Hand warmers contain iron powder. When the pack is opened and exposed to air, the iron reacts with oxygen in an exothermic reaction, releasing energy to warm the hands. Explain, in terms of bonds and energy transfer to the surroundings, why this reaction is exothermic, and describe what happens to the temperature of the surroundings during the reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Hand warmers contain iron powder. When the pack is opened and exposed to air, the iron reacts with oxygen in an exothermic reaction.
Model answer (5 marks)
In an exothermic reaction, energy is transferred to the surroundings.
1. Breaking the Fe–Fe bonds in the iron powder requires energy (endothermic).
2. Forming Fe–O bonds in iron oxide releases energy (exothermic).
3. The energy released in forming Fe–O bonds is greater than the energy required to break the Fe–Fe bonds.
4. Therefore the net energy change is negative, so energy is given off.
5. This energy is transferred to the surroundings, raising their temperature.
1. Breaking the Fe–Fe bonds in the iron powder requires energy (endothermic).
2. Forming Fe–O bonds in iron oxide releases energy (exothermic).
3. The energy released in forming Fe–O bonds is greater than the energy required to break the Fe–Fe bonds.
4. Therefore the net energy change is negative, so energy is given off.
5. This energy is transferred to the surroundings, raising their temperature.
Examiner tips
- Use the phrase "energy is transferred to the surroundings" for the first point.
- Show the energy balance: bond breaking > bond making.
- Mention the temperature rise explicitly.
- Keep the answer concise and use the exact terminology.
Mark scheme (5 marks)
- In an exothermic reaction, energy is transferred to the surroundings
- Bond breaking is an endothermic process (requires energy input)
- Bond making is an exothermic process (releases energy)
- The energy released in bond making is greater than the energy required for bond breaking / more energy is released than taken in overall
- The temperature of the surroundings increases
Key terms in this question
exothermic reaction · surroundings
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