Heat capacity of 242PuO2 from 12 to 350°K and of 244PuO2 from 4 to 25°K. Entropy, enthalpy, and Gibbs energy of formation of PuO2 at 298.15°K

Abstract

Heat capacity measurements have been made on a 22.5 g sample of 242PuO2 from 12 to 350°K and on a 3.6 g sample of 244PuO2 from 4 to 25°K. The heat capacity curve has the normal sigmoid shape. At 298.15°K the heat capacity C°p, entropy S°, enthalpy H°−H°0, and tempered Gibbs energy (G°−H°0)/T are, respectively, (66.25±0.26) J °K−1⋅mole−1, (66.13±0.26)J °K−1⋅mole−1, (10 784±43) J mole−1, and −(29.96±0.12) J °K−1⋅mole−1. This value of the entropy is significantly lower than the one previously derived from heat capacity measurements on 239PuO2. At 298.15°K the standard entropy of formation and the standard Gibbs energy of formation are −(195.2±0.4) J °K−1⋅mole−1 and −(998.0±0.7) kJ mole−1, respectively.