Kinetics of the reactions of active nitrogen with oxygen and with nitric oxide

Abstract

The rate of decay of nitrogen atoms in a fast-flow system in the presence of oxygen has been studied between 412 and 755°K. Nitrogen atom concentrations were estimated by titration with nitric oxide. The slow primary step can be represented by N + O 2 = NO + O, (1) while the much more rapid secondary reaction (2) removes the nitric oxide formed in reaction (1) N + NO = N 2 + O. (2) Reaction (1) was found to be first order in both nitrogen atom and oxygen molecule concentrations, and k 1 could be represented by the expression k 1 = 8.3 x 10 12 exp (— 7100/ RT ) cm 3 mole -1 s -1 between 412 and 755 °K. Under conditions of large oxygen flow rates and at high temperatures the air afterglow continuum was observed with low but easily measurable intensity in the gaseous products of reaction of oxygen with active nitrogen. Both nitric oxide and oxygen atoms are therefore present, and not all the nitric oxide formed in reaction (1) is consumed in reaction (2). These nitric oxide concentrations were determined by measuring the intensity of the air afterglow with a photomultiplier cell, which was calibrated by observation of the increase in the air afterglow intensity when known quantities of nitric oxide were added between the first mixing point and the photomultiplier. In this way a value of k 2 = 3.0 x 10 13 exp( — 200/ RT ) cm 3 mole -1 s -1 was determined. The mean value of k 2 between 476 and 755 °K was 2.5 x 10 13 cm 3 mole -1 s -1 , and was practically independent of temperature over this range, corresponding to a reaction occurring at about one sixth of the bimolecular collision frequency. It can be shown that both reactions (1) and (2) are expected to proceed through transition complexes having very similar molecular constants and vibration frequencies to those of nitrogen dioxide. However, the ratio of the frequency factors calculated on this basis, A 1 / A 2 = 1.4, was much larger than the experimentally determined value of 0.3, and this discrepancy is outside the limits of experimental error.