A student uses a cold pack to reduce swelling after a sports injury. When the cold pack is activated, the contents dissolve in water and the pack becomes noticeably cold. Explain why the pack becomes cold, using your knowledge of endothermic reactions and bond breaking and bond making.

OCR GCSE Chemistry A: Gateway Science (J248) — C3.3 Types of chemical reactions · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Cold packs are used by sports teams to treat minor injuries on the pitch. When activated, the pack rapidly drops in temperature without any external cooling source.

Model answer (4 marks)

The reaction in the cold pack is endothermic. Energy is absorbed from the surroundings – the pack and the skin – to break the bonds in the reactants. The energy required to break these bonds is greater than the energy released when new bonds are formed in the products. Consequently, the excess energy is taken from the surroundings, lowering their temperature. This decrease in temperature makes the pack feel cold.

Examiner tips

  • Use the word ‘endothermic’ early to show understanding of the reaction type.
  • Explain the energy flow: energy absorbed > energy released, leading to cooling.
  • Mention that the temperature drop is felt by the pack and the skin.
  • Keep the answer concise – 4 points can be covered in 3–4 short sentences.

Common mistakes

  • Saying the reaction is exothermic – the pack actually absorbs heat.
  • Failing to mention that more energy is needed to break bonds than is released when new bonds form.
  • Not linking the energy absorption to a drop in temperature of the surroundings.

Mark scheme (4 marks)

  1. The reaction taking place is endothermic
  2. Energy is taken in from the surroundings (the pack / skin)
  3. More energy is required to break bonds in the reactants than is released when bonds are formed in the products
  4. This causes the temperature of the surroundings to decrease, making the pack feel cold

Key terms in this question

endothermic reaction · bond breaking · bond making

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