A student adds magnesium ribbon to a solution of copper sulfate. The magnesium ribbon gradually disappears and a reddish-brown solid forms on the surface of the metal. The student also notices that the test tube becomes warm to the touch. Explain why this reaction is classified as both an oxidation reaction and an exothermic reaction.

OCR GCSE Chemistry A: Gateway Science (J248) — C3.3 Types of chemical reactions · Explain · 4 marks · View as Markdown

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

Magnesium is more reactive than copper and displaces it from copper sulfate solution. During the reaction, magnesium atoms lose electrons and copper ions gain electrons.

Model answer (4 marks)

Magnesium is oxidised – it loses electrons to form Mg²⁺.
Copper ions are reduced – they gain the electrons lost by magnesium.
Because electrons are transferred, the reaction is an oxidation–reduction (redox) reaction.
The reaction releases energy; the surroundings (test tube) warm up, showing it is exothermic.

Examiner tips

  • Use the word ‘oxidised’ and ‘reduced’ to show electron transfer. Mention the temperature rise to prove exothermicity. Keep each point short – 1 mark each.
  • common_mistakes
  • :
  • Confusing oxidation with reduction. Failing to link the temperature change to exothermicity. Using vague terms like ‘heat’ without showing the test tube warming.

Mark scheme (4 marks)

  1. Oxidation involves the loss of electrons (by magnesium)
  2. Magnesium is oxidised / the oxidising agent is the copper ion which gains electrons
  3. The reaction is exothermic because energy is transferred to the surroundings
  4. The temperature of the surroundings increases, shown by the test tube becoming warm

Key terms in this question

oxidation · exothermic reaction

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