N2 + O2 ----> 2 NO
Kp = [p(NO)]2 t [p(N2).p(O2 )]
- the equilibrium constant for this reaction Kp is about 5 x 104 at 1800 C (the hottest part of a bunsen flame), which corresponds to about 1% conversion of NO from atmospheric N2 and O2
The reaction is far too slow to attain equilibrium in a flame, and some of the NO formed decomposes in the cooler parts of the flame, but enough NO escapes from flames (e.g. in internal combustion engines) to contribute to acid rain and the formation of smog in sunshine via aerial oxidation to NO2.
Small amounts of NO are also formed in lightning discharges.