6.7 Bond Enthalpies

Estimate ΔH from average bond energies: ΔH = Σ bonds broken − Σ bonds formed.

If the products have stronger bonds than the reactants, the reaction is exothermic.

Broken (reactants) − formed (products). Not the reverse.

Worked example: H₂ + Cl₂ → 2HCl: (436 + 243) − 2(431) = −183 kJ.