T H E
J O U R N A L
OF
PHYSICAL CHEMISTRY Registered in U.S. Patent Ofice @ Copyright, 1969, by the American Chemical Society
VOLUME 73, NUMBER 5 MAY 1969
Absolute Measurements of the 0
+ C,H, Rate Coefficient
by A. A. Westenberg and N. deHaas Applied Physics Laboratory, The Johns Hopkins University, Silver Sprinp., Maryland
20910
(Received November 88, 1968)
A wide-temperature-range, fast-flow reactor with esr detection has been used to measure the rate coefficient for the reaction 0 C2H2 + CO CH2 over the range 195-616°K. Values of k1 at room temperature are in good agreement with previous results. The Arrhenius plot of kl is curved, but in the region 230-450°K where the plot is approximately linear the expression kl = 2.0 X 1013exp(-3200/RT) cm3 mol-’ sec-’ is obeyed. Stoichiometric measurements by combined esr and mass spectroscopy with C2H2 in excess show that two 0 atoms are consumed per C2H2 independent of temperature. The reaction mechanism is discussed and compared CzH4, revealing some interesting similarities and differences. The moderately pronounced temwith 0 perature dependence of kl found in this work may be significant for the interpretation of other high-temperature results in flames and shock waves.
+
+
+
Introduction The reaction between 0 atoms and C2H2 presumably enters into the chemistry of acetylene-oxygen mixtures reacting under various conditions. The consumption of C a z in low-pressure, premixed, laminar flames was attributed largely to reaction with 0 by Fenimore and Jones,’ although later evidence of Westenberg and Fristrom2 cast some doubt on this. A fast 0 C2H2 reaction was postulated by Bradley and Kistiakowskya as an important step in the high-temperature oxidation of C&Z in shock waves. The 0 C2H2 reaction has been studied directly in fast-flow systems a t room temperature by several worker^.^' Despite the amount of study the reaction has received, additional confirmation of the mechanism, as well as determination of the temperature dependence of the reaction rate coefficient, is of interest. A fast-flow reactor capable of operation over a wide temperature range (200-1000°K) has been used in several studies reported from this laboratory.&l* Electron spin resonance (esr) spectroscopy is the tool for quantitative detection of atoms and radicals in this apparatus, and absolute atom-molecule rate coefficients can be measured with good accuracy (usually
+
+
to about &lo%). The present paper reports results of the application of this technique to the 0 CZHZ reaction over the temperature range 1954316°K. Rate coefficients are quoted in units of cm* mol-’ sec-l.
+
Experimental Technique The basic technique was described in detail previously8 and only a few brief comments are needed here. (1) C. P. Fenimore and G. W. Jones, J . Chem. Phys., 39, 1514 (1963). (2) A. A. Westenberg and R. M. Fristrom, “Tenth Symposium on Combustion,” The Combustion Institute, Pittsburgh, Pa., 1965, p 473. (3) J. N . Bradley and G. B. Kistiakowsky, J . Chern. Phys., 35, 264 (1961). (4) C. A. Arrington, W. Brennen, 0. P. Glass, J. V. Michael, and R. Niki, ibid., 43, 526 (1966). (5) J. 0.Sullivan and P. Warneck, J . Phys. Chern., 69, 1749 (1965). (6) J. M. Brown and B. A. Thrush, Trans. Faraday Soc., 63, 630 (1967). (7) K. Hoyermann. H. G . Wagner, and J. Wolfrum, 2. Physik. Chem. (Frankfurt), 5 5 , 72 (1967). ( 8 ) A. A. Westenberg and N. deHaas, J . Chem. Phys., 46, 490 (1967). (9) A. A. Westenberg and N . deHaas. ibid., 47, 1393 (1967). (10) A. A. Westenberg and N . deHaas, Ibid., 47, 4241 (1967). (11) A. A. Westenberg and N. deHaas, ibid.. 48. 4405 (1968). (12) A. A. Westenberg and N. deHaas, “Twelfth Symposium on Combustion,’’ The Combustion Institute. Pittsburgh, Pa., in press. (la) A. A. Westenberg and N . deHaas, J . Chem. Phys., 50,707 (1969).
