1772
The Journal of Physical Chemistry, Vol. 82, No. 16, 1978
C. L. Lin, M. T. Leu, and W. B. DeMore
Rate Constant for the Reaction of Atomic Chlorine with Methane C.
L. bin, M. 1. Leu, and W.
B. DeMore*
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 9 I 103 (Received March 2, 1978) Publication costs assisted by Jet Propulsion Laboratory
The rate constant and temperature dependence of the C1+ CHdreaction have been investigated by the techniques of competitive chlorination of CH4/CzH6mixtures and by discharge-flow/mass spectroscopy. The objectives were to determine an accurate value for the rate constant for use in stratospheric modeling, and to clarify discrepanciesin results previously obtained by different techniques. The results deduced from the competitive chlorination study are in good agreement with the absolute values measured by the mass spectrometric method, and at temperatures above 300 K are in good agreement with measurements by other techniques based on resonance fluorescence detection of atomic chlorine. However, in the 220-300 K region, the competitive experiments indicate lower rate constants than those obtained by resonance fluorescencemethods, and do not reproduce the curved Arrhenius plots seen in some of those studies.
I. Introduction The reaction of atomic chlorine with methane C1+ CH4 HC1+ CH3 -+
has recently been the subject of several direct rate constant studies,l* largely because of its importance in connection with stratospheric ozone depletion by chlorofluorocarbons. Early models7-10of stratospheric chlorine chemistry used exp(-1790/T) for the the rate constant hCH4 = 6.1 X reaction, as determined by Clyne and Walker2 using a discharge-flow/mass spectrometer (DF/MS) method. However, the latter rate constant was measured in the temperature range 300-686 K, whereas temperature relevant to the stratosphere are about 220-270 K. The more recent direct measurements of hCH4,3+used in subsequent assessments of chlorine effects,11J2have been made in lower temperature ranges, encompassing the temperature conditions of the stratosphere. These experiments, which were based on the methods of flash photolysis/resonance and discharge-flow/resonance f l u o r e s ~ e n c e ~(FP/RF) -~ f l u o r e ~ c e n c e(DF/RF), ~ ~ ~ ~ have yielded hCHl values significantly higher a t stratospheric temperatures than the extrapolated results of Clyne and Walker. Owing to the fact that the C1+ CH4 reaction is the major path by which C10, is converted to the inactive reservoir HC1, the different hCH4values have some impact on calculated ozone depletion, and on other quantities such as the predicted stratospheric HCl/C10 ratio. An additional discrepancy is that at temperatures above J*~ slightly ~ ~ higher than 300 K, the DF/MS r e s ~ l t s ~ are those obtained by the techniques using resonance fluorescence d e t e ~ t i o n . ~ -To ~ Jaid ~ in the determination of accurate values of hCH4over a wide temperature range, we have conducted further studies of the reaction by the techniques of competitive chlorination and DF/MS. Competitive chlorination of CH4/CzH6mixtures was chosen as a means of investigating the temperature dependence of the C1+ CH4reaction for several reasons. The ratios of the products, CH3Cl and C2H5C1,can be accurately measured by gas chromatographic methods, and thus the temperature dependence of the product ratio provides an accurate measure of the activation energy difference of the respective rate constants. This gives, in effect, a lower limit to the activation energy of the CH4 reaction. Further, the C1 + Cz& reaction has a known, weak temperature dependence which has now been measured in two independent absolute studies5J3over the 0022-3654/78/2082-1772$01 .OO/O
relevant temperature range. Most of the previous work on competitive chlorination has been reviewed by Fettis and Knox.14 We felt it was necessary to conduct additional experiments because much of the earlier work was a t higher temperatures. Knox15 did study the CH4/C2H6system a t temperatures as low as 230 K; however, there were relatively few data points at low temperatures, and also there is a discrepancy between the data points and the derived expressions listed in the paper15 and in the review.14 (See Results section.) The competitive chlorination of CH4/C2H6mixtures has also recently been studied by Lee and Rowland16 using a radiotracer technique. The results of this work were consistent with a temperature dependence for the C1 + CH4 reaction similar to that measured by Clyne and Walker? and also with lower values of hcb at stratospheric temperatures than those of the recent direct measurements.
