Cold packs are used in sports medicine to treat injuries. When a cold pack is activated, ammonium nitrate dissolves in water inside the pack. The temperature of the pack drops significantly, making the surrounding skin feel cold. Explain why the temperature drops when the cold pack is activated, referring to the overall energy change of the reaction and the processes of bond breaking and bond making.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Cold packs are single-use devices used by physiotherapists on sports pitches. When squeezed, the pack activates and rapidly becomes cold enough to reduce swelling in an injured joint.
Model answer (5 marks)
1. The dissolution of ammonium nitrate in water is an endothermic process.
2. Energy is absorbed from the surroundings – the skin and the air around the pack.
3. The energy required to break the ionic bonds in solid NH₄NO₃ and the hydrogen bonds in water is greater than the energy released when new bonds form between NH₄⁺, NO₃⁻ and water molecules.
4. Bond breaking is endothermic; it takes in energy.
5. Because the overall ΔH is positive, the products have more energy than the reactants, so the temperature of the surroundings falls.
2. Energy is absorbed from the surroundings – the skin and the air around the pack.
3. The energy required to break the ionic bonds in solid NH₄NO₃ and the hydrogen bonds in water is greater than the energy released when new bonds form between NH₄⁺, NO₃⁻ and water molecules.
4. Bond breaking is endothermic; it takes in energy.
5. Because the overall ΔH is positive, the products have more energy than the reactants, so the temperature of the surroundings falls.
Examiner tips
- Use the word ‘endothermic’ early to show understanding of the process.
- Show the energy flow – from surroundings into the reaction – to justify the temperature drop.
- Mention bond breaking vs bond making to explain the positive ΔH.
- Keep the answer concise – 5 points, one sentence each.
Mark scheme (5 marks)
- The dissolving process is an endothermic reaction / change
- Energy is taken in from the surroundings (the skin / the environment)
- The energy needed to break bonds in the reactants is greater than the energy released when bonds in the products are formed
- Bond breaking is an endothermic process (requires / takes in energy)
- The overall energy change is positive / the products have more energy than the reactants, so the temperature of the surroundings decreases
Key terms in this question
bond breaking · bond making · overall energy change
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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