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114 CHAPTER 2

The overall reaction is

Now:

However, the entropies of H and Cl in the gas phase can easily be calculated so
2 2
–
+
that the sum of the entropies of H and Cl in solution can be obtained. The value
in solution is known (2.15.8). It is fairly easy to devise these kinds of cells, which have
been used in obtaining much data.
2.15.9. Individual Ionic Properties: A Summary

In Section 2.15, methods for obtaining the properties of individual ions (their
hydration numbers, heats, and entropies) have been considered. Starting with a general
method—extrapolation to eliminate the effect of a partner on the value of the (easily
obtainable) corresponding electrolyte property—two special cases were dealt with:
how one obtains the individual values of the heat of hydration of the proton and then
its entropy.
Values of the thermal properties of individual ions do not have the same status as
thermodynamic properties (for these are assumption-free). For thermodynamic prop-
erties, only the accuracy of the experimental determination can be questioned. For
example, in electrochemical cells, are they being operated in equilibrium as required?
Thus, determining the values of the properties of individual ions always involves some
assumption—that it is appropriate to extrapolate according to a certain law, for
example—and therefore the values will always be open to improvement. Some of the
newer values increasingly refer to nonaqueous solutions.

2.15.10. Model Calculations of Hydration Heats
In the 1970s Bockris and Saluja developed models incorporating and extending
ideas proposed by Eley and Evans, Frank and Wen, and Bockris and Reddy. Three
basic models of ionic hydration that differ from each other in the structure in the first
coordination shell were examined. The features of these models are given in Table
2.16. The notations chosen for the models were 1A, 1B, 1C; 2A, 2B, 2C; and 3A, 3B,
3C, where 1, 2, and 3 refer to three basic hydration models, and A, B, and C refer to
the subdivision of the model for the structure-broken (SB) region. These models are
all defined in Table 2.16. A model due to Bockris and Reddy (model 3 in Table 2.16
and Fig. 2.37) recognizes the distinction between coordination number (CN) and
solvation number (SN).