Neutralisation is an exothermic reaction. When dilute hydrochloric acid is added to sodium hydroxide solution, the temperature of the mixture rises. Explain, in terms of bond breaking and bond forming, why the temperature of the mixture increases during this neutralisation reaction.

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)

Energy is required to break the bonds in the reactants.
Energy is released when new bonds are formed in the products.
The energy released from forming new bonds is greater than the energy needed to break the existing bonds.
The surplus energy is transferred to the surroundings, raising the temperature.

Examiner tips

  • Use the word "explain" to show cause and effect, not just state facts.
  • Mention both bond breaking and bond forming explicitly.
  • Show the energy balance: released > required.
  • Link the surplus energy to the temperature rise.

Common mistakes

  • Failing to mention both bond breaking and bond forming.
  • Saying the reaction is exothermic without explaining the energy balance.
  • Confusing the direction of heat flow (e.g., saying heat is absorbed instead of released).

Mark scheme (4 marks)

  1. Energy is required / 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 new bonds is greater than the energy needed to break existing bonds
  4. This surplus / excess energy is transferred to the surroundings, causing the temperature to increase

Key terms in this question

exothermic reaction · neutralisation · bond breaking · bond forming

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