Induced heterogeneity, a novel technique for the study of gas‐phase reactions. Part I. Determination of the arrhenius parameters for CC bond scission in propane

It is shown that, by deliberate activation of the reaction vessel, heterogeneous reaction at the wall can be made to dominate chain termination in a complex gas‐phase reaction. For a homogeneous process, characterized, as is often the case, by multiple terminations, this has the effect of simplifying the mechanism and allowing explicit solution of the relevant steady‐state equations so that the rate constants of some individual steps can be evaluated without assumption as to the values of those of others. The pyrolysis of propane, in the vicinity of 500°C, has been used as an example of this approach. Enhancement of the wall activity leads to the reactionproviding, almost exclusively, chain termination. As a result, rate constants for the initiation stepcan be directly determined. The results of this study provide the Arrhenius equation\documentclass{article}\pagestyle{empty}\begin{document}$$ \log k_1 (s^{ - 1}) = 16.71 \pm 0.54 - 83400 \pm 1950{\rm cal}/{\rm mol}/2.303RT $$\end{document}In combination with current thermochemical values this result gives k −1 = 10 13.40 cm 3 /mol·s which, in turn, implies, via the geometric mean rule, k Et‐Et = 10 12.9 cm 3 /mol·s for ethyl–ethyl recombination, in good accord with the most recent determinations and compatible with the newly proposed value of the enthalpy of formation of ethyl. The first‐order wall constant k 8 has been evaluated as k 8 <10 4.2 s −1 . This appears to be the first occasion on which a wall constant has been evaluated from data for a high‐temperature complex gas reaction.