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

362 18. Plutonium group 17 (halogen) compounds and complexesCs 3 PuCl 6 (cr), from 603 to 735 K; ii) Cs 3 PuCl 6 (cr) + CsPu 2 Cl 7 (cr), from 667 to 769 K;and iii) CsPu 2 Cl 7 (cr) + PuCl 3 (cr), from 722 to 846 K, using a platinum effusion cellattached to a mass-spectrometer. Since the absolute pressures of CsCl(g) were notdetermined, but the experiments were all carried out successively in the same Knudsencell, the best way to treat these data is to consider the totally condensed phase reactions0.4Cs 3 PuCl 6 (cr) Å 0.2CsPu 2 Cl 7 (cr) + CsCl(cr, bcc) (18.6)CsPu 2 Cl 7 (cr) Å 2PuCl 3 (cr) + CsCl(cr, bcc) (18.7)The Gibbs energies of these reactions can be derived by subtracting the log(IT) valuesgiven by [93WIL/KLE] for region i) from those for regions ii) and iii) respectively: r G ◦ m (18.6, T) = ((23586 ± 2950) − (4.00 ± 4.13)T ) J·mol−1from 667 to 769 K, and r G ◦ m (18.7, T) = ((37869 ± 3160) − (0.57 ± 4.12)T ) J·mol−1from 722 to 846 K.The small differences in the temperature ranges in three series have been ignored,since the small error introduced is very much within the uncertainty of the Gibbs energies.These have then been reduced to give the enthalpies and entropies of thesereactions at 298.15 K, using the heat capacities of the ternary halides estimated below.In fact, since these estimates are based on r Cp,m ◦ ∼ 0 for the formation from CsCl andPuCl 3 , the corrections from the mean temperature of the measurements to 298.15 K areessentially zero and we have r Hm ◦ (18.6, 298.15 K) = (23.58 ± 2.95) kJ·mol−1 r Sm ◦ (18.6, 298.15 K) =−1(4.00 ± 4.13) J·K−1·mol r Hm ◦ (18.7, 298.15 K) = (37.87 ± 3.16) kJ·mol−1 r Sm ◦ (18.7, 298.15 K) =−1(0.57 ± 4.12) J·K−1·molFor reaction (18.6), we have also ignored the transition in Cs 3 PuCl 6 (cr) at 683 Knoted by Benz and Douglass [61BEN/DOU]. Since the crystal structure is not knownfor either the low- or high-temperature phases, it is difficult to estimate the enthalpyof this transition. A correction should therefore be applied to this treatment when therelevant data become available.The above values give, after rounding, the selected values f Hm ◦ (Cs 3PuCl 6 , cr, 298.15 K) = −(2364 ± 8) kJ·mol −1Sm ◦ (Cs 3PuCl 6 , cr, 298.15 K) =−1(455 ± 11) J·K−1·mol f Hm ◦ (CsPu 2Cl 7 , cr, 298.15 K) = −(2399 ± 6) kJ·mol −1Sm ◦ (CsPu 2Cl 7 , cr, 298.15 K) =−1(424 ± 7) J·K−1·mol