Vapour pressures and the enthalpies and entropies of sublimation of five dicarboxylic acids

Abstract

The Knudsen mass-loss effusion technique was used to measure the vapour pressures of five crystalline dicarboxylic acids as a function of temperature. From the temperature dependence of the vapour pressure, the standard molar enthalpies of sublimation were derived by using the Clausius–Clapeyron equation and the standard molar entropies of sublimation were calculated. The standard molar enthalpies and entropies of sublimation atT= 298.15 K were also calculated by using the estimated value of the standard molar heat capacity of sublimationΔcrgCp,mo=−50J · K−1· mol−1:The sublimation enthalpy increments for aCH2group in straight-chain dicarboxylic acids with less than nine methylene groups were calculated as (7.9 ± 0.4)kJ · mol−1. The available values for the enthalpies of sublimation of straight-chain dicarboxylic acids with more than eight methylene groups are discussed in terms of partial cyclization. The standard molar enthalpies of formation of dicarboxylic acids in the gaseous phase have been derived.