Further studies on the dissociation of HBr behind shock waves

Previously reported shock tube studies of the dissociation of HBr in the temperature range of 2100–4200°K have been extended to lower temperatures (1450–2300°K) in pure HBr. The course of reaction was followed by monitoring the radiative recombination emission in the visible spectrum from Br atoms. The results imply that, in the lower range of temperatures, the activation energy of dissociation, E in the expression AT^?2e^?E/RT, can be approximated by the HBr bond energy (88 kcal/mole). It was also found that, in this temperature range, the rate of HBr dissociation is sensitive to the Br₂ dissociation rate and the HBr + Br exchange rate. When these rates were adjusted to bring computed reaction profiles into agreement with experimental ones, it was found that the higher-temperature data could also be fitted reasonably well with an HBr dissociation activation energy of 88 kcal/mole, contrary to the conclusions of our previous work, which favored an activation energy of 50 kcal/mole. The “best value” for k₁^Ar, the rate coefficient for HBr dissociation in the presence of Ar as chaperone, appears to be 10^{21.78 ± 0.3} T^?2 10^?88/θ cc/mole sec, where θ = 2.3 RT/1000; that for k₁^HBr, is 10^22.66T^?210^?88/θ. A detailed review is given of the rate coefficients for the other pertinent reactions in the H₂–Br₂ system, viz., Br₂ dissociation and reactions of HBr with H and Br.