Question detail
For an AQA Energy changes calculation on 2H2O -> 2H2 + O2, which result would earn the mark?
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
Exothermic and endothermic reactions
Question
- A. 482 kJ, endothermic
- B. -482 kJ, exothermic
- C. 0 kJ, no net energy change
- D. Incorrect bond-energy interpretation 4
Answer
The correct option is 482 kJ, endothermic.
Explanation
The correct option is 482 kJ, endothermic. Bonds broken: 4 × 463 = 1852 kJ. Bonds formed: 2 × 436 + 1 × 498 = 1370 kJ. Energy transferred = 1852 - 1370 = 482 kJ. The sign shows the reaction is endothermic. This exam-check question supports the approved objective to use reaction profiles to identify whether a reaction is exothermic or endothermic and keeps bond breaking separate from bond making in Reaction profiles.
Common mistake
Misidentifying the overall energy change on a reaction profile
Students often read the vertical drop or rise between the reactant and product baselines as the activation energy, rather than the overall energy change (ΔE).
Explain that the activation energy is the height of the peak above the reactant baseline, while the overall energy change is the difference between the reactant and product baselines; a downward drop indicates an exothermic reaction and an upward rise indicates an endothermic reaction.
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