Lead(II) bromide can be electrolysed when it is melted. During electrolysis, lead(II) ions gain electrons at the negative electrode to form lead metal, and bromide ions lose electrons at the positive electrode to form bromine gas. Explain which species is oxidised and which is reduced in this process, and state what is meant by the terms oxidation and reduction in terms of electron transfer.

Eduqas A-Level Chemistry — 1.8 Redox · Explain · 5 marks · View as Markdown

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

During the electrolysis of molten lead(II) bromide, lead metal is deposited at the negative electrode and bromine gas is produced at the positive electrode.

Model answer (5 marks)

Lead(II) ions are reduced – they gain electrons at the cathode to form lead metal.
Bromide ions are oxidised – they lose electrons at the anode to form bromine gas.
Oxidation means loss of electrons; reduction means gain of electrons.
This is a redox reaction because both oxidation and reduction occur simultaneously.

Examiner tips

  • Use the terms ‘oxidised’ and ‘reduced’ with the correct species. State the electron transfer definition for each. Show the simultaneous nature of the process to earn the full 5 marks.

Common mistakes

  • Confusing which ion is oxidised/reduced. Forgetting to define oxidation/reduction. Not linking the simultaneous electron transfer to the redox description.

Mark scheme (5 marks)

  1. Oxidation is the loss of electrons
  2. Reduction is the gain of electrons
  3. Bromide ions are oxidised (because they lose electrons to form bromine)
  4. Lead(II) ions are reduced (because they gain electrons to form lead metal)
  5. This reaction is described as a redox reaction because both oxidation and reduction occur simultaneously

Key terms in this question

oxidation · reduction · electron transfer

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