Iron rusting is an example of an oxidation reaction. A student investigates the reaction between iron(III) oxide and carbon monoxide to produce iron and carbon dioxide. Explain why this reaction is a redox reaction, identifying which substance is oxidised and which is reduced.

OCR A-Level Chemistry A (H432) — 2.3 Acid-base and redox reactions · Explain · 5 marks · View as Markdown

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

Iron(III) oxide reacts with carbon monoxide as shown: Fe₂O₃ + 3CO → 2Fe + 3CO₂

Model answer (5 marks)

Oxidation is the loss of electrons, which in this reaction is equivalent to the loss of oxygen atoms. In the equation Fe₂O₃ + 3CO → 2Fe + 3CO₂, the carbon in CO gains oxygen atoms to form CO₂, so CO is oxidised. Reduction is the gain of electrons, or loss of oxygen atoms. The iron in Fe₂O₃ loses oxygen atoms to form elemental Fe, so Fe₂O₃ is reduced. Because both oxidation (CO → CO₂) and reduction (Fe₂O₃ → Fe) occur simultaneously, the process is a redox reaction.

Examiner tips

  • Use the terms ‘oxidised’ and ‘reduced’ explicitly; link them to loss/gain of oxygen.
  • Show the simultaneous nature of the two half‑reactions to justify the redox classification.

Common mistakes

  • Confusing oxidation with the gain of oxygen instead of loss of electrons.
  • Failing to identify both the oxidised and reduced species in the same sentence.

Mark scheme (5 marks)

  1. Oxidation is the gain of oxygen / carbon monoxide gains oxygen
  2. Carbon monoxide (CO) is oxidised
  3. Reduction is the loss of oxygen / iron(III) oxide loses oxygen
  4. Iron(III) oxide (Fe₂O₃) is reduced
  5. Both oxidation and reduction occur simultaneously / at the same time, making it a redox reaction

Key terms in this question

oxidation · redox reaction

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