Thermochemistry of gaseous UF5 and UF4

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Abstract
Thermochemical data for gaseous UF5 and UF4 were obtained from equilibrium and electron impact measurements made by high temperature mass spectrometry. Equilibrium measurements yielded for the gaseous reaction Ag+UF5=AgF+UF4 the enthalpy change ΔH298=16.9±1.5 kcal by second law analysis. Additionally, the sublimation of UF4(s) was studied by the torsion–effusion method, leading to ΔH298(sub) =76.2±0.5 kcal/mol. These results were combined with auxiliary thermochemical data to derive the standard heats of formation ΔHf °298(UF5, g) =−460.6±3.4 kcal/mol and ΔHf °298(UF4, g) = −377.8±0.6 kcal/mol, and the bond dissociation energies D0(F5U–F) =68.3±3.5 kcal and D0(F4U–F) =101.0±3.4 kcal. Threshold energies for formation of UF5 + and UF4 + by dissociative ionization of UF6 were determined along with the ionization potentials of UF6(14.00 eV), UF5 (11.29 eV), and UF4 (9.96 eV), leading to the BDE values D0(F5U–F) =68.0±3.2 kcal and D0(F4U–F) =102.4±3.2 kcal, in agreement with the equilibrium data. Uncertainties associated with the measurements and the interpretation of the data are discussed. The entropy of UF5(g) inferred from the equilibrium measurements was used in a third law treatment of published UF5 sublimation data, giving thermochemical results in good agreement with the above. The UF4 sublimation measurements lead to a vapor entropy that seems more compatible with a molecular configuration of lower symmetry than the obvious tetrahedral structure.

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