Comparison of isochoric and isobaric heat capacities of liquid organic non-electrolytes and their vaporization enthalpies

Abstract

The present paper is devoted to the study of the relationship between the difference of isochoric heat capacities of liquid and ideal gas phase organic non-electrolytes and their vaporization enthalpy at 298.15 K. This investigation is intended to more deeply analyze the origin of the recent finding that the difference of isobaric heat capacities of organic compounds in liquid and ideal gas phase is in a linear correlation with the vaporization enthalpy. We showed that the difference of isobaric and isochoric heat capacities of liquid aromatic compounds is constant within 3 J·K−1·mol−1 and does not depend on the vaporization enthalpy. A direct proportionality between isochoric heat capacity change and the vaporization enthalpy was established for 84 compounds, which included not only aromatic liquids but other rigid and relatively inflexible molecules (root-mean-square deviation equaled 2.5 J·K−1·mol−1). The slope agreed with that of isobaric heat capacity change – vaporization enthalpy correlation. Statistical significance of the established relationship far exceeded correlations with other molecular size-related parameters, namely the heat capacities, molar volume, and molecular weight.