J. Phys. Chem. 1980, 84, 1674-1681
1674
A plot of In x’vs. t will have slope -h’, the true first-order rate constant. If x’is calculated by using lorather than I,,’, plots of In x’vs. t will be nonlinear, and the rate constant thus obtained will be in error. When the product absorber comes from the second of two consecutive first-order reactions, e.g. CH302 NO CH30 + NOz
+
(12) E. Sanhueza, R. Simonaitis, and J. Heickien, Int. J. Chem. Kinet., 11, 907 (1979). (13) D. A. Parkes, D. M. Paul, C. P. Quinn, and R. C. Robson, Chem. Phys. Lett., 23, 425 (1973). (14) D. A. Parkes, Int. J. Chem. Kinet., 9, 451 (1977). (15) R. A. Cox and G. S.Tyndali, Chem. Phys. Lett., 65, 357 (1979). (16) I. C. Plumb, K. R. Ryan, J. R. Steven, and M. F. R. Muicahy, Chem. Phys. Lett., 63, 255 (1979). (17) (a) C. J. Howard and K. M. Evenson, Geophys. Res. Lett., 4, 437 (1977); (b) C. J. Howard, W. M. 0. Symposium on the Geophysical Aspects and Consequences of Changes in the Composition of the Stratosphere, Toronto, June 1978. (18) CODATA Task Group on Chemical Kinetics, J. Phys. Chem. Ref. Data, in press. (19) W. P. L. Carter, A. M. Winer, K. R. Darnall, and J. N. Pitts, Jr., Environ. Sci. Techno/., 13, 1094 (1979). (20) R. T. Watson, S.P. Sander, and Y. L. Yung, J. Phys. Chem., 83, 2936 (1979). (21) A. Savitsky and M. J. E. Goiay, Anal. Chem., 36, 1627 (1964). (22) High pressure limit based on the work of Hochanadei et al. (ref 8). Allowance has been made for the fact that the reaction is in the transition region between second- and third-order kinetics between 50 and 700 torr. (23) R. Renaud and L. C. Leitch, Can. J. Chem., 32, 545 (1954). (24) High pressure limit based on the work of L. Batt and G. N. Rattray, to be published. The uncertainty in this rate constant may be as high as a factor of 3 but the exact value does not affect our results. See ref 22. (25) J. 0. Caivert and J. N. F‘itts, Jr., “Photochemistry”, Wiley, New York, 1966. (26) R. A. Graham, A. M. Winer, and J. N. Pitts, Jr., Geophys. Res. Lett., 5, 909 (1978). (27) R. A. Graham, A. M. Winer, and J. N. Pitts, Jr., J. Chem. Phys., 68, 4505 (1978). (28) R. Simonaitis and J. Heickien, Chem. Phys. Lett., 65, 362 (1979). (29) R. F. Hampson, Jr., and D. Garvin, Natl. Bur. Stand. U . S . Spec. Pub/., No. 513 (1978). (30) k , , is estimated from the data of Burrows et ai. (ref 18) and R. A. Cox and J. P. Burrows, J. Phys. Chem., 83, 2560 (1979). (31) I.Plumb, M. Mulcahy, personal communications. (32) J. Troe, J. Phys. Chem., 83, 114 (1979). (33) K.Luther and J. Troe, “Weak Coiilsion Effects in DissociationReactions at High Temperatures”, presented at the 17th International Symposium on Combustion, Leeds, Aug, 1978. (34) P. J. Robinson and K. A. Holbrook, “Unimoiecular Reactions”, Wiley, London, 1972. (35) J. Troe, J. Chem. Phys., 66, 4758 (1977). (36) NASA Panel for Data Evaluation, “Chemical, Kinetic and Photochemicai Data for Use in Stratospheric Modeling, EvaluationNo. 2”. Jet Propulsion Laboratory Publication 79-27, Apr, 1979. (37) T. Shimanouchi, J. Phys. Chem. Ref. Data, 6, 993 (1977).
