Rate Constant for the Reaction H + C2H5 at T = 150−295 K

Abstract

The reaction between the hydrogen atom and the ethyl (C2H5) radical is predicted by photochemical modeling to be the most important loss process for C2H5 radicals in the atmospheres of Jupiter and Saturn. This reaction is also one of the major sources for the methyl radicals in these atmospheres. These two simplest hydrocarbon radicals are the initial species for the synthesis of larger hydrocarbons. Previous measurements of the rate constant for the H + C2H5 (1) reaction varied by a factor of 5 at room temperature, and some studies showed a dependence upon temperature while others showed no such dependence. In addition, the previous studies were at higher temperatures and generally higher pressures than that needed for use in planetary atmospheric models. The rate constant for the reaction H + C2H5 has been measured directly at T = 150, 202, and 295 K and at P = 1.0 Torr He for all temperatures, and additionally at P = 0.5 and 2.0 Torr He at T = 202 K. The measurements were performed in a discharge−fast flow system. The decay of the C2H5 radical in the presence of excess hydrogen was monitored by low-energy electron impact mass spectrometry under pseudo-first-order conditions. H atoms and C2H5 radicals were generated rapidly and simultaneously by the reaction of fluorine atoms with H2 and C2H6, respectively. The total rate constant was found to be temperature and pressure independent. The measured total rate constants at each temperature are: k1(295 K) = (1.06 ± 0.25) × 10-10, k1(202 K) = (1.05 ± 0.23) × 10-10, and k1(150 K) = (0.94 ± 0.21) × 10-10, all in units of cm3 molecule-1 s-1. The total rate constant derived from all the combined measurements is k1 = (1.07 ± 0.18) × 10-10 cm3 molecule-1 s-1. At room-temperature our results are about a factor of 2 higher than the recommended rate constant and a factor of 3 lower than the most recently published study.