Electrochemistry

Syllabus Unit: Physical Chemistry

Why it matters

Chapter overview

Electrochemistry connects chemical reactions to electrical energy in both directions: electrolysis uses electricity to force a nonspontaneous reaction to occur, quantified precisely by Faraday's laws, while galvanic cells generate electricity from a spontaneous redox reaction, measured as EMF. This topic builds from that foundation through the Nernst equation — which extends cell potential to real, non-standard concentrations and links directly to the equilibrium constant — to the everyday applications JEE tests: batteries, fuel cells and corrosion.

Chemistry Syllabus Structure Physical Chemistry ~33% weightage

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1Ions MigrateCations to cathode, anions to anode

2Oxidation at AnodeAnions/atoms lose electrons

3Reduction at CathodeCations gain electrons

4m = (E/96500)ItFaraday's first law quantifies the deposit

1Electrolytic Conduction & Faraday's Laws

Electrolysis forces a nonspontaneous reaction to happen by paying for it with electrical current — and Faraday's laws say exactly how much you get for that price.

Learning Objectives

Distinguish electrolytic conduction (ion migration) from metallic conduction (electron flow).
State Faraday's first law: mass deposited/liberated is proportional to the quantity of charge passed.
State Faraday's second law: for equal charge, mass deposited is proportional to equivalent weight.
Calculate the mass of substance deposited or liberated at an electrode given current and time.

Key Terms

Electrolytic ConductionConduction of electric current through a molten or dissolved electrolyte via the physical migration of ions.

Faraday's First LawThe mass of substance deposited or liberated at an electrode is directly proportional to the quantity of charge passed.

Faraday Constant (F)96500 C/mol — the electric charge carried by one mole of electrons.

Equivalent WeightMolar mass divided by the number of electrons transferred per ion or atom in the electrode reaction.

Electrolysis drives a nonspontaneous redox reaction by forcing current through an electrolyte using an external power source — the exact reverse of what a galvanic cell does spontaneously.

Electrolytic vs. metallic conduction

Metallic conduction — electrons flow through a fixed metal lattice; no mass transfer; conductivity decreases as temperature rises.
Electrolytic conduction — ions physically migrate to oppositely charged electrodes; involves mass transfer and chemical change; conductivity increases as temperature rises (ions move faster, viscosity drops).

Faraday's first law

m = Z·I·t = (E/96500)·I·t, where E is equivalent weight, I is current (A), t is time (s)

Faraday's second law

When the same quantity of charge passes through different electrolytes connected in series, the masses of substances deposited or liberated are directly proportional to their equivalent weights.

m1/E1 = m2/E2 (for the same charge passed through different cells)

Exam TipWorked example: to deposit Cu (M = 63.5, n = 2) from a 2 A current for 965 s: Q = It = 1930 C, moles e⁻ = 1930/96500 = 0.02, moles Cu = 0.01, mass = 0.01 × 63.5 = 0.635 g.

Common MistakeAlways identify n (electrons transferred per formula unit) correctly from the half-reaction before computing equivalent weight — a frequent source of factor-of-2 or factor-of-3 errors.