Faraday’s Laws of Electrolysis

Faraday’s first law of electrolysis

The amount of chemical reaction which occurs at any electrode during electrolysis by a current is proportional to the quantity of electricity passed through the electrolyte (solution or melt).

Or

The amount of the substance deposited or liberated at cathode is directly proportional to the quantity of electricity passed through electrolyte.

W ∝ I × t

or W = I t Z = Q Z

Here, I is the current in amp, t is time in sec, Q is quantity of charge (coulomb) and Z is a constant known as electrochemical equivalent.

When I = 1 amp, t = 1 sec then Q = 1 coulomb, then W = Z.

Thus, electrochemical equivalent is the amount of the substance deposited or liberated by passing 1A current for 1 sec (i.e. 1 coulomb, I × t = Q).

Faraday’s second law of electrolysis

The amounts of different substances liberated by the same quantity of electricity passing through the electrolytic solution are proportional to their chemical equivalent weights or

Hence, 

electrochemical equivalent ∝ eq. weight

The amount of electricity (or charge) required for oxidation or reduction depends on the stoichiometry of the electrode reaction.

For example, in the reaction,

Ag⁺(aq) + e^– → Ag(s)

One mole of the electron is required for the reduction of one mole of silver ions.

Now, the charge on one electron is 1.6021 × 10^–19 C. Therefore, the charge on one mole of electrons

= N_A × 1.6021 × 10^–19 C

= 6.02 × 10²³ mol–1 × 1.6021×10^–19 C

= 96487 C mol^–1 ≈ 96500 C mol^–1

This quantity of electricity is called Faraday and is represented by the symbol F.

For the electrode reactions

Mg²⁺ (l) + 2e^– → Mg(s)

One mole of Mg²⁺ requires 2 moles of electrons (2F).

And for

Al³⁺ (l) + 3e^– → Al(s)

One mole of Al³⁺ requires 3 moles of electrons (3F).

The charge passed through the electrolytic cell during electrolysis is equal to the product of current in amperes and time in seconds.

In commercial production of metals, current as high as 50,000 amperes is used that amounts to about 0.518 F per second.