ACCURACY OF STANDARD THERMODYNAMIC DATA AND SOME INFLUENCING FACTORS
Authors: O.G. Polyachenok, E.I. Ashmyanskaya, S.I. Voitenko, E.N. Dudkina, L.D. Polyachenok, Department of Chemistry, Mogilev State University of Foodstuffs, Mogilev, Belarus, and others
Abstract: Four factors influencing the accuracy of the thermodynamic data obtained in thermochemical or equilibrium measurements are discussed: metastable state of a substance, in particular its high dispersity; stoichiometric purity of a substance; exact stoichiometry of the reaction investigated; approximate account of the heat capacity and entropy change in equilibrium measurements. General thermodynamic relations for the systems anhydrous salt – its lowest hydrate – water vapour are considered to show the influence of a metastable (superdisperse) phase formation of the lowest hydrate studied on the results of tensimetric equilibrium measurements. Suggestion is advanced that the presence of such phase may have an influence also on the results of other physicochemical, i.e. thermochemical measurements. High emphasis is placed on the fact that appreciable errors in the results of thermodynamic investigations may originate from insufficient attention to the minute details of synthesis and analysis of stoichiometrically pure substances. It is concluded that thermochemical and thermogravimetrical studies of metal salt hydrates demand the water content accuracy not less than 1%. New experimental results for the lowest hydrates of calcium and barium chlorides are given, illustrating the problems emerging sometimes in tensimetric and derivatographic practice, when the assumed scheme of the process may prove to be incorrect. Thermodynamic quantities are derived for CaCl2·H2O (α, β) decomposition. The formation of a new lowest hydrate 3BaCl2·2H2O is found out when the effect of sintering the substance is eliminated.