Non-isothermal cell potentials and evaluation of entropies of ions and of activation for single electrode processes in non-aqueous media

Abstract

Measurement of cell potentials for the 1 2 H 2/H +, Ag/AgCl, Ag/AgBr and Ag/Ag +( S) reference electrodes in several aqueous and non-aqueous solvent systems, S, have been made non-isothermally in order to derive half-cell reaction entropy changes and related individual ionic entropies in the solvents, S, ultimately for the purpose of deriving “true” entropies of activation for proton discharger at the Hg electrode from various non-aqueous solvents, based on previously published kinetic measurements. The non-isothermal cell potential measurements have been made on cells with two identical reference electrodes at different temperatures with and without a salt bridge thermal junction. The effect of the thermal liquid-junction potential, when present, is thereby evaluated for the systems studied and shown to be substantial in most cases when measurements, as in the present work, are made over an appreciable range of temperature. When the salt bridge thermal junction is included in the measurement system, non-isothermal cell potentials can be obtained that give reliable results for half-cell reaction entropies and corresponding individual standard ionic partial molar entropies S 0 ion. Ionic activity and ion-association effects in the various solvents are taken into account. The resulting S 0 ion values for several ions in some of the solvents used are not in agreement with respective values estimated by the empirical “correspondence principle” method, the assumptions in which must therefore be questioned. Reasons are suggested. The results enable some absolute values of entropies of activation for proton discharge in the cathodic H 2 evolution reaction at the Hg electrode to be determined and compared in several non-aqueous solvents containing various proton donors. The mechanistic significance of the results obtained is discussed.