NOTES
1749
other. Further calculations are in progress which will elucidate the interaction and accumulation of such “single-bond defects” in n-paraffins and polyethylene.
Rate Constant for the Reaction of Oxygen Atoms with Acetylene’
by J. 0. Sullivan and Peter Warneck Geophysics Corporation of America, Bedford, Mas8achuset2s (Received September 11, 1964)
The oxidation of acetylene has been the subject of several recent investigations involving a variety of experimental methods. Fenimore and Jones2 studied the process in flames, Norrish, et aL18applied the method of flash photolysis, and Kistiakowsky and collaborator^,^--^ in a series of papers, reported on experiments with shock tubes. From these studies it has become apparent that a key reaction in the over-all mechanism of acetylene oxidation is the reaction of acetylene with atomic oxygen. Until recently,’ no information has been available concerning the associated rate constant or the ensuing reaction products, and the present work was undertaken in an effort to provide some of the desired data. The reaction of oxygen atoms with acetylene was studied in a rather direct manner employing a mass spectrometer coupled to a fast flow system. A detailed description of the apparatus has been given elsewhere.8 Briefly, it consisted of a microwave discharge to produce oxygen atoms, a cylindrical flow reactor incorporating a movable inlet for the admixture of acetylene, and a continuous sampling mass spectrometer equipped with an ion source specifically designed to favor the detection of free radicals. The capability of the instrument in this respect has been demonstrated previously.8 The oxygen atom concentration was monitored by the corresponding signal a t mass number 16 improving a technique first reported by Phillips and Schiff.9 The contribution of 0 2 to the total signal a t this mass number was minimized in two ways: (a) by the use of a 10: 1 mixture of helium and oxygen or (b) by discharging nitrogen and adding sufficient amounts of nitric oxide behind the discharge to convert the resulting N atoms quantitatively into oxygen atoms by virtue of the rapid reaction, N NO ---t N2 0. The feasibility of these procedures was demonstrated recently also by Klein and Herron.1°
+
+
Signal sensitivities were determined a t the beginning and a t the end of each series of runs, making use of the established titration techniquess for the measurement of oxygen atom concentrations. Variations of the sensitivities and/or of the involved partial pressures were occasionally experienced and all results in which such errors could not be taken into account by corrections were discarded. The reaction between 0 atoms and acetylene was accompanied by the well-known associated chemiluminescence. Carbon monoxide was found to be the major reaction product, with hydrogen and water vapor being formed in smaller amounts. Carbon dioxide was not observed except as a product of the discharge. The amount of CO production is thought to be most significant. With a 1:1 ratio of acetylene to oxygen atom concentration, about 1.5 times as much CO was produced as acetylene was consumed, but with acetylene in excess, the amount of CO production was in all cases found to equal that of acetylene consumption, even when O2 was the major constituent in the reaction system. This is exemplified in Figure 1, which shows the time dependence of reactant and CO concentration for a series of experiments employing a helium-oxygen mixture and initial concentrations of 28 and 4.4 p , respectively, for acetylene and atomic oxygen. In addition, it is evident from Figure 1 that despite the large excess of acetylene concentration, the consumption of oxygen atoms is appreciably greater than that of acetylene. A similar behavior has been observed also in all other experiments, which indicates that oxygen atoms are consumed in one or more subsequent reactions. Figure 2 shows a plot of the time-averaged excess consumption of 0 atoms, f, vs. the initial acetylene to oxygen atom concentration ratio. Since increasing this ratio favors the initial O-C2H2 reaction over all subsequent reactions, and since f exhibits an asymptotic decline approaching zero for a large excess of acetylene, it can be con(1) This work wm partially supported by the Advanced Research Projects Agency under Contract No. AF19(628)-3320. (2) C. P. Fenimore and G. W. Jones, J. Chem. Phys., 39, 1514 (1963). (3) R. G. W. Norrish, G. Porter, and B. A. Thrush, Proc. Roy. SOC. (London), A216, 165 (1953). (4) J. N. Bradley and G. B. Kistiakowsky, J . Chem. Phys., 35, 264 (1961). (5) C. W. Hand and G. B. Kistiakowsky, ibid., 37, 1239 (1962). (6) G. B. Kistiakowsky and L. W. Richards, ibid., 36, 1707 (1962). (7) C. A. Arrington, W. Brennen, G . P. Glass, J. V. Michael, and H. Niki, Discussion Comment, Tenth Symposium on Combustion, Cambridge, Aug. 1964. (8) J. 0. Sullivan and P. Warneck, Ber. Bunsenges, in press. (9) L. F. Phillips and H. I. Schiff, J . C h . Phys., 36, 1509 (1962). (10) F. S. Klein and J. T. Herron, ibid., 41, 1285 (1964).
