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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
                 38
            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

            38
             This energy can be obtained spectroscopically.
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