Kinetics of the Homogeneous Exchange Reaction: 14–14N2+15–15N2⇋214— 15N2. Single-Pulse Shock-Tube Studies

The kinetics of the homogeneous exchange reaction in molecular nitrogen highly diluted in argon was studied in a single-pulse shock tube over a wide range of temperatures, initial pressures, and compositions. The reaction order with respect to the total nitrogen, argon, and the over-all reaction order were determined. A rate law: Rate=kb[Ar][N214]+kb[Ar ][N215], which is a sum of two parallel reactions, was found to be compatible with the experimental results kb=1013.82±0.31 exp[−(116±5)×103/RT) cc mole−1·sec−1. It was shown that the exchange reaction does not proceed via a radical chain. A mechanism based on vibrational excitation of the nitrogen molecule to a critical vibrational level as a rate determining step is proposed. A rate constant, computed from available vibrational relaxation data in nitrogen agrees very well with the observed rate constant. It was found that slight amounts of oxygen, up to O2/N2∼0.01, had very little effect on the reaction rate. In the presence of higher oxygen concentration, O2/N2∼0.5, the exchange proceeds mainly via an atom displacement mechanism. An approximate rate constant for the reactionN*+N2→ lim k3N+N2*, k3=3×1011 cc mole−1·sec−1at 3400°K was obtained. The temperature range involved in this study, i.e., 3200°—3800°K, is the highest ever reported in a single-pulse shock-tube study. The gas-dynamic conditions and an innovation in the design of the shock tube are briefly described and discussed.