Reaction of hydrogen atoms with ethane

Abstract

The reaction between hydrogen atoms and ethane has been studied in a flow-discharge system over the wide temperature range 503–753 K at pressures between 8 and 16 Torr. The major products are methane and ethane, reformed by methyl recombination. Minor products are propane and ethylene with traces of n-butane.

The detailed mechanism has been established and computer calculations have been used to derive the set of best-fit rate parameters which reproduce all the experimental results. The results of this work yield the result k₁/cm³ mol^–1 s^–1= 10^{14.27 ± 0.13} exp (–40.9 ± 1.6 kJ mol^–1/RT); H + C₂H₆→ H₂+ C₂H₅. (1)

A survey of all published data some of which have been revised by us to take account of wrongly assumed stoichiometry in the original work, shows that, over a range of 1000 K, the data can be represented by the Arrhenius expression, k₁/cm³ mol^–1 s^–1= 10^{14.12 ± 0.09} exp (–39.2 ± 0.9 kJ mol^–1RT). There is thus no reason to suppose curvature in this plot as has been suggested.

Values of the rate constants for the reactions H + C₂H₅→ 2CH₃(2), CH₃+ H → CH₄(5), and, H + C₂H₅→ H₂+ C₂H₄(11), are found to be k₂/cm³ mol^–1 s^–1= 10^13.57, k₅/cm³ mol^–1 s^–1= 10^12.04 at 8 Torr, 10^12.20 at 12 Torr and 10^12.34 at 16 Torr and k₁₁/cm³ mol^–1 s^–1= 10^12.23.

We have reassessed our earlier data on the reaction of hydrogen atoms with ethylene in the light of the recent “low” values for the rate constant of ethyl recombination. From this, we find values for k₂ and k₅ at 290 K which are, respectively, lower and higher than the corresponding values in the range 503–753 K. It is shown that the slight temperature dependence observed is consistent with the order of reactions (2) and (5).