A LINEAR FREE-ENERGY RELATIONSHIP FOR CRYSTALLINE SOLIDS AND AQUEOUS IONS

QUANTITATIVE chemical modelling of geochemical, environmental and industrial processes is often severely limited because of large gaps in experimentally derived thermodynamic databases for Gibbs free energies of crystalline solids and aqueous ions. Methods proposed previously for estimation of the free energies or enthalpies of formation of crystalline solids 1-9 are subject to large uncertainties, typically greater than +/- 5-10 kcal mol-1. Here we present an empirically based linear free energy equation 10 applicable to cations of any charge, radius or chemical type, which allows estimates of the free energies of solids with uncertainties of less than +/- 1 kcal mol-1. Our equation is analogous to the linear free energy relations of Hammett and others 11,12 for aqueous organic reactions, but applies instead to crystalline solids. We apply our equation to experimentally derived standard Gibbs free energies of formation of isostructural families of divalent oxides, hydroxides, carbonates, fluorides, chlorides and sulphates. This new level of accuracy for predicting free energies of crystalline solids opens up opportunities for chemical modelling of many processes not previously amenable to thermodynamic analysis.