When magnesium burns in oxygen, combustion occurs and a white powder is produced. Explain, in terms of bond breaking and bond forming, why combustion reactions are exothermic.

AQA GCSE Chemistry (8462) — 4.5.1 Exothermic and endothermic reactions · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

1. Energy must be supplied to break the bonds in the reactants.
2. Energy is released when new bonds are formed in the products.
3. In an exothermic reaction the energy released from forming bonds is greater than the energy required to break bonds.
4. The overall energy change is transferred to the surroundings, raising their temperature.

Examiner tips

  • Use the exact wording from the mark scheme – e.g. "energy must be supplied to break bonds". Show the sequence: break → form → net release → temperature rise.

Common mistakes

  • Confusing endothermic with exothermic; writing that energy is absorbed overall. Omitting the step that energy is released when bonds are formed.

Mark scheme (4 marks)

  1. Energy must be supplied to break bonds in the reactants
  2. Energy is released when bonds are formed in the products
  3. In an exothermic reaction, the energy released from forming bonds is greater than the energy needed to break bonds
  4. The overall energy change means energy is transferred to the surroundings, so the temperature of the surroundings increases

Key terms in this question

bond breaking · bond forming · combustion

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