Chemistry / Unit 6 Chem 1024 worked example, 60 to 90 seconds
Estimating Reaction Enthalpy from Bond Energies
The idea
Estimating a gas-phase reaction's enthalpy from bond energies means summing the energy to break every bond in the reactants, then subtracting the energy released forming every bond in the products.
In practice
For the reaction H2 + Cl2 → 2 HCl, first name the quantities in kilojoules per mole from the anchor's bond-energy table: the energy to break one H–H bond, the energy to break one Cl–Cl bond, and the energy released forming two H–Cl bonds. Substituting these into the bonds-broken-minus-bonds-formed formula, the estimated enthalpy equals the sum of the two bond-breaking energies minus twice the H–Cl bond-forming energy, carrying kJ/mol through every term of the subtraction rather than dropping the units partway through. The anchor's tabulated values push that difference negative, and a negative sign in kJ/mol is the physically sensible result here, matching the reaction's known exothermic character rather than signaling a sign error.
Where people slip
Using average tabulated bond energies gives only an estimate, not an exact enthalpy — real bond energies vary slightly by molecule.
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