Thermodynamic study on the sublimation of the three iodobenzoic acids and of 2-fluoro- and 3-fluorobenzoic acids

The Knudsen mass-loss effusion technique was used to measure the vapour pressures at different temperatures of the following substituted benzoic acids: 2-iodobenzoic acid, between T = 345.20 K and T = 359.17 K, 3-iodobenzoic acid, between T = 347.15 K and T = 363.17 K, 4-iodobenzoic acid, between T = 363.15 K and T = 379.11 K, 2-fluorobenzoic acid, between T = 309.15 K and T = 323.15 K and 3-fluorobenzoic acid between T = 303.17 K and T = 317.16 K. From the temperature dependence of the vapour 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, p(0) = 10(5) Pa, molar enthalpies, entropies and Gibbs energies of sublimation at T = 298.15 K, were derived: [GRAPHICS] (C) 2000 Academic Press.