Hydrogen fluoride is produced industrially by the reaction
H₂(g) + F₂(g) → 2 HF(g)
At 298 K the reaction has ΔH°rxn = −542 kJ. Average bond energies (kJ mol⁻¹):
H—H 436 F—F 158 H—F ?
The exceptionally large magnitude of ΔH°rxn is often cited as evidence for the unusually strong H—F bond.
(a) Without carrying out a full numerical calculation, use the data provided to explain why the average H—F bond energy must be significantly greater than 400 kJ mol⁻¹.
(b) The actual tabulated average bond energy for H—F is 565 kJ mol⁻¹. Describe two factors related to bond energy averaging or molecular environment that could cause the value you inferred in part (a) to differ from the tabulated average.