5.1.5—Bond energies and ΔHr

Average bond enthalpies estimate the energy change for a reaction by comparing the energy needed to break the bonds in the reactants with the energy released when new bonds form in the products. Use the balanced equation and count bonds, not whole molecules.

Use this fixed sequence:

1. Balance the equation and draw or inspect the relevant structural changes.
2. Count every reactant bond broken, including stoichiometric multiplicity.
3. Count every product bond formed, including multiplicity.
4. Align the two totals in the bond-energy relationship and substitute the supplied values.
5. Report the result with its energy unit and reaction amount.
6. Check the sign and whether the counted bonds match the equation.

Worked-example scaffold (use the supplied equation and data):

• Reactant bonds broken: [bond type] × [coefficient] = [energy total].
• Product bonds formed: [bond type] × [coefficient] = [energy total].
• Substitute: ΔHᵣ ≈ [broken-bond total] − [formed-bond total] = [result] [unit].
• Interpret the sign as net energy release or input, then state that the result is an estimate.

Average bond enthalpies are model values, so the result is approximate rather than an exact experimental enthalpy. Common errors are using an unbalanced equation, counting unchanged bonds, adding product bond energies instead of subtracting them, confusing bond order, or reporting no unit/sign.