships between the quantity of electricity passed and the amount of substance liberated or dissolved at an electrode. These relationships are known as “Faraday’s Laws of Electricity”.

Two laws were formulated by him to explain the quantitative aspects of electrolysis also called Faraday’s laws of electrolysis. They are explained below:-

Faraday’s First law of Electrolysis :

This law states that the amount of a substance liberated or dissolved at an electrode is proportional to the quantity of electricity passed through the electrolyte solution.

Suppose W grams of substance is liberated or dissolved at the electrode when Q Coloumbs of electricity is passed through an electrolyte solution then according to Faraday’s first law,

W a Q
or
W=ZQ…………………………………………(1)

Where the proportionality constant Z is called the electrochemical equivalent of the substance. When Q=1 Coloumb, then we have

W=Q …………………………………………..(2)

Thus, the proportionality constant Z is defined as the amount of a substance liberated or dissolved due to the passage of 1 Coloumb of electricity. In order to know the quantity of electricity passing through an electric circuit, the strength of the current and the time for which the current is passed are required. If the strength of the current(I) is measured in amperes and time(t) in seconds, then

Q=I X t……………………………………….(3)
or
W=1 x t……………………………………..(4)

The quantity of electricity that generates or dissolves 1 gram equivalent weight of any substance is called ‘Faraday’. The value of Faraday has been determined by electrolyzing the AgNO₃ solution. Very precise experiments on electrolysis of AgNO₃ solution showed that 0.001118 grams of Ag are deposited when 1 Coloumb of electricity is passed. It means that 1 gram equivalent of Ag (107.88) will be deposited by the passage of 107.86/0.001118  =  96493.1 Coloumbs.

Thus, 1 Faraday = 96493.1 Coloumbs. But for most practical purposes, the value of Faraday is taken to be 96500 Coloumbs.

Faraday’s Second law of Electrolysis:

This law states that if the same quantity of electricity is passed through different electrolyte solutions, then the amounts of substances liberated or dissolved at the electrodes are proportional to their chemical equivalent weights. For example, the electro-chemical cell of the type:

Cu(s), Cu²⁺ (1M)|| Ag⁺(1M), Ag(s). It will be seen that-

i.e. the amounts of copper and silver deposited at the copper and silver electrodes are in the ratio of their equivalent weights in accordance with Faraday’s second law of electrolysis. It may be noted that Faraday’s second law of electrolysis is strictly applicable only if the whole of the conduction is electrolytic in character.