Enthalpy and Gibbs energy of formation of lanthanum dicarbide

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 spec trometer. The LaC2(s) phase was generated in situ starting from a mixture o f LaO1.5(s) and C(s). The enthalpy of the reaction LaO1.5(s) + 3.5C(s) = La C2(s) + 1.5CO(g) at 298 K derived by the second-law and third-law methods u sing thermal functions of LaC2(s) derived from those of UC1.94 and ThC1.94 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 LaC2(s) was derived from the enthalpy of reaction and the enthalpies of formation of LaO1.5(s) and CO(g) taken from the liter ature. The Gibbs energy of formation of LaC2(s) was derived from the enthal py 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 U C1.94 and ThC1.94) are -(58.6 +/- 10.1) and -(62.4 +/- 10.1) kJ mol(-1), re spectively. (C) 2001 Elsevier Science B.V. All rights reserved.