A student adds a small amount of citric acid powder to a glass of water and stirs it. The glass feels noticeably colder on the outside after the powder has dissolved and reacted. Explain why the glass feels colder, using your knowledge of endothermic reactions and bond breaking and bond making.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Citric acid is a weak acid found naturally in lemons and limes. It is often used in fizzy drink powders.
Model answer (4 marks)
The dissolution of citric acid is an endothermic reaction. Energy is absorbed from the surroundings – the water and the glass – to break the bonds in the citric acid molecules. The energy required to break these bonds is greater than the energy released when new bonds form in the products. Consequently the net energy taken from the surroundings is negative, lowering the temperature of the water and the glass. The glass therefore feels colder.
Examiner tips
- Use the word 'endothermic' early to show you know the type of reaction.
- Explain that energy is absorbed from the surroundings and link this to a drop in temperature.
- Mention that bond breaking requires more energy than bond making releases.
Common mistakes
- Saying the reaction is exothermic or ignoring the energy balance of bonds.
- Failing to connect the absorbed energy to a temperature drop.
- Using vague terms like 'heat is lost' without explaining the source of the heat.
Mark scheme (4 marks)
- The reaction is endothermic
- Energy is taken in from the surroundings (the water/glass)
- More energy is required to break bonds in the reactants than is released when bonds are formed in the products
- This causes the temperature of the surroundings to decrease, so the glass feels colder
Key terms in this question
endothermic reaction · bond breaking · bond making
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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