Enthalpy and Gibbs energy of formation of lanthanum dicarbide

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.