Absolute rate constant for the reaction of atomic chlorine with hydrogen peroxide vapor over the temperature range 265–400 K

TL;DR: In this article, the rate constants for the reaction of atomic chlorine with hydrogen peroxide were measured from 265-400 K using the flash photolysis-resonance fluorescence technique.

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

INTRODUCTION

• 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.

2 EXPERIMENTAL

• 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.

RESULTS

• 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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X-681-77434
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ABSOLUTE RATE CONSTANT FOR THE
REACTION OF ATOMIC CHLORINE
WITH HYDROGEN PEROXIDE VAPOR
OVER THE TEMPERATURE
RANGE 265-400K
f
J. V. MICHAEL
D. A. W HYTOCK
J. H. LEE
W. A. PAYNE
L. J. STIFF
MAY 1977
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GODDARD SPACE FLIGHT CENTER
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(NASA-TM
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X-71340) ABSOLUTE RATE CONSTANT
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N77-26248
FOR THE REACTION OF ATOMIC CHLORINE WITH
HYDROGEN PEROXIDE VAPOR OVER THE TEMPERATURE
RANGE 265-400 K (NASA) 20 p HC A02/MF A01
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CSCL 07D G3/25 37031
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ABSOLUTE RATE CONSTANT FOR THE REACTION OF
ATOMIC CHLORINE WITH HYDROGEN PEROXIDE VAPOR
OVER THE TEMPERATURE RANGE 265 - 400 K
o..
J. V. Michael
*
, D. A. Whytock
t
, J. H. Leett^
W. A. Payne and L. J. Stief
Astrochemistry Branch
Laboratory for Extraterrestrial Physics
NASA/Goddard Space Flight Center
Greenbelt, Maryland 20771
*
NAS/NRC Senior Resident Research Associate
t
Department of Chemistry, University of Essex,
Colchester, Essex; on leave 1975-76 at Catholic
University of America, Washington, D. C. 20017
tt
NAS/NRC Resident Research Associate
+
Adjunct Professor of Chemistry, Catholic
University of America, Washington, D. C. 20017

ABSTRACT
Rate constants for the reaction of atomic chlorine with
hydrogen peroxide have been measured from 265-400 K using the
flash photolysis-resonance fluorescence technique. Analytical
techniques were developed to measure [H
2
O
2
3
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:
k
l
= (3.14 + 0.56) x 10
-13
cm molecule
-1
s
-1
independent
of temperature or k
l
= (1.24 + 0.74) x 10
-12
exp (-384 + 168,1T)
3
cm molecule
-1
s
-1
.
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 + H
2
O 2
reaction at 298 and 359 K are compared.
with earlier results from this laboratory and give a slightly
revised bimolecular rate constant of (4.89 + 0.70) x 10-12
exp (-2780 + 280/RT) cm molecule
-1
sec-1.
{
=_

-

INTRODUCTION
The rate constants for chlorine atom reactions with hydrogenous
molecules have become increasingly important due to the
current interest in the potential modification of strato-
spheric 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.
Due to the known abundance of CH in the stratosphere, one
such important reaction is Ct + CH -"HCt, + CH302
Watson, et al have reviewed other important abstraction
reactions
including
Ct + H
2
-+ HCt, + H
4
and have
explicitly
discussed the stratospheric implications. One possibly
important substrate is hydrogen peroxide which is presumably
formed from the self disproportionation of H0
2
0
These
authors have additionally studied
Ct, + H
2
0
2
-> products

(1)
and have supplied one of the two published room temperature
values to date, the other being by Leu and DeMore.
5
These
two studies show adequate agreement within claimed uncertainties.
On the basis of these determinations and a
presumed temperature
f
_,

-1

t