Determination of the Heats of Atomization of the Molecules RhC2, RhC, and TiC2 by High Temperature Mass Spectrometry

The mass spectrometric Knudsen effusion technique has been used in the determination of the thermodynamic properties of the gaseous molecules RhC, TiC₂, and RhC₂ over a TiP–Rh–C system. The third law enthalpies, ΔH₀^o, in kcal of the reactions (1) $\mathrm{Rh}(\mathrm{g}\mathrm{)+}\mathrm{C}(\mathrm{graph}\mathrm{)=}\mathrm{RhC}(\mathrm{g})$ , (2) $\mathrm{Ti}(\mathrm{g}\text{)+2}\mathrm{C}(\mathrm{graph}\mathrm{)=}{\mathrm{TiC}}_2(\mathrm{g})$ , (3) $\mathrm{Rh}(\mathrm{g}\text{)+2}\mathrm{C}(\mathrm{graph}\mathrm{)=}{\mathrm{RhC}}_2(\mathrm{g})$ , (4) ${\mathrm{RhC}}_2(\mathrm{g}\mathrm{)+}\mathrm{Rh}(\mathrm{g}\text{)=2}\mathrm{RhC}(\mathrm{g})$ , (5) ${\mathrm{TiC}}_2(\mathrm{g}\mathrm{)+}\mathrm{Rh}(\mathrm{g}\mathrm{)=}{\mathrm{RhC}}_2(\mathrm{g}\mathrm{)+}\mathrm{Ti}(\mathrm{g})$ , and (6) $\mathrm{RhC}(\mathrm{g}\mathrm{)=}\mathrm{Rh}(\mathrm{g}\mathrm{)+}\mathrm{C}(\mathrm{g})$ have been determined as $\text{33.1± 0.5,\hspace{0.17em}62.0± 0.8,\hspace{0.17em}93.6± 1.1,\hspace{0.17em}−26.0± 0.7,\hspace{0.17em}32.2± 1.3}$ , and $\text{137.4± 0.9}$ , respectively. The second law value of ΔH₀^o corresponding to Reaction (1) is 29.7±0.6 kcal. These reaction enthalpies have been combined with appropriated literature data to yield the dissociation energy $D_0^{\mathrm{o}}(\mathrm{RhC}\text{)=138.0± 2.0\hspace{0.17em}}\mathrm{kcal}\hspace{0.17em}{\mathrm{mole}}^{\text{−1}}$ and the heats of atomization, ${\mathrm{\Delta \; H}}_{\mathrm{atm}\text{,0}}^{\mathrm{o}}[{\mathrm{RhC}}_2(\mathrm{g}\text{)]=247± 5.0\hspace{0.17em}}\mathrm{kcal}\hspace{0.17em}{\mathrm{mole}}^{\text{−1}}$ and ${\mathrm{\Delta \; H}}_{\mathrm{atm}\text{,0}}^{\mathrm{o}}[{\mathrm{TiC}}_2(\mathrm{g}\text{)]=277± 2.0\hspace{0.17em}}\mathrm{kcal}\hspace{0.17em}{\mathrm{mole}}^{\text{−1}}$ . The bond dissociation energies, D₀^o, for Rh–C₂ and Ti–C₂, have been determined as $\text{105± 5}$ and $\text{135± 4\hspace{0.17em}}\mathrm{kcal}\hspace{0.17em}{\mathrm{mole}}^{\text{−1}}$ , respectively. The standard heats of formation, $H_{\text{f,298}}^{\mathrm{o}}$ , of gaseous RhC, TiC₂, and RhC₂ have been derived as $\text{164.1± 2.3,\hspace{0.17em}174.4± 2.1}$ , and $\text{225± 5}\mathrm{kcal}\hspace{0.17em}{\mathrm{mole}}^{\text{−1}}$ , respectively.