When hand warmers are used, an exothermic reaction occurs inside the pack. Explain, in terms of bond breaking and bond forming, why the temperature of the hand warmer increases during this reaction.

Edexcel GCSE Chemistry (1CH0) — 7.2 Heat energy changes in chemical reactions · Explain · 4 marks · View as Markdown

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

Hand warmers are single-use packs that become hot when activated. They are used by athletes and outdoor workers in cold weather.

Model answer (4 marks)

Energy is required to break the bonds in the reactants. Energy is released when new bonds are formed in the products. The energy released from forming new bonds is greater than the energy required to break the existing bonds. This means there is a net transfer of energy to the surroundings, so the temperature of the hand warmer increases.

Examiner tips

  • Use the word ‘explain’ to describe the energy flow, not just state it.
  • Show the sequence: bond breaking → energy absorbed, bond forming → energy released.
  • Mention that the released energy exceeds the absorbed energy, giving a net exothermic effect.
  • Link the net energy transfer to the rise in temperature of the pack.

Common mistakes

  • Failing to mention that bond breaking requires energy.
  • Confusing the direction of energy transfer (saying energy goes into the pack instead of out).
  • Not recognising that the net energy released heats the surroundings rather than the reactants themselves.

Mark scheme (4 marks)

  1. Energy is required / taken in to break 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 existing bonds
  4. This means there is a net transfer of energy to the surroundings, so the temperature of the hand warmer increases

Key terms in this question

exothermic reaction · bond breaking · bond forming

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