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

The Knudsen mass-loss effusion technique was used to measure the vapour pre ssures 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 equ ation and the standard molar entropies of sublimation were calculated. The standard molar enthalpies and entropies of sublimation at T = 298.15 K were also calculated by using the estimated value of the standard molar heat ca pacity of sublimation Delta(cr)(g)C(p,m)(O), = -50 J K-1 mol(-1): [GRAPHICS] The sublimation enthalpy increments for a CH2 group in straight-chain dicar boxylic 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 enthalpi es of formation of dicarboxylic acids in the gaseous phase have been derive d. (C) 1999 Academic Press.