Bond enthalpiesEdexcel A-Level Chemistry: Mind map

Exam questions on Bond enthalpies

1. A student is estimating the enthalpy change for the reaction H₂(g) + Cl₂(g) → 2HCl(g) using mean bond enthalpies (kJ mol⁻¹): H–H 436, Cl–Cl 243, H–Cl 432.
   Use the bond enthalpy values to explain why this reaction is exothermic.2 marks
2. Ethene reacts with hydrogen in the presence of a nickel catalyst: C₂H₄(g) + H₂(g) → C₂H₆(g). Mean bond enthalpies (kJ mol⁻¹): C=C 612, C–C 347, C–H 413, H–H 436. The experimental enthalpy change for the reaction is −137 kJ mol⁻¹.
   Calculate the enthalpy change for the reaction using the mean bond enthalpies.2 marks
3. Ammonia has a standard enthalpy change of formation of −46 kJ mol⁻¹. Mean bond enthalpies (kJ mol⁻¹): N≡N 945, H–H 436. Hydrazine, H₂N–NH₂, which contains one N–N bond and four N–H bonds, decomposes: N₂H₄(g) → N₂(g) + 2H₂(g), ΔH = −95 kJ mol⁻¹.
   Calculate the mean bond enthalpy of the N–H bond from the standard enthalpy change of formation of ammonia, for ½N₂(g) + 3/2 H₂(g) → NH₃(g).3 marks