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 center dot + HBr -> C2H6 + Br-center dot (R1) and C2H5 center dot + (HI) -> C2H6 + I-center dot (R2) has been performed and led to the following Arrhenius expressions: k(1) = 3.69(+/- 0.95) x 10(-11) exp(-10.62(+/- 0.66)/RT), k(2) = 1.20(+/- 0.38) x 10(-11) exp(-7.12(+/- 1.059)/RT) in the temperature range 293-623 K (A/cm(3) 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 center dot free radicals. A minor correction to k(1) and k(2) for the wall loss of C2H5 center dot (k(w)) has been applied throughout the temperature range. The obtained results are consistent regarding both the disappearance of C2H5 center dot 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 center dot free radical resulting from the present kinetic measurements are discussed and point toward a slightly lower value for the standard heat of formation Delta H-f(298)degrees(C2H5 center dot) 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 Delta H-f(298)degrees(C2H5 center dot) = 117.3 +/- 3.1 kJ/mol resulting from an average of third law evaluations using S-298 degrees(C2H5 center dot) = 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 (Delta H-f(298)degrees(C2H5 center dot) = 119 +/- 2 kJ/mol) despite using two very different experimental techniques.