Hand warmers are small packets used by skiers and hikers to keep their hands warm. Inside a hand warmer, iron powder reacts with oxygen from the air. Explain why a hand warmer becomes warm when it is activated, and describe what this tells us about the overall energy change of the reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Hand warmers are single-use devices that produce heat when a packet is opened and exposed to air. The iron powder inside reacts with oxygen to form iron oxide.
Model answer (4 marks)
The reaction of iron with oxygen releases energy to the surroundings, so the packet heats up.
When the iron powder oxidises, the temperature of the surrounding air rises.
Because heat is given off, the reaction is exothermic.
The energy released when new bonds are formed in the iron oxide is greater than the energy required to break the bonds in iron and oxygen, giving a negative overall ΔE.
When the iron powder oxidises, the temperature of the surrounding air rises.
Because heat is given off, the reaction is exothermic.
The energy released when new bonds are formed in the iron oxide is greater than the energy required to break the bonds in iron and oxygen, giving a negative overall ΔE.
Examiner tips
- Use the word ‘exothermic’ to link heat release with the reaction’s energy change.
- Show the chain: energy released → surroundings warm → reaction exothermic → ΔE negative.
- Mention bond breaking/formation to justify the negative ΔE.
Common mistakes
- Failing to state that the reaction is exothermic.
- Confusing the direction of heat flow (saying heat goes into the packet instead of out).
- Omitting the bond‑formation/breaking explanation for the negative ΔE.
Mark scheme (4 marks)
- The reaction transfers energy to the surroundings
- The temperature of the surroundings increases
- This means the reaction is exothermic
- The energy released when bonds form in the products is greater than the energy needed to break bonds in the reactants / the overall energy change is negative
Key terms in this question
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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