The approach to equilibrium in the reaction H + C2H4 ⇌ C2H5 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 fluorescence. Rate coefficients ka and kd 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 ≤ P/Torr ≤ 600. Values of the equilibrium constant, Kp, were combined with the entropy of reaction (calculated from literature data), ΔH°f,298(H), and ΔH°f,298(C2H4) to give a value of ΔH°(C2H5) = 120.2 ± 0.9 kJ mol-1. The Troe factorization technique was applied to values of ka 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 reverse 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 both master equation models, and the values of ka returned agreed to better than 1%. © 1993 American Chemical Society.
