ABSTRACT
The branching cycle involving the radicals H, OH and O in the H2+O2 reaction involves the three elementary steps
H + O2 → OH + O (A3.14.1) O + H2 → OH + H (A3.14.2)
OH + H2 → H2O + H. (A3.14.3) In steps (1) and (2) there is an increase from one to two ‘chain carriers’. (For brevity, step (x) is used to refer to equation (A3.14.x) throughout.) Under typical experimental conditions close to the first and second explosion limits (see section A3.14.2.3), steps (2) and (3) are fast relative to the rate determining step (1). Combining (1) + (2) + 2× (3) gives the overall stoichiometry
H + 3H2 + O2 → 3H + 2H2O so there is a net increase of 2 H atoms per cycle. The rate of this overall step is governed by the rate of step (1), so we obtain
d[H]/dt = +2k1[O2][H] where the + sign indicates that the rate of production of H atoms increases proportionately with that concentration.
