Page 62 - Instant notes

Physical Chemistry 48

Fig. 2. Use of the enthalpy of
formation in calculating the enthalpy
of a reaction.

Enthalpy of combustion

In the same way as it is possible to usefully combine the enthalpy of formation and
Hess’s law, it is also possible to combine Hess’s law with the enthalpy of combustion
(Table 1). Taking the previous example, sufficient oxygen may be added to both sides of
the equation to formally combust the reactants and products (Fig. 3).

Fig. 3. Use of the enthalpy of
combustion in calculating the enthalpy
of a reaction.

The overall enthalpy of reaction is unaffected by this alteration, but ∆H reaction may now be
calculated using Hess’s law (note the change of sign as compared to the previous
expression):
∆H reaction=−Σ∆H c(products)+Σ∆H c(reactants)

The advantage of this method is that enthalpies of combustion are more readily obtained
than heats of formation. The disadvantage is that it can only be applied to reactions
involving combustible substances, a restriction which generally also excludes materials in
solution.

The Born-Haber cycle

The Born-Haber cycle is a specific application of the first law of thermodynamics
using Hess’s law. The cycle allows indirect determination of the lattice enthalpy of an
ionic solid. This is the enthalpy associated with the direct combination of gaseous ions to
form an ionic lattice:

Because direct measurement of this process is generally impractical, an indirect path is
created. If the example of KCl is taken, the processes illustrated in Fig. 4 is obtained.