11. Experimental Section A. Competitive Chlorination Experiments. The principles of the competitive chlorination method used in this work have been described in detail in previous papers by Knox and c o - ~ o r k e r s . ~ ~The J ~ Jbasic ~ mechanism for photochlorination of a hydrocarbon, RH, is Clz + hu C1+ C1 R + HC1 C1+ RH R + Cl2 RC1+ C1 (termination steps)
-
-+
+
Since the experimental conditions are such that the termination steps for R are slow compared to the chain propagating reaction with Clz,the chains are very long and the amount of RC1 formation is a measure of the rate at which C1 reacts with RH compared to competing paths. For the case of mixtures of two hydrocarbons, such as CHI and C2H6, the relative rate constant expression is ~ P H B [CzH&1] [CH41 _ _ -(1) ~ C H [CH&11 ~ [C2H61 where [C2H5Cl]and [CH3C1] are the net product yields and [CH,] and [CzH6]are the concentrations of methane and ethane. The initial CH4/CZ& ratio and the product ratio can be measured with great accuracy (combined error estimated to be 7% or less), using standard techniques or pressure measurements and gas chromatography. 0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 16, 1978 1773
Reaction of Atomic Chlorine with Methane
The following factors must also be considered in evaluating possible error in the competitive chlorination method. Product Loss bv Further Reaction of CH&1 or C7H5C1. Even though atimic chlorine reacts slighdy faster with CH3Cl than with CH4,14loss of CH3C1is not a problem because of the very high CH4/CH3Clratios in all of the experiments. On the other hand, the C2H6/C2H5C1ratio was typically much smaller, owing to the necessity of using low CzH6 concentrations in order to obtain comparable yields of CH3Cl and CzH5C1. Even so, loss of C2H5C1was negligible in most of the experiments because of the fact that C1 reacts six times faster with C2H6 at room temperature, and even faster with CzH6 (relatively) a t lower temperatures.14 Where necessary, a first-order correction was applied to the C2H5C1yield by use of a multiplicative factor, f , where f=1+
[CZH5C11av kEtCl [CZH61av k 2 H e
(11)
In this equation, [C2H5C1],, and [C2H6],,are the average concentrations of ethyl chloride and ethane during the experiment, and kEtC1/kCzH6is the rate constant ratio. Corrections were required only in the room temperature experiments, and even in those cases were in the range of 1-690, Depletion of Reactants. The CH4 was in such excess that no appreciable change in its concentration occurred by conversion to CH3C1. However, because of the much lower CzHG concentrations, significant decrease of the C2He occurred in some of the experiments. In most cases, this was in the range of about 10%. Corrections, which were usually quite small, were applied by use of average concentrations in eq I. Using this method of correction, the results showed no dependence on CzH6 conversions, even when these were as high as 50%. Impurities in the Reaction Mixture, The only significant problem in this connection was the presence of a trace of impurity in the CH4. Although the concentration was only a few parts per million, as determined by gas chromatographic analysis, the use of very high CH4/CzH6 ratios in some of the experiments necessitated a small correction to obtain the true C2H6 concentration in the reaction mixture. Other impurities were small, and would not cause a problem in any case because they would not be expected to lead to the formation of either CH&l or C2H6C1. T h e Back Reaction R + HC1- R H Cl. This reaction and its possible effects were discussed by Knox and Nelson,17who showed that it is unimportant because the reaction of R with Clz is much faster than with HCl, and the ClZ/HCl ratio is typically high in the experiments. Possible effects were tested for in our work by conducting experiments at different percent conversions,with no effect being observed. The experiments were carried out in a 5 cm long, 66 cm3, vacuum-jacketed brass cylindrical cell. One end of the cell was sealed with a brass plate and the other was fitted with a quartz window for receiving light. A magnetic stirrer was in operation during the photolysis to aid in obtaining uniform temperature throughout the cell. Two chromel-constantan thermocouples were used to measure the temperature. One was attached to the side wall of the cell and the other to the quartz window. The cell was cooled in a separate chamber surrounding the cell with three types of coolants: dry ice-ethanol a t -75 "C, liquid COP (maintained at constant pressure by regulation) at -53 and -40 "C, and circulating ethanol at -20,0, and 23 O C (by
