Absolute Rates of the Reactions H+C2H4 and H+C2H5

Abstract

The hydrogen atom–ethylene system was studied at 298°K employing the methods of resonance fluorescence and absorption by hydrogen atoms of Lyman $\alpha$ radiation at 1216 Å. The contribution of hydrogen atom–radical reactions was evaluated under varying experimental conditions, and the rate of disappearance of H atoms in ethylene was measured under conditions where stoichiometric corrections became significant. Measurements in the literature of reaction rates for H+C₂H₄ at low total pressure are now in good agreement; however the limiting high‐pressure absolute rate constants thus far reported differ depending on the assignment of stoichiometric factors. Our results indicate that stoichiometric factors obtained under low‐pressure conditions may not be applicable to high pressure. Furthermore, extrapolations based on plots of inverse rate constant vs inverse pressure may be in error due to significant curvature in such plots. Our high‐pressure limiting rate constant for H+C₂H₄, extrapolated from data at pressures higher than those used by other workers, is free of stoichiometric corrections and is in agreement with our earlier measurements. Absolute rate constants obtained in this work are $$\mathrm{H}+{\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{\to k}}_1{\text{\hspace{0.17em}=\hspace{0.17em}13.6\hspace{0.17em}±\hspace{0.17em}1.9\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−13}}({\mathrm{cm}}^3{\mathrm{molecule}}^{\text{−1}}·{\sec }^{\text{−1}}\mathrm{)\; (}\mathrm{extrapolated\; to\; infinite\; He\; pressure}\mathrm{),}$$ $$\mathrm{H}+{\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{\to k}}_4{\text{\hspace{0.17em}=\hspace{0.17em}6.0\hspace{0.17em}±\hspace{0.17em}2.0\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−11}}({\mathrm{cm}}^3{\mathrm{molecule}}^{\text{−1}}·{\sec }^{\text{−1}}\text{) (50}\mathrm{torr\; He}\mathrm{).}$$ The latter rate has been estimated from the variation of the rate of disappearance of H atoms as a function of initial H atom concentration at 50 torr total pressure.