Zinc oxide can be reduced by heating it with carbon. The equation for this reaction is: ZnO + C → Zn + CO. Explain what has been oxidised and what has been reduced in this reaction, giving a reason for each answer.

Edexcel A-Level Chemistry (9CH0) — 3.1 Oxidation and reduction · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Carbon has been oxidised because it gains oxygen to form carbon monoxide.
Zinc oxide has been reduced because it loses oxygen to form zinc.
This is a redox reaction as oxidation and reduction occur simultaneously.

Examiner tips

  • Use the word ‘oxidised’ for the species that gains oxygen and ‘reduced’ for the species that loses oxygen.
  • Show the change in oxidation state (C: 0 to +2, O in ZnO: –2 to 0).
  • Mention that the reaction is a redox because both processes happen together.

Common mistakes

  • Confusing ‘oxidised’ with ‘reduced’ for carbon or zinc oxide.
  • Failing to explain the reason (e.g., not stating the gain/loss of oxygen).
  • Omitting that the reaction is a redox example.

Mark scheme (5 marks)

  1. Carbon has been oxidised
  2. Because carbon gains oxygen (to form carbon monoxide)
  3. Zinc oxide has been reduced
  4. Because zinc oxide loses oxygen (to form zinc)
  5. This reaction is an example of a redox reaction because oxidation and reduction occur simultaneously

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