Enthalpy and Gibbs energy of formation of lanthanum dicarbide

Abstract

Equilibrium CO partial pressures over the phase field LaO 1.5(s)–C(s)–LaC 2(s) between 1343 and 1638 K were obtained by employing a quadrupole mass spectrometer. The LaC 2(s) phase was generated in situ starting from a mixture of LaO 1.5(s) and C(s). The enthalpy of the reaction LaO 1.5(s)+3.5C(s)=LaC 2(s)+1.5CO(g) at 298 K derived by the second-law and third-law methods using thermal functions of LaC 2(s) derived from those of UC 1.94 and ThC 1.94 was found to be (656.6±8.4) kJ mol −1 and (636.3±8.4) kJ mol −1 (second-law), and (673.0±3.6) kJ mol −1 (third-law), respectively. The enthalpy of formation of LaC 2(s) was derived from the enthalpy of reaction and the enthalpies of formation of LaO 1.5(s) and CO(g) taken from the literature. The Gibbs energy of formation of LaC 2(s) was derived from the enthalpy of formation at 298 K and the standard entropies of LaC 2(s), La(s) and C(s) from the literature. The enthalpy and Gibbs energy of formation of LaC 2(s) at 298 K (based on thermal functions of LaC 2(s) derived from those of UC 1.94 and ThC 1.94) are −(58.6±10.1) and −(62.4±10.1) kJ mol −1 , respectively.