+
a Haake circulator). At the lowest temperature, the uncertainty was about =t3 deg, and correspondingly less a t higher temperatures. Matheson research grade methane was used directly without further purification. This batch of methane contained 21 ppm ethane, which has been considered in calculating the total ethane concentration used in the experiments. Research grade ethane and chlorine were obtained from Philips-66 and Matheson, respectively. The light source was a low pressure mercury lamp. A Corning No, 5874 filter was used which allowed only 3650and 3340-A lines to pass through. Since the 3650-A line is much stronger than the 3340-w line, the photolysis of Clz is largely due to 3650-A light. The compositions of the reaction mixtures were made in such a way that the CHI to CzH6 ratio would produce comparable amounts of the respective chlorides. In this manner, the optimum accuracy in analyzing the products would be obtained. Typical reaction mixtures were composed of 5 Torr of Clz, 4 Torr of CzH6, and 1520 Torr of CH4 at 296 K. For lower temperature experiments, the amount of CHI was increased to accomodate the slower reaction rate for C1+ CHI. At -75 "C, up to 10 atm of CH4 was used. The preparation of gas mixtures was done at room temperature with the cell shielded from light to prevent room light from photolyzing chlorine. The temperature was then lowered to the desired temperature for photolysis. In most experiments, the light intensity was adjusted so that, for 30-min photolysis, the CzHSC1yield was not more than about 10% of the initial CzH6 concentration. After an experiment, the mixture was frozen out at liquid nitrogen temperature and the noncondensable gases pumped off. The condensable gases (C2H5C1,CHSC1, CzH6, and Clz) were then transferred to the U-tube of the chromatography apparatus for analysis. An 8-ft (0.25 in.) Porapak Q (Waters Associates) column, operated at 100 "C, was used for product analysis. Helium was used as the carrier gas and a thermal conductivity gauge as the detector. The peak areas or heights of CH&1 and C2HsClwere calibrated against their respective authentic samples, and their reproducibility was within 3% for the typical amount (0.2 Torr) of products. B. Discharge Flow/Mass Spectrometer Experiments. The experimental principle and the apparatus have been described in detail in a previous publication.18 All rate constant measurements were made by monitoring the decay of methane ( m / e 16) in the presence of a large excess of atomic chlorine, under pseudo-first-order conditions in a Pyrex flow tube 120 cm in length and 2.5 cm i.d. The temperature of the flow tube was regulated by means of a surrounding jacket, through which appropriate liquids were pumped by means of a high capacity Haake circulator. The temperature was measured by a chromelconstantan thermocouple, which was inserted inside the jacket. The reactant CH4 was admitted to the flow tube through a sliding Pyrex injector (0.6 cm 0.d.) with multiple holes around the tip. The pressure in the reaction zone was measured by a calibrated MKS Baratron gauge (Model AHS-10) connected to the sliding injector (with no gas flowing through the injector). Flow rates of gases were measured by linear mass flowmeters (Teledyne Hastings-Raydist). Flowmeters used for helium were calibrated by a Hastings bubble-type calibrator (Model HBM-1). The flow rates of Clz and NOCl were calibrated by the method of pressure drop at constant volume and temperature. Atomic chlorine was generated in a side arm of the flow tube by passing a small amount ( 1 4 % ) of chlorine in a helium carrier through a microwave discharge with ap-
1774
The Journal of Physical Chemistty, Vol. 82, No. 16, 1978
C. L. Lin, M. T. I
l
o
i
o
W
Leu, and W. B. DeMore I
I
I
W
4
1
lWW = 1930 cm/s
[CH41 = I . 2 x I O l 3 ~ r n - ~
0 KNOX
0 PRITCHARD, PYKE AND
TROTMAN-DICKENSON
IO 1 . 5 2.0
2.5
3.0 I OW/ 3.5T( K) 4.0
4.5
5.0
5.5
Figure 1. Arrhenius plot of CH,/C,H, competitive chlorination data. The line represents a least-squares fit to our data points in the range 220-296 K, showing that this line is a satisfactory representation of all the combined data in the range 198-633 K.
