Standard Enthalpies, Entropies, and Gibbs Functions of Sublimation of Four Alkyl-Substituted Malonic Acids

Abstract

The Knudsen mass-loss effusion technique was used to measure the vapor pressures at different temperatures of the following crystalline dicarboxylic acids: methylmalonic acid, between 341.13 K and 354.74 K, dimethylmalonic acid, between 347.18 K and 363.26 K, ethylmalonic acid, between 347.52 K and 362.34 K and butylmalonic acid, between 348.20 K and 362.18 K. From the temperature dependence of the vapor pressure, the standard molar enthalpies of sublimation were derived by the Clausius−Clapeyron equation and the molar entropies of sublimation at equilibrium pressures were calculated. Using estimated values for the heat capacity differences between the gas and the crystal phases of the studied compounds the standard (pressure p0 = 105 Pa) molar enthalpies, entropies and Gibbs functions of sublimation at T = 298.15 K, were derived.