Heats of Hydration and Absolute Heats of Formation of Aqueous Ions

Accurate calculations are made for the hydration energies of the alkali halides, using the most recent data. These results, and the average of theoretical calculations made by others for the individual ions, are used to determine the most probable hydration energy of the individual ions. These in turn are the basis for calculating the absolute enthalpy of formation of aqueous H: 98.6 kcal/g-equiv. A Born-Haber cycle can then be used to calculate the hydration energy of any other ion for which the enthalpy of the gaseous ion is known. HEATS OF HYDRATION OF THE ALKALI HALIDES Although computations of the heats of hydration at infinite dilution of the alkali halides have already been made, the accumulation of more accurate data warrants a recalculation. The data come from critical compilations by the National Bureau of Standards (1952, 1965). The results of these calculations are given in table 1. The very high precision and internal consistency of these data can be demonstrated by obtaining the differences between values of adjacent columns and rows, thereby obtaining differences for halides and alkalis, respectively. TABLE 1 Standard enthalpies of solution of gaseous ionic alkali halides in kcal/mole at 25°