chemical thermodynamics of neptunium and plutonium - U.S. ...

18.1 Plutonium halide compounds 359Gruen and DeKock [67GRU/DEK] made similar measurements for the reaction involvingPuCl 3 (l) from 1052 to 1187 K, using the optical density of the principal bandin the spectrum of PuCl 4 (g) as a monitor of the pressure. There is some confusion inthe reporting of the equilibrium constants for the reaction in both the table and figure,but the tabulated values, when divided by 100, (as required to correspond to the givenGibbs energy of the reaction) agree excellently with the extrapolation of the data ofBenz [62BEN], using the selected fusion data for PuCl 3 . A comparison of the experimentaland calculated equilibrium constants is given in the report by Rand and Fuger[2000RAN/FUG]. The values for the stabilities of PuCl 4 (cr) and PuCl 4 (g) give a hypotheticalenthalpy of sublimation of 176.7 kJ·mol −1 at 298.15 K, somewhat smallerthan those for NpCl 4 and UCl 4 , which are both close to 200 kJ·mol −1 .18.1.3.5.b Standard entropy and heat capacityThe thermal functions of PuCl 4 (g) were calculated using the rigid-rotator, harmonicoscillator approximation. Following the work of Haaland et al.[95HAA/MAR] ontheelectron diffraction and infrared spectroscopy of UCl 4 (g), the molecule was assumedto be an undistorted tetrahedron, with a Pu-Cl distance of 2.50 × 10 −10 m, cf. r(U-Cl)=2.503 × 10 −10 minUCl 4 (g) [95HAA/MAR]. The vibration frequencies were takento be very similar to those determined for UCl 4 (g) by these authors [95HAA/MAR].The electronic contribution to the entropy was adjusted to reproduce fairly closely theexperimental entropy of the reactionPuCl 3 (cr) + 0.5Cl 2 (g) Å PuCl 4 (g)derived from the measurements of the equilibrium constant from 674 to 1024 Kby Benz [62BEN]. In fact with the first five electronic levels of UCl 4 (g) suggestedby Hildebrand et al. (which are a simplified version of those suggested by[73GRU/HEC] from their the spectral measurements), the calculated entropy of reaction(120.7 J·K −1·mol−1 ) is very close to the experimental value, 118.9 J·K −1·mol−1 ,and these electronic states were used. The full data used are given in the reportby Rand and Fuger [2000RAN/FUG]. They are very similar to those suggested byKonings and Hildenbrand [98KON/HIL], which appeared after the current assessmentwas completed. Clearly other combinations of molecular parameters and electroniccontributions could provide as good agreement, but the current set has the merit ofbeing consistent with the broad corpus of other data adopted for the gaseous actinidehalide species.The selected data giveSm ◦ (PuCl −14, g, 298.15 K) = (409.0 ± 10.0) J·K−1·molCp,m ◦ (PuCl −14, g, 298.15 K) = (103.4 ± 5.0) J·K−1·molwhere the uncertainties include that arising from the molecular parameters.