Example Question - 6

Hydrogen and chlorine react as shown. Use the data to calculate the mean bond enthalpy of the H-Cl bond.H₂(g) + Cl₂(g) → 2HCl(g)   ΔH = -184 kJ mol^-1Bond enthalpies: H-H = 436 kJ mol^-1, Cl-Cl = 243 kJ mol^-1.Bonds broken: 1 H-H + 1 Cl-Cl = 436 + 243 = 679 kJ mol^-1. Bonds formed: 2 H-Cl = 2x, where x is the H-Cl bond enthalpy.ΔH = Σ(bonds broken) - Σ(bonds formed), so -184 = 679 - 2x. Rearranging gives 2x = 679 + 184 = 863, so x = 431.5 kJ mol^-1.Bond enthalpies are always positive, because breaking a bond always needs energy. A value found this way applies to one compound only, so it can differ slightly from the data book mean, which is averaged over many compounds.