Cationic and anionic contributions to the crystal lattice energy and the heat of solution of salts

Abstract

Up to now values of δH, δS, and δG have been determined for individual ions in the gaseous state and in aqueous solution at infinite dilution. Values of S have also been determined for individual ions entering into a crystal lattice. The Born-Mayer-Huggins crystal lattice theory has been used to calculate the enthalpy of individual ions in the crystal lattice of alkali metal halides. A formula has been derived on the basis of this theory for the difference between the enthalpies of the cation and anion in the crystal lattice. This difference depends on the van der Waals forces resulting from dipole-dipole and dipole-quadrupole interactions and on the quantum mechanical repulsive forces between the electron clouds of the ions. The enthalpy difference between ions in the lattice varies from +3.1 (LiCl) to -10.1 (KCl) kJ/g-ion. Values of δH and δG for ions in the crystal lattices of twenty alkali metal halides are presented in tabular form. It is found that the lattice energy is almost equally divided between the cations and anions.