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

346 18. Plutonium group 17 (halogen) compounds and complexes r H m (18.2, 1417 K) = (120.0 ± 10.0) kJ·mol −1 . The corrections to 298.15 Kusing the thermal functions discussed in the next section, are respectively +34.1 and+23.7 kJ·mol −1 , giving r Hm ◦ (18.1, 298.15 K) = (132.0 ± 20.0) kJ·mol−1 and r Hm ◦ (18.2, 298.15 K) = (143.7 ± 11.0) kJ·mol−1 . With the relatively precisevalues of f Hm ◦ (Pu, g, 298.15 K) = (349.0 ± 3.0) kJ·mol−1 (Section 15.2) and f Hm ◦ (PuF 3, g, 298.15 K) =−(1167.8 ± 3.7) kJ·mol −1 (Section 18.1.2.3), these givethe selected values f Hm ◦ (PuF, g, 298.15 K) = −(112.6 ± 10.0) kJ·mol−1 f Hm ◦ (PuF 2, g, 298.15 K) = −(614.3 ± 6.0) kJ·mol −1where the uncertainties have been increased to allow for uncertainties in the thermalfunctions and in the fraction of the overall PuF + 2ion due to the parent PuF 2(g) andPuF 3 (g) species.18.1.2.1.b Standard entropy and heat capacityThe thermal functions of PuF(g) and PuF 2 (g) were calculated assuming the molecularparameters discussed in Rand and Fuger [2000RAN/FUG]. These are essentiallythose suggested by Hildenbrand et al. [85HIL/GUR], namely values similar to thoseestimated for UF(g) and UF 2 (g) by Glushko et al. [82GLU/GUR], but with no electroniccontributions. The Pu-F interatomic distances were taken to be 2.03 × 10 −10and 2.10 × 10 −10 m respectively for PuF(g) and PuF 2 (g). These assumptions giveSm ◦ (PuF, g, 298.15 K) =−1(251 ± 5) J·K−1·molCp,m ◦ (PuF, g, 298.15 K) =−1(33.5 ± 3.0) J·K−1·molSm ◦ (PuF 2, g, 298.15 K) =−1(297 ± 10) J·K−1·molCp,m ◦ (PuF 2, g, 298.15 K) =−1(51.5 ± 5.0) J·K−1·molFull tables of the estimated thermal functions of these species up to 2000 K aregiven by Rand and Fuger [2000RAN/FUG].18.1.2.2 PuF 3 (cr)18.1.2.2.a Enthalpy of formationThe enthalpy of formation of PuF 3 (cr) is based on the enthalpy of precipitation of (hydrated)PuF 3 from 1.5 M HCl by HF · 1.925H 2 O (−(30.1 ± 1.2) kJ·mol −1 ), measuredby Westrum and Eyring [49WES/EYR]. The precipitate is assumed to contain 0.4H 2 O,based on the detailed study by Jones [53JON]. The value for the integral enthalpy offormation of the precipitating solution HF · 1.925H 2 O, was interpolated from the dataof Johnson et al. [73JOH/SMI] by a quadratic function of m 1/2 , for the range m 1/2 =5.0−5.6form 1/2 = 5.3699 to be −320.11 kJ·mol −1 . As noted by Parker [76PAR], nocorrection is required for the fact that the value for f Hm ◦ (HF, aq, 298.15 K) used byJohnson et al. [73JOH/SMI], −(335.64 ± 0.29) kJ·mol −1 , was slightly different from