LECTURE PHYSICAL CHEMISTRY 3rd EDITION ROBERT G. MORTIMER

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The error of 9 kJ mol −1 in the previous example is
typical. It is larger than the difference between ∆H
and ∆U, so that the value of ∆U obtained from
average bond energy values is ordinarily used for ∆H
without correction for the value of ∆(PV). Average
bond energies are used only if accurate enthalpy
changes of formation are not available, since the
results are only approximately correct. Average
bond energies cannot be used for reactions involving
solids or liquids, where intermolecular forces make
important contributions.
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