Reinvestigation of the Elementary Chemical Kinetics of the Reaction C2H5 • + HBr (HI) → C2H6 + Br• (I•) in the Range 293–623 K and Its Implication on the Thermochemical Parameters of C2H5 • Free Radical

A reinvestigation of the absolute rate constants of the metathesis reactions C2H5 • + HBr → C2H6 + Br• (R1) and C2H5 • + HI → C2H6 + I• (R2) has been performed and led to the following Arrhenius expressions: k 1 = 3.69(±0.95) × 10–11 exp(−10.62(±0.66)/RT), k 2 = 1.20(±0.38) × 10–11 exp(−7.12(±1.059)/RT) in the temperature range 293–623 K (A/cm3 molecule–1 s–1, E a/kJ mol–1). The study has been performed using a Knudsen reactor coupled to single-photon (VUV) photoionization mass spectrometer (SPIMS). Hydrocarbon free radicals have been generated externally before admission into the Knudsen reactor according to two different chemical schemes, enabling the generation of thermalized C2H5 • free radicals. A minor correction to k 1 and k 2 for the wall loss of C2H5 • (k w) has been applied throughout the temperature range. The obtained results are consistent regarding both the disappearance of C2H5 • and the formation of closed shell products (n-C4H10, C2H4, C2H6), indicating that the chemical mechanism is largely understood and complete. Thermochemical parameters for C2H5 • free radical resulting from the present kinetic measurements are discussed and point toward a slightly lower value for the standard heat of formation Δf H 298 °(C2H5 •) compared to some presently recommended values. On the basis of the present results and suitable data on the reverse reaction taken from the literature, we recommend Δf H 298 °(C2H5 •) = 117.3 ± 3.1 kJ/mol resulting from an average of “third law” evaluations using S 298 °(C2H5 •) = 242.9 ± 4.6 J/K mol. The present work yields a standard heat of formation in satisfactory agreement with the results obtained by W. Tsang (Δf H 298 °(C2H5 •) = 119 ± 2 kJ/mol) despite using two very different experimental techniques.