Thermochemistry of uranium compounds XII. Standard enthalpies of formation of the α and β modifications of uranium pentafluoride The enthalpy of the β-to-α transition at 298.15 K

Abstract

A solution-calorimetric determination of the enthalpies of reaction of the α and β modifications of UF 5 with Ce(SO 4) 2 + H 2SO 4 is described. Auxiliary measurements were made of the enthalpies of reaction of UO 2, γ-UO 3, and HF(aq). From these results are calculated the standard enthalpies of formation ΔH f o(298.15 K) of β-UF 5, −(2083.0 ± 6.3) kJ·mol −1 and of α-UF 5, −(2075.5 ± 6.7) kJ·mol −1. The enthalpy of the (β-to-α) transition is (7.5 ± 2.1) kJ·mol −1 at 298.15 K. Suggested ΔH f o values are also given for U 4F 17 and U 2F 9.