Question detail
Show the working for the energy transferred in H2 + Cl2 -> 2HCl using average bond energies in The energy change of reactions (HT only).
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 H2 + Cl2 -> 2HCl using average bond energies in The energy change of reactions (HT only).
Answer
-184 kJ. The reaction is exothermic.
Explanation
The correct option is -184 kJ, exothermic. Bonds broken: 1 × 436 + 1 × 242 = 678 kJ. Bonds formed: 2 × 431 = 862 kJ. Energy transferred = 678 - 862 = -184 kJ. The sign shows the reaction is exothermic. This supports the approved objective to (HT only) Distinguish bond energy calculations from reaction profile interpretation and keeps bond breaking separate from bond making in The energy change of reactions (HT only).
Common mistake
Confusing bond‑energy calculations with reaction‑profile interpretation
Students think that the shape of a reaction profile can be used to calculate the exact energy change of a reaction, or that the activation energy shown on the profile is the same as the bond energies used in calculations.
Explain that a reaction profile is a qualitative diagram showing the relative energies of reactants, transition state and products, and that it cannot provide numerical bond‑energy values. Bond‑energy calculations use tabulated average energies for specific bonds to determine the overall energy change, while a reaction profile only indicates whether a reaction is exothermic or endothermic and the relative size of the activation energy.
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