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150 CHAPTER 2
Fig. 2.53. A plot of the heat of hydration of
2+
2+
2+
Ca , Mn , and Zn vs. atomic number.
the same as that calculated from a model that neglects interactions affecting the filling
of orbitals. Similarly, for with no 3d electrons and with a completely filled
3d shell, the heat of hydration does not become more negative than would be expected
from the electrostatic theory of ion–solvent interactions developed in Section 2.4.3. It
can be concluded, therefore, that the experimental heats of hydration of these three
ions should vary in a monotonic manner with atomic number as indeed they do (Fig.
2.53).
All the other transition-metal ions, however, should have contributions to their
heats of hydration from the field stabilization energy produced by the effect of the
field of the water molecules on the electrons in the 3d orbitals. It is these contributions
that produce the double-humped curve of Fig. 2.54. If, however, for each ion, the
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energy corresponding to the water-field stabilization is subtracted from the experi-
mental heat of hydration, then the resulting value should lie on the same smooth curve
yielded by plotting the heats of hydration of and versus atomic
number. This reasoning is found to be true (Fig. 2.54).
The argument presented here has been for divalent ions, but it is equally valid
(Fig. 2.55) for trivalent ions. Here, it is and which are similar to
manganese in that they do not acquire any extra stabilization energy from the field of
the water molecules acting on the distribution of electrons in their d levels.
Thus, it is the contribution of the water-field stabilization energy to the heat of
hydration that is the special feature distinguishing transition-metal ions from the
alkali-metal, alkaline-earth-metal, and halide ions in their interactions with the solvent.
This seems quite satisfying, but interesting (and apparently anomalous) results
have been observed by Marinelli and Squire and others concerning the energy of
interaction of successive molecules as the hydration shell is built up in the gas phase.
Thus, it would be expected that the first hydrating water would have the greatest heat
of binding, because there are no other molecules present in the hydration shell with
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This energy can be obtained spectroscopically.