Topic 5 - Energetics - 5.3 Bond enthalpies

Nature of science: Models and theories-measured energy changes can be explained based on the model of bonds broken and bonds formed. Since these explanations are based on a model, agreement with empirical data depends on the sophistication of the model and data obtained can be used to modify theories where appropriate. (2.2)


Bond-forming releases energy and bond-breaking requires energy.

Average bond enthalpy is the energy needed to break one mol of a bond in a gaseous molecule averaged over similar compounds.

Applications and skills

Calculation of the enthalpy changes from known bond enthalpy values and comparison of these to experimentally measured values.

Sketching and evaluation of potential energy profiles in determining whether reactants or products are more stable and if the reaction is exothermic or endothermic.

Discussion of the bond strength in ozone relative to oxygen in its importance to the atmosphere.


Bond enthalpy values are given in the data booklet in section 11.

Average bond enthalpies are only valid for gases and calculations involving bond enthalpies may be inaccurate because they do not take into account intermolecular forces.