Explain why the temperature of the surroundings decreases during an endothermic reaction. Use ideas about bond breaking and bond forming in your answer.

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)

During an endothermic reaction energy is absorbed from the surroundings. Breaking the bonds in the reactants requires energy input. Although forming new bonds in the products releases energy, the energy required to break the reactant bonds is greater than the energy released when the product bonds are formed. Consequently the net energy taken from the surroundings is positive, so the surroundings lose energy and their temperature falls.

Examiner tips

  • Use the word ‘absorbed’ to show energy taken from surroundings
  • Mention both bond breaking and bond forming
  • Explain the energy imbalance clearly
  • Keep the answer concise and to the point

Common mistakes

  • Confusing endothermic with exothermic; writing that temperature rises
  • Omitting the comparison of bond energies
  • Using vague terms like ‘energy changes’ without specifying source or sink

Mark scheme (4 marks)

  1. Energy is taken in / absorbed from the surroundings during the reaction
  2. Breaking bonds requires/takes in energy
  3. Forming bonds releases energy
  4. The energy needed to break bonds in the reactants is greater than the energy released when bonds in the products are formed, so the surroundings lose energy and the temperature decreases

Key terms in this question

endothermic reaction · bond breaking · bond forming · surroundings

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