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

402 20. Plutonium group 15 compounds and complexes298.15 K indicate that the earlier estimates [68SPE/LEI] were sound. The experimentalvalues with estimated uncertainties are recommended;C ◦ p,m(PuN, cr, 298.15 K) = (49.6 ± 1.0) J·K−1·mol−1S ◦ m(PuN, cr, 298.15 K) = (64.8 ± 1.5) J·K−1·mol−120.1.1.1.c High temperature enthalpy incrementsMeasurements have been made of the enthalpy increments for PuN(cr) in the temperaturerange 457-1562 K using a drop calorimeter [72OET/LEI]. The experimental dataare represented by:(HT ◦ − H◦ 298.15 ) = 45.002 T + (7.710 × 10−3 ) T 2 − (1.410 × 10 4 ) J·mol −1C ◦ p = 45.002 + (1.542 × 10 −2 −1)T J·K−1·molWith the values of Cp,m ◦ at 298.15 K [78HAL/LEE], and the data for the enthalpyincrements [78OET] thermal functions for PuN(cr) were calculated up to 3000 K[87MAT/OHS].20.1.1.1.d Enthalpy of formationThe enthalpy of formation, f Hm ◦ (PuN, cr , 298.15 K) has been determined by oxygenbomb calorimetry [67LAP/BUN], [69JOH/DEV]. The samples used in the earlierwork contained small quantities of carbon and oxygen; the exact chemical form ofthe impurities was unknown, and the value given for the enthalpy of formation is f Hm ◦ (PuN, cr, 298.15 K) =−(293.72 ± 6.28) kJ·mol−1 derived using the enthalpyof formation of PuO 2 (cr) [58HOL/MUL], f Hm ◦ (PuO 2, cr, 298.15 K) =−(1058.01±1.59) kJ·mol −1 .The later measurements [69JOH/DEV] on a well characterised sample of PuN(cr)gave f Hm ◦ (PuN, cr, 298.15 K) =−(299.20 ± 2.5) kJ·mol−1This value which is recommended here was derived with the enthalpy of formationof PuO 2 (cr) [69JOH/DEV] which is the recommended value given in Section 17.2.1.2 f Hm ◦ (PuO 2, cr, 298.15 K) =−(1055.8 ± 1.0) kJ·mol −1If the recommended value for f Hm ◦ (PuO 2, cr, 298.15 K) were used to recalculatethe earlier experimental data [67LAP/BUN] the derived value for f Hm ◦ (PuN, cr,298.15 K) would be even more positive than the recommended value.20.1.1.1.e The Gibbs energy of formationThe recommended two term equation for the Gibbs energy of formation of PuN(cr) inthe temperature range 298-2000 K is: f G ◦ m (PuN, cr) =−((302825 ± 221) − (97.27 ± 0.15)T) J·mol−1