J. Phys. Chem. 1996, 100, 4111-4119
4111
Kinetics of the Reactions of Chlorine Atoms with C2H4 (k1) and C2H2 (k2): A Determination of ∆Hf,298° for C2H3 E. W. Kaiser* and T. J. Wallington* Ford Motor Company, Research Laboratory, Mail Drop 3083/SRL, Dearborn, MI 48121-2053 ReceiVed: October 30, 1995; In Final Form: NoVember 20, 1995X
The rate constant (k1) for the reaction of Cl atoms with C2H4 has been measured as a function of pressure (0.2-100 Torr) at 297 K using the relative rate technique in two reactors with either FTIR or GC analysis. The results of these and previous experiments (100-3000 Torr) can be described to within (10% by a Troe expression with the limiting rate constants k1(∞) ) (3.2 ( 0.15) × 10-10 cm3 molecule-1 s-1; and k1(0) ) (1.42 ( 0.05) × 10-29 cm6 molecule-2 s-1 using Fcent ) 0.6. The stated uncertainties are statistical only, and the true uncertainties in the limiting rate constants must include uncertainty in Fcent. Temperature-dependent (297-383 K) measurements of k1 were carried out in the low-pressure regime (0.2-2 Torr) yielding the rate constant expression k1(0) ) (1.7 ( 0.3) × 10-29(T/298)-3.28 cm3 molecule-1 s-1. The rate constant, k1b, for the abstraction channel of reaction 1 to form the vinyl radical was determined at 300, 343, and 383 K. This measurement of k1b in combination with literature values of k-1b allows a determination of the heat of formation of the vinyl radical by the third law method [∆Hf,298°(C2H3) ) 70.6 ( 0.4 kcal/mol]. Using the same data with the second law yields ∆Hf,298°(C2H3) ) 69.6 ( 1.6 kcal/mol. These measurements agree satisfactorily with a recent negative-ion photoelectron spectroscopy determination of this quantity (71.6 ( 0.8 kcal mol-1). The pressure dependence of the rate constant (k2) for the reaction of Cl with C2H2 was determined over the range 0.3-700 Torr. The results of these and previous experiments (100-6000 Torr) can be described to within (10% by a Troe expression with the limiting rate constants k2(∞) ) (2.0 ( 0.1) × 10-10 cm3 molecule-1 s-1 and k1(0) ) (6.1 ( 0.2) × 10-30 cm6 molecule-2 s-1 using Fcent ) 0.6.
Introduction The pressure-dependent reactions of Cl atoms with both C2H4 and C2H2 have been examined at 295 K in our laboratory previously.1 However, data were not obtained at pressures below 25 Torr. In the present experiments, we have carried out relative rate measurements of these rate constants at total pressures from 0.2 to 700 Torr using both FTIR and GC analyses to determine their pressure dependence relative to pressureindependent reference reactions. The primary motivation for carrying out measurements at low-pressure was to examine the possibility that the abstraction channel for the reaction of Cl with C2H4 (reaction 1b) might be observed at sufficiently low total pressure:
C2H4 + Cl (+M) ) C2H4Cl (+M)
(1a)
C2H4 + Cl ) C2H3 + HCl
(1b)
A measurement of k1b will assist in determining the heat of formation of the vinyl radical. The heat of formation of C2H3 has been the subject of experiments by different experimental techniques. However, as discussed by Ervin et al.2 and by Berkowitz et al.,3 there remains a discrepancy of ∼5 kcal mol-1 in the heat of formation of C2H3 determined by radical kinetics (66.9 ( 0.3 kcal mol-1) and by negative-ion photoelectron spectroscopy (71.6 ( 0.8 kcal mol-1). The kinetics determination is based in part on measurements of k1b by Parmar and Benson4 and by Dobis and Benson5 using the very low pressure reactor (VLPR) technique. The rate constants obtained in these two studies were k1b ) (5.66 ( 0.7) × 10-13 cm3 molecule-1 s-1 at 293 K and (5.0 ( 0.17) × 10-13 cm3 molecule-1 s-1 at 298 K, respectively. On the basis of these measurements and X
Abstract published in AdVance ACS Abstracts, February 1, 1996.
0022-3654/96/20100-4111$12.00/0
our previous measurement of the pressure dependence of reaction 1a, the rate of reaction 1b will become equal to that of (1a) at a pressure of approximately 1 Torr. Thus, the pressure dependence of reaction 1 would be influenced significantly by the abstraction reaction at pressures below approximately 5 Torr, and at sufficiently low-pressure the rate constant of reaction 1 (k1 ) k1a + k1b) will become pressure independent at the value of k1b. In addition, the low-pressure measurements on reactions 1 and 2, reported herein,
C2H2 + Cl (+M) ) C2H2Cl (+M)
(2)
provide a 100-fold extension of the pressure range over which measurements have been made previously and consequently give better determinations of the low-pressure limiting rate constants for both reactions. Experiment Two experimental setups were used; both have been described previously. The first system6 consisted of a Mattson Instruments Inc. Sirius 100 FT-IR spectrometer interfaced to a 140 L, 2 m long, evacuable Pyrex chamber. White-type multiple reflection optics were mounted in the reaction chamber to provide a total path length of 26.6 m for the IR analysis beam. The spectrometer was operated at a resolution of 0.25 cm-1. Infrared spectra were derived from 32 coadded interferograms. Reagents and products were quantified by fitting reference spectra of the pure compounds to the observed product spectra using integrated absorption features. Reference spectra were obtained by expanding known volumes of the reference material into the long pathlength cell. C2H4, C2H2, 1,2-C2H4Cl2, C2H3Cl, CH4, C2H5Cl, CH3Cl, CHCl3, and C2H6 were identified and quantified using features over the following wavelength ranges: 8501050 and 1850-1950; 660-800; 700-770 and 1200-1300; © 1996 American Chemical Society
4112 J. Phys. Chem., Vol. 100, No. 10, 1996 850-1000; 1200-1400; 1240-1520; 1300-1600; 700-1250; and 800-980 cm-1, respectively. Systematic uncertainties associated with quantitative analyses using these reference spectra are estimated to be
