A student adds a spatula of ammonium nitrate crystals to a test tube containing a small volume of water and stirs the mixture. The outside of the test tube feels noticeably cold to the touch. Describe what this observation tells us about the energy change occurring, and explain, in terms of bonds, why the temperature of the surroundings decreases.

Cambridge International IGCSE Chemistry (0620) — 5.1 Exothermic and endothermic reactions · Describe and 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.
Breaking the bonds of ammonium nitrate requires more energy than the energy released when new bonds are formed.
The net absorption of energy causes the temperature of the surroundings to fall.

Examiner tips

  • Use the term "endothermic" for the first point. Show the energy flow: surroundings → reaction. Explain bond breaking vs bond forming. Keep the answer to 4 points, no extra words.

Common mistakes

  • Saying the reaction is exothermic. Forgetting to mention that energy is taken from the surroundings. Confusing the direction of energy flow or the bond energy comparison.

Mark scheme (4 marks)

  1. The reaction is endothermic
  2. Energy is transferred from the surroundings to the reaction / reactants
  3. Energy is required to break bonds (in the reactants)
  4. The energy needed to break bonds is greater than the energy released when new bonds form, so there is an overall net absorption of energy

Key terms in this question

surroundings

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