Vapor pressures and standard molar enthalpies, entropies, and Gibbs free energies of sublimation of 2,4- and 3,4-dinitrobenzoic acids

Abstract

The vapor pressures of the solid and liquid 2,4- and 3,4-dinitrobenzoic acids were determined by torsion–effusion and thermogravimetry under both isothermal and non-isothermal conditions, respectively. From the temperature dependence of vapor pressure derived by the experimental torsion–effusion and thermogravimetry data the molar enthalpies of sublimation Δ cr g H m ∘ (〈 T〉) and vaporization Δ l g H m ∘ (〈 T〉) were determined, respectively, at the middle 〈 T〉 of the respective temperature intervals. The melting temperatures and the molar enthalpies of fusion of these compounds were measured by d.s.c. Finally, the results obtained by all the methods proposed were corrected at the reference temperature of 298.15 K using the estimated heat capacity differences between gas and liquid for vaporization experiments and the estimated heat capacity differences between gas and solid for sublimation experiments. Therefore, the averages of the standard ( p° = 0.1 MPa) molar enthalpies, entropies and Gibbs free energies of sublimation at 298.15 K, have been derived: Compound Δ cr g H m ∘ ( 298.15 K ) /(kJ · mol –1) Δ cr g S m ∘ ( 298.15 K ) /(J · mol –1 · K –1) Δ cr g G m ∘ ( 298.15 K ) /(kJ · mol –1) 2,4-Dinitrobenzoic acid 134 ± 3 244 ± 8 61 ± 5 3,4-Dinitrobenzoic acid 129 ± 3 234 ± 8 59 ± 5