Average Bond Enthalpy: Enthalpy change when one mole of atoms of gaseous covalent bonds is
broken. Use average bond enthalpies to calculate enthalpy changes of reaction ∆rH without
experiments. To use must have energy required to break bond = endothermic = positive enthalpy
value. Never negative values, always positive.
• Exothermic = bond forming. Endothermic = bond breaking.
• Why Reaction Exo / Endo: Exothermic when more energy released due to bonds forming than
energy required from bond breaking. Endothermic when more energy required to break bondsbond enthalpy.
• Calculating Enthalpy Changes from Average Bond Enthalpies: For a reaction involving gaseous
molecules of covalent substances
• ∆rH or ∆H of reaction = ∑(bond enthalpies in reactants) – ∑(bond enthalpies in products). In units
kJ mol-1
. Can use for enthalpy changes of combustion etc as well.
• With each molecule in equation draw out displayed formula so can see all the bonds. Add up for
each bond. In CO2, it is C=O=C, so there are 2 C=O bonds. For organic molecules like alkenes,
remember the C-C bonds.
Limitations…
– Average Bond Enthalpy: Average bond enthalpies not be the same as actual bond enthalpy
as bonds have different strengths in different chemical environment. Average bond enthalpy
calculated from actual in different chemical environments. Given either.
– Gaseous Molecules: Average bond enthalpies must need all gaseous molecules, so for things
like H2O, calculated ∆rH is not standard enthalpy change. Could work out standard enthalpy
change using average bond enthalpies, but would need to take into account the enthalpy
change for H2O (g) condensing into H2O (l).