Hand warmers are single-use packets used by hikers in cold weather. When a hand warmer is activated, a chemical reaction occurs inside the packet and the surface of the packet becomes warm to the touch. Explain why the packet becomes warm, using the terms bond breaking and bond making in your answer. Also state whether the overall energy change of this reaction is positive or negative, and justify your answer.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Hand warmers are single-use packets used by hikers in cold weather. When a hand warmer is activated, a chemical reaction occurs inside the packet and the surface of the packet becomes warm to the touch.
Model answer (5 marks)
Bond breaking is endothermic – energy is absorbed to break the bonds in the reactants. Bond making is exothermic – energy is released when new bonds form in the products. In the hand‑warmer reaction more energy is released in bond making than is taken in during bond breaking, so the net effect is that energy is transferred to the surroundings. This transfer of energy raises the temperature of the packet surface, making it feel warm. The overall energy change of the reaction is negative (exothermic) because more energy is released than absorbed.
Examiner tips
- Use the terms ‘bond breaking’ and ‘bond making’ explicitly. Show the energy flow: endothermic for breaking, exothermic for making. Explain that the net release of energy heats the packet. State the overall ΔE is negative and justify it as exothermic.
Common mistakes
- Confusing endothermic with exothermic for the whole reaction. Forgetting to mention that the packet heats up because energy is transferred to the surroundings. Using vague language such as ‘energy is released’ without linking it to bond making.
Mark scheme (5 marks)
- Bond breaking is an endothermic process (energy is taken in to break bonds in the reactants)
- Bond making is an exothermic process (energy is released when new bonds form in the products)
- More energy is released in bond making than is taken in during bond breaking
- Therefore energy is transferred to the surroundings, causing the temperature of the surroundings (the packet surface) to increase
- The overall energy change is negative, because more energy is released than is taken in (net energy is transferred to surroundings in an exothermic reaction)
Key terms in this question
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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