proximately 30-W power. Throughout the experiment, a discharge by-pass technique was used with about 80% of the carrier gas being admitted through a downstream inlet in the side arm of the flow tube. Two methods were used to measure the C1 atom concentrations: (1)The decrease of Clz signal upon activation of the microwave discharge, so that the C1 concentration was given by [Cl] = 2([ClZloff- [Clz]on), This method measures [Cl] at the sampling hole of the mass spectrometer, and would be subject to error if there were a gradient in C1 concentration in the flow tube resulting from surface loss. To minimize this loss, both the inside surface of the flow tube and the outside surface of the injector were coated with phosphoric acid, aged under vacuum. (2) The titration of C1 atoms by NOCl19 C1 NOCl Clz + NO The end point is determined by observing the rise of Clz with the mass spectrometer. Method (2) gives [Cl] at the upstream end of the reaction tube, since the NOCl is admitted through an inlet which is 70 cm from the sampling hole. This method is possibly subject to slight error due to the relatively large flow rate (up to about 5%) of NOC1, which increases the total pressure by as much as 10%. Resulting changes in the flow characteristics, both in the flow tube and in the molecular beam nozzle, could lead to errors in the [Cl] measurement, which we estimate to be approximately 5% or less. In general, [Cl] measured by the titration method was found to be greater than that obtained by the discharge on/off method, as expected for a nonzero gradient of [Cl]. The average differences between the two methods were as follows: 9.7% at 268 K, 2.4% at 296 K, 0% at 355 K, 5.8% at 371 K, and 3.7% at 423 K. The average value was used for the rate constant calculations. At temperatures below about 250 K, the gradient in [Cl] increased rapidly, and for that reason, rate constant measurements are not reported in that temperature region.
+
-+
111. Results A. Competitive Chlorination. The rate constant ratios for the reactions of atomic chlorine with CH4 and CzHs which were obtained at temperatures in the range of 198-298 K are shown in Table I. The average values at each temperature are shown as an Arrhenius plot in Figure 1, which also includes the data points of Knox15 and Pritchard et a1.20 Altogether, the data cover the tem-
ICPI x 1 0 - l ~crn-3
-
2.93 4.95
0
x 7.59 A 9.47
I021 0
1
I
IO
20
I 30
Q
I
I
40
50
60
(4
Figure 2. First-order decay of CH4 in the presence of excess atomic chlorine, observed in the flow dischargelmass spectrometer experiments.
