Comparative Study of the Absolute Values of Enthalpy and Gibbs Free Energy of Solvation of Proton from Cluster-ion Solvation data and Direct Determination of the Thermodynamic Parameters of Proton in Aqueous and Non-aqueous Solvents

Abstract

Absolute values of enthalpy and Gibbs free energy of hydration (h) or solvation (s) of H⁺ ion, ΔH⁰(H⁺)_{h or s} and ΔG⁰(H⁺)_{h or s} in aqueous and non-aqueous solvents (methanol, ethanol, n-propanol, iso-propanol, n-butanol, t-butanol, ethylene glycol, propylene carbonate, n-methyl formamide, acetone, tetrahydro furan, 1,4-dioxan, acetonitrile) were determined directly using a single standard state [i.e.H₂(g) at 1bar and 298.15K]. A comparative study of the methods of Tissandier et al. and the present one has been made. The values −1299.4 kJ mol⁻¹(−1303.9 kJ mol⁻¹) and −1284.5 kJ mol⁻¹(−1288.9 kJ mol⁻¹) for the absolute enthalpy [∆H⁰_h(H⁺)] and Gibbs free energy [∆G⁰_h(H⁺)] of hydration determined in the present work have been found to be much lower than the corresponding values −1150.1±0.9 kJ mol⁻¹ and −1104.5±0.3 kJmol⁻¹ determined by Tissandier et al. using cluster-ion solvation data. The values of Tissandier et al. have been acclaimed to be the most accurate values of these quantities by most of the workers. However, the method is based on approximations and assumptions and uses a number of conventional standard states. The calculations use the principle of ionic additivity and Klot’s equation which are open to question. The equations, based on the difference between several sets of energy values of different ion-pairs of similar magnitude, have been used. Thus, the method is insensitive and many of the important energy terms characterizing the ions and the structure of the solvents are eliminated. Thus the accuracy of the energy values are not without question.Our method, on the other hand, is a direct one using a single standard state. The most important contributory factor for the determination of ΔG⁰_h(H⁺) is the Gibbs free energy of charging and the value is accurately known. Thus, the values for ΔH⁰_h(H⁺) and ΔG⁰_h(H⁺) determined by us though much lower than those of Tissandier et al. can be regarded to be reasonable.