Ionic radii based on electron-density distribution in crystalline sodium chloride and interionic distances in alkali halides of Bl structure have been used for estimation of theabsolute standard enthalpies of hydration of individual alkali metal and halide ions at 298.15 K. The absolute standard partial molar entropies of hydration have also been assessed, and used in conjunction with the results for change in heat content to yield absolute standard Gibbs free energies of ionic hydration. Lattice enthalpies of alkali halides have been derived on the basis of more recent thermochemical data. The magnitudes of thermodynamic properties for salts in solution extrapolated to infinite dilution are strictly additive, with individual contributions from cation and anion. However, there is no purely thermodynamic way to separate the sum into the constituent parts[l]. Many attempts have been made to achieve the correct divisions and in the case of some properties apparently reliable results have been achieved. A critical review of the subject has been presented quite recently by Conway[2].
Many procedures for circumventing the formal thermodynamic problem have been based on relating experimental data for salts in solution to ionic radii. Use of "crystal radii" in this connection appears justifiable because ion-solvent interactions in solution in the case of a highly polar solvent such as water are of comparable energy and involve similar forces to those between cations and anions in crystals. Aqueous ions may be expected to be "compressed" to much the same extent as those in crystals. A problem arises here, however, in that scales of crystal radii based on electron density distribution in sodium chloride determined by X-ray diffraction[3-81 differ markedly from the scales of Pauling[9] and Goldschmidt[lD], Table 1. A discussion of these systems of radii has been