perature range from 198 to 633 K. As can be seen from Figure 1, the data from all three groups are in excellent agreement and are well represented by a straight line. Table I1 summarizes the relative Arrhenius parameters obtained in the three studies in the indicated temperature ranges. It should be pointed out that the Knox value for the preexponential ratio A C p b / A C H 4 listed in Table I1 (4.4) is not the same as that given by Knox in the original paper16 (3.63), or in the later review by Fettis and Knox14 (3.84). However, we find that neither of the latter numbers fits the actual Knox data, and therefore, we have calculated the number 4.4 as a better fit to the original data of Knox. There are two measurements of kc,, which may be used in combination with the relative rate data to calculate kCH,. The measurement of Manning and Kurylo5 was based on the FP/RF method and covered the temperature range value is 222-322 K. Their derived kCZH6 kCzH6 = (7.29 f 1.23) X exp[-(61 f 44)/T] (111) The measurement by Watson et a l l 3 was by the DF/RF method and covered the range 220-600 K. Their result is kCzH6 = (7.84 f 0.3) X exp[-(126 f 23)/T] (Iv) These two measurements of kczfIa.are in good agreement and show no evidence of significant curvature in the Arrhenius plots. Combining the Arrhenius parameters from eq IV with our relative results from Table I1 yields k C H 4 = (1.88 f 0.17) X exp[-(1564 f 31)/m (v) A similar expression would have resulted using eq 111; the Watson expression was chosen merely because it covers a wider temperature range. Based on the linearity of data in Figure 1 and the absence of appreciable curvature in kCzHa,it appears that an Arrhenius-type expression is an adequate representation of kCH4in the temperature range 220-600 K. Our lowest temperature point (198 K) lies slightly below the line in Figure 1, which may indicate non-Arrhenius behavior or, equally likely, the effect of slight temperature rises resulting from failure of the cell
The Journal of Physical Chemistry, Vol. 82, No. 16, 1978
Reaction of Atomic Chlorine with Methane
1775
TABLE I: Data from the Competitive Chlorination Experiments temp,
K
296
273
a
[CH,linit, Torr
[CZH, linit, Torr
[Clz linit, Torr
[CH,C11, Torr
753 753 753 753 753 753 7 53 753 1520 1520 1520 1520 1520 1520 1520
2.036 2.036 2.036 2.036 2.036 2.036 2.036 2,036 4.072 4.072 4.072 4.034 4.034 4.034 4.034
5 5 5 5 5 5 5 5 5 5 5 5 5 5 5
1.095 1.005 0.215 1.145 0.780 0.421 0.111 0.613 0.284 0.179 0.261 0.202 0.816 0.382 0.171
2585 2585 2585 2585 2781 2585
4.047 4.057 4.057 4.057 4.061 4.057
5 5 5 5 5 5
0.299 0.298 0.241 0.332 0.320 0.305
c
2 H 5 c1I, Torr 1.055 0.980 0.281 1.134 0.870 0.541 0.159 0.693 0.376 0.263 0.341 0.280 0.974 0.452 0.239
kC,H,/kCH,
Av
509 501 525 541 541 562 555 521 522 570 514 54 3 524 478 546 5 3 0 + 24a
Av
861 789 809 807 801 867 8 2 2 i 33
0.385 0.353 0.295 0.400 0.358 0.395
253
3878 3878 3878 3878 3878 3878
4.086 4.086 4.086 4.086 4.086 4.086
5 5 5 5 5 5
0.215 0.279 0.229 0.217 0.261 0.287
0.289 0.339 0.309 0.229 0.323 0.345
1328 1205 1331 1266 1225 1191 Av 1258 i 6 1
233
5946 5946 5946 6023
4.131 4.131 4.131 4.131
5 5 5 5
0.246 0.237 0.202 0.127
0.323 0.317 0.265 0.186
1963 2002 1951 2192 Av 2 0 2 7 i 1 1 2
220
7600 7610 7605 7600 7600 7600 7610 7605
4.168 4.168 4.168 4.168 4.168 4.168 4.178 4.168
5 5 5 5 5 5 5 5
0,099 0.190 0.161 0.219 0.166 0.190 0.208 0.226
0.167 0.270 0.209 0.298 0.27 1 0.303 0.329 0.322
3145 2680 2430 2569 3077 3018 3007 2701 Av 2828 .c 266
198
7600 7600 7626 7600 7600 7600
4.168 4.168 4.168 4.168 4.168 4.168
5 5 5 5 5 5
0.173 0.177 0.170 0.102 0.092 0.123
0.490 0.506 0.467 0.271 0.260 0.318
5503 5565 5324 5007 5326 4892 Av 52702 268
The uncertainty represents first standard deviation,
