The thermodynamic stability of gaseous molybdenum pentafluoride

The heterogeneous reaction equilibrium (5/6)MoF6(g) + (1/6)Mo(s) = MoF5(g) was studied by mass spectrometry over the range 460 to 525 K, and values of Δsub(r)H0/R at 298.15 K derived by the second-law and third-law procedures are in close agreement. From the preferred second-law result, Δsub(f)H0/R of MoF5(g) at 298.15 K is found to be -(149.3 +- 0.5) kK, in good agreement with a value obtained previously from gaseous equilibrium measurements in the Mo + S + F system. Another value for MoF5(g) derived from the vapor pressure of MoF5(l) appears to be in error because of an incorrect evaluation of the partial pressure of monomer. The primary bond-dissociation function is found to be 47.6 kK, compared with the average value of 53.8 kK in MoF6. Gas-solid equilibrium in the MoF6 + MoF5 + Mo system is attained rapidly at the low temperatures and pressures of these experiments, once a 'clean' Mo surface is generated. (author)