Free energies and entropies of transfer of hydrogen halides from water to aqueous alcohols and the structure of aquo-organic solvents

Abstract

Free energies ΔG⁰_t and entropies ΔS⁰_t of transfer of HCl from water to ethanol + water and propan-2-ol + water mixtures have been determined from the standard potentials E⁰ of the cell Pt, H₂(g, 1 atm)|HCl (m), solvent|AgCl–Ag. ΔG⁰_t values for hydrogen and halide ions have been obtained by a simultaneous extrapolation procedure applied to ΔG⁰_t(HCl), ΔG⁰_t(HBr) and ΔG⁰_t(HI), values of the last two being taken from previous papers. The chemical parts of the ionic ΔG⁰_t values in these and two other aquo-alcoholic systems have been interpreted in terms of ion-solvent interactions of the acid–base type modified by solvent structural effects.

Characteristic humps observed in ΔS⁰_t(HX)-composition profiles for aqueous ethanol, propan-2-ol and t-butanol mixtures have been shown, by means of a semi-quantitative theory, to result from a well-known feature common to these solvents, viz., promotion of water structure in highly aqueous compositions. This theory has then been used to obtain information regarding solvent structure in other aquo-organic solvent systems and to stress the usefulness of ΔS⁰_t as a probe for solvent structuredness.