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) Explain that bond energies can be used to calculate energy needed to break bonds and keeps bond breaking separate from bond making in The energy change of reactions (HT only).
Common mistake
Misunderstanding Bond Energies
Students often confuse bond energies with the energy changes in reaction profiles, thinking they are the same concept.
Clarify that bond energies specifically refer to the energy required to break bonds, while reaction profiles illustrate the overall energy changes during a reaction.
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