Bond-enthalpy calculations did not appear in 91390 from 2021 to 2025 — enthalpy questions used Hess cycles and ΔfH° data every year instead. It is still legitimate chemistry and worth understanding, but revise it after the Hess and ΔfH° methods, not before. Average bond enthalpy is the energy to break one mole of a particular bond in the gas phase. Because breaking absorbs and forming releases energy: Reaction enthalpy from bonds ΔH = Σ(bonds broken) − Σ(bonds formed) Broken − formed. Only works if everything is gaseous. Worked example H₂ + Cl₂ → 2HCl Bond enthalpies (kJ mol⁻¹): H–H 436, Cl–Cl 242, H–Cl 431. Bonds broken = 1(H–H) + 1(Cl–Cl) = 436 + 242 = 678 Bonds formed = 2(H–Cl) = 2 × 431 = 862 ΔH = 678 − 862 = −184 kJ mol⁻¹ ✓ ΔH = −184 kJ mol⁻¹. ✕
Bond-enthalpy values are averages
Bond enthalpy answers are approximate because they use averaged values across many molecules: they won’t exactly match ΔHf data. Always state this if asked to compare methods.