A student mixes sodium hydroxide solution with hydrochloric acid in a polystyrene cup. The temperature of the mixture rises. Explain why the temperature rises and why a polystyrene cup is used instead of a glass beaker.

AQA A-Level Chemistry (7405) — 3.1.4 Energetics · Explain · 5 marks · View as Markdown

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

Exothermic reactions release energy to the surroundings, causing a temperature rise. Polystyrene is a thermal insulator.

Model answer (5 marks)

The reaction between NaOH and HCl is exothermic, so energy is released as heat.
The heat produced is transferred from the reacting chemicals to the surrounding solution, raising its temperature.
The energy released is greater than the energy required to break the bonds in the reactants because the bonds formed in the products (NaCl and H₂O) are stronger.
A polystyrene cup is a good thermal insulator, so it limits heat loss to the external environment.
This insulation gives a more accurate and reliable measurement of the temperature change of the reaction mixture.

Examiner tips

  • Use the word "exothermic" and mention heat transfer to the solution. Explain bond energy difference. State that polystyrene reduces heat loss and improves accuracy.

Common mistakes

  • Saying the reaction is endothermic. Claiming the cup is used for safety rather than insulation. Forgetting to mention bond energy or heat transfer to the solution.

Mark scheme (5 marks)

  1. The reaction is exothermic
  2. Energy is transferred from the reaction/chemicals to the surroundings/solution
  3. The bonds formed in the products release more energy than is required to break bonds in the reactants
  4. Polystyrene is a good thermal insulator / reduces energy transfer to the surroundings
  5. This gives a more accurate/reliable temperature change measurement

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