Absolute rate constant for the reaction of atomic chlorine with hydrogen peroxide vapor over the temperature range 265–400 K
Abstract: Rate constants for the reaction of atomic chlorine with hydrogen peroxide were measured from 265-400 K using the flash photolysis-resonance fluorescence technique. Analytical techniques were developed to measure H2O2 under reaction conditions. Due to ambiguity in the interpretation of the analytical results, the data combine to give two equally acceptable representations of the temperature dependence. The results are compared to previous work at 298 K and are theoretically discussed in terms of the mechanism of the reaction. Additional experiments on the H + H2O2 reaction at 298 and 359 K are compared with earlier results from this laboratory and give a slightly revised bimolecular rate constant.
Summary (1 min read)
- The rate constants for chlorine atom reactions with hydrogenous molecules have become increasingly important due to the current interest in the potential modification of stratospheric ozone concentration.
- Modelers have shown that hydrogen abstraction reactions which produce HCt constitute significant sinks for chlorine atoms, and the potential depletion of ozone from the U -Ct0 chain is moderated over that which is predicted if such reactions are not operative.
- Within five minutes after collection, a known volume of 1M H2 sO4 was introduced into the sample bulbs, and the bulbs were thoroughly shaken in order to extract the gaseous H2O Hydrogen peroxide (90% from FMC Corporation) was thoroughly outgassed and was pumped at 273 K for at least 30 minutes to remove H 2O.
- The sample was then further pumped for 5-10 minutes at 298 K before making mixtures.
- Ctl so that pseudo-first order kinetics are applicable, and the decay of chlorine atoms is given by EQUATION With the described apparatus, the inixturo inlet and exit ports to the cell assembly are not symmetric.
- Thus, samples are always preheated or cooled before entering the cell.
- Values of kbi are extracted from (a) the assumption that the arithmetic mean is valid and (b) the assumption that the "after the cell" measurement is appropriate.
- The authors note that the experimental results, either (a) or (b), are more compatible with the latter approach and may suggest important contributions from reactions (5) and/or (6) .
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Cites background from "Absolute rate constant for the reac..."
...The A-factor reported by Michael et al. (1977) is considerably lower than expected from theoretical considerations and may possibly be attributed to decomposition of H,O, at temperatures above 300 K....
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