The standard partial molal volume change ΔV° for the ionization of acetic acid in water has been determined over the range 25-225°C and 1-3000 bars from measurements of the pressure dependence of the dissociation constant. The standard partial molal volumes and partial molal compressibilities of ionization at atmospheric pressure ΔV°1 and Δχ°1 were obtained from linear plots of [RT/(P– 1)] In (KP/K1) against (P– 1). Both ΔV° and Δχ° became more negative with increasing temperature at constant pressure, and less negative with increasing pressure at constant temperature. These changes can be correlated qualitatively with the temperature and pressure dependence of the dielectric constant of water. At constant volume, the volume of ionization and pressure becomes more negative with increasing temperature, but the isochoric temperature dependence is smaller than the isobaric one. The difference between the isobaric and isochoric standard partial molal expansibilities of ionization is considered in terms of the equation ΔE°p–ΔE°V=αΔχ°T/β.
Approximately linear correlations have been demonstrated between the volumes and compressibilities of ionization, and between the volumes and entropies of ionization at constant pressure. At constant volume, however, the entropy of ionization shows a maximum at about 80°C, which is attributed to the existence of a minimum in the temperature dependence of the isothermal compressibility of water. Suggestions that the isochoric parameters for chemical reactions in solution are more fundamental than the corresponding isobaric parameters are considered, but it is concluded that both should be studied.
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Transactions of the Faraday Society
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