Atom–Molecule Kinetics using ESR Detection. IV. Results for Cl + H2 ⇄ HCl + H in Both Directions

Abstract

The technique described in previous papers of this series has been used to measure separately the rate coefficients $k_1$ for the reaction Cl + H₂ → HCl + H over the temperature range 251°–456°K, and $k_{\text{−1}}$ for the reverse reaction over 195°–497°K. Both sets of data are Arrhenius linear and obey (cm³ mole^− 1·sec^− 1) $$k_1{\text{\hspace{0.17em}=\hspace{0.17em}1.2\hspace{0.17em}×\hspace{0.17em}10}}^{13}\exp {\text{\hspace{0.17em}(\hspace{0.17em}−\hspace{0.17em}4300\hspace{0.17em}/\hspace{0.17em}RT); k}}_{\text{\hspace{0.17em}−\hspace{0.17em}1}}{\text{\hspace{0.17em}=\hspace{0.17em}2.3\hspace{0.17em}×\hspace{0.17em}10}}^{13}\exp \text{\hspace{0.17em}(\hspace{0.17em}−\hspace{0.17em}3500\hspace{0.17em}/\hspace{0.17em}RT)}$$ . Interpretation of the individual rate coefficients is made in terms of absolute rate theory with no tunneling, using a potential surface constructed by the semiempirical Sato method for a linear transition complex. The properties of the complex give a theoretical pre‐exponential factor for the forward reaction about 2.5 times higher than experiment, while that for the reverse reaction is in fair agreement with experiment. It is shown that the ratio $k_1{\mathrm{\hspace{0.17em}/\hspace{0.17em}k}}_{\text{\hspace{0.17em}−\hspace{0.17em}1}}$ is a factor of 2–3 below the thermodynamic equilibrium constant over the experimental temperature range. A possible qualitative explanation for this deviation from the strict prediction of microscopic reversibility is offered in terms of the greater probability of obtaining rotationally excited product HCl in the forward reaction than excited product H₂ in the reverse.