Bond enthalpiesEdexcel International A Level Chemistry: Flashcards

Exam questions on Bond enthalpies

1. A gas hob burns methane in air: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g). An engineer estimates the enthalpy change using mean bond enthalpies (kJ mol⁻¹): C–H 413, O=O 498, C=O (in CO₂) 805, O–H 464.
   The experimental standard enthalpy change of combustion of methane is –890 kJ mol⁻¹. Give two reasons why the value calculated from mean bond enthalpies is different.2 marks
2. Chemists use bond enthalpy data to predict which bonds break most easily and so how quickly a compound reacts at room temperature. Mean bond enthalpies (kJ mol⁻¹): C–Cl 346, C–Br 290, C–I 228, N≡N 945, O=O 498.
   The compound BrCH₂CH₂Cl contains a C–Br bond and a C–Cl bond. State which bond breaks first and explain why. Predict, with a reason, whether iodoethane reacts faster or slower than bromoethane.2 marks
3. In the Haber process nitrogen and hydrogen react to form ammonia: N₂(g) + 3H₂(g) → 2NH₃(g). A student uses mean bond enthalpies (kJ mol⁻¹): N≡N 945, H–H 436, N–H 391.
   Define the term mean bond enthalpy and explain why bond enthalpy data tables quote mean values.3 marks