Direct observation of equilibration in the system hydrogen atom + ethylene .dblharw. ethyl radical. Standard enthalpy of formation of the ethyl radical

Abstract

The approach to equilibrium in the reaction H + C[sub 2]H[sub 4] [r equilibrium] C[sub 2]H[sub 5] has been studied at 800 K in He. Exciplex laser flash photolysis at 193.3 nm of ethene-helium mixtures was used to generate H atoms, which were detected by time-resolved resonance florescence. Rate coefficients k[sub a] and k[sub d] for the forward and reverse reactions were deduced from measurements of the equilibrium constant and relaxation rate coefficient at nine pressures in the range 97 [le] P/Torr [le] 600. Values of the equilibrium constant, K[sub P], were combined with the entropy of reaction (calculated from literature data), [Delta]H[degrees](H), and [Delta]H[degrees](C[sub 2]H[sub 5]) = 120.2 [+-] 0.9 kJ mol[sup [minus]1]. The Troe factorization technique was applied to values of k[sub a] from this work and the literature to find Troe parameters for the forward reaction over the range 285 [<=] T/K [<=] 800. Master equation methods were developed to describe the association reaction under both reversible and irreversible conditions. Consideration of a model three-level (Lindemann) system demonstrated that association rate coefficients obtained from an analysis of an irreversible association reaction and from decomposition of the relaxation rate coefficient for a reversible reaction into forward and reversemore » rate coefficients are equivalent provided the steady-state approximation can be applied to the energized adduct. A similar conclusion was reached analytically from consideration of an extended distribution of energized molecules with a strong collision assumption and numerically for weak collisions using a master equation analysis. The fall-off data were analyzed using a master equation models, and the values of k[sub a] returned agreed to better than 1%. 38 refs., 6 figs., 7 tabs.« less