Magnesium ribbon is burned in oxygen to produce magnesium oxide. In this reaction, magnesium is oxidised and oxygen is reduced. Explain what happens to magnesium and to oxygen in terms of electron transfer, and state what type of overall reaction this is.

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).

Magnesium burns vigorously in oxygen. The word equation for the reaction is: magnesium + oxygen → magnesium oxide.

Model answer (5 marks)

Magnesium loses two electrons to form Mg²⁺.
Oxygen gains two electrons to form O²⁻.
Loss of electrons is oxidation – therefore magnesium is oxidised.
Gain of electrons is reduction – therefore oxygen is reduced.
The overall process is a redox reaction, as oxidation and reduction occur simultaneously.

Examiner tips

  • Use the OIL‑RIG mnemonic to link electron loss/gain to oxidation/reduction.
  • Show the electron transfer explicitly (Mg → Mg²⁺ + 2e⁻, O₂ + 4e⁻ → 2O²⁻).
  • State the overall reaction type at the end.
  • Keep the answer concise – 5 marks only.

Common mistakes

  • Writing ‘oxygen is oxidised’ instead of ‘reduced’. Failing to mention that magnesium loses electrons. Not identifying the overall reaction as redox.

Mark scheme (5 marks)

  1. Magnesium loses electrons (during the reaction)
  2. Oxygen gains electrons (during the reaction)
  3. Loss of electrons is oxidation (OIL) — magnesium is oxidised because it loses electrons
  4. Gain of electrons is reduction (RIG) — oxygen is reduced because it gains electrons
  5. The overall reaction is a redox reaction (because oxidation and reduction occur simultaneously)

Key terms in this question

electron transfer

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