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Thermochemistry 47

−
+
Enthalpy of Dissolution of a substance in a ∆H sol NaCl (s)→Na aq +Cl aq
solution specified quantity of solvent
+
−
+
−
Enthalpy of Solvation of gaseous ions ∆H solv Na (g)+Cl (g)→Na aq +Cl aq
solvation
enthalpy change associated with the burning of carbon to form carbon monoxide. It is
practically impossible to prevent formation of some carbon dioxide if the enthalpy
change is measured directly. The reaction may be written as either a direct (one-step) or
an indirect (two-step) process (Fig. 1).
Hess’s law indicates that the total enthalpy change by either path is identical, in which
case ∆H 1=∆H 2+∆H 3, so allowing to be obtained a value for ∆H 1 without the need for
direct measurement.

Fig. 1. Two possible chemical
pathways to the formation of CO from
its elements.

Enthalpy of formation

The usefulness of the concept of enthalpy of formation (Table 1) is readily appreciated
when it is used in conjunction with Hess’s law. Tables listing the enthalpies of formation
of a wide range of materials may be found in the literature, and are more readily available
than the enthalpy change associated with a specific reaction. For any reaction, it is
possible to construct a reaction pathway which proceeds via the elemental components of
both the reactants and the products (Fig. 2a). The value for ∆H reaction is readily calculated
from:
∆H reaction=Σ∆H f(products)−Σ∆H f(reactants)

Hence for the example reaction in Fig. 2b, the reaction enthalpy is given by:
∆H reaction=[∆H f(CH 3CO 2CH 3)+∆H f(H 2O)]−[∆H f(CH 3CO 2H)+∆H f(CH 3OH)]