O Levels Chemistry (5070)•5070/11/M/J/25
Explanation
Bond Energies Determine Enthalpy Change
Steps:
- ΔH negative indicates an exothermic reaction, releasing energy.
- Breaking bonds absorbs energy; forming bonds releases energy.
- Overall ΔH = (energy of bonds broken) - (energy of bonds formed).
- For ΔH < 0, energy of bonds formed exceeds energy of bonds broken.
Why B is correct:
- Matches ΔH = bonds broken - bonds formed < 0, where bonds formed > bonds broken, defining an exothermic process.
Why the others are wrong:
- A: Energy of bonds broken > formed means ΔH > 0 (endothermic), not exothermic.
- C: Energy of bonds broken > formed correctly describes endothermic (ΔH > 0), irrelevant to ΔH < 0.
- D: Energy of bonds broken < formed means exothermic (ΔH < 0), but wrongly labeled endothermic.
Final answer: B
Topic: Exothermic and endothermic reactions
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