+ -
CH30 + NO M C H 3 0 N 0 + M the rate constant and the simple relationship derived above no longer apply, and the correction to the first-order rate constant must be obtained by numerical techniques. Note Added in Proof. Another study of the CH3O2+ NOz + M reaction has recently been published (H. Adachi and N. Basco, Int. J. Chem. Kinet.,12, 1 (1980)). While the value for M = O2 a t 108 torr total pressure ((1.57 f 0.30) X cm3 molecule-’ s-’) is in excellent agreement with the results of this study a t 100 torr for M = Nz, no pressure dependence over the range 108-583 torr is observed, which is contrary to our measurements. References and Notes (1) (a) T. A. Hecht, J. H. Seinfeki, and M. C. Dodge, Environ. Sci. Techno/., 8, 327 (1974); (b) J. A. Logan, M. J. Prather, S. C. Wofsy, and M. 6. McEiroy, Phibs. Trans. R. Soc. London, Ser. A, 290, 187 (1978); (c) W. L. Chameides, Geophys. Res. Lett., 5, 17 (1978). (2) C.W. Spicer, A. Villa, H. A. Wiebe, and J. Heickien, J. Am. Chem. SOC., 95, 13 (1973). (3) R. Simonaitis and J. Heickien, J. Phys. Chem., 78, 2417 (1974). (4) C. T. Pate, 6. J. Finiayson, and J. N. Pitts, Jr., J. Am. Chem. Soc., 96, 6554 (1974). (5) R. A. Cox, R. G. Derwent, P. M. Holt, and J. A. Kerr, J. Chem. SOC., Faraday Trans. 1, 72, 2444 (1976). (6) R. Simonaitis and J. Heickien, J . Phys. Chem., 79, 298 (1975). (7) (a) R. Atkinson, B. J. Finiayson, and J. N. Pitts, Jr., J. Am. Chem. Soc.,96, 6554 (1974); (b) N. Wash& and K. D. byes, Int. J. Chem. Kinet., 8, 777 (1976). (8) C. J. Hochanadei, J. A. Ghormley, J. W. Boyie, and P. J. Ogren, J. Phys. Chem., 81, 3 (1977). (9) C. S. Kan, R. D. McQuigg, M. R. Whitbeck, and J. G. Caivert, Int. J. Chem. Kinet., 11, 921 (1979). (10) H. Adachi and N. Basco, Chem. Phys. Lett., 63, 490 (1979). (1 1) C. Anastasi, I. W. M. Smith, and D. A. Patkes, J. Chem. Sm., Faraday Trans. 1 , 74, 1693 (1978).
Kinetics Study of the CI(,P) 4- C1,O G. W. Ray, L. F. Keyser, and
I?.
-
CI, 4- CIO Reaction at 298 K
T. Watson”
Molecular Physics and Chemistry Section, Jet Propulsion Laboratory, California Institute of Techno/ogy, Pasadena, California 9 1 103 (Received January 3, 1980) Publication costs assistedby the NationalAeronautics and Space Adrninistratlon
The low-pressure discharge flow technique has been used in conjunction with collision-free sampling mass spectrometry (DF/MS) and atomic resonance fluorescence (DF/RF) to study the kinetic behavior of atomic chlorine with chlorine monoxide (ClzO) at 298 K. In order to minimize complications caused by secondary kinetic processes, we used pseudo-first-order conditions in each study: excess atomic chlorine in the DF/MS experiments and excess ClzO in the DF/RF experiments. The reaction and the two values measured for the rate coefficient can be written as follows: C1(T) ClzO Clz + C10 (l),kl = (9.33 f 0.54) X lo-” cm3molecule-’ s-’ (DF/MS), kl = (10.3 f 0.8) X lo-’’ cm3 molecule-'^^^ (DF/RF). The reported value for k l is (9.8 f 0.8) x cm3molecule-’ s-’. This is the unweighted average of the two measurements. These results are compared with previous measurements for hl and used to reevaluate the results of some studies of the flash photolysis of OClO + ClzO mixtures.
+
Introduction In recent years there has been considerable interest in the kinetic behavior of the clO(2n) radical. Most of this interest has been stimulated by the need to understand the role of this radical in the atmospheric chlorine cycle 0022-3654/80/2084-1674$01 .OO/O
-
where it is currently thought to play a major role in photochemically controlling the stratospheric ozone layer.’ Laboratory studies of c10 kinetic behavior require methods of generating the radical in a rapid, clean manner. There are several possible reactions which can be used to 0 1980 American Chemical Society
The Journal of Physical Chemistry, Vol. 84, No. 13, 1980 1675
Kinetics Study of Atomic Chlorine
generate the ClO(Q) radical in a discharge flow system, including c1 ClZO c 1 0 c1, (1)
+
C1+ O3
--
+
C10
+ O2
(2)
k2(298 K) = 1.2 X lo-'' cm3 molecule-' s-''
c1 -I- OClO
c10
k3(298 K) = 5.9 X
+ c10
(3)
cm3 molecule-' s-13
Lg ----*--E ;;iGJ-
In principle reactions 1-3 are ideal sources of C10, because C10 does not further react with atomic chlorine. In contrast, reactions such as
-
+ ocio cio + o2
o(3~)
k4(298 K) := 5 X
NO .t OClO
(4)
cm3 molecule-' s-l -+
k5(298 K) = 3.4 X
C10
+ NO2
(5)
cm3 molecule-' s-l
are not clean sources of C10 because of the reactivity of C10 towards O(3P) and NO: o(3~)c i o c1 o2 (6)
+
-
-
+
k&98 K) = 5.0 X lo-'' cm3 molecule-'
NO
+ C10
NO2
s-I2
+ C1
(7)
