Kinetics of the Reactions of Chlorine Atoms with C₂H₄ (<i>k</i>₁) and C₂H₂ (<i>k</i>₂): A Determination of Δ<i>H</i>_f,298° for C₂H₃

Abstract

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 [ΔHf,298°(C2H3) = 70.6 ± 0.4 kcal/mol]. Using the same data with the second law yields ΔHf,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.