1181
A. A. WESTENBERG AND N. DEHAAS
1182
A small concentration (((1%) of the reactant 0 atom was usually furnished from a microwave discharge of a trace of O2 in an inert carrier gas. A few check runs were performed with 0 furnished by titrating a trace of discharged N2 with NO according to the well-known reaction N NO +X2 0, to prove that the presence of any residual 0 2 did not affect the results. A flow of C2H2in excess (-10: 1) compared to the 0 was metered into a uniformly heated or cooled reactor through a movable injector, and the mixture then flowed to the esr cavity located at a fixed position outside the reactor. With this arrangement any firstorder atom wall losses cancel out, and the pseudo-firstorder gas-phase rate coefficient nkl[CzHJ at the reactor temperature could be derived from the slope of a logarithmic plot of [O] (ie., the esr signal proportional to [ O ] ) vs. distance x using the simple relation
+
dln
+
([010/[01)- nhCC2H21 dx
C2H2 was taken directly from commercial tanks (Matheson 99.60j0)and used without further purificw tion. The acetone content of a full tank (250 psi) was analyzed mass spectrometrically to be about 0.5%, and a tank was never depleted enough (or a t a high enough rate) to increase this appreciably. The presence of this small an amount of impurity could have no significant effect on rate measurements in a fast-reacting system such as this. All of the other gases were of 99.9% purity or better.
Results and Discussion The logarithmic plots of measured 0 decay were of excellent linearity over the entire range of temperature, as the examples in Figure 1 illustrate. As usual, several values of nkl were obtained at each temperature under various conditions to show its independence of [C2H2], pressure, and flow velocity.
(1)
V
Table I: Summary of Measurements of the Stoichiometric
v is the linear-flow velocity in the reactor, kl is the rate coefficient for the primary 0 C2Hzreaction, and n is the number of 0 atoms consumed per C2H2reacted. n > 1 if 0 atoms are consumed in any other reactions
+
subsequent to the primary step, and the value of n was determined as previously describeds by auxiliary experiments using a mass spectrometer to measure the consumption of C2H2which caused the [O] to vanish (ie., [ 0 ] / [ 0 ] 0< 0.01) a t the esr cavity. The C2H2 "on-off" procedure8 was not necessary for the rate measurements since movement of the injector did not affect the 0 signals with the C2Hzturned off, so that the C2Hzcould be left on continuously during the kinetic runs. The apparatus was the version incorporating the improvements discussed in our most recent paper."
Coefficient n (Helium Carrier) Temp, Pressure, OK mm
195 297
436
604
2.73 0.86 0.86 1.22 1.22 1.66 1.66 1.21 1.95 1.95 2.00 2.00 1.75 2.34 2.34
lO~o[CtHt]o,
mol/cms
ICeHdo/lOlo
n
19.3 4.95 2.60 3.12 3.12 4.24 2.24 3.10 3.80 1.47 3.90 2.24 1.02 1.36 0.88
2.80 3.60 1.06 2.57 2.57 2.57 1.35 2.58 3.68 1.42 3.76 2.16 2.78 2.66 1.72
1.7 2.0 2.3 2.0 1.9 1.9 2.0 2.0 1.7 1.8 1.9 1.7 1.9 1.9 2.0 Av 1 . 9 f 0 . 1