k7(298 K) = 1.8 X 10-l' cm3 molecule-' Although reactions 2 and 3 have been used in numerous discharge flow experiments4 to generate C10 radicals, the C1+ C120 reaction has not. This is mainly due to the fact that Basco and Dogra5 reported the reaction to be quite slow, Le., k1(298 K) = 6.7 X cm3 molecule-l d. With such a low rate constant, the C1+ C120reaction would be an unsuitable source of C10 in many discharge flow experiments, because an extremely high atomic chlorine concentration would be required to generate C10 before the addition of the other reagent of interest. For example, if the time required for the complete generation (-99% conversion) of C10 from C1 C120 is lO-'s, the atomic chlorine concentration required would be 1 X 1015 for kl = 5 X cm3 molecule-l s-', This contrasts to an atomic chlorine concentration of only 1 X lOI3 cm-3 for the Cl OClO reaction due to its larger rate constant. Clearly, routine generation of atomic chlorine concentrations of 1015cm-3 would be inconvenient in a discharge flow system. In addition, such concentrations may cause kinetic complications in certain instances. During a discharge flow/mass spectrometric study of the NO + C10 reaction,6 the C1 C120 reaction was used as an alternate source for C10 in order to verify that the resultv were independent of the source of C10. It was soon apparent that significantly lower atomic chlorine concentrations were necessary for complete generation of the C10 radical in the time required than the reported rate constant would suggest. Consequently, a study of the C1 ClzO reaction was undertaken by using the discharge flow/mass spectrometric technique (DF/MS) to monitor the decay of Cl20 in the presence of an excess concentration of atomic chlorine. However, accurate (better than 10%) determinations of atomic chlorine concentrations in the range (0.5-10) X 11012 are difficult and normally present several problems in a DF/MS system. These problems can include the finite rate constant of the titration reaction, heterogeneous removal of atoms, and the presence of impurity atoms. Although these problems were effectively minimized in the present DF/MS study, it was decided also to determine the C1+ ClzO rate constant by using the
+
-
+
-
-
+
+
Figure 1. Schematic diagram of the discharge flow/mass spectrometric system: (---) vacuum chamber; E.I.I., electron impact ionizer; Q.M.F., quadrupole mass filter; CEM, channeltron electron multiplier; GV, gate valve; N, liquid nitrogen trap; N,/Ti, combined liquid nitrogen titanium sublimator trap; DP, diffusion pump; BV, ball valve, MP, mechanical pump; ZT, zeolite trap; CWT, copper wool trap; CT, cryogenic trap; EN, baratron pressure gauge; RI, reagent inlets; MD, microwave discharge; EM, electrometer; PC, photon counter; MCA, multichannel analyzer; RE, recorder; TTY, teletypewriter.
discharge flow/resonance fluorescence (DF/RF) technique. This technique complements the mass spectrometric measurement by using C120 in place of C1 as the excess species, Thus, it does not require an absolute atomic chlorine concentration measurement but relies instead on an accurate determination of the concentration of chlorine monoxide to arrive at the C1 + C120 rate coefficient.
Experimental Section ( a ) Discharge FlowlMass Spectrometric Study. The discharge flow/mass spectrometric system will be outlined only briefly here as it will be described in greater detail in a forthcoming publication.6 A schematic diagram of the apparatus is shown in Figure 1. The essential features include a 2.5-cm diameter Pyrex flow tube with ten fixed inlet jets spaced at 5-cm intervals, interfaced to a modular multistage (three or four) vacuum chamber. A three-stage configuration was employed in this study. (The fourth pinhole shown in Figure 1 was removed, and the ionizer connected to the quadrupole rod assembly.) Efficient collision-free sampling of the reaction mixture results in high sensitivity for both stable and labile species. The detection limit for C120 ( m / e86) without beam modulation and with analogue signal processing is I1 X lo8 molecule ~ m - All ~ . experiments were performed at 298 K and 1torr total pressure, with helium as the carrier gas. Flow velocities were varied from 1200 to 2000 cm s-'. Chlorine atoms were produced by flowing dilute Clz in He mixtures through a 2450-MHz microwave discharge. In order to minimize potential kinetic complications from hydrogen and oxygen atoms, the discharge region and the flow tube were both unpoisoned. Compared to a phosphoric acid coated discharge region, use of an uncoated discharge effectively reduces production of these impurity atomic species. Concentrations of hydrogen and oxygen atoms were measured by adding NO2 to the flow system and mass spectrometrically monitoring the concentration of NO' removed upon initiation of the discharge via the following reactions: o ( 3 ~+) NO' NO + o2 (8) k8(298 K) = 9.3 X
