Explain what happens during the electrolysis of molten lead bromide, including what is produced at each electrode and why.

AQA GCSE Chemistry (8462) — 4.4.3 Electrolysis · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Lead bromide must be molten (or dissolved) so that the ions are free to move and carry charge.

At the cathode (negative electrode) the positively charged Pb²⁺ ions are attracted and gain two electrons:

Pb²⁺ + 2e⁻ → Pb(s)

Thus lead metal is deposited.

At the anode (positive electrode) the negatively charged Br⁻ ions are attracted and lose one electron each:

2Br⁻ → Br₂(g) + 2e⁻

Hence bromine gas is evolved.

The cathodic reaction is a reduction and the anodic reaction is an oxidation.

Examiner tips

  • Show the molten state requirement first, then describe each electrode’s reaction with the correct half‑equations.
  • Use the words ‘reduction’ and ‘oxidation’ to link the reactions to the electrodes.
  • Include the movement of ions (Pb²⁺ to cathode, Br⁻ to anode).

Common mistakes

  • Forgetting that Pb²⁺ is reduced at the cathode.
  • Writing the wrong half‑reaction for bromine (e.g. Br₂ → 2Br⁻).
  • Omitting the requirement that the lead bromide must be molten or dissolved.

Mark scheme (4 marks)

  1. Lead bromide must be molten (or dissolved) so that the ions are free to move / carry charge
  2. Positively charged lead ions move to the negative electrode (cathode) and gain electrons, producing lead metal
  3. Negatively charged bromide ions move to the positive electrode (anode) and lose electrons, producing bromine
  4. The reaction at the negative electrode (cathode) is reduction / the reaction at the positive electrode (anode) is oxidation

Key terms in this question

electrolysis

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