Direct determination of the rate constant for the reaction of oxygen atoms with carbon monosulphide

Volume 32, number 1 ..

CJJEMICAL‘PHYSICSLE-M-ERS .’ ..’ : .’

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.. DETERM!NAT!ONOFTHERATECONSTANT. ; F~~~~~E~~A~ION~~OX~GENATO~S~IT~.C~RBON~[ONOSWLPHIDE

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1.R. SLAGLE, R.E. GRAHAM, J.R. GILBERT* and D. GUTMAEr Depurtment of Chemkiry. IflinoisInstih& olicago, hWnoir 606!6. U,%l

of Techsology.

”

Received 13 December,1974

mu overal! rate constant f!owearlor

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for the reaction of oxygen atoms with &don

experiments at 305 I( and found to be (2.0~+0.14,)x10-‘~*,cm3

hfoduction

monosulphide has been directly determined m&ecule-’ s-l,.

in

2. Experimental ‘...

0

In recent years the CS, t 02, CS2.+0,and CS i 0 systeiis have received considerable attention because they produce CO with an inverted vibrhtionai popula-

The overall rate constant for reaction (1). was-measured using a conventional fast-flow reactor coupled to z phc?oioriization mass spectrometer [5]. Oxygen at-

tion distribution (hereafter referred to as COT) capable of producing pulsed or contiriuous laser emission in a variety of experimental arrangements [ 1,2]. In 2.l.l cases the source of COT is solely (or principauy) the reaction,

oms in great excess over CSconcentratio,ns were generated by passing 02 t.He or N2 t-He gas mixtures +!otigh

a microwave

discharge (in the latter case the

N atoms producedwere converted to 0 atoms by ti: trating with NO). The procedures used for measuring absolute O-atom concentrations have been described otcs+coi+S.’ ‘, (1) ., [5 $1. Carbcn monosulphide was-generated by flow‘ing 0.2% CS, in He gas mixtures through a short dc The factors governing the performance of these chemidischarge similar t,o one described by-Richardson-[7]: cal lasers are being studied using models of the chemiExcess CS2 and solid products were removed by subcal and energy,exchange processes governing the dysequent passage of the discharge products through a namics of.these.systems [3] ; The rate constant for re-133°C co&l-trap. The.He f CS gas mixture emerging action (1) which is needed in the:calculations using from the toll-trap was directed, down the movable gas these models has not.been directly‘determined so a .’ inlet tube to vary reaction time before sampling. The. “lower limit” repo&d by Hanccck and Smith is gendata obtained from a representative experiment are’ ..erally used instead [4].: .’ : plotted in fig. 1. Such first.order decay.pJots,.together We have begun a.sttidy of reaction (1) and have with ths measured O-atom concentrations and calc,unow,&ec,tly measured its rate constant at 305 K. The kted flow velocity yielded the rate constants for the 0 ,+ethod used and the results.ara reported here. : ..’ + CS reactioulisted in table i. Estimates of the CS ‘. indicate [0] / [CS] 0 2 20 in al! expen2 merits, wbicT;was substantiated by’the fact that O-atom depletior. due to reaction with CSIwas usually unconcentration

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reaction \yith 0,‘ (or NO) was +so undetectable!< 3%).: Eleven cx&,iments_werk performed at. 305k2 K ,c,over-. .I. ‘. ,:,..,. . . . .. ‘. .‘.:.‘...: ‘,._ ;;_: ,_; I,.:, :‘ ,,, ; .: ,,._, .I’ ,,,.., ‘, -. .._‘_ .,,_I ‘, ‘. .., . m’,.

Volume 32, number 1, 10 9-

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Table i Reaction conditions

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0.728 0.721 0.721 0.742 1.41 1.41 1.40 1.40 1.40 1.41 1.41 1.91 1.94c) 1.94

[ojx10-‘3

klxlO”

Caricm”)

(cm3molec-~s-‘)

SEC)

kl

2.93 169 L59,

15.2 IS.? 15.7 13.8 13.6 .’ 13.6 13.8 13.6 13.6 12.5 13.0 9.84 9.52 9.52

2.25 d) 1.19 0.593 1.78 1.03 1.01 1.36dI 2.41 d, 1.27 0.84i 0.854

‘averagekI andstandard deviation

14

1975

tid results of experiments to determine

Flow veIocityb! (m/s)

a)

(torr) :B

5

1 A@

CHEMICAL PHYSICS LETTERS

1.94 2.I4 2.25 1.90 2.09 2.03 2.18 2.36 2.18 L-9.5 2.08 1.89 2.04 -1.84 2.06cO.14

