Temperature Coefficients of the Rate of Cl Atom Reaction with HBr in the 228−368 K Range at Millitorr Pressures

Abstract

Temperature coefficients for the kinetics of reaction Cl + HBr → HCl(v,j) + Br have been measured in the range 228−368 K using the very low-pressure reactor (VLPR) system. This experimental technique permits measurement of both the reactant consumption and product formation rates under second-order conditions and achieves mass conservation of 98 ± 2% for the overall chemical change in the reactor. Thermalization of vibrationally and rotationally excited HCl product is well attained in this system. A simple Arrhenius function describes the thermal rate constant as k = (1.99 ± 0.10) × 10-11 exp[−(710 ± 29)/RT] cm3/(molecule s). The measured activation energy is in good agreement with E a = 0.72 kcal/mol obtained from total energy balancing of energy partition among the reaction products, while the A factor indicates a bent configuration of the transition state.

Hydrogen Atom Reactions with Molecular Halogens: The Rate Constants for H + F2 and H + Cl2 at 298 K

“…The difference between our result, which is unaffected by wall effects, and Dobis and Benson's 17 is not clear. We note that Dobis and Benson's 34,35 application of their VLPR apparatus to the Cl + HBr reaction also resulted in a rate constant that was a factor of 2 smaller than the majority of previous results. [36][37][38]…”

Kinetic Study of the Reaction of Hydrogen Atoms with Molecular Chlorine at MilliTorr Pressures

“…In our earlier publication 20 we have noted that our H 3 PO 4coated microwave discharge tube produces a small H atom flow which is independent of the applied microwave power. It maintains a low, permanent background H atom concentration, on the order of 10 10 atoms/cm 3 , in the reaction cell which is too small to measure with mass spectrometry at low voltages due to the small ionization cross section of H atoms.…”

“…The VLPR system used for current measurements is the same 20,21 three-stage, all-turbo-pumped equipment we have used for all previous experiments. It has been described in detail earlier.…”

Temperature Coefficients of Rates of Ethyl Radical Reactions with HBr and Br in the 228−368 K Temperature Range at Millitorr Pressures

“…In terms of H atom metathesis, it cannot be negative, and even the application of a contact transition state mechanism 43 (representing the highest rate for a bimolecular H atom transfer) would result in a rate constant k 7 that is about 1 order of magnitude less than reported by Seakins et al . Some of our reservations concerning the techniques that have been employed have been discussed in earlier papers, ,, and we shall not consider them here.…”

“…For k 1 and k 2 values are given above in connection with eq 1, and k 3 = (2.00 ± 0.06) × 10 -12 cm 3 /(molecule s) is taken from our earlier VLPR study 25 of Cl + C 2 H 6 reaction. k 6 comes from our recent study of reaction 6 in the same system.…”