Kinetic studies of the reaction of C 2 H 5 with O 2 at 295 K

The reaction between C 2 H 5 and O 2 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 cm 3 /s as [He] was increased from 2 × 10 16 to 3.4 × 10 17 cm −3 . The importance ofhas been determined from a knowledge of the initial C 2 H 5 concentration together with a measurement of the C 2 H 4 produced in reaction (5). F , the fraction of the C 2 H 5 radicals removed by path (5), was found to decrease from 0.15 to 0.06 as [He] increased from 2 × 10 16 to 3.4 × 10 17 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 cm 3 /s. The variation in F reflects the fact that k 1b increases as [He] increases. These observations are taken as evidence for a direct mechanism for C 2 H 4 production and a collision‐stabilized route for C 2 H 5 O 2 formation. Calculations indicate that the high‐pressure limit for reaction (1b) is ∼4.4 × 10 −12 cm 3 /s and that in the polluted troposphere the branching ratio for reactions (1b) and (5) will be ∼l20.