A student places a cold pack on a sports injury. The cold pack contains ammonium nitrate and water in separate compartments. When the pack is squeezed, the compartments break and the chemicals mix, causing the pack to feel cold against the skin. Explain why the cold pack feels cold, referring to bonds and energy transfer.

OCR GCSE Chemistry A: Gateway Science (J248) — C3.2 Energetics · Explain · 4 marks · View as Markdown

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

Cold packs are used in sports medicine to reduce swelling after an injury. They work through a chemical reaction that occurs when the pack is activated.

Model answer (4 marks)

The dissolution of ammonium nitrate is an endothermic reaction. Breaking the ionic bonds in solid NH4NO3 and the H–O bonds in water requires more energy than is released when new bonds are formed in the aqueous solution. The excess energy is taken from the surroundings – the skin and the air around the pack – so the temperature of the surroundings falls. Because the pack absorbs heat, it feels cold to the skin.

Examiner tips

  • Use the word ‘endothermic’ and explain energy is absorbed from surroundings
  • Mention bond breaking requires more energy than bond forming
  • Show that the surroundings lose heat, lowering temperature
  • Link the temperature drop to the sensation of cold

Common mistakes

  • Saying the reaction releases energy (exothermic)
  • Forgetting to mention bond breaking vs forming
  • Not connecting the heat absorption to a drop in surrounding temperature

Mark scheme (4 marks)

  1. The reaction is endothermic
  2. Energy is taken in from the surroundings (the skin / the environment)
  3. More energy is required to break bonds in the reactants than is released when bonds form in the products
  4. The temperature of the surroundings decreases, so the pack feels cold

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