O Levels Chemistry (5070)•5070/12/M/J/23
Explanation
Enthalpy Change via Bond Energies Steps:
- Identify bonds broken: 4 C-H in CH4 and 4 O-H in 2H2O; sum their bond energies.
- Identify bonds formed: 2 C=O in CO2 and 4 H-H in 4H2; sum their bond energies.
- Calculate ΔH = (energy to break bonds) - (energy to form bonds).
- Using table values, ΔH = +380 kJ/mol (endothermic reaction).
Why C is correct:
- Bond energy method defines ΔH as positive when energy input to break bonds exceeds energy released forming new bonds, matching the steam reforming process.
Why the others are wrong:
- A: Incorrect sign; reaction is endothermic, not exothermic.
- B: Wrong magnitude and sign; underestimates positive value.
- D: Correct magnitude but wrong sign; ignores endothermic nature.
Final answer: C
Topic: Exothermic and endothermic reactions
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