When citric acid is dissolved in water and sodium hydrogencarbonate powder is added, the temperature of the mixture decreases. Explain what this temperature decrease tells us about the energy change in this reaction, and describe what is happening to energy at the particle level.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Citric acid reacts with sodium hydrogencarbonate in solution. The temperature of the surrounding solution is observed to fall during the reaction.
Model answer (4 marks)
The reaction is endothermic. Energy is absorbed from the surroundings, so the solution cools. More energy is required to break the bonds in the reactants than is released when bonds in the products are formed. Consequently the overall ΔE is positive – the products are at a higher energy level than the reactants.
Examiner tips
- Use the term "endothermic" to gain a mark for the reaction type.
- Explain that heat is taken from the solution, giving a temperature drop.
- Mention that bond breaking requires more energy than bond forming releases.
- State that ΔE is positive, meaning products have higher energy than reactants.
Common mistakes
- Saying the reaction is exothermic or ignoring the temperature drop.
- Failing to link the temperature change to energy being absorbed from the surroundings.
- Not mentioning the positive ΔE or the higher energy of the products.
Mark scheme (4 marks)
- The reaction is endothermic
- Energy is taken in from the surroundings (the solution / water), causing the temperature of the surroundings to decrease
- More energy is required to break bonds in the reactants than is released when bonds in the products are formed
- The overall energy change of the reaction is positive (the products have more energy / are at a higher energy level than the reactants)
Related
- All Pearson Edexcel International GCSE Chemistry (4CH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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