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ION–SOLVENT INTERACTIONS 53

Now, only (the heat of solution) is experimentally measured, and hence
the evaluation of the heat of solvation of the salt, by means of Eq. (2.3) involves
trusting the reliability of information on the heat of sublimation of the salt, In
some cases, is known reliably (±1%) from calculations. Alternatively, it can be
determined experimentally. From then, and the measured heat of dissolution,
the heat of hydration of the salt concerned in the solvent can be deduced.
Fajans (1962) was the first scientist to put these thoughts into practice. One finds
that the determined heats of solvation are relatively small and endothermic (+) for
some salts but exothermic (–) for others. However, lattice energies are known to be in
the region of several hundreds of kilojoules so that, in rough terms [Eq. (2.3)],
heats of solvation should not be more than a dozen kilojoules (numerically)
different from lattice energies. In Table 2.4 a compilation is given of the quantities
9
mentioned earlier in the case of the alkali halides. Now, the method described here
gives the sum of the heat of hydration of the ions of a salt. The question of how to
divide this sum up into individual contributions from each of the ions of a salt requires
more than the thermodynamic approach that has been used so far. The way this is done
is described in later sections (e.g., in Section 2.6.2 or 2.15.9).

2.5.3. Obtaining Experimental Values of Free Energies and Entropies
of the Solvation of Salts

In the preceding section it was shown how to obtain, a little indirectly, the heat of
solvation of a salt. However, it is the free energies of the participants in a chemical reaction
that determine the state of equilibrium so that one cannot leave the situation with only the
determined. Free energy and entropy changes have to be dealt with also.
How can the free energy of a solution be obtained? Consider a saturated solution
of a 1:1 salt of the type MA. Because the solid salt lattice is in equilibrium with its
ions in solution, the chemical potential of the salt, can be expressed in terms of
its individual chemical potentials and activities and in solution,

In thermodynamic reasoning, there has to be a standard state. The standard state
for the solid crystal is the substance in its pure state at 298 K. It follows that the standard
chemical potential of solution is:

9
The alkali halides are chosen as good examples because the lattice energy is particularly well known and
reliable.