Thermodynamic Properties of Octafluoropropane from 14°K to Its Normal Boiling Point. An Estimate of the Barrier to Internal Rotation from the Entropy and Heat Capacity of the Gas

Abstract

The heat capacity of octafluoropropane has been measured from 14°K to its normal boiling point of 236.42°K for a sample of greater than 99.99 mole % purity. The following heats of transition and transition temperatures were found: Crystal I—Crystal II, 849.8 cal/mole, 99.37°K; Crystal I—liquid, 114.1 cal/mole, 125.46°K; and liquid—vapor (1 atm), 4723 cal/mole, 236.42°K. The vapor pressure up to 1 atm pressure is represented by the equation log10Pmm=20.85981−(1584.677/T)−4.750509 log10T,and the density of the liquid between 197° and 234°K by the equation d(g/ml)=2.471−0.003628T. The value of the entropy of the ideal gas at 1 atm pressure at the normal boiling point calculated from the calorimetric data is 88.75 cal/ (mole·°K). An average potential barrier of 3300 cal/mole restricting the rotation of the trifluoromethyl groups is estimated from the entropy computed from molecular data. A reasonable but not an entirely satisfactory interpretation of the heat capacity of the gas is also obtained with a barrier of the above order of magnitude.