Temperature coefficients of rates of ethyl radical reactions with HBr and Br in the 228-368 K temperature range at millitorr pressures

The rates of the reactions of ethyl radicals with HBr (k(7)) and with Br atoms (k(8)) have been measured in the temperature range 228-368 K at millitorr pressures using the very low pressure reactor (VLPR) technique. The Arrhenius function for the H atom abstraction reaction is found to be k(7)=(1.43+/-0.06) x 10(-12) exp-[-(444 +/- 26)/RT] cm(3)/(molecule s), while the ethyl radical disproportionation with Br atom shows no temperature dependence. Its average value over the entire temperature range is k(8)=(1.18+/-0.05) x 10(-11) cm(3)/(molecule s). Reaction 7 is significantly slower than has been reported in the only other two direct measurements, both finding a negative activation energy for k(7) of from -1.0 to -1.1 kcal/mol. The small positive activation energy found in this work for k(7) fits standard models for H atom metathesis. Combination with all known kinetic information for ethane bromination gives an average reaction enthalpy of Delta H-7(0)=13.0+/-0.2 kcal/mol using both the second-and third-law thermochemical calculations. It sets the heat of ethyl radical formation to Delta(f)H(0)(C2H5)=28.40+/-0.25 kcal/mol and the bond dissociation enthalpy, DH0-(C2H5-H)=100.5+/-0.3 kcal/mol.