4 3
s 4 s
2
\
1
0
20 40 DISTANCE Z(cm)
+
60
Figure 1. Examples of 0 decay in the 0 CzHz reaction: (A) 195"K, 1.16 mm, e, = 1880 cm/e.ec, [C2Ha] = 1.31 X 10-e mol/cms; (B) 195"K, 1.72 mm, e, = 1290 cm/sec, [CZH~] = 1.75 X mol/cm~;(C) 369"K, 1.39 mm, v = 3700 cm/sec, [C&] = 2.50 X lO-"Jmol/cms; (D) 597'K, 2.21 mm, v = 6870 cm/sec, [C~HZ] = 2.11 X 10-lOmol/cms. The J O U T of~ Phgaical L ~ ~ ChemiatTy
The stoichiometric coefficient n was measured (see Table I) in 15 runs at four different temperatures over the 195-600'K range and was found to be 1.9 f 0.1 with no temperature dependence. This is in good agreement with the room-temperature value of 1.8 f 0.2 obtained by Brown and Thrusha in a somewhat different way in their esr study of this reaction and with the value 2.0-2.1 measured by Arrington, et using a technique essentially the same as our own (but with no esr detection involved). In view of these excellent cross checks, the even value n = 2 was used to derive the absolute kl values given in Table I1 from the measured nkl data. It should be noted that Sullivan and Warnecks found n > 1 and dependent upon the initial CJ12:0 ratio at rather short reaction times (a few milliseconds), where it is likely that the secondary reaction consuming 0 had not yet become
ABSOLUTE MEASUREMENTS OF THE 0
+ C2H2 RATECOEFFICIENT
Table 11: Summary of Measurements of the Rate Coefficient kl for the Primary 0
kr, cma mol-1 sec-1
1.16 0.67 1.72 0.89 0.92 1.32
1880 1440 1290 1000 1700 1010
1.31 X 1.70 X 1.75 x 1.03 x 1.85 X 3.15 x
10-9 10-9 10-9 10-0 10-9
6.6 X lo9 6.7 x 109 6 . 5 x 109 6.8 X lo9 6.8 X lo9 6.0 X log Av (6.6 f 0 . 2 ) X 100
228
0.76 0.76 1.20 1.17 0.78 1.74
1900 1820 1200 1200 1800 1390
1-64 x 2.16 X 2.46 x 0.85 x 0.57 X 1.19 x
10-9 10-9 10-9 10-9 10-9 10-9
1.91 x 1010 1.70 X l O l o 1.62 X 1Olo 1.85 X l O l o 2.00 x 1010 1.86 X 101O Av ( 1 . 8 2 f 0 . 1 1 ) X 100
298
0.84 0.84 0.85 1.45 1.84 2.43 1.21 2.97 1.22 1.26" 1.25" 1.24"
2510 2540 2480 1450 1140 1440 2890 1180 2870 2810 2860 2920
5.08 X 7.72 X 3.15 X 5.30 X 6.84 X 5.44 x 2.71 X 6.62 X 2.65 X 3.72 X 7.39 x 10.58 X
10-10 10-10 10-10 1O-IO
9.0 x 10'0 8.8 X 101O 9.2 X 1Olo 9.0 x 1010 8 . 5 X 1O1O 8.8 X 1Olo 9.2 X 10'O 8.5 X 1Olo 8.8 X 1Olo 9.6 X 1O1O 9.0 x 1010 8.4 X 101O Av (8.9 f 0 . 3 ) X 1O1O
0.94 1.39 1.39 1.65
3130 3700 3700 4040
369
2.99 2.50 3.76 4.91
X X X X
loWg
1O-Io
10-10 10-10 10-lo 10-1O 10-10 10-10 10-lO 10-lo 10-lo 10-10 10-lo
2.45 X 1O1l 2.50 X 1011 2.50 X 10" 2.48 X 10" Av (2.48 =k 0.02) X
loll
0.86 1.22 1.72 2.16 2.16 0.92 1.24
3700 4340 5070 5290 5290 3820 4370
3.95 x 3.16 X 2.71 X 2.56 X 3.36 X 2.02 x 1.76 X
10-10 10-10 10-10
597
2.10 1.51 1.50 2.21
6900 6110 6060 6870
1.24 X 1.38 X 2.43 X 2.11 x
10-10 10-10 10-10 10-10
1.79 X 10la 1.80 x 10'2 2.10 x 10'2 1.92 X 10l2 Av (1.90 f 0 . 1 0 ) X 1012
616
2.11 1.52
6960 6180
1.01 x 10-10 1I12 x 10-10
1.93 X 10l2 2.06 X lo'* Av (2.00 f0.07) X 10l2
450
Nz
10-10
10-10 10-lo lO-Io
5.7 x IO" 5.8 X 1011 6.1 X 10" 5.9 x 10" 5.6 X 10" 6.0 X 101l 5.9 x 10" Av (5.9 f O . 2 ) X 1Ol1
+ 0.
