Question detail
Using the supplied average bond energies, what is the energy transferred for H2 + Cl2 -> 2HCl 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
MCQ
Type
practice
Style
Topic
Exothermic and endothermic reactions
Question
- A. -184 kJ, exothermic
- B. 184 kJ, endothermic
- C. 0 kJ, no net energy change
- D. Incorrect bond-energy interpretation 4
Answer
The correct option is -184 kJ, 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) Calculate the overall energy change as energy needed to break bonds minus energy released when bonds form and keeps bond breaking separate from bond making in The energy change of reactions (HT only).
Common mistake
Mis‑applying the sign convention
Students often add the energy released when bonds form to the energy required to break bonds, giving a positive value for an exothermic reaction.
Remember the formula: ΔE = ΣE(bonds broken) – ΣE(bonds formed). For an exothermic reaction the second term is larger, so ΔE is negative, indicating energy is released.
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