Faraday’s Second Law of Electrolysis

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Faraday’s second law of electrolysis states that when the same quantity of electricity is passed through different electrolytes, the relative numbers of moles of element discharged are inversely proportional to the charges on the ion of the element respectively.

Whenever 1 mole of electrons is involved in an electrolytic cell, the total quantity of electricity involved is 96,500 Coulombs. This quantity is known as one Faraday and is denoted by F.

(i) 1 faraday of electricity is needed to discharge 1 mole of monovalent element during electrolysis:

Ag⁺ _(aq) + e^–   → Ag_(s)

Na⁺ _(aq) + e^–    →   Na_(s) (1 mole of atom)

(ii) 2 faradays of electricity are needed to discharge 1 mole of divalent elementAn atom, with a valency of two, which thus can form two covalent bonds is called divalent. For example, oxygen (O), magnesium (Mg), etc. have a valency of two. More during electrolysis:

Cu²⁺ _(aq) + 2e^–  →  Cu_(s) 

Zn²⁺ _(aq) + 2e^–  → Zn_(s) (2 moles of atoms)

(iii) 3 faradays of electricity are needed to discharge 1 mole of trivalent elementAn atom, ion, or element with a valence of three, is called trivalent. Examples of trivalent atoms are boron, aluminium, gallium, indium, etc. More during electrolysis: 

Fe³⁺ _(aq)   + 3e^–  → Fe

Al³⁺ _(aq) + 3e^–  → Al (3 moles of