fully effective. Hoyermann, et al.,s reported n = 4 with a large excess of 0 over C2H2, i.e., the opposite situation from our work. The average values of LIare plotted in Arrhenius form in Figure 2. The plot shows definite curvature in a manner similar to that exhibited by the 0 C2H4 rate coefficient12 also shown in Figure 2. The central portion (103/T 2.3-4.3) may be regarded as reason-
-
[CZHZ]. mol/cma
Velocity, cm/sec
195
+ NO+
+ CZHZReaction (Helium Carrier)
Pressure, mm
Temp, OK
"0from N
1183
+
ably linear, and the simple Arrhenius expression valid in this region only is kl = 2.0 X 10laexp( -3200/RT). Obviously, this relation should not be used for extrapolation outside its limited temperature range. Several significant comparisons of these IC1 data with other results may be made. As noted in Table 11, the present work at room temperature gives the value LI = (8.9 f 0.3) X lolo. Brown and Thrushe used a Volume Wt Number 6 May 1868
A. A.
1184
.+
13
.
12
-
CI
r
-kc
'Ti E
3 11 -i
I
Y
10
-
- 9 0
4
2
6
103 A (OK)
+
Figure 2. Arrhenius plot of data on kl for 0 C ~ H-+Z CO 0,this work; Fenimore and Jones.' Also shown is the smoothed plot of data on the 0 CZH,reaction from ref 12 (broken line).
+;
+ CHz:
+
similar fast-flow technique with esr detection, the essential difference being that they did not inject C2Hz in large excess compared to 0, so that [C2H2] was not constant and a second-order kinetic analysis was necessary. Kevertheless, their room-temperature value of (9.2 f 0.4) X 1O'O (determined with n = 2 also) is in excellent agreement with ours. Arrington, et u Z . , ~ reported a number of room-temperature data taken in a fast-flow system by monitoring various species. These scatter considerably, but their runs obtained with 0 in excess and monitoring C2H2 disappearance mass spectrometrically are probably the simplest and most reliable,I4 and these average kl = 8.4 X 1Olo in fair agreement with our value. The average of all of their data by monitoring 0 disappearance, C2H2 disappearance, and CH chemiluminescence (the latter presumably being proportional to [C2H2I3) is (5.3 f 1.7) X 1010, which is considerably lower than the present result. More importantly, they found essentially no change in kl from 195 to 295°K (based on a single measurement at 195°K). In view of the present work, where kl was found to vary more than an order of magnitude over this temperature range, the lack of temperature dependence reported by Arrington, et aE., seems clearly in error. Sullivan and Warnecks measured kl mass spectrometrically in a fast-flow system from the C2H2consumption and the integrated LO] down the flow tube, which is a valid procedure providing C2H2 is not consumed in any reaction other than with 0. This assumption is probably good in this case, and they obtained kl = (9.0 f 1.8) X lolo, again in excellent agreement with our result and that of Brown and Thrush. Hoyennann, et uL17reported a room-temperature kl = 9.6 X 1O'O from a fasbflow system with The Journal of Physical Chemistry
WESTENBERG AND
N.
