5.1.6—Some bond energies are exact and some bond

A bond enthalpy is the enthalpy required to break one mole of a specified bond in gaseous molecules. Some values are exact for a particular chemical environment, but many table values are average bond enthalpies taken across different compounds.

The same nominal bond can have slightly different strengths because its surrounding atoms, bond order and molecular environment differ. An average value is therefore useful for estimating a reaction enthalpy but does not exactly describe every bond of that type in every molecule.

When average bond enthalpies are used in ΔHᵣ = Σ(bonds broken) − Σ(bonds formed), the result is an estimate based on the supplied averages. It is appropriate for comparing the direction or approximate size of energy change, not for claiming an exact experimental value.

Do not treat every tabulated bond enthalpy as exact, or interpret a difference from an experimental value automatically as a counting error. Keep the limitation visible: the equation and bond count may be correct while the average-bond model remains approximate.