Combustion is an example of an exothermic reaction. Explain, in terms of bond breaking and bond forming, why the overall energy change for a combustion reaction is exothermic.

Eduqas GCSE Chemistry — C5 Energy changes · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Energy must be supplied to break the bonds in the reactants.
Energy is released when new bonds are formed in the products.
The energy released from forming bonds is greater than the energy required to break bonds.
Therefore the overall energy change is exothermic and energy is transferred to the surroundings, raising their temperature.

Examiner tips

  • Use the word ‘exothermic’ early to show understanding of the concept.
  • Show the energy flow: break bonds (energy in) → form bonds (energy out).
  • Explain that the net energy out > energy in, giving a negative ΔH.
  • Mention that the excess energy is released as heat to the surroundings.

Common mistakes

  • Failing to mention that energy is required to break bonds.
  • Confusing the direction of energy flow (e.g., saying energy is taken in when bonds form).
  • Not linking the greater energy release to the exothermic nature of the reaction.

Mark scheme (4 marks)

  1. Energy must be supplied / is taken in to break bonds in the reactants
  2. Energy is released when new bonds are formed in the products
  3. The energy released from forming bonds is greater than the energy needed to break bonds
  4. Therefore the overall energy change transfers energy to the surroundings, so the temperature of the surroundings increases

Key terms in this question

exothermic reaction · bond breaking · bond forming · overall energy change

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