TABLE 11: Relative Arrhenius Parameters for c1 t CH, and Cl + C,H,
~~
4.16 f 0.34' 4.4b 4.67
1438 i: 21 1414 t 23 1 4 3 4 i 190
220-296' 232-633 349-563
~
this work 15 20
The uncertainty represents first standard deviation, Corrected, See text. For the range 198-296 K the corresponding parameters are 4.94 i: 0.61 and 1392 i: 29. a
to maintain the sample uniformly cooled to 198 K. B. Mass Spectrometer Experiments. During all experiments, [Cl] was always at least one order of magnitude greater than [CH,] in order to satisfy first-order conditions. Figure 2 shows examples of data taken under the following
conditions: T = 296 K, P = 1.255 Torr, u = 1930 cm s-l, [CH,], = 1.2 X 1013molecules/cm3, [Cl] = 2.93 X 1014-9.47 X 1014atoms/cm3. The logarithmic decay of methane vs. reaction length, I , measured from the tip of the injector to the sampling hole of the mass spectrometer, can be represented as straight lines. A least-squares program was then used to calculate the first-order constant kI from the relationship where u is the flow velocity. The first-order rate constants (hI) for five sets of data in the range 268423 K are shown in Figure 3, as a function of [Cl]. The data can be represented approximately by straight lines with zero intercept (i.e., ha, = /q/[Cl]). However, we also used a least-squares fitting program to
1776
The Journal of Physical Chemistry, Vol. 82, No. 16, 1978
350
7Lj,K
(COMPETITIVE CHLORINATION,
250
1
1 0 AV
0 SLOPE
2
3 I OOOA(K)
4
5
Flgure 4. Arrhenius plots of competitive chlorination and discharge flowlmass spectrometer results for kc,,,. All points shown are from the mass spectrometer experiments. ICPI
IO-^^\^^-^^
Figure 3. First-order rate constants, k I ,as a function of [CI], for five sets of data in the range 268-423 K. The lines drawn correspond to
k , = k,,[CI].
TABLE 111: Summarv of the Data bv DF/MS 10' %lope
T, K
1013k,, cm3/s
268.4 296.0 335.0 371.1 423.1
(0.52 i 0.03)a (0.96 i 0.09) (1.41 f 0.09) (2.19 f 0.11) (3.88 ?r 0.34)
a
cm3/s (0.57 (0.96 (1.47 (2.04 (4.37
no. of
experiments
0.03) 0.09) 0.06) 0.10) -r 0.28)
7 17 12 7 19
k i i i
The uncertainty represents la of standard deviation.
calculate the slope of the straight line with the best fit of the data. All results are listed in Table 111. Note that the uncertainty represents the first standard deviation. The same least-squares fitting program was used to calculate the Arrhenius expressions kCH4 = (1.05 f 0.25) X lo-'' exp[-(1420 f 8 0 ) / T ] (VII) (average value of the data) kCH4
= (1.07 f 0.40) X
exp[-(1410
f
120)/T] (VIII)
(slope of the data)
It is apparent that these two expressions are not significantly different. The systematic errors in all experiments are estimated as f l % in absolute pressure measurement and f2% in helium, chlorine, and NOCl flow rate measurements, respectively. Axial diffusion corrections (55%)were made using the method of Poirier and Carr.22 Undoubtedly, the most significant error in these experiments is that associated with the measurement of [Cl], which is of the order of &lo%. Combining all the errors, we assign an overall uncertainty of about 15% to the rate constant measurements by this method.
A few experiments were carried out at 296 K with Teflon coating on the inside surface of the flow tube and the outside surface of the injector. The purpose was to test for possible surface effects on kCH4,and also to attempt to reduce the surface loss of C1. The Teflon used was Dupont Clear Finish 852-201, following the coating procedure of Berg and Kleppner.*l Although the rate constant obtained under these conditions was in agreement (20%) with that obtained with phosphoric acid coating, the C1 average concentration gradient was large (13%), and the method was not used at other temperatures.
