Thermal Effects in Ion-Exchange Reactions: Heats of Ion Exchange

Abstract

Ion-exchange reactions between aqueous electrolyte solutions and organic ion exchangers are temperature dependent, and therefore they must be accompanied by the evolution or the absorption of heat. However, the magnitudes of these thermal effects generally are small so that they are measured best with a calorimeter rather than estimated by application of the van’t Hoff equation to the variation of the thermodynamic equilibrium constant with temperature. Calorimetrically determined ion-exchange reaction enthalpy changes are useful further in that they may be combined with free energies computed from equilibrium measurements to obtain the change in entropy. The sign and magnitude of the enthalpy and entropy changes afford an insight into the factors which govern the selective uptake of one ion in aqueous solution over another. For example, the entropy change frequently suggests that ion-solvent interactions are of central importance, while the enthalpy change may indicate that “site binding” or ion-pair formation by the ion exchanger governs the observed selective ion absorption.