DEIlAAS
esr detection (after sampling through a probe). However, it seems likely that this should be divided by n = 2, since it was apparently measured with CZHZin excess (ie., [C~HZ]was assumed constant), and their stoichiometry should have been the same as in our own work. They noted only that they found n = 4 with 0 in large excess. The only results for kl in the literature at other than room temperature are the flame data of Fenimore and Jones.' These were not direct determinations, of course, but involved a number of assumptions and reliance on other rate coefficients to infer kl. The disappearance of C2H2 in a number of flames with various C2H2-02-CO-Ar mixtures was assumed (after showing that reaction with H and OH was unreasonable) to be caused by reaction only with 0 atoms, and at temperatures in the range of about 1000-1600OK values for IC1 of (1-2) X l O I 3 were obtained. These values are indicated by the large cross in Figure 2 extending over the measured ranges in T and kl. While, no doubt, quite approximate, the flame data do lie reasonably well on the curve which might be smoothly extrapolated from our data. Figure 2 gives a smoothed plot of the data for the 0 C2H4 rate coefficient (ICs) obtained previously12 by the present method. It is of interest to compare the ratio ka/kl from these separate absolute measurements with the relative data of Saunders and Heicklen.16 The latter were obtained from relative reaction rates of 0 atoms (from Hg-sensitized N2O decomposition) with various hydrocarbons in competition with C2F4. From their relative data for C Z H ~C2F4 : and CzH2:CzF4 at 297, 343, and 398OK one can derive the values k3/kl = 4.7 f 0.6, 3.9 f 0.4, and 2.4 f 0.2 at the three temperatures, respectively. The ratios taken from the plots in Figure 2 at the same temperatures are 4.7,3.2, and 2.3. This is generally excellent agreement and strongly supports the validity of our stoichiometric measurements ( n = 2 in both the C Z H ~and CzH4 reactions) particularly. The difference in activation energies from Saunders and Heicklen's data in their temperature range is El - Ea = 1.5 kcaI/mol, while our absolute measurements in this same Arrhenius linear range give El - Ea = 3.2 - 1.5 = 1.7 kcal/mol, which is also good agreement. I t should be emphasized that the absolute measurements of kl reported here do not depend in any way on a knowledge of the products of the primary 0 C2Hz reaction nor of the subsequent mechanism. These aspects of the reaction are of interest, however, and the evidence we have will now be presented. Considerable discussion is also given in ref 4 and 6. The initial step is generally thought to be
+
+
0
+ C2Hz
4
GO
+ CH2
(1)
(14) H. Niki, private communication. (16) D. Saunders and J. Heicklen, J . Phys. Chem., 7 0 , 1960 (1986).
ABSOLUTEMEASUREMENTS
OF THE
and there seems no reason to doubt this. It is strongly exothermic ( A H = -45 kcal/mol) , whereas all of the other conceivable products give fairly endothermic reactions, including the hydrogen abstraction to yield CzH OH proposed by Bradley and Kistiakowsky.a The observed stoichiometry of two 0 atoms consumed per CzH2then requires a rapid second step
+
O+CHZ+CO+ZH
(11)
which is exothermic by 77 kcal/mol. Brown and Thrush6 have outlined the reasons why CO 2H are the preferred products rather than HCO H, the point being that the exothermicity of the latter step is so high that the HCO would probably rapidly dissociate to H CO anyway, giving the same result as (11). The formation of appreciable HCO would also mean that 0 HCO +OH CO followed by 0 OH + Oz H would occur, which would violate the observed n = 2 stoichiometry. Our mass spectrometric analyses taken at very long reaction times and with CzHz in excess showed only CO and H2 as major products in agreement with previous finding^,^^^ with no C02, H20, or other species formed in appreciable quantity. One H2 (approximately) was formed for each C2H2 consumed. Reaction I1 is written as indicated rather than with CO Hz as products, however, because there is considerable evidence for it. References 4,6, and 7 all agree that H is a major product, and we have verified this by esr also. ,4lthough attempts were made to measure the rate of H formation quantitatively, they were not successful in our work. It is always difficult to measure a product atom reliably in our apparatus, particularly when its loss rate on the walls is quite large as with H in this case. Brown and Thrush6 appear to have measured the H:O stoichiometry successfully, however, by working atj very low C2Hz flows, and found one