Volume 69,Number 6 M a y 1966
NOTES
1750
TIME (millirreondr) 1.0
I
t
3
4
5
6
7
1
1
I
1
1
I
I
providing justification for the applied procedure. In Table I are shown the averaged rate constants determined from eleven series of runs, together with the employed experimental parameters. These k values exhibit a larger spread than has been noted in the evaluation of a single series of runs, but again no significant trends are noticeable. The averaged rate concc./moIecule sec., is in stant, k = 1.5 i 0.3 X approximate agreement with that reported by Arrington, et aL7
JET INLET POSITION lcm)
Table I: Rate Constant for the Reaction of 0 Figure 1. Concentration-time dependence in the oxygen atom-acetylene reaction: W, 0 atom concentration; A, co concentration; 0, acetylene consumption; [C2H2]o= 28 p.
Partial pressure, (CzHdo (He)
(010
3.3 4.5 4.4 4.4 4.6 2.3 6.0 2.6 3.5 11 10.8
42.2 28 4.56 28 17.4 6.1 8.1 31 1.68 29 5.62
544 462 435 435
... ... ... ... ...
...
...
~.r
(02)
... ... ...
... ...
... .. . ... ...
,
500 270 530 245 215 940 765
Av.
Figure 2. Excess 0 atom consumption, f, as a function of the initial acetylene :oxygen atom concentration ratio.
eluded that the reaction between 0 atoms and acetylene involves the reactants in a l :l ratio. Evidence for bimolecular kinetics was obtained also from the determination of the associated rate constant. In view of the excess 0 atom consumption, use was made of the integrated rate expression
which is applicable regardless of the rate law governing the removal of oxygen atoms, so long as the reaction between oxygen atom and acetylene is the major acetylene-consuming reaction. In each series of experiments, rate constants were determined for various reaction times, but none of the sets of values obtained showed any trends. The lack of trends substantiates (a) the above conclusion concerning bimolecular kinetics, and (b) the assumption that acetylene is consumed mainly in the reaction with atomic oxygen, The Journal of Physical Chemistry
-
...
15 13.5 13.0 13.0
...
(Nz)
=
+ CeHz
b, cc./molecules sea.
x lox* 1.13 1.29 1.79 1.37 1.85 1.88 1.83 1.24 1.22 1.83 1.35
1.5 i 0.3 X 10-1*
The observation that with excess acetylene the amount of CO production is equal to that of acetylene consumption, regardless of the acetylene concentration or the amount of 0 atom consumption, strongly indicates that CO is a direct product of the O-CZH, reaction, and the additional evidence for bimolecular kinetics suggests the reaction path
0
+ CzHz+CO + CH2 - 53 kcal.
This reaction has previously, been proposed by Fenimore and Jones.2 It is worth noting that ketene is a probable reaction intermediate. By use of the matrix isolation method a t a temperature of 20°K., Haller and Pimentel" have shown that ketene formation occurs due to the attachment of oxygen atoms to acetylene. From photolysis work,12 energy-rich ketene is known to decompose, yielding CO and methylene, so that the pathway of product formation in the reaction can be qualitatively understood. (11) I. Haller and G. C. Pimentel, J. Am. Chem. SOC.,84, 2855 (1962). (12) H. M. Frey in "Progress in Reaction Kinetics, 11," G. Porter, Ed., The Maomillan Co., New York, N. Y., 1963,p. 139.
