J. Phys. Chem. 1995, 99, 16192-16193
16192
COMMENTS Comment on “Kinetics of Chlorine Atom Reaction with Hydrogen Bromide” Avigdor Persky Department of Chemistry, Bar-Ilan University, Ramat Gan 52900, Israel Received: July 5, 1995 In a recent paper,’ Dobis and Benson report on the reinvestigation of the reaction C1+ HBr ---, HC1+ Br (1) at 298 K, employing an improved VLPR (very low pressure reactor) method. Their new value for the rate constant is (6.16 f 0.07) x cm3/(molecule s), which is about 60% higher than their earlier2VLPR value. In their paper they review other studies of this reaction, including the study from our lab~ratory.~ The significantly higher values obtained in most of the other ~ t u d i e s ~are - ~considered by them to be inaccurate. On the other hand, the single published result6 which seems to support their own results is presented by Dobis and Benson with very small error limits, (5.9 f 0.15) x lo-’, cm3/(molecule s), ignoring the statement made by the authors of ref 6 concerning the relatively large uncertainty (f25%) which is attributed to the dark reaction between C12 and HBr. In our study3 a competitive method was employed using as a reference the reaction C1+ C2H6
-
HC1+ C2H5
(2)
Values of the ratio of rate constants kl/k2 which were determined over a wide temperature range (222-504 K) were combined with available data for k2 in order to calculate values for k ~ . The value thus obtained for 298 K was kl = (1.05 f 0.10) x 10-I I cm3/(molecule s), without including the uncertainty in k2. Including the uncertainly in k2, the estimated uncertainty in kl was about twice as large. The value recommended for k2 in recent kinetic data evaluation^^.^ is nearly the same as the one used by us, and therefore our calculated value for kl remains unchanged. Dobis and Benson refer to our competition scheme as “an oversimplified competition scheme” and state that “since the conversion of C2H6 was set between 55% and 85% and all secondary reactions of ethyl radicals were neglected, this study may incur a serious error”. It is rather disappointing to see that they have completely ignored our detailed discussion of possible effects of secondary reactions of C2H5 radicals, as well as of Br atoms, on the results, and our conclusion that they are negligible under the conditions of our experiments. The secondary reactions which were considered in our paper are
+ C2H, - HBr + C2H5 C2H5+ HBr - C2H6+ Br C2H5+ C2H5- C2H6 + C2H, C2H5+ C1, - C,H,Cl + C1 Br
(3) (4)
(6)
(7) In our experiments we measured relative concentrations of HBr and C2H6, and therefore the secondary reactions which might affect our results are reactions 3-5 which either consume or produce these reagents. As discussed in our paper, the main concern is reaction 4, since reaction 3 has a very low rate constantg and reaction 5 is much slower than the competing reaction 7 (k5lk7 % 0.14; see ref 10, Table V). As will be shown below, reactions 6 and 7, which compete with reaction 4 on the C2H5 radicals, have higher rate constants than the rate constant for reaction 4. Another reaction which competes efficiently with reaction 4 and which was not included in our paper is the reaction In addition to reactions 6-8 which compete efficiently with reaction 4, C2H5 radicals are probably removed also by surface recombination reactions between two C2H5 radicals or between a C2H5 radical and a halogen atom. The most convincing proof that effects of secondary reactions were negligible under the conditions of our experiments was the fact that results at a given temperature were unchanged, within the reported error limits, on changing the experimental conditions significantly. At room temperature a large number of experiments were carried out in which flow rates and ratios of flow rates of the reagents HBr, C2H6, and Cl2 were varied over wide ranges without affecting