When estimating a reaction enthalpy from average bond enthalpies, ΔH is calculated as

Core Concept

Breaking bonds always demands energy input, so that term enters with a positive sign. Forming bonds always releases energy, so that term is subtracted. The net, broken minus formed, matches the ΔH sign convention: negative when the new bonds release more than the old ones cost. For H2 + Cl2 → 2HCl this gives 679 - 862 = -183 kJ, correctly predicting an exothermic reaction.