Iron can be oxidised and reduced in different chemical reactions. In a thermite reaction, iron(III) oxide reacts with aluminium to produce aluminium oxide and iron metal. Explain what is happening in terms of oxidation and reduction, identifying which substance is oxidised and which is reduced, and explain why aluminium is able to displace iron in this reaction.

OCR A-Level Chemistry B: Salters (H433) — 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).

The thermite reaction is used industrially to weld railway tracks together. It produces molten iron at very high temperatures. The word equation for the reaction is: iron(III) oxide + aluminium → aluminium oxide + iron.

Model answer (5 marks)

In the thermite reaction iron(III) oxide is reduced: the Fe³⁺ ions gain electrons to form Fe⁰ metal, so the iron is reduced.
Aluminium is oxidised: Al⁰ loses electrons to form Al³⁺, so aluminium is oxidised.
Oxidation and reduction occur simultaneously – it is a redox reaction.
Aluminium is more reactive than iron (it lies higher in the reactivity series).
Because a more reactive metal can displace a less reactive metal from its compound, aluminium displaces iron from iron(III) oxide, forming aluminium oxide and iron metal.

Examiner tips

  • Show the electron transfer (Fe³⁺ + 3e⁻ → Fe⁰, Al⁰ → Al³⁺ + 3e⁻) to justify oxidation/reduction. Mention the reactivity series to explain displacement. Use the exact terms ‘oxidised’ and ‘reduced’ as the scheme expects. Keep the answer concise – 5 marks, so one sentence per point.
  • common_mistakes
  • :
  • Saying ‘iron is oxidised’ instead of ‘iron is reduced’. Forgetting to state that aluminium is more reactive. Using vague terms like ‘iron changes’ without specifying oxidation states.

Mark scheme (5 marks)

  1. Iron(III) oxide is reduced (because iron gains electrons / iron ions gain electrons / oxygen is removed from iron(III) oxide)
  2. Aluminium is oxidised (because aluminium loses electrons / oxygen is gained by aluminium)
  3. Oxidation and reduction occur simultaneously / this is a redox reaction
  4. Aluminium is more reactive than iron (aluminium is higher in the reactivity series than iron)
  5. A more reactive metal displaces a less reactive metal from its compound / oxide (so aluminium displaces iron from iron(III) oxide)

Key terms in this question

oxidation · reduction

Related

More Acid-base and redox reactions questions

▶ Try answering this question with AI marking (free) →