Faraday's Laws of ElectrolysisJEE Main

Charge in, metal out — by the mole — interactive Chemistry simulation for IIT-JEE.

Concept

Electrolysis is stoichiometry with electrons. First law: mass deposited ∝ charge passed. Second law: for the same charge, different substances deposit in the ratio of their equivalent weights (M/z). One mole of electrons — a faraday, 96 485 C — deposits exactly one equivalent.

Key formula

m = \frac{Q}{F}\cdot\frac{M}{z} = \frac{It}{96485}\cdot\frac{M}{z}

Derivation

Each ion M^{z+} needs z electrons. Q coulombs carry Q/F mol of electrons → Q/zF mol of metal → mass = (Q/zF)·M.

Second law follows: same Q, masses ∝ M/z (equivalent weights). Series cells: same charge through all — deposit ratio = ratio of equivalents, regardless of solution concentration.

Scenarios to explore

Faraday's Electrolysis — m = (It/F)(M/z) — watch the cathode deposit grow.

Real-world applications

Electroplating (chrome, silver, gold).
Aluminium smelting (Hall–Héroult, ~13 kWh/kg).
Copper refining to 99.99% purity; coulometers.

JEE exam tips

1 F deposits: 108 g Ag, 31.75 g Cu, 9 g Al — the classic ladder.
Series cells: m₁/m₂ = E₁/E₂ (equivalents), instantly.
Water electrolysis: 2F per mole O₂... careful — 4F per mole O₂, 2F per mole H₂.

Common mistakes

Forgetting z: Cu²⁺ needs TWO faradays per mole.
Time in minutes fed into Q = It (needs seconds).
Using atomic mass instead of equivalent mass in second-law ratios.

Exam traps to avoid

Current efficiency < 100%: multiply deposited mass by the efficiency factor.
At the cathode REDUCTION deposits metal; anode may dissolve (active) or evolve O₂ (inert).