VERY-LOW PRESSURE REACTOR STUDY OF THE H-REVERSIBLE-ARROW-H-2+BR REACTION(HBR)

The reaction of H atoms, generated by microwave discharge of H-2/Ar ga s mixtures, with HBr has been studied employing the very low pressure reactor (VLPR) technique. Reactants and reaction products were followe d by mass spectrometry. Rate constants of (5.4 +/- 0.7) x 10(-12), (6. 3 +/- 0.8) x 10(-12), and (6.8 +/- 0.4) x 10(-12) cm(3) molecule(-1) s (-1) were extracted for the title reaction at 268, 298, and 333 K, res pectively. Difficulties were encountered in obtaining mass balance clo sure for molecular hydrogen, owing to its production from the internal surfaces of the phosphoric acid coated quartz microwave tube. Formati on of very small amounts of molecular bromine, detected at mle 158, 16 0, and 162, appeared due to some back-diffusion of HBr into the microw ave tube. Br-2 formation was observed to be a function of discharge po wer and, at sufficiently high powers, could be reduced below detection limits. Under these conditions, bromine mass balances (Br-2, Br, and HBr) were around (95 +/- 5)%. A transition state calculation provides an estimated preexponential factor for the reaction in good agreement with our experimental results and with those available from the other most recent temperature-dependent studies. The temperature dependence of reaction 1 can be expressed by k(1)/cm(3) molecule(-1) s(-1) = 9.96 x 10(-13)T(0.5) exp(-300/T) over the temperature range 200-1000 K.