The torsion-effusion method, with simultaneous torque angle and weight loss measurements, was used to study the thermal decomposition of MgSO4 and CaSO4. The initial step in the decomposition/vaporization process appears to be the evolution of an SO3 molecule from the lattice, followed by conversion to SO2 and O2 on the surface. Measurements of decomposition pressure as a function of orifice area show that both steps are kinetically hindered. The experimental results can be correlated very satisfactorily by the Whitman–Motzfeldt model, so that reliable equilibrium data can be derived despite large degrees of undersaturation of the vapors. With MgSO4 it was possible to evaluate both the SO3 and SO2+O2 equilibrium pressures, the latter by addition of a small amount of Fe2O3 catalyst to the sample. Analysis of the derived equilibrium pressures yields values of −310.3±0.5 and −344.8±0.5 kcal/mol for the standard enthalpies of formation of MgSO4(s) and CaSO4(s), respectively, at 298 K.
The gaseous species ThF, ThF2, ThF3, and ThF4 were generated in a heated effusion beam source, and were identified and characterized by mass spectrometry. An oxygen impurity in the sample also led to the characterization of the gaseous oxyfluoride ThOF. Reaction equilibria involving these species were studied over large temperature ranges, leading to the derivation of both enthalpy and entropy data for these species. The vapor pressure and vaporization thermodynamics of ThF4(s) were also determined. Derived bond dissociation energies in ThF4 oscillate about a mean value of 670±25 kJ mol−1, with a distinct pattern similar to other metal tetrafluorides. Absolute entropies calculated from the equilibrium data indicate moderate electronic level contributions for the odd-electron molecules ThF and ThF3, but not for the even-electron species ThF2 and ThF4. Furthermore, the experimental entropy of ThF4 is compatible with a regular tetrahedral configuration, rather than the distorted configuration of lower symmetry found earlier for UF4 and UCl4. Thermochemical properties of the Th–F species are presented and the comparison with the corresponding uranium fluorides is discussed.
Because of new studies questioning an earlier determination of the thermochemistry of UF5(g), we have reexamined the gaseous equilibrium Ag+UF5=AgF+UF4 by mass spectrometry over a broader temperature range, and have obtained more definitive results, leading to Δf H0298 (UF5,g)=−1929±10 kJ mol−1. Included was an independent thermochemical study of AgF which yielded Δf H0298 (AgF,g)=7.5±6 kJ mol−1 and D00 (AgF)=3.66±0.06 eV. Additionally, second-law measurements of the gaseous reaction U+UF2=2UF corroborated earlier results showing that D(U–F)>D(FU–F), while the reverse holds in the U–Cl and U–Br systems. A revised set of bond dissociation energies and enthalpies of formation for the gaseous UFn species is presented, consistent with all key thermochemical values. This new information is discussed in terms of other results in the literature.
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