Extrapolation of enthalpies of reaction in electrolyte systems to infinite dilution

Abstract

The use of theory-based equations for the calculation of standard enthalpies of reaction from experimental data is discussed. The reactions used as examples have been studied in the presence of an ionic medium, or in the presence of a large excess of one reactant, which then serves as the ionic medium. The results show that precise estimates of the standard enthalpies of reaction may be obtained, by using both the Pitzer and the Brensted-Guggenheim-Scatchard specific ionic interaction models. 'Empirical' equations based on a simple Debye-Hückel model should be avoided. The validity of the Vasil'ev-Yasinskii equation has also been tested. It is shown that this ignores the contribution of the enthalpies of mixing of the reaction participants and the ionic medium electrolyte to ΔrHm. Published data obtained by using this procedure should therefore be revised. Literature data on the standard enthalpies of formation of Al3+(aq) and Al(OH)4-(aq) at 298.15 K, have been revised, and new values ΔfHm°[Al3+(aq)] = -539.2 ± 1.0 kJ mol-1 and ΔfHm°[Al(OH)4-(aq)] = -1502.7 ± 1.0 kJ mol-1, are proposed. © Acta Chemica Scandinavica 1996.