Kinetic studies of the reaction of C2H5 with O2 at 295 K

The reaction between C2H5 and O2 at 295 K has been studied with a flow reactor sampled by a mass spectrometer. With helium as the carrier gas the rate coefficient was found to increase from (1.2 ± 0.3) × 10−12 to (3.6 ± 0.9) × 10−12 cm3/s as [He] was increased from 2 × 1016 to 3.4 × 1017 cm−3. The importance of equation image has been determined from a knowledge of the initial C2H5 concentration together with a measurement of the C2H4 produced in reaction (5). F, the fraction of the C2H5 radicals removed by path (5), was found to decrease from 0.15 to 0.06 as [He] increased from 2 × 1016 to 3.4 × 1017 cm−3. The rate coefficient for reaction (5) was found to be independent of [He] and to have a value of (2.1 ± 0.5) × 10−13 cm3/s. The variation in F reflects the fact that k1b increases as [He] increases. These observations are taken as evidence for a direct mechanism for C2H4 production and a collision‐stabilized route for C2H5O2 formation. Calculations indicate that the high‐pressure limit for reaction (1b) is ∼4.4 × 10−12 cm3/s and that in the polluted troposphere the branching ratio for reactions (1b) and (5) will be ∼l20.