IB Chemistry home > Syllabus 2016 > Energetics > Bond Energy

Syllabus ref: 5.3

Atoms are held together in covalent compounds by forces, which we call chemical bonds. Breaking these bonds requires energy and conversely energy is released in the form of heat when they are made.

The bond energy can be defined in terms of a specific bond or more generally as an average. This section examines the usefulness of each definition.

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.


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.

In Chapter 4.4