Question detail

For The energy change of reactions (HT only), use bond energies to answer this exam-check check: what is the energy transferred for H2 + I2 -> 2HI?

Try the question, check the answer, then read the explanation to understand the curriculum point.

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At a glance

MCQ

Type

practice

Style

Topic

Exothermic and endothermic reactions

Question

A. -9 kJ, exothermic
B. 9 kJ, endothermic
C. 0 kJ, no net energy change
D. 184 kJ, endothermic

Answer

The correct option is -9 kJ, exothermic.

Explanation

The correct option is -9 kJ, exothermic. Bonds broken: 1 × 436 + 1 × 151 = 587 kJ. Bonds formed: 2 × 298 = 596 kJ. Energy transferred = 587 - 596 = -9 kJ. The sign shows the reaction is exothermic. This exam-check question 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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