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=k_b[Ar][N₂¹⁴]+k_b[Ar ][N₂¹⁵], which is a sum of two parallel reactions, was found to be compatible with the experimental results $k_{\mathrm{b}}{\text{=10}}^{\text{13.82±0.31}}\hspace{0.17em}\exp {\text{[−(116±5)×10}}^3\mathrm{/RT)\hspace{0.17em}}\mathrm{cc}\hspace{0.17em}{\mathrm{mole}}^{\text{−1}}·{\sec }^{\text{−1}}$ . 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 reaction $${\mathrm{N}}^{*}+{\mathrm{N}}_2\to {\displaystyle {lim}^{k_3}}\mathrm{N}+{\mathrm{N}}_2^{*}{\mathrm{,\hspace{0.17em}k}}_3{\text{=3×10}}^{11}\hspace{0.17em}\mathrm{cc}\hspace{0.17em}{\mathrm{mole}}^{\text{−1}}·{\sec }^{\text{−1}}$$ at 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.