Metal ions hydration enthalpy

Table 9 Single-ion Hydration Enthalpies (kJmol-1) for Metal Cations (at 298.2K)a 1 ...

The overall strength of interaction between a metal ion and its hydration shell, which will parallel the strength of bonding within the [M(OH2 ),]" , can be assessed from ion hydration enthalpies. The values listed in Table are based on the single ion assumption of Halliwell and Nyburg... 

Crystal field stabilization energy is a factor that contributes to the thermodynamic stability of complexes with predominantly ionic metal-ligand interactions, and also to the variation in properties of d metals and their compounds. Some of these properties are the size of di- and trivalent ions, hydration enthalpies, lattice energies, and stability of oxidation states. 

The enthalpy changes AH involved in this equilibrium are (a) the heat of atomisation of the metal, (b) the ionisation energy of the metal and (c) the hydration enthalpy of the metal ion (Chapter 3). 

Hydration of the metal ions produces an enthalpy change that is commensurate with the size and charge of the ion with the addition of the number of Dq units shown in the weak field column in Table 17.4. For d°, d5, and d10 there is no additional stabilization of the aqua complex since these cases have no ligand field stabilization. Figure 17.10 shows a graph of the heats of hydration for the first-row + 2 metal ions. 

Fig. 18. Rate constants kn and enthalpy of hydration for the solvent substitution reactions in water for various metal ions...

It has been pointed out that various alkali metal salts in water are more associated than is expected on the basis of the electrostatic theory and also that metal ions of equal size and equal charge but of different electronic structure exhibit different enthalpies of hydration and different acidity constants in water 11 ... 

It, thus, follows that the electrode potential in equilibrium of metal ion transfer is given by the free enthalpy for the formation of a solid metal from both hydrated metal ions and standard gaseous electrons as shown in Eqn. 4—25 ... 

Inclusion of the absolute value of the standard enthalpy of hydration of the proton, Ahyd// (H +, g) = — 1110 kJ mol 1 (derived in Chapter 2), gives the absolute values for the enthalpies of hydration of the transition metal ions. The estimated values are given in Table 7.5. 

Figure 7.13 Plots of the sums of the first two ionization energies of the metal and its enthalpy ol atomization and the negative enthalpy of the emhalpy of hydration of the + 2 ion, to which has been added 840 kJ mol"1...

Although there is no space to develop a detailed discussion of the solubilities of compounds of the transition elements, the general insolubility of their + 2 and + 3 hydroxides is important. The rationale underlying their insolubility can be summarized (i) the hydroxide ion is relatively small (152 pm ionic radius) and the ions of the +2 and +3 transition metals assume a similar size if their radii are increased by 60-80 pm, and (ii) the enthalpy of hydration of the hydroxide ion (—519 kJ mol ) is sufficiently negative to represent a reasonable degree of competition with the metal ions for the available water molecules, thus preventing the metal ions from becoming fully hydrated. Such effects combine to allow the lattice enthalpies of the hydroxides to become dominant. 

By contrast, the chlorides of the metal ions are soluble because the chloride ion (181 pm ionic radius) is considerably larger than the hydroxide ion, and its enthalpy of hydration ( — 359 kJ mol l) is less negative than that of OH. This allows the metal cations to exert more nearly their full effect on the solvent molecules, thus overcoming the lattice enthalpy terms, and this leads to their general solubility as chlorides. 

The enthalpies of hydration of a selection of transition metal ions were derived. 

Somewhat better data are available for the enthalpies of hydration of transition metal ions. Although this enthalpy is measured at (or more property, extrapolated to) infinite dilution, only six water molecules enter the coordination sphere of the metal ion lo form an octahedral aqua complex. The enthalpy of hydration is thus closely related to the enthalpy of formation of the hexaaqua complex. If the values of for the +2 and +3 ions of the first transition elements (except Sc2, which is unstable) are plotted as a function of atomic number, curves much like those in Fig. 11.14 are obtained. If one subtracts the predicted CFSE from the experimental enthalpies, the resulting points lie very nearly on a straight line from Ca2 lo Zn2 and from Sc to Fe3 (the +3 oxidation state is unstable in water for Ihe remainder of the first transition series). Many thermodynamic data for coordination compounds follow this pattern of a douUe-hunped curve, which can be accounted for by variations in CFSE with d orbital configuration. 

---