TEMPERATURE COEFFICIENTS OF RATES OF ETHYL RADICAL REACTIONS WITH HBRAND 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) tech nique. The Arrhenius function for the H atom abstraction reaction is f ound 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 s hows no temperature dependence. Its average value over the entire temp erature range is k(8)=(1.18+/-0.05) x 10(-11) cm(3)/(molecule s). Reac tion 7 is significantly slower than has been reported in the only othe r two direct measurements, both finding a negative activation energy f or k(7) of from -1.0 to -1.1 kcal/mol. The small positive activation e nergy found in this work for k(7) fits standard models for H atom meta thesis. Combination with all known kinetic information for ethane brom ination gives an average reaction enthalpy of Delta H-7(0)=13.0+/-0.2 kcal/mol using both the second-and third-law thermochemical calculatio ns. 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.