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.

Guidance:

• 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.

• Stratospheric ozone depletion is a particular concern in the polar regions of the planet, although the pollution that causes it comes from a variety of regions and sources. International action and cooperation have helped to ameliorate the ozone depletion problem.

Utilization:

• Energy sources, such as combustion of fossil fuels, require high ΔH values.
  • Syllabus and cross-curricular links:
  • Topic 4.3—covalent structures

Aims:

• Aim 6: Experiments could be enthalpy of combustion of propane or butane.
• Aim 7: Data loggers can be used to record temperature changes.
• Aim 8: Moral, ethical, social, economic and environmental consequences of ozone depletion and its causes.