Thermochemistry of gaseous UF/sub 5/ and UF/sub 4/

Journal Article · Wed Jun 01 00:00:00 EDT 1977 · J. Chem. Phys.; (United States)

OSTI ID:7313674

Hildenbrand, D L

Thermochemical data for gaseous UF/sub 5/ and UF/sub 4/ were obtained from equilibrium and electron impact measurements made by high temperature mass spectrometry. Equilibrium measurements yielded for the gaseous reaction Ag+UF/sub 5/=AgF+UF/sub 4/ the enthalpy change ..delta..H/sub 298/=16.9 +- 1.5 kcal by second law analysis. Additionally, the sublimation of UF/sub 4/(s) was studied by the torsion--effusion method, leading to ..delta..H/sub 298/(sub) =76.2 +- 0.5 kcal/mol. These results were combined with auxiliary thermochemical data to derive the standard heats of formation ..delta..Hf degree/sub 298/(UF/sub 5/, g) =-460.6 +- 3.4 kcal/mol and ..delta..Hf degree/sub 298/(UF/sub 4/, g) = -377.8 +- 0.6 kcal/mol, and the bond dissociation energies D/sub 0/(F/sub 5/U--F) =68.3 +- 3.5 kcal and D/sub 0/(F/sub 4/U--F) =101.0 +- 3.4 kcal. Threshold energies for formation of UF/sub 5/ /sup +/ and UF/sub 4/ /sup +/ by dissociative ionization of UF/sub 6/ were determined along with the ionization potentials of UF/sub 6/(14.00 eV), UF/sub 5/ (11.29 eV), and UF/sub 4/ (9.96 eV), leading to the BDE values D/sub 0/(F/sub 5/U--F) =68.0 +- 3.2 kcal and D/sub 0/(F/sub 4/U--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 UF/sub 5/(g) inferred from the equilibrium measurements was used in a third law treatment of published UF/sub 5/ sublimation data, giving thermochemical results in good agreement with the above. The UF/sub 4/ sublimation measurements lead to a vapor entropy that seems more compatible with a molecular configuration of lower symmetry than the obvious tetrahedral structure.

• Research Organization: Stanford Research Institute, Menlo Park, California 94040
• OSTI ID: 7313674
• Journal Information: J. Chem. Phys.; (United States), Journal Name: J. Chem. Phys.; (United States) Vol. 66:11; ISSN JCPSA
• Country of Publication: United States
• Language: English

Related Subjects

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEAR CHEMISTRY
400702* -- Radiochemistry & Nuclear Chemistry-- Properties of Radioactive Materials
ACTINIDE COMPOUNDS
ENTHALPY
ENTROPY
EQUILIBRIUM
FLUORIDES
FLUORINE COMPOUNDS
FORMATION HEAT
HALIDES
HALOGEN COMPOUNDS
PHYSICAL PROPERTIES
REACTION HEAT
THERMOCHEMICAL PROCESSES
THERMODYNAMIC PROPERTIES
URANIUM COMPOUNDS
URANIUM FLUORIDES
URANIUM TETRAFLUORIDE