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124 CHAPTER 2

Substitution of values from Eq. (2.81) into Eq. (2.76) gives

Thus, for where (the number of molecules in the structure-breaking region per
ion; see later discussion) is 24, the net contribution of will be –41 kJ. Similar
calculations can be carried out for other ions. The net contribution from (model
C) comes to

for SN = CN, and

for SN CN, where in the first term is obtained from Eq. (2.82). The values
of (model C) are listed in Table 2.17.
2.15.11.8. The total number of molecules in the SB region can be
calculated by consideration of the close packing of water molecules in the area of a
sphere consisting of the ion plus the first layer. Thus, the number of molecules of
cross-sectional area will be given as

Values of are listed in Table 2.17.
2.15.11.9. Numerical Evaluation of The heats of hydration of
monovalent ions have been calculated for the various models by using Eqs. (2.61) and
(2.62) and the parameters listed in Table 2.17. The results are shown in Figs. 2.40 and
2.41. From Figs. 2.40 and 2.41 it can be seen that the experimental data for cations fit
model 3C of Table 2.16 best, while for the anions, the best fit is with model 3A.
Both models of best fit assume that there is a distinction between the coordination
shell and the solvation shell. The difference between cations and anions is that the
anion calculations are more consistent with H bonding from the first coordination shell
around the ions and the water in the structure-broken region; with the cations, the better
model fit is to stress the librating properties of water in the structure-broken region.
This difference may arise from the smaller peripheral field strengths of the anions
(larger radius) so that there is more time for orientation and H bonding with the (larger
amount of) structure-broken waters after an ion arrives in a given region.
All these conclusions must be tempered by continuous reminders that solvation
is a dynamic matter and that water molecules are constantly being attracted by the