Hand warmers contain chemicals that react when the pack is activated. Explain why a hand warmer feels warm to the touch, and describe what happens to the temperature of the surroundings during this type of reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student activates a disposable hand warmer and notices it quickly becomes hot. She also observes that when she dissolves ammonium nitrate in water to make a cold pack, the outside of the container feels cold.
Model answer (4 marks)
A hand warmer contains chemicals that react exothermically.
When the pack is activated the reaction releases energy as heat.
This heat is transferred to the surroundings (the hand and air), so the
surroundings become warmer.
In contrast, the cold pack reaction is endothermic – it absorbs energy from
the surroundings, so the surrounding temperature falls.
When the pack is activated the reaction releases energy as heat.
This heat is transferred to the surroundings (the hand and air), so the
surroundings become warmer.
In contrast, the cold pack reaction is endothermic – it absorbs energy from
the surroundings, so the surrounding temperature falls.
Examiner tips
- Use the word "exothermic" for the hand warmer and "endothermic" for the cold pack. Show the energy flow: reaction → surroundings → temperature change. Mention the temperature change of the surroundings explicitly.
Common mistakes
- Confusing exothermic with endothermic. Forgetting to state that the surroundings are the hand or the air. Not linking the energy transfer to the change in temperature.
Mark scheme (4 marks)
- The hand warmer undergoes an exothermic reaction
- Energy is transferred to the surroundings (from the reaction)
- The temperature of the surroundings increases (in an exothermic reaction)
- The cold pack reaction is endothermic because energy is taken in from the surroundings, so the temperature of the surroundings decreases
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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