A hand warmer is used by a person on a cold day. When the hand warmer is activated, it releases energy to the surroundings. Explain, in terms of bond breaking and bond forming, why the reaction in the hand warmer causes the temperature of the surroundings to increase.

AQA GCSE Chemistry (8462) — 4.5.1 Exothermic and endothermic reactions · 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 bonds is greater than the energy needed to break bonds.
4. Therefore, the overall reaction gives off energy to the surroundings, raising their temperature.

Examiner tips

  • Use the word "explain" to describe the energy flow; mention bond breaking and forming. Show the comparison of energy required vs released. Link the excess energy to the temperature rise of the surroundings. Keep the answer concise and use correct terminology (exothermic, bond energy).

Common mistakes

  • Confusing the direction of energy flow (saying energy is absorbed by surroundings). Omitting the step that the released energy exceeds the energy used to break bonds. Using vague terms like "heat up" without linking to bond energy.

Mark scheme (4 marks)

  1. Energy is required / supplied to break bonds in the reactants
  2. Energy is released when new bonds are formed in the products
  3. The energy released from forming bonds is greater than the energy needed to break bonds
  4. So overall energy is transferred to the surroundings, causing the temperature of the surroundings to increase

Key terms in this question

bond breaking · bond forming · surroundings

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