When a cold pack is applied to a sports injury, a chemical reaction occurs inside the pack and the pack feels cold. Explain why the pack feels cold, referring to energy transfer 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, so it absorbs energy from the surroundings.
The energy taken in is used to break bonds in the reactants.
Because the energy released when new bonds form in the products is less, the excess energy is taken from the pack.
Thus the pack loses heat and its temperature falls, making it feel cold.

Examiner tips

  • Use the word "endothermic" to show you know the type of reaction. Explain that energy is absorbed to break bonds and that this energy comes from the pack. Mention that the temperature of the surroundings (the pack) decreases. Link the bond‑breaking and bond‑forming energies to the endothermic nature.

Common mistakes

  • Saying the reaction is exothermic or ignoring the energy transfer. Failing to mention that the energy for bond breaking comes from the pack. Using vague terms like "heat is absorbed" without explaining bond energies.

Mark scheme (4 marks)

  1. The reaction is endothermic
  2. The reaction takes in energy from the surroundings
  3. The temperature of the surroundings decreases
  4. The energy needed to break bonds in the reactants is greater than the energy released when bonds in the products are formed

Key terms in this question

bond breaking · bond forming

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