Question detail
Show the working for the energy transferred in 2HI -> H2 + I2 using average bond energies in Reaction profiles.
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
Question
Type
exam_style
Style
Topic
Exothermic and endothermic reactions
Question
Show the working for the energy transferred in 2HI -> H2 + I2 using average bond energies in Reaction profiles.
Answer
9 kJ. The reaction is endothermic.
Explanation
The correct option is 9 kJ, endothermic. Bonds broken: 2 × 298 = 596 kJ. Bonds formed: 1 × 436 + 1 × 151 = 587 kJ. Energy transferred = 596 - 587 = 9 kJ. The sign shows the reaction is endothermic. This 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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