Combustion is an exothermic reaction. Explain, in terms of bond breaking and bond forming, why the temperature of the surroundings increases when a fuel burns completely in oxygen.

Edexcel GCSE Chemistry (1CH0) — 7.2 Heat energy changes in chemical 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. In the reactants (fuel and oxygen) bonds must be broken.
2. Breaking bonds requires energy.
3. In the products, new bonds are formed.
4. Energy is released when these bonds form, and this released energy is greater than the energy absorbed to break the reactant bonds, so the excess energy is transferred to the surroundings, raising their temperature.

Examiner tips

  • Use the sequence: bond breaking → energy absorbed, bond forming → energy released, compare energies to explain temperature rise.
  • Include the word "exothermic" to show you understand the overall energy flow.
  • Show the energy balance clearly: released > absorbed.
  • Mention that the excess energy is transferred to the surroundings as heat.

Common mistakes

  • Failing to state that bond breaking requires energy.
  • Confusing the direction of energy flow (e.g., saying energy is taken from the surroundings).
  • Not recognising that the released energy exceeds the absorbed energy.

Mark scheme (4 marks)

  1. Bond breaking occurs in the reactants (fuel and oxygen)
  2. Bond breaking requires/takes in energy
  3. Bond forming occurs in the products and releases energy
  4. The energy released from forming bonds in the products is greater than the energy needed to break bonds in the reactants, so energy is transferred to the surroundings

Key terms in this question

exothermic reaction · bond breaking · bond forming · surroundings

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