Shock Tube Investigation of Bromine Dissociation Rates in the Presence of Helium, Neon, Argon, Krypton, and Xenon

Abstract

The rates of dissociation of Br2 in the presence of He, Ne, Ar, Kr, and Xe are reported. The following rate constant expressions were obtained from experimental data measured by the light absorption method behind incident shock waves: kD, He = (2.15 ± 0.86) × 1011 T1/2exp[(− 31.9 ± 1.2 kcal/mole) / RT] cm3/mole·sec, kD, Ne = (1.04 ± 0.5) × 1011 T1/2exp[(− 29.1 ± 1.4 kcal/mole) / RT] cm2/mole·sec, kD, Ar = (2.14 ± 0.78) × 1011 T1/2exp[(− 31.3 ± 1.1 kcal/mole) / RT] cm3/mole·sec, kD, Kr = (3.32 ± 0.60) × 1011 T1/2exp[(− 32.8 ± 0.5 kcal/mole) / RT] cm3/mole·sec, kD, Xe = (6.92 ± 1.73) × 1011 T1/2exp[(− 35.5 ± 0.7 kcal/mole) / RT] cm3/mole·sec. In obtaining these rate constants, corrections were made for dissociation due to Br2–Br2 collisions and for the effects of boundary layer buildup. Molecular bromine was less than four times as effective as any of the noble gases in Br2 dissociation over the temperature range investigated, 1200–2000°K. Therefore, in the 1% Br2–99% noble gas mixtures studied, the corrections for the contributions of Br2–Br2 collisions were small. Boundary layer corrections shifted the rate constants to somewhat higher values, the percentage increase being greatest at the lower temperatures. In every single case the activation energy, Ea, was well below the dissociation energy of Br2, D0. The boundary layer corrections lowered Ea making the discrepancies between the Ea's and D0 even greater. The experimental data were used as a basis for evaluating five theoretical dissociation-recombination models (Nielsen–Bak, Light, Bunker–Davidson, Benson–Fueno–Berend, and Keck–Carrier). No single model excelled in predicting all three criteria: the rate constants, the effect of temperature upon kD, and the collision partner efficiencies.