A student mixes citric acid and sodium hydrogencarbonate in a beaker. The temperature of the mixture decreases. Explain why the temperature decreases, in terms of energy transfer to or from the surroundings and bond breaking and bond forming.

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)

The reaction is endothermic.
Energy is absorbed from the surroundings, so the beaker cools.
Energy is required to break the bonds in citric acid and sodium hydrogencarbonate.
The energy needed to break the bonds is greater than the energy released when new bonds form in the products.

Examiner tips

  • Use the term "endothermic" to gain a point. Explain that heat is taken from the surroundings. Mention bond breaking and bond forming. Keep the answer concise and use correct terminology.

Common mistakes

  • Saying the reaction is exothermic. Forgetting to mention that heat is taken from the surroundings. Using vague phrases like "energy is used" without specifying bond breaking and forming.

Mark scheme (4 marks)

  1. The reaction is endothermic
  2. Energy is taken in from the surroundings (causing the temperature to decrease)
  3. Energy is required / supplied to break bonds in the reactants
  4. The energy needed to break bonds is greater than the energy released when new bonds are formed (in the products)

Key terms in this question

surroundings · bond breaking · bond forming

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