IV. Discussion Figure 4 shows the Arrhenius line for kCH4(eq V), which was calculated from our competitive chlorination results using kc,, as measured by Watson et al.I3 Also shown are the rate constants measured in the present study by the DF/MS method. In general, the results are in good agreement. Table IV shows a summary of the recent measurements of KCHI. Many of the important factors in the comparison of different results have been discussed in recent papers1?= and need not be reiterated here. However, certain factors which are unique to the present study should be pointed out. Previous measurements of kCHr by the DF/MS method2J8,24gave results at room temperature and above which were somewhat high (-25-75%) compared to measurements by other methods. By contrast, the present mass spectrometer measurements are in much better agreement with other methods in this temperature range. The source of the variation in DF/MS results is not known with certainty, but may be associated with measurement of the atomic chlorine concentration in the flow tube. The latter is more difficult than the measurement of a substrate concentration such as [CH,], which may account for the higher precision and possibly higher accuracy of the DF/RF and FP/RF methods relative to the DF/MS method.
The Journal of Pbysical Chemistry, Vol. 82, No. 16, 1978
Reaction of Atomic Chlorine with Methane
TABLE IV: Comparison of Recent Measurements of h c ~ . rate parameters t e m p range,
10"A
method
K
cm3/s
FP/RF FP/RF FP/RF
218-401 218-322 200-299 299-500a 200-300 300-500b 220-298 298-423 300-686 295-490 268-423 220-633
0.79 0.79 0.65 1.84 0.82 2.2 0.74 1.65 5.08 1.84 1.06
DF/RF DF/RF DF/MS DF/MS DF/MS
ccc
1.88
E/R, K 1260 1273 1229 1545 1320 1639 1291 1530 1792 1409 1415 1564
ref
1 5 4 6 23 2 24 present work present work and 1 3 , l l
a T h e overall rate constant recommended for t h e temperature r a n e 200-500 K in r e f 4 is 5.44 X 10-19T2.50. exp(- 608/T). T h e overall rate constant recommended for t h e temperature range 200-500 K in r e f 6 is 8.6 X 10-18TZ.1 exp(- 795/T). Competitive chlorination.
#
Table IV shows that in the region of 300-500 K, the competitive chlorination result for k C H agrees remarkably well with the recent measurements b y DF/RF6*23and FP/RFa4 It thus appears that all measurements of ~ C H ( by various techniques are now in substantial agreement in the range of about 300-500 K. The situation is less satisfactory in the range 200-300 K. In this range, the F P / R F and (to a lesser extent) DF/RF measurements show hCH4declining less rapidly with temperature than is indicated by the competitive chlorination experiments. This effect is manifested in the form of lower apparent Arrhenius parameters from the direct measurements, as seen in Table IV, and also by curvature in the Arrhenius plots when these plots cover the entire region from 200 to 500 K. For example, both Zahniser et a1.6 and Whytock et alS4found their data to be best represented by three-parameter expressions of the form AT" exp(-E/T), with n = 2.11 and 2.50 in the respective studies. Unfortunately, our DF/MS measurements were not useful in resolving this discrepancy, owing to the onset of large gradients in [Cl] in the low temperature region. As previously mentioned, the competitive chlorination studies show no significant curvature over the range 220-600 K, and are compatible with a temperature dependence for hCH4 in the form exp(-1564/T) for the entire range. It is recognized, of course, that strictly linear Arrhenius behavior is not to be expected. However, the degree of nonlinearity is not easily predicted on theoretical grounds. From the transition state point of view, this requires detailed information on the structure, or entropy, of the transition state, which is not available. The apparent near-linear Arrhenius behavior of h C H 4 deduced from the competitive chlorination experiments cannot be ascribed to simultaneous curvature in the kCZH6 data, because the magnitude of the temperature dependence of that reaction5J3 is clearly too small to mask curvature in the kc, data. Further, the observation cannot be attributed to unidentified experimental or mechanistic errors in the chlorination experiments. The most obvious source of error, local heating, would tend to accentuate curvature. The chlorination studies are direct measurements of the corresponding rate constant ratios, involving relatively simple experimental techniques which are capable of high accuracy. The results should, in principle,
