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9701 · 24.1

Electrolysis — practice questions

Practice and worked examples for 9701 Electrolysis. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

Predict the products and write the half-equations for the electrolysis of molten lead(II) bromide (PbBr₂) using inert graphite electrodes.

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  1. Identify ions present: In molten PbBr₂, the ions are Pb²⁺(l) and Br⁻(l).
  2. Identify electrode processes:
    • Cations (Pb²⁺) move to the negative cathode and are reduced.
    • Anions (Br⁻) move to the positive anode and are oxidised.
  3. Write half-equations:
    • At the Cathode (Reduction): Pb²⁺(l) + 2e⁻ → Pb(l)
    • At the Anode (Oxidation): 2Br⁻(l) → Br₂(g) + 2e⁻
  4. State the products: Liquid lead metal is produced at the cathode, and bromine gas is produced at the anode.

Worked example 2

Calculate the mass of copper deposited at the cathode during the electrolysis of aqueous copper(II) sulfate, when a current of 1.50 A is passed for 40.0 minutes. (Ar of Cu = 63.5)

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  1. Convert time to seconds: t = 40.0 min × 60 s/min = 2400 s
  2. Calculate the total charge passed (Q): Q = I × t = 1.50 A × 2400 s = 3600 C
  3. Calculate the moles of electrons transferred: n(e⁻) = Q / F = 3600 C / 96500 C mol⁻¹ = 0.0373 mol
  4. Use the half-equation to find the mole ratio: The reduction of copper is: Cu²⁺(aq) + 2e⁻ → Cu(s) The mole ratio of electrons to copper is 2:1.
  5. Calculate the moles of copper deposited: n(Cu) = n(e⁻) / 2 = 0.0373 mol / 2 = 0.01865 mol
  6. Calculate the mass of copper: Mass = moles × Ar = 0.01865 mol × 63.5 g mol⁻¹ = 1.18 g (to 3 s.f.)
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