The Exchange of Energy Between Organic Molecules and Solid Surfaces Part I. Accommodation Coefficients and Specific Heats of Hydrocarbon Molecules

Abstract

A molecular beam method primarily devised for the investigation of heterogeneous thermal decomposition of hydrocarbon-molecules (to be published later) was used to determine the accommodation coefficients of various hydrocarbon molecules at 20°C on an ordinarily clean nickel surface at 140°C and at pressures in the region of 10-6 mm Hg. The absolute values of the accommodation coefficients were obtained by relative measurements with the help of the following simple relation that was found to exist for paraffin hydrocarbons between the accommodation coefficient α, the specific heat per mole at constant pressure for the ideal gas state Cv0, and the molecular weight M,α(Cv0+R/2)/M=constant=0.352,where R is the gas constant per mole. This equation supports a further relation which can be set up for the mean specific heat between 20° and 140°C for the paraffins by using a few reliable Cv0 values: Cv0 = 0.352M +1.80. A combination of both relations gives the simple equation for α of the paraffins: α=c1M/(c1M+c2),where c1 = 0.352 and c2 = 1.80+R/2. The accommodation coefficient for hydrogen was found to be 0.310, in excellent agreement with earlier experimental values. The accommodation coefficients of ethane and ethylene whose specific heats are known from normal frequencies were found to be equal within a few tenths of a percent; acetylene was found to be 3 percent lower. Specific heats have been calculated for paraffin gases and vapors up to n-heptane by a combination of the results of this work and the specific heat of ethane from the normal frequencies. The specific heats are given to 300°C. Under the assumption of equal accommodation coefficients for paraffins and corresponding olefines based on the accommodation coefficient measurements of ethane and ethylene and on their specific heats, specific heats are given for propylene and n- and isobutylene at 300°C. It was found that the absolute difference between the specific heats of a paraffin and the corresponding olefine decreases with the chain length. The Cv0 values of the following pairs were found to be equal within the experimental error of one percent: n- and isobutane, n- and isobutylene, methyl acetylene and allene. Approximate values for the mean specific heats between 20° and 140 C are given for methyl acetylene and allene. A discussion is given of the relationship of the present work to spectroscopic work in the field.