A student places a piece of magnesium ribbon into a beaker of dilute hydrochloric acid. The beaker feels warm to the touch. Explain what type of reaction is occurring and why the beaker feels warm.

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).

Magnesium reacts vigorously with dilute hydrochloric acid, producing magnesium chloride and hydrogen gas. The student notices the beaker becomes noticeably warm during the reaction.

Model answer (4 marks)

The reaction is exothermic.

Energy is transferred to the surroundings.

The energy released when bonds form in the products (MgCl₂ and H₂) is greater than the energy required to break the bonds in the reactants (Mg and HCl).

This excess energy raises the temperature of the surroundings – the beaker and the solution – so the beaker feels warm.

Examiner tips

  • Use the word ‘exothermic’ early to show you know the reaction type.
  • Explain that bond formation releases more energy than bond breaking requires.
  • Mention that the excess energy is transferred to the surroundings, raising temperature.
  • Keep the answer concise – 4 points can be covered in 4 short sentences.

Common mistakes

  • Saying the reaction is ‘endothermic’ or ‘neutral’ – it is exothermic.
  • Failing to mention that energy is released when bonds form in the products.
  • Not linking the energy release to the beaker feeling warm.

Mark scheme (4 marks)

  1. The reaction is exothermic
  2. Energy is transferred to the surroundings
  3. The energy released when bonds form in the products is greater than the energy needed to break bonds in the reactants
  4. This causes the temperature of the surroundings (the beaker/solution) to increase

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