Mass spectrometric study of the stabilities of gaseous carbides of vanadium, niobium, and molybdenum

The gaseous molecules VC, NbC, NbC2, MoC, and MoC2 have been identified in a mass spectrometric study of the Knudsen effusate from the niobium--graphite and vanadium--niobium--molybdenum--graphite systems at high temperatures. Third-law enthalpies of the various reactions involving these species were determined from the experimental equilibrium measurements and estimated thermodynamic functions. The reaction enthalpies in conjunction with published thermodynamic data yielded the thermochemical values (kJ mol-1, kcal mol-1): D00(VC)=419 +- 24, 101.1 +- 5.7; ΔH0/sub at,0/(VC2)=1178 +- 17, 281.1 +- 4.1; D00(NbC)=564 +- 13, 134.8 +- 3.1; ΔH0/sub at,0/(NbC2)= 1257 +- 13, 300.4 +- 3.1; D00(MoC)=478 +- 16, 114.2 +- 3.8; and ΔH0/sub at,0/(MoC2)=1101 +- 20, 263.1 +- 4.8. In addition, the bond energies D00(V--C2)=572 +- 19 kJ mol-1, or 136.6 +- 4.6 kcal mol-1; D00(Nb--C2)=650 +- 16 kJ mol-1, or 155.4 +- 3.8 kcal mol-1; and D00(Mo--C2)=494 +- 22 kJ mol-1, or 118.1 +- 5.4 kcal mol-1 were also calculated. The results are discussed in relation to the stabilities of previously known gaseous mono- and dicarbides of transition metal