Example Question - 5

Use the mean bond enthalpies below to calculate the enthalpy change for the combustion of methane.CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)Bond enthalpies in kJ mol^-1: C-H = 413, O=O = 498, C=O = 805, O-H = 464.Bonds broken (endothermic): 4 C-H + 2 O=O = 4(413) + 2(498) = 1652 + 996 = 2648 kJ mol^-1. Bonds formed (exothermic): 2 C=O + 4 O-H = 2(805) + 4(464) = 1610 + 1856 = 3466 kJ mol^-1.ΔH = Σ(bonds broken) - Σ(bonds formed) = 2648 - 3466 = -818 kJ mol^-1.This differs from the value found using enthalpies of formation, because mean bond enthalpies are averages taken from many different compounds. The method also only works when all species are gases, which is why the water here is H₂O(g).