PYQ Vault

Playbook

Electrochemistry

Cell potentials and the Nernst equation, Gibbs energy from a cell, conductance and Kohlrausch's law, and Faraday's laws. Sign conventions and units decide the answer.

Questions in the bank
125
q/paper in 2025–26
1.17
Calculation
58%
Notes pages
7

Strand: Calculate

When you’ll see it

A table of reduction potentials, a cell emf at non-standard concentrations, ΔG° or K from E°, a conductivity or molar conductivity, or a current passed for a time.

How this chapter is tested

The Nernst equation is the chapter's centre of gravity, so the sign of its log term must be automatic: E = E° − (0.059/n) log Q at 298 K, with Q as products over reactants and the powers from the same balanced equation that fixes n. Many questions run it backwards for a concentration, a ratio or a pH.

Most numericals need only a handful of relations: E°cell = E°cathode − E°anode, ΔG° = −nFE° with F = 96500 C mol⁻¹, log K = nE°/0.059, Λm = 1000κ/c with Kohlrausch's law, and Faraday's m = MIt/nF. Marks slip on joules against kilojoules, on minutes left unconverted, on the factor 1000 in Λm, and on n.

The rest is recall: the electrochemical series, what forms at each electrode in water, and the named batteries and fuel cells. Those questions are multiple choice and fast if the two tables are learnt.

The sub-skills

The distinct skills inside the chapter, in the order to learn them.

  • Galvanic cells and the series

    E°cell = E°cathode − E°anode; a more negative E° is a stronger reducing agent; E° does not scale with the equation.

  • Nernst equation

    E = E° − (0.059/n) log Q; run it backwards for an unknown; the hydrogen electrode gives −0.059 pH plus a pressure term.

  • Gibbs energy, K and combined potentials

    ΔG° = −nFE°, log K = nE°/0.059; a new E° from two others weights each by its electrons: n₃E₃ = n₁E₁ ± n₂E₂.

  • Conductivity and molar conductivity

    κ = G*/R from the cell constant; Λm = 1000κ/c in S cm² mol⁻¹ with c in mol L⁻¹.

  • Dilution and Kohlrausch's law

    Λm = Λm° − A√c for strong electrolytes; Λm° = ν₊λ₊° + ν₋λ₋°; α = Λm/Λm° gives Ka and solubility.

  • Electrolysis and Faraday's laws

    m = MIt/nF with t in seconds; water is reduced before Na⁺; an active anode dissolves.

  • Batteries, fuel cells and corrosion

    Dry, mercury, lead storage and Ni–Cd cells; H₂–O₂ fuel cell; rusting as a small galvanic cell; zinc protects sacrificially, tin only as a barrier.

Traps to expect

Distractor shapes this chapter reuses. The Traps page covers the ones that cut across chapters.

  • Q upside down or powers dropped

    Q is products over reactants, and in Zn + 2Ag⁺ the Ag⁺ is squared. Each slip moves the emf by a multiple of the correction.

  • Potentials subtracted directly

    E°(Fe³⁺/Fe) − E°(Fe²⁺/Fe) is not E°(Fe³⁺/Fe²⁺). Weight each potential by its electrons, then divide by the total electrons.

  • Four electrons for oxygen

    2H₂O → O₂ + 4H⁺ + 4e⁻: one mole of O₂ needs 4 F, not 2 F. Gold in AuCl₄⁻ is +3, so 3 electrons per atom.

  • Dropping the 1000

    With κ in S cm⁻¹ and c in mol L⁻¹, Λm = 1000κ/c. Without the 1000, or with SI units and the 1000 kept, every answer is a thousand off.

  • Joules reported as kilojoules

    nFE comes out in joules. A blank asking for kJ mol⁻¹ needs a division by 1000.

Learn it before you drill it

This chapter has full teaching notes — foundations, worked examples, self-checks and a mastery check for each page. Read the notes once, then drill page by page below.

Electrochemistry notes

Drill every Electrochemistry question

125 questions from the bank, across 7 subtopics.

Drill the 125 questions

Drill one subtopic at a time

The 7 subtopics, in teaching order.

Related playbooks

Often paired with this one — the technique or the trap overlaps. Drill these next.