The Low Temperature Gaseous Heat Capacities of C2H6

Abstract

The heat capacities of gaseous C₂H₆ and C₂D₆ have been measured down to 93°K with the low pressure thermal conductivity apparatus previously described. An improved cryostat and better designed conductivity cells were used. A new method of obtaining the heat capacities from the thermal conductivity data is described which does not require a knowledge of the accommodation coefficient α. It is based on the assumption that when the accommodating efficiency of the wire is changed, α changes in the same direction for the gases being compared and further that the percentage change in α is greater the more its absolute value deviates from unity. It is then possible to show that the unknown α for the gas being studied can be bracketed within narrow limits by the α's of comparison gases. The heat capacity results for light and heavy ethane indicate a potential barrier restricting internal rotation of essentially sinusoidal shape and with a depth of 2750 cal./mole.