Thermodynamic study on the sublimation of six methylnitrobenzoic acids

The Knudsen mass-loss effusion technique was used to measure the vapour pressures at different temperatures of the following six compounds: 2-methyl-3-nitrobenzoic acid, between T = 357.16 K and T = 371.16 K; 2-methyl-6-nitrobenzoic acid, between T = 355.16 K and T = 369.16 K; 3-methyl-2-nitrobenzoic acid, between T = 371.16 K and T = 385.14 K; 3-merhyl-4-nitrobenzoic acid, between T = 363.21 K and T = 379.16 K; 4-methyl-3-nitrobenzoic acid, between T = 363.10 K and T = 377.18 K; 5-methyl-2-nitrobenzoic acid, between T = 355.18 K and T = 371.08 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(o) = 10(5) Pa, molar enthalpies Delta H-g(cr)m(o), entropies Delta S-g(cr)m(o) and Gibbs energies Delta (g)(cr)G(m)(o) of sublimation at T = 298.15 K, were derived: [GRAPHICS] (C) 2001 Academic Press.