the results. Moreover, part of the experiments were carried out under such surface conditions that the dissociation of Cl2 molecules to produce C1 atoms was much less efficient than in other experiments (or much more recombination of C1 atoms took place), and therefore several times higher flow rates of C12 had to be used, again without affecting the results. Large changes in the degree of conversion of the competing reactions 1 and 2 were also without any noticeable effect. At the time when our paper3 was published, the kinetic information concerning most of the reactions of C2H5 radicals mentioned above was rather scarce and very inaccurate. Since then, more accurate data have become available for all these reactions. Reported values for k4 at room temperature are (9.4 f 0.5) x and (8.1 f 1.6) x cm3/(molecule s).I1 Higher values were reported for k6, and they are (1.85 f 0.35) x lo-” l 2 and (1.65 f 0.25) x lo-” cm3/(molecule s).I3 Reported rate constants for reaction 7 are in the range (1.42.0) x lo-’] cm3/(molecule s) (see ref 10, Table Vl). Several determinations of kg gave very high values, in good agreement with each other, (2.4 f 1.2) x 10-10,’4 (2.9 f 0.6) x 10-’0,15 and (3.0 f 1.0) x cm3/(molecule s).I6 An exception is the much lower value (1.20 f 0.08) x lo-’ cm3/(molecule s) determined by Dobis and Benson’O using their VLPR method. An examination of the kinetic data for reactions 4 and 6-8 shows clearly that the rate constant for reaction 4 is much smaller than for the competing reactions 6-8, and therefore under the conditions of our experiments reaction 4 was not significant. This conclusion is consistent with the observation mentioned above concerning the insensitivity of the results to considerable changes in flow rates, ratios of flow rates, and the degree of conversion of the reagents. It should also be noted that our value for kl is in very good agreement with the value (1.02 f 0.15) x lo-” cm3/(molecule s) reported by Dolson
0022-3654/95/2099-16192$09.00/0 0 1995 American Chemical Society
Comments and Leone4 and also in reasonable agreement with the somewhat lower value (8.4 f 0.5) x lo-'* cm3/(molecule s) reported by Mei and Mooree5 We believe that our results were not affected by secondary reactions to any significant extent and that they are accurate to within the stated error limits.
References and Notes (1) Dobis, 0.;Benson, S. W. J . Phys. Chem. 1995, 99, 4986. (2) Lamb, J. J.; Kondo, 0.;Benson, S. W. J . Phys. Chem. 1986, 90, 941. (3) Rubin, R.; Persky, A. J . Chem. Phys. 1983, 79, 4310. (4) Dolson, D. A,; Leone, S. R. J . Phys. Chem. 1987, 91, 3543. ( 5 ) Mei, C. C.; Moore, C. B. J . Chem. Phys. 1977, 67, 3936. (6) Nicovich, J. M.; Wine, P. H. Int. J . Chem. Kinet. 1990, 22, 379. (7) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, Jr., R. F.; Ken, J. A.; Troe, J. J . PhJs. Chem. Ref. Data 1992, 21, 1125. (8) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina,
J. Phys. Chem., Vol. 99, No. 43, 1995 16193 M. J. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling; Evaluation Number 11 of the NASA Panel for Data Evaluation, JPL Publication 94-26, 1994. (9) Seakins, P. W.; Pilling, M. J.; Niiranen, J. T.; Gutman, D.; Krasnoperov, L. N. J . Phys. Chem. 1992, 96, 9847. (10) Dobis, 0.; Benson, S. W. J . Am. Chem. SOC. 1991, 113, 6377. (1 1) Nicovich, J. M.; Van Dijk, C. A,; Kreutter, K. D.; Wine, P. H. J . Phys. Chem. 1991, 95, 9890. (12) Timonen, R. S.; Gutman, D. J . Phys. Chem. 1986, 90, 2987. (13) Kaiser, E. W.; Wallington, T. J.; Andino, J. M. Chem. Phys. Lett. 1990, 168, 309. (14) Kaiser, E. W.; Rimai, L.; Wallington, T. J. J . Phys. Chem. 1989, 93, 4094. (15) Maricq, M. M.; Szente, J. J.; Kaiser, E. W. J . Phys. Chem. 1993, 97, 7970. (16) Seakins, P. W.; Woodbridge, E. L.; Leone, S. R. J . Phys. Chem. 1993, 97, 5633.
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