Enthalpy and Gibbs energy of formation oflanthanum dicarbide

Abstract

Equilibrium CO partial pressures over the phase field LaO1:5 (s)±C(s)±LaC2 (s) between 1343 and 1638 K were obtained by employing a quadrupole mass spectrometer. The LaC2 (s) phase was generated in situ starting from a mixture of LaO1:5 (s) and C(s). The enthalpy of the reaction LaO1:5 (s) ( 3:5C)(s)(LaC2)(s)(1:5CO)(g) at 298 K de- rived by the second-law and third-law methods using thermal functions of LaC2 (s) derived from those of UC1:94 and ThC1:94 was found to be (656:6 ± 8:4)kJ/ mol and (636:3± 8:4)kJ/mol (second-law), and (673:0±3:6) kJ/ mol (third-law), respectively. The enthalpy of formation of LaC2 (s) was derived from the enthalpy of reaction and the enthalpies of formation of LaO1:5 (s) and CO(g) taken from the literature. The Gibbs energy of formation of LaC2 (s) was derived from the enthalpy of formation at 298 K and the standard entropies of LaC2 (s), La(s) and C(s) from the literature. The enthalpy and Gibbs energy of formation of LaC2 (s) at 298 K (based on thermal functions of LaC2 (s) derived from those of UC1:94 and ThC1:94 ) are (-58:6±10:1) and (-62:4 ±10:1)kJ/ mol, respectively.