Some Complexities in the Reaction of Hydrogen Atoms Generated in H2 Discharge with Molecular Chlorine

The kinetics of the reaction H + Cl2 HCl + Cl has been studied in the very low-pressure reactor (VLPR) system at 298 K using two different H atom generation sources. The first one is the microwave decomposition of HCl giving k 1 = (8.66 ± 0.18) × 10-12 cm3/(molecule-s). No traces of HCl(v) excited product side reaction could be found in the system due to the much longer residence time in the reactor than the spontaneous decay of excited HCl. The second H atom source is the microwave decomposition of H2. This technique produces 5% H2(v) which initiates the reaction Cl + H2(v) HCl + H, hydrogen atom recovery and the quenching reaction, H + H2(v) H2 + H. They are identified by differences in the HCl and Cl yields, as well as from the extra H2 consumption. Rate constants k 2 = (2.88 ± 0.13) × 10-11 and k tr = (2.95 ± 0.17) × 12-12 cm3/(molecule-s) are measured along with excellent mass balances between reactant consumption and product formation. Consideration of the high value found for k tr leads to the conclusion that its mechanism is by atom transfer rather than collisional deactivation. Both rate constants agree well with the corresponding A-factors reported for thermal reactions indicating that the vibrational excitation energy of H2(v) provides the activation energies of the thermal reactions.