Faraday's laws of electrolysis

In 1833 M. Faraday studied the quantitative aspects of electrolysis, and postulated two laws named after him.

Faraday's First Law of Electrolysis

This law states:

The mass of any substance deposited or dissolved at any electrode during electrolysis is directly proportional to the quantity of electricity passed through the solution.

Mathematically, if W gram of any material gets deposited or dissolved when Q coulombs of electricity is passed, then

W µ Q

But Q = I x t

Therefore,� W µ I t

or

W = Z It

where, Z is the electrochemical equivalent of the substance.

If I = 1 unit (say,1 ampere) and t = 1 unit (say,1 second). Then Z = W, i.e., the electrochemical equivalent of a substance is equal to the mass of that substance deposited or dissolved, when a current of one ampere is passed for one second.

The electrochemical equivalents of some common elements are given below.

The electrochemical equivalents of some elements

electrochemical equivalents of some elements

Faraday's Second Law of Electrolysis

The second law of electrolysis is defined thus:

The masses of different substances, liberated or dissolved by the same amount of electricity are proportional to their equivalent masses.

Thus, when the same quantity of electricity is passed through a number of electrolytic solutions connected in series, then the masses of the different materials liberated or dissolved at the respective electrodes are in the ratio of their equivalent masses.

For example, when the same current is passed through the solutions of sulphuric acid (H2SO4), copper sulphate (CuSO4) and silver nitrate (AgNO3) for the same period of time,

faraday s second law of electrolysis

and

faraday s second law of electrolysis

Experimental set up for the verification of the Second Law of Electrolysis

Fig: 9.9 - Experimental set up for the verification of the Second Law of Electrolysis

According to the second law of electrolysis, the same quantity of electricity will produce or dissolve chemically equivalent amounts of all the substances. Therefore, the same quantity of electricity is required to liberate or dissolve one equivalent of any material during electrolysis. This quantity of electricity is called Faraday (F). One Faraday is equal to 96487 coulomb per mole of electronic charges. For the sake of convenience, we generally use a value 96500 C mol-1 for the Faraday constant (F).

So,

1 Faraday = 1F = Electrical charge carried by one mole of electrons.

1F = Charge on an electron x Avogadro's number

or

1F = e- x NA = (1.602 x l0-19 C) x (6.02 x 10-23 rnol-1)

1F = 96488 C mol-1

The Faraday's laws of electrolysis are applicable only,

  • When the whole electrical conduction is electrolytic in nature, i.e., the current is carried only by the ions.
  • No other side-reaction takes place after the electrode reaction has taken place.

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