H formed per 0 consumed in the over-all reactions Thus the 2H product in reaction I1 seems well confirmed, and all of the H recombines to Hz in our analyses a t long reaction times. Thus the simple sequence (I) followed by (11) giving the over-all reaction 2 0 C2Hz 2CO 2H advocated by Brown and Thrushe seems to account for nearly all of the observations. There is one difficulty in that our CO analysis (seven runs at four temperatures over the range 195-600°K) gave an average value of 1.6 f 0.1 CO molecules produced per C2H2 consumed, which is not quite the value 2 predicted by also roughly the mechanism. Arrington, et estimated a value of 1.5 for this ratio (with 0 in excess over C ~ H Z ) .Sullivan and Warneck6 obtained 1.5 with C2Hz:0 = 1 but about one CO per C2Hzwhen CzHz was in excess, which disagrees with our finding under similar CZHZexcess conditions. There is general agreement, however, that the CO production is somewhat below what one would expect from the operation
+
+
+ +
+
+
+
+
+
1185
0 f C2H2 RATECOEFFICIENT
---f
+
of (I) and (11) exclusively. We have no good explanation for this, except to note that there was a hint of possible product formation at mass peaks 29 and 30 in addition to 28 in some of our runs. This may have been due to a minor formation of H2CO which could account for some of the CO discrepancy. The results were not definite enough to be sure about this, however. H2C0 is a major product of the 0 C2H4reaction,l2JB but it is not easy to see how it could be formed in the 0 CzH2system. Comparison of the 0 CzHzmechanism with that of 0 CZH4 is of some interest. I n both cases n = 2 with the hydrocarbon in excess as it is for the kinetic runs in our type of experiment, although it has been shown12 that n > 2 when 0 is in excess over cZH4, and there is some indication of this being true in the CzHz case also7 as noted above. The initial step in the 0 CZH4 reaction is still a matter of some controversy.12J6 For reasons discussed elsewherelZ we tend to favor the slightly exothermic ( A H = - 4.5kcal/mol) reaction
+
+ +
+
+
rather than the kcal/mol) reaction
more
exothermic
(AH
=
-43
which is perhaps more frequently ~uggested.’~J~ The step following (111) and giving n = 2 is then considered to be reaction 11, the same as in the 0 CzH2 mechanism. The sequence (111) and (11) explains all of the major experimental findings in the 0 CzH4 system and gives the over-all reaction 2 0 CzH4-+ HzCO CO 2H, although there are complications with CZH4 which are probably not found with CzHz (possible H CzH4 reaction, for example). Comparison of the (111)-(11)mechanism for 0 CZH4 with (I)-(11) for 0 CZHZmakes them appear very similar. The details of the primary reactions would be quite different, however. From the viewpoint of transition-state theory, the intermediate complex in reaction I requires the migration of an H atom from one carbon to the other to give a ketenelike structure, i.e.
+
+ +
0
+ H-CEC-H
+ + + +
+
[
+
I>=C=O]
+ CH2
+ CO (1)
(16) H.Niki, E. E. Daby, and B. Weinstock, “Twelfth Symposium on Combustion,” The Combustion Institute, Pittsburgh, Pa., in press. (17) R. J. Cvetanovik, J. Chem. Phys., 23, 1375 (1955). Volume 78,Number 6 M a y io60
A. A. WESTENBERG AND N. DEHAA0
1186
The complex in reaction I11 must be like ethylene oxide
H
H
\
/
H
/ \ / \
H
+ H2CO
+ CH2
(111)
which would not require an H migration. The "entropy of activation" AS* would be more negative for 111than for I. Regarding the complexes as having the entropies of their respective normal molecules, the entropy changes are A&* = -29.4 eu and AS3" = -32.8 eu. On this basis one might expect the Arrhenius preexponential factor of (1) to be larger than that of (111), since the transition-state theoryls predicts that the factor A in the simple expression k = A exp( -E/RT) should be related to AS* by means of A a exp(AS*/R), so that A1/A3 = exp[(ASI* - AS,*)/R] NN 5. As noted in Figure 2, the Arrhenius plot for the 0 C Z H ~rate coefficient is curved like that for 0 C2H2. The Arrhenius expression in the midrange where it may be regarded as approximately linear is k3 = 5.0 X 10l2exp( - 1500/RT), while as previously noted the Arrhenius expression for reaction I in the same range is L1 = 2.0 X lOl3 exp( -3200/RT). Therefore the experimental ratio A1/Aa = 4 is in rough agreement with the prediction of transition-state theory. Although this is not a rigorous result, it tends to support reaction I11 as the primary 0 C2H4 step rather than (IV), since if the latter occurred, the complex would be like acetaldehyde after the migration of an H atom, Le.
