Free energies and entropies of transfer of hydrobromic and hydroiodic acids from water to t-butyl alcohol + water mixtures from electromotive force measurements at different temperatures (5—35°C)

Abstract

Free energies, ΔG⁰_t and entropies, ΔS⁰_t of transfer of hydrobromic and hydroiodic acids from water to t-butyl alcohol + water mixtures containing 10, 20, 30 and 50 % t-butyl alchol by weight have been evaluated using the corresponding values of the standard potentials, E⁰, of silver–silver bromide and silver–silver iodide electrodes in these solvents obtained from electromotive force measurements at seven different temperatures ranging from 5 to 35°C performed on the cell Pt, H₂(g, 1 atm)|HX(m), solvent|Ag-AgX, where X = Br or I. These and the corresponding values for HCl obtained from E⁰ data given in literature were used to evaluate ΔG⁰_t for individual ions by the “extrapolation procedure”. In contrast with other aqueous organic solvents, humps or kinks in the ΔG⁰_t against composition profiles occur at water-rich compositions reflecting the strong structural characteristics of this solvent system in that region. The variation of entropies of transfer with composition has been interpreted in terms of marked structure-promotion of water and consequent ion-dehydration at small concentrations of t-butyl alcohol and of the formation of alcohol–water complexes at somewhat higher contents of t-butyl alcohol. The irregular order Br^– > I^– > Cl^– in water-rich regions for the entropy profiles is suggested to be due to the formation of charge-transfer complexes of the iodide ion with t-butyl alcohol.