Free Energies and Entropies of Transfer of Hydrogen Halides from Water to Aqueous Alcohols and the Structure of Aquo-organic Solvents

Abstract

Free energies ΔG0t and entropies ΔS0t 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. ΔG0t values for hydrogen and halide ions have been obtained by a simultaneous extrapolation procedure applied to ΔG0t(HCl), ΔG0t(HBr) and ΔG0t(HI), values of the last two being taken from previous papers. The chemical parts of the ionic ΔG0t 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 ΔS0t(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 ΔS0t as a probe for solvent structuredness.