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

360 18. Plutonium group 17 (halogen) compounds and complexes18.1.3.6 PuOCl(cr)18.1.3.6.a Enthalpy of formationWestrum and Robinson [49WES/ROB] have studied the dissolution in 6 M HCl. Theirenthalpy of dissolution −(101.13 ± 0.42) kJ·mol −1 gives with auxiliary data (seePuCl 3 (cr) section), the following selected value f Hm ◦ (PuOCl, cr, 298.15 K) = −(931.0 ± 1.7) kJ·mol−118.1.3.6.b Standard entropy and heat capacityThe standard entropy has been calculated from the equilibrium constants for the reactionPuOCl(cr) + 2HCl(g) Å PuCl 3 (cr) + H 2 O(g)measured by Sheft and Davidson [49SHE/DAV] from 816 to 969 K and Weigel et al.[77WEI/WIS] from 736 to 885 K, with excellent agreement. With the above enthalpyof formation and estimated heat capacities for PuOCl(cr), the Gibbs energies measuredin these studies require r Sm ◦ −1(298.15 K) = −(128.90 ± 1.95) J·K−1·molfrom [49SHE/DAV], and r Sm ◦ −1(298.15 K) = −(128.70 ± 0.50) J·K−1·molfrom [77WEI/WIS].The weighted mean is −(128.71 ± 0.49) J·K −1·mol−1 , which givesSm ◦ −1(PuOCl, cr, 298.15 K) = (105.6 ± 3.0) J·K−1·molwhere the uncertainty is dominated by that in the (estimated) standard entropy ofPuCl 3 (cr).This value is slightly greater than that for UOCl (102.5 ± 8.4) J·K−1·mol−1[92GRE/FUG] and that estimated from the revised Latimer method 103.9 J·K−1·mol−1[93KUB/ALC].18.1.3.6.c High temperature heat capacityThese have been estimated from those for UOCl(cr) [92GRE/FUG] and from the sumof Cp,m ◦ (Pu 2O 3 + UCl 3 )/3[82GLU/GUR, 47GIN/COR]Cp,m ◦ (PuOCl, cr, T) = (73.03 + 17.1 × 10−3 T − 5.83 × 10 5 T −2 −1) J·K−1·molfrom 298.15 to 1100 K, and hence,Cp,m ◦ −1(PuOCl, cr, 298.15 K) = (71.6 ± 4.0) J·K−1·mol