Thermodynamics of the dimerization of gaseous UF5

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Abstract
The gaseous dimerization equilibrium 2 UF5=U2F10 was studied by high temperature mass spectrometry over the range 600 to 870 K. Gaseous UF5 monomer and dimer were generated by the reaction of UF6(g) with a solid phase that was initially UF4(s), and the abundances of the two species were monitored with both the parent and major fragment ions. Measurements of the U2F10/UF5 ratio as a function of pressure established that gaseous equilibrium was achieved. The average of several second-law determinations yielded a value of −39.0±1.0 kcal/mole for the enthalpy of dimerization of UF5 at 750 K corrected to −40.0±1.0 kcal/mole at 298 K. From an absolute pressure calibration, the equilibrium constant was evaluated, leading to an entropy of dimerization of −40.0 cal/mole deg at 750 K. The results indicate that previously reported transpiration studies on UF5(s) should be interpreted in terms of vapor transport by U2F10 (g) rather than UF5 (g). The enthalpies of sublimation of UF5 (g) and U2F10 (g) from UF5 (s) at 298 K are calculated to be 39.0 and 38.5 kcal/mole, respectively, from the corrected transpiration vapor pressure data and the UF5 dimerization data reported here. The electron impact mass spectra of UF6 and UF5 at several ionizing energies are reported. A weak U2F+11 signal was observed at high UF6 flow rates, apparently the product of the secondary ion source reaction of UF+5 and UF6.