3) Pressure is average value along flow tube: pressure drop

Fig. 1. Plot of CS ion counts versus reaction :ime. For conditions of experiment see table 1. The bars on each data point indicate,the standard deviation of net CS ion signal.

over a 3-fold range in [0] and nearly a ftictor of changes in totai gas density and flow velocity. No systematic variation in rate constantwith these panmeters or the O-atom source ‘wasapparent. The average value of k, from all experiments together with the standard deviation is

,ing

along tube was always less than 8% b! Flow veIocity calculated using average pressure. c) Data from this experiment displayed&n fi. L. d) 0 atoms generated by N + NO - Nak 0 reaction. In&l other experiments 0 atoms were generated by passing 02 + He through a ,microwave discharge.

three

k, = (2.0650.14) X lo-Ii

cm3 molecule-l

s-l.

The estimatec! accuracy is -C20%. :

3. Di&ssioti Thk only quantitative estimate of k, was reported’ by Hancock and Smith who measured the ratio of Icl to the rate constant for the reaction (2) 0+NOi-+NO+Ci2 by a competitive n&hod [+I _The experiments yield-, ed the result k /k; = 2.3 at 298 K. Using a value of. k2 =,6.(3X 10-l= ‘,m3.mo1ecu1e-1 S-J iHancock and Smith determined k,. 7 1.4 X I O-l1 cm3 molecule-! ‘..s-1 which they &&ted.be considered as a lotier l&it since it tias possible’ that CS.tias also : remdved, -, : . ; .. . ., :’

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heterogeneously in their experiments. This “lower limit” is somewhat below our determined value but would be tirtually identical if one uses instead more recently determined values of k, = 9.1XiO-12 reported by Davis et al. [S], or 95X1,0-12 reported by Bernand et,al. [9]. The thermally averaged cross section (a) for reartion (1) at 305 K (calculated frOM k, = @kr/Tg)“’ a) is 2.8 .A2 which suggests avery.small activation energy fur this rea,ction. An activation energy oFless than i kcaljmole has been suggested .by Schofield in his evaluation of the Limited information available on reaction (1) ‘[lo]. We are preparing to study reaction (1) at several temperatures to de&nine its actual activation enerfl.

Acknowledgement ,.

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The authprs gratefully acknowledge the support of this, work, by th& Nati&! Science Focndation and the Do$ors.of the Petroleum,Research Fund administered by the A.ine;ican~_en+l Sotie*. .I85 : .

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CHEMICriLPHYSICS ,,

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LETTERS.

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1 epril197.5, .‘,

_,. ,_ ‘, 1_4! G. Hticock and I.W.M. Smith, Trans. Farhdai Sot..67 (1971) 25.86: ., ‘.: ;. .’ ._. -, .[5] I.R. Slagle, FJ..Pruss Jr. and D. Gutmkn, Intern. I.. . ” ‘I [l],‘IS,.J.Ar&ld.and‘H. Rojeska;.Appl. Opt. 12(1973) l69. ’ ” Chem. Kinetic: 6 (1974).111.’ [ 21;W.Q: Jeffq:, C.E. .Wi.sw& J.6. Kelley & R.:J: Richard261 I.R. Slagle, Ji. Sai&ska, F.J. Pruss Jr. and D.‘Cutman, ..I’ son, AppL Phys.‘Letters 22 (1973) 587. : [3] P.H. Dawsq

‘.8ag;

Gd.W.G. Tam,Can.

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‘,..G: Han&k, B.A. Ridley and r.\ir.%i. S&h, J, Chem. -. -, Sot. F&day11 68 (1972) 2117; ‘.. : D-W. Howgate and T.A. Barr J;., J. Chem. Phys. 59 : 11973)2815;- -” .. S. TsucZya; N. Nicls& and S.H. Bauer; J. Phys. Chim. .., 77 (1973).2455; N. Nielsen, Co&U University, private communication.

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‘J. Phys. Chem. 78 (1974) 208. [7] R-J,. Richardson, MtiD&neU~~o&as Research Labbratorics, St;Lou@, pr?vate communication. .. [S) D-D. Davis, J.T. Her-ran a.nd‘RB. Huie, J. Chem. Phys. : ,’ 58 !1573)530. [9] P-P. Bernand, 1IA.A. Clyne and R.T.‘\iatson, J. Chem. Sot. Faraday :i 70 (1974) 564. [lo] K. Schofield, J. Phys. Chem,Ref. Data 2 (1973) 25.

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