Paul W. Gilles

The Vapor Pressure and Heat of Sublimation of Graphite

The heat of sublimation of graphite and the heats of dissociation of CO and C2, which have been the subject of much controversy in recent years, have been unambiguously established by the direct determination of the total vapor pressure of graphite by an equilibrium effusion method and by the determination of the partial pressure of C2(gas) in equilibrium with graphite.The heat of sublimation of graphite to C(g) is found to be ΔH0=170.39±0.20 kilocalories per mole at 0°K. The heat of sublimation of graphite to C2(g) is found to be ΔH0=233.1±7 kilocalories per mole. The heats of dissociation of C2 and CO have been shown to be 4.7±0.3 and 11.109±0.01 electron volts, respectively. The accommodation coefficient of carbon gas on graphite at high temperatures is found to be about 0.3 and vaporized carbon gas is shown to be in the 3P ground electronic state.