Hand warmers are an everyday example of an exothermic reaction. Explain, in terms of bond breaking and bond forming, why the temperature of the surroundings increases when a hand warmer is used.

Eduqas GCSE Chemistry — C5 Energy changes · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

1. Energy is required to break the bonds in the reactants.
2. Energy is released when new bonds are formed in the products.
3. The energy released from forming new bonds is greater than the energy needed to break the existing bonds.
4. The surplus energy is transferred to the surroundings, raising their temperature.

Examiner tips

  • Use the exact terms ‘break bonds’ and ‘form bonds’. Show the energy flow: break → release → surplus → surroundings. Keep the answer concise – one sentence per point.
  • Use the word ‘exothermic’ to link the reaction to heat release.
  • Mention that the temperature rise is due to energy transfer to the surroundings.

Mark scheme (4 marks)

  1. Energy is released when new bonds are formed in the products
  2. Energy is taken in / supplied to break bonds in the reactants
  3. The energy released from forming new bonds is greater than the energy needed to break existing bonds
  4. This surplus / extra energy is transferred to the surroundings, causing the temperature to increase

Key terms in this question

exothermic reaction · bond breaking · bond forming · surroundings

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