A student dissolves ammonium nitrate crystals in water in a beaker. The outside of the beaker feels noticeably colder after mixing. Explain what type of energy change is occurring and why the beaker feels cold.

OCR GCSE Chemistry A: Gateway Science (J248) — C3.1 Introducing chemical reactions · Explain · 4 marks · View as Markdown

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

Ammonium nitrate is used in instant cold packs for treating sports injuries. When ammonium nitrate dissolves in water, an energy change occurs.

Model answer (4 marks)

The dissolution of ammonium nitrate is an endothermic process. Energy is absorbed from the surroundings, so the beaker and the water lose energy and their temperature falls. The process requires more energy to break the ionic bonds in the solid and the hydration bonds than is released when new bonds form, giving a net positive energy change.

Examiner tips

  • Use the word "endothermic" to show you know the type of reaction.
  • Explain that energy is taken from the surroundings and that this reduces the temperature.
  • Mention the bond‑breaking vs bond‑forming energy difference to justify the endothermic nature.
  • Keep the answer concise – 4 marks are enough for a short explanation.

Common mistakes

  • Saying the reaction is exothermic or that the beaker heats up.
  • Failing to mention that energy is taken from the surroundings. Not explaining the bond energy difference or the net positive ΔH.

Mark scheme (4 marks)

  1. The reaction/process is endothermic
  2. Energy is taken in from the surroundings
  3. The surroundings (beaker/water) lose energy / the temperature of the surroundings decreases
  4. More energy is required to break bonds in the reactants than is released when bonds form in the products / overall energy change is positive

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