1777
agree with the ratios of direct measurements, as in fact is the case at temperatures above 300 K. The precision and accuracy of the chlorination experiments is demonstrated by the excellent agreement among different experimenters (Figure l),which has been substantially greater than that among the various direct measurements of ~ c H ~ . Similar conclusions with regard to the temperature dependence of KCH are found in the competitive chlorination results for Ck4/H2 mixtures, as measured by Knox and Nelson17 and Pritchard et aL20 These comparisons have previously been discussed by Watson et aL3 These studies covered the range 200-600 K. The results of the two studies, which were in excellent agreement, gave kHz/hCH4 = (3.2 f 0.1) exp(-830/T). Combining this ratio exp(-2290/n, as recwith the value hH2= 3.5 X ommended by Watson1 for the range 213-1071 K, yields k C H 4 = 1.1 X exp(-1460/T) (IN Thus, the CH4/H, results are also consistent with linear Arrhenius behavior for hCH4 over a wide range of temperature. The values of kCH4 calculated from eq IX are in good agreement with those from eq V, particularly at low temperatures. We conclude, therefore, that the temperature dependence of h C H 4 seen in the direct measurements is not presently suitable as a test of reaction rate theory. The hcH4 value currently used in many stratospheric models is hCHl 7.29 X 10-l' exp(--1260/T) (X) This is an evaluated expression1 based primarily on the three FP/RF measurements3r5 and the DF/RF data of Zahniser et a1.6 At a characteristic stratospheric temcm3/s, perature of 235 K, this yields kCH4= 3.4 X which is about 40% higher than that indicated by the competitive chlorination results.
Acknowledgment. This work was supported by the National Aeronautics and Space Administration under Contract No. NAS7-100. References and Notes For a review, see R. T. Watson, J . Pbys. Cbem. Ref. Data, 6, 371 I1 9771. M. A. 'A. Clyne and R. F. Walker, J . Cbem. Soc., Faraday Trans. 7 , 69, 1547 (1973). R. T. Watson, G. Machado, S . Fischer. and D. D. Davis, J. Cbem. Phys., 65, 2126 (1976). D. A. Whytock, J. H. Lee, J. V. Michael, W. A. Payne, and L. J. Stief, J . Cbem. Pbys., 66, 2690 (1977). R. G.Manning and M. J. Kurylo, J. Pbys. Cbem., 81, 291 (1977). M. S. Zahniser, 8. M. Berquist, and F. Kaufman, Int. J. Cbem. Klnet., in press. F. S.Rowland and M. J. Molina, Rev. Geopbys. Space Pbys., 13,
1 (1975). P. J. Crutzen, Geopbys. Res. Lett., 1, 205 (1974). S . Wofsy and M. McElroy, Can. J . Cbem., 52, 1582 (1974). R. J. Cicerone, R. S. Stolarski, and S . Walters, Science, 185, 1165
(1974). "Halocarbons: Effects on Stratospheric Ozone", Panel on Atmospheric Chemistry, National Academy of Sciences, Sept, 1976. "Chlorofluoromethanes and the Stratosphere", NASA reference publication no. 1010,Aug, 1977. R. T. Watson, R. Lewis, S. Sander, and S.Wagner, to be published. G. C. Fettis and J. H. Knox, Prog. React. Kinet., 2, 1 (1964). J. H. Knox, Chem. Ind. (London), 1631 (1955). F. S. C. Lee and F. S. Rowland, J . Phys. Cbem., 81, 86 (1977). J. H. Knox and R. C. Nelson, Trans. Faraday Soc., 55, 937 (1959). M. T. Leu and W. 0 . DeMore, Cbem. Phys. Lett., 41, 121 (1976). M. A. A. Clyne and H. W. Cruse, J . Chem. SOC.,Faraday Trans. 2, 68, 1281 (1972). H. 0.Pritchard, J. B. Pyke, and A. F. Trotman-Dickinson, J . Am. Cbem. SOC.,77, 2629 (1955). H. C. Berg and D. Kleppner, Rev. Sci. Instrum., 33, 248 (1962). R. V. Pokier and R. W. Carr, J . Phys. Cbem., 75, 1593 (1971). L. F. Keyser, to be published. G. Poulet, G. LeBras, and J. Combourieu, J . Cbim. Phys., 71, 101 (1974).
