Bond enthalpiesAQA A-Level Chemistry: Mind map

Exam questions on Bond enthalpies

1. Hydrogen reacts with chlorine in the gas phase: H₂(g) + Cl₂(g) → 2HCl(g). The mean bond enthalpies, in kJ mol⁻¹, are: H–H 436, Cl–Cl 243 and H–Cl 432.
   Explain, in terms of bonds, why this reaction is exothermic.2 marks
2. Ethene reacts with hydrogen in the gas phase in the presence of a nickel catalyst: C₂H₄(g) + H₂(g) → C₂H₆(g). The mean bond enthalpies, in kJ mol⁻¹, are: C=C 612, C–C 347, C–H 413 and H–H 436.
   Define the term mean bond enthalpy.2 marks
3. Methane burns completely in oxygen with all species in the gas phase: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g). The mean bond enthalpies, in kJ mol⁻¹, are: C–H 413, O=O 496, C=O 805 and O–H 463. The standard enthalpies of formation, in kJ mol⁻¹, are: CH₄(g) −74.8, CO₂(g) −393.5 and H₂O(g) −241.8.
   Use the mean bond enthalpies to calculate the enthalpy change for the combustion of methane.3 marks