+
+
+
H 0 -I-
\
C=C
/"
\
I
H
+ H-C-C=O
-+ CHs
+ HCO (IV)
Since A&* = -27.7 eu, this mechanism would predict A l / A 4 ~ 0 . 4which is markedly in conflict with the ratio 4 found experimentally. It should be mentioned that the intense chemiionization knownl9 to accompany the 0 CzHz reaction was manifested in the present experiments by way of the cyclotron resonance signal from free electrons20 detected with the esr spectrometer. Since the electron signal is many times broader than the 0 atom signal, there was no interference with the kinetic measurements themselves. The electron presence simply showed up as a measurable, smaoth shift in the zero position on the recorder as the CZHZinjector was moved out to longer reaction distances.
+
Implications for Other Work The present measurements of LI,and particularly its temperature dependence, may require the reevaluation of some earlier work. For example, a previously published paper by Westenberg and Fristrom2 seemed to offer evidence that the consumption of CzHz in premixed flames with 0 2 might not be attributed in an important way to reaction with 0 atoms. This was based on probe-sampling data from flames, with ew detection used to infer concentration profiles for 0 and H atoms. From a comparison of the CzHz disappearance rate and the minimum detectable concentration of 0 atoms, it was concluded that the 0 CZHZ rate coefficient at about 1000°K would have to be at least (5-10) X 10la to account for the observed CzHz consumption. This was regarded as a rather high value (although not as high as in the C2He and C2H4 cases) and cast some doubt on the importance of thiu route for C2Hz consumption. It now appears that the kl data in Figure 2 might easily extrapolate to a value well in excess of 10laat lOOO"K, so that the 0 C2H2 reaction could be important in these flames. This is in agreement with the original suggestion of Fenimore and Jones.' These data may also have significance for the interpretation of C2Hr02 shock-wave results. In the Bradley-Kistiakowsky mechanism3lz1the reaction 0 CzHz + C2H OH is assumed to occur in the hightemperature ( 1500-2000°K) chain-branching process, even though this step is probably endothermic by 10-11 kcal/mol. Reaction I was considered too slow to be irnportantlz2 based on the room-temperature result of Arrington, et al.,14and their finding essentially no temperature dependence for kl. It is now clear, however, that kl may be as high as 1014 at 2000°K, which is about three orders of magnitude larger than was supposed. In view of this, another look at the shock-wave mechanism seems in order.
+
+
+
+
Acknowledgment. The authors thank J. T. O'Donovan for assisting in the experimental aspects of this work. (18) 8. Glasstone, K. J. Laidlor, and H. Eyring, "The Theory of Rate Processes," McGraw-Hill Book Go., Inc., New York, N. Y., 1941, p. 21. (19) A. Fontijn, W. J. Miller, and J. M. Hogan, "Tenth Symposium on Combustion,'' The Combustion Institute, Pittsburgh, Pa., 1965.
p 545. (20) A. A. Westenberg. J . Chem. P h y s . , 43, 1544 (1965). (21) G. P. Glass, G. B . Kistiakowsky. J. V. Michael, and H. Nikl, ( b i d . , 42, 608 (1965). (22) Discussion by G. B. Kistiakowsky. "Tenth Symposium on Combustion," The Combustion Institute, Pittsburgh, Pa., 1965, p 520.
