Second law of Faraday

Faraday’s Second Law of Electrolysis

Faraday’s Second Law of Electrolysis extends the understanding of electrochemical reactions. It deals with the amount of different substances deposited or liberated when the same quantity of electricity passes through different electrolytes.

Statement of the Law

Faraday’s Second Law states:

If an identical amount of electric charge flows through various electrolytes arranged in series, the mass of material deposited or released at each electrode is directly related to its equivalent weight.

Mathematical Expression

This principle can be represented as:

m₁ / m₂ = E₁ / E₂

Where:

• m₁, m₂ = Masses of substances deposited
• E₁, E₂ = Equivalent weights of the substances

Combined Form of Faraday’s Laws

By applying both Faraday’s First and Second Laws, the deposited mass can be determined using:

m = (E × Q) / F = (E × I × t) / F

Where:

• m = Mass of substance deposited (g)
• E = Equivalent weight of the substance
• Q = Total electric charge passed (Coulombs)
• I = Current in amperes
• t = Time in seconds
• F = Faraday constant (96,500 C/mol)

Key Implication

The law highlights that different substances require different amounts of charge to deposit the same number of equivalents, depending on their chemical nature.

Example

If the same current is passed through solutions of AgNO₃ and CuSO₄, the mass of silver and copper deposited at their respective cathodes will be in the ratio of their equivalent weights:

m_Ag / m_Cu = E_Ag / E_Cu