Vaporization Thermodynamics of the Lanthanum Carbon System. Mass Spectrometric Determination of the Dissociation Energy of LaC2, LaC3, and LaC4

Abstract

The vaporization of the lanthanum dicarbide–carbon system has been studied by Knudsen‐effusion mass spectrometry over the temperature range from 2267 to 2600°K. Experimentally determined reaction enthalpies were combined with published thermodynamic data to yield the atomization energies $D_{\text{0.atom}}\mathrm{°(}{\mathrm{LaC}}_2\text{)\hspace{0.17em}=\hspace{0.17em}1257\hspace{0.17em}±\hspace{0.17em}8}{\mathrm{kJ\; mole}}^{\text{−1}}{\mathrm{,\; D}}_{\text{0.atom}}\mathrm{°(}{\mathrm{LaC}}_3\text{)\hspace{0.17em}=\hspace{0.17em}1759\hspace{0.17em}±\hspace{0.17em}31}{\mathrm{kJ\; mole}}^{\text{−1}}$ , and $D_{\text{0.atom}}\mathrm{°(}{\mathrm{LaC}}_4\text{)\hspace{0.17em}=\hspace{0.17em}2500\hspace{0.17em}±\hspace{0.17em}16}{\mathrm{kJ\; mole}}^{\text{−1}}$ . Ion intensities of the parent molecular ions La⁺, LaC₂⁺, LaC₃⁺, and LaC₄⁺ were combined with absolute effusion measurements from the literature to obtain vapor pressure equations (1900–2600°K): $\log P_{\mathrm{La}}{\text{\hspace{0.17em}=\hspace{0.17em}(−\hspace{0.17em}2.606\hspace{0.17em}×\hspace{0.17em}10}}^4\text{\hspace{0.17em}/\hspace{0.17em}T)\hspace{0.17em}+\hspace{0.17em}10.162,}\log P_{{\mathrm{LaC}}_2}{\text{\hspace{0.17em}=\hspace{0.17em}(−\hspace{0.17em}3.327\hspace{0.17em}×\hspace{0.17em}10}}^4\text{\hspace{0.17em}/\hspace{0.17em}T)\hspace{0.17em}+\hspace{0.17em}12.813,}\log P_{{\mathrm{LaC}}_3}{\text{\hspace{0.17em}=\hspace{0.17em}(−\hspace{0.17em}4.414\hspace{0.17em}×\hspace{0.17em}10}}^4\text{\hspace{0.17em}/\hspace{0.17em}T)\hspace{0.17em}+\hspace{0.17em}14.557}$ , and $\log P_{{\mathrm{LaC}}_4}{\text{\hspace{0.17em}=\hspace{0.17em}(−\hspace{0.17em}4.301\hspace{0.17em}×\hspace{0.17em}10}}^4\text{\hspace{0.17em}/\hspace{0.17em}T)\hspace{0.17em}+\hspace{0.17em}15.067}$ , where the units of pressure are newtons per square meter.