Vapor pressures and standard molar enthalpies, entropies and Gibbs energies of sublimation of two hexachloro herbicides using a TG unit

The vapor pressures above the solid hexachlorobenzene (HCB) and above both the solid and liquid 1,2,3,4,5,6-hexachlorocyclohexane (lindane) were determined in the ranges 332-450 K and 347-429 K, respectively, by measuring the mass loss rates recorded by thermogravimetry under both isothermal and nonisothermal conditions. The results obtained were compared with those taken from literature. From the temperature dependence of vapor pressure derived by the experimental thermogravimetry data the molar enthalpies of sublimation Delta(g)(cr)H(m)degrees(< T >) were selected for HCB and lindane as well as the molar enthalpy of vaporization Delta(g)(1)H(m)degrees(< T >) for lindane only, at the middle < T > of the respective temperature intervals. The melting temperatures and the molar enthalpies of fusion Delta(1)(cr)H(m)degrees(T(fus)) of lindane were measured by differential scanning calorimetry. Finally, the standard molar enthalpies of sublimation Delta(g)(cr)H(m)degrees D(298.15 K) were obtained for both chlorinated compounds at the reference temperature of 298.15 K using the Delta(g)(cr)H(m)degrees(< T >), Delta(g)(1)H(m)degrees(< T >) and Delta(1)(cr)H(m)degrees(T(fus)) values, as well as the heat capacity differences between gas and liquid and the heat capacity differences between gas and solid, Delta(g)(1)C(p,m)degrees and Delta(g)(cr)C(p,m)degrees, respectively, both estimated by applying a group additivity procedure. Therefore, the averages of the standard (p degrees = 0.1 MPa) molar enthalpies, entropies and Gibbs energies of sublimation at 298.15 K, have been derived: [GRAPHICS] (C) 2009 Elsevier B.V. All rights reserved.