H(2S) + NO2 k9(298 K) = 1.4 X
-
cm3 molecule-l
NO
s-12
+ OH
cm3 molecule-' s-17
(9)
1676
The Journal of Physical Chemistry, Vol. 84, No. 13, 1980
Impurity atom concentrations were found to be 2 X 10l2 ~ m - Consequently, ~. significant errors may have resulted had a poisoned discharge been employed. The upper limit for impurity atom concentrations was 5 X 1O'O cm-3 when an unpoisoned discharge tube was used. Atomic chlorine concentrations were measured by using three different approaches for each experimental run. In the first method (I) a known concentration of molecular chlorine, [C1z]o,was established and the mass spectrometric signal at mle 70 noted. The microwave discharge was then initiated, and the fractional change in the m l e 70 signal (AC12)measured. The atomic chlorine concentration was derived from the following equations:
Ray et ai. TO MOVABLE INLET JET OF FLOW REACTOR
N
[C1lproduced
=
2[C121removed
[C1lproduced = 2(Ac1Z)[c1210
-+
l/,C12
(10)
-
unimportant. The second and third methods of determining the atomic chlorine concentrations were performed simultaneously. A large known excess concentration of molecular bromine, [Brzlo,was used to stoichiometrically remove the atomic chlorine via reaction 12. Molecular
-
BrCl
+ Br
(12)
k12(298 K) = 1.9 X lO-'O cm3 molecule-' s-19 bromine could be added at any jet between the discharge region and the sampling pinhole. In method I1 the fractional change in the Br2signal (ABrz)was measured at mle 160 after initiation of the discharge. The atomic chlorine concentration was determined from the following equations: [C1lprduced = [Br21 removed [C1lproduced = (ABr2)[BrZ10
(11)
The molecular bromine was normally added through three different inlet jets, and, invariably, ABrz was constant. This indicates either that chlorine atoms were not being appreciably removed on the flow-tube walls or that this effect was being masked by heterogeneous recombination of the atomic bromine produced in reaction 12, thus underestimating the true atomic chlorine concentration at the point of Brz addition: Br
+ wall
-
l/zBrz
1
t-j
-4
This assumes that heterogeneous removal of atomic chlorine by reaction 11, as suggested by Martin et is C1 + wall HC1 (11)
C1+ Br,
4
STORAGE FLASKS
(1)
Method I represents a measurement of the atomic chlorine concentration at the flow tube sampling pinhole. Consequently, this may represent a lower limit to the atomic chlorine concentration if any atomic chlorine is removed heterogeneously on the flow-tube walls between the discharge region and the sampling pinhole via reaction 10. C1 + wall
NEEDLE NEEDLE VALVE VALVE
(13)
Method I11 involved the measurement of AClz in the presence of Brz which was added at the inlet jet closest to the discharge region. Consequently, the chlorine atoms were scaveneged by Brz (within -1 ms of the Br, inlet jet) before they could decay heterogeneously on the flow-tube walls: [ClIppoduced = 2(AC12)[Clz]~ (with Brz added) (111) This method measures the atomic chlorine concentration in the flow tube close to the discharge region. The agreement between the different methods for determining
FLOW METER
-4
-
t
*t
-4
*4
Kinetics Study of Atomic Chlorine Pyrex flasks which had been conditioned by previous exposure to C&O. The C120/He mixtures were added to the reactor through a movable inlet jet. Flow rates between 6 and 12 cm3 atm m i d (at 293 K) were measured with a calibrated flow meter. During the actual rate measurement, the ~ L D Wmeter was bypassed to prevent thermal decomposition of the ClzO. Pressures upstream of the flow meter were greater than 500 torr. Under this condition no significant change (6.8 X lo"' (9.3 i: 0 . 6 ) X lo-'' (10.2 i: 1.4) X lo-'' thisstudy (DF/RF) (10.3 i: 0 . 8 ) X lo-'' (10.0 i: 0 . 8 ) x lo-'' Errors are twice the standard deviation.
(slope) (mean) (slope) (mean)
at 298 K was 0.71 atm cm2 s-l. Both axial diffusion and pressure drop corrections to hl were less than 3%. No corrections were made for radial diffusion since estimates indicate that it is small under the conditions used. Use of an uncoated discharge tube for production of atomic chlorine has been shown to reduce significantly the production of impurity atoms and, thus, minimize interference from secondary reactions.1° Concentrations of atomic oxygen and hydrogen were determined by resonance fluorescence under conditions similar to those used in the present experiments and found to be less than 2 X 1O1O ~ m - This ~ , is similar to the results obtained with the mass spectrometric system. At these atom concentrations secondary reactions such as H Cl2 HC1+ C1 (14)
+
--
0 + Cl20 c10 + c10 0 + c10 c 1 + 0 2
(17) (6)
do not interfere with the present measurements. At the low impurity concentration of OClO in ClzO (
