Mass Spectral Studies of Kinetics behind Shock Waves. II. Thermal Decomposition of Hydrazine

Abstract

The decomposition of hydrazine diluted in argon has been studied over the temperature range 1200° to 2500°K and pressure range 0.04 to 0.25 atm using a shock tube coupled to a time‐of‐flight mass spectrometer. The time‐resolved mass spectra (25 μsec) enable the simultaneous identification and determination of N₂H₄, NH₃, N₂, H₂, and the NH₂ radical, the five major species observed. The material balance is within the experimental error (10%) during reaction up to complete decomposition. The primary process in the dissociation of hydrazine is shown to be the rupture of the N–N bond to give NH₂ radicals. The observed rate constants for the disappearance of hydrazine are those expected of a unimolecular reaction: $${\mathrm{N}}_2{\mathrm{H}}_4+\mathrm{M}\to {\displaystyle {lim}^{k_1}}{\mathrm{N}}_2{\mathrm{H}}_4^{*},$$ $${\mathrm{N}}_2{\mathrm{H}}_4^{*}\to {\displaystyle {lim}^{k_2}}2{\mathrm{NH}}_2,$$ at or near the second‐order region where the collisional activation (1) is rate controlling. At the higher temperatures, the results are consistent (within experimental error) with the very simple reaction scheme of Reaction (1) and (2) followed by $${\mathrm{NH}}_2+{\mathrm{NH}}_2\to {\displaystyle {lim}^{k_3}}{\mathrm{NH}}_3+\mathrm{NH},$$ $$\mathrm{NH}+\mathrm{NH}\to {\displaystyle {lim}^{k_{4\mathrm{a}}}}{\mathrm{N}}_2{\mathrm{H}}_2^{*}\to {\displaystyle {lim}^{k_{4\mathrm{b}}}}{\mathrm{N}}_2+{\mathrm{H}}_2.$$ At the lowest temperatures the reaction NH₂+N₂H₄→N₂H₃+NH₃ appears to become important. From the experimental concentrations, a value of $$k_3{\text{=2.5×10}}^{13}\hspace{0.17em}\mathrm{cc/mole-sec}{\text{\hspace{0.17em}(2000}}^{\circ }\mathrm{K})$$ is obtained. An order‐of‐magnitude estimate gives $$k_4{\text{∼10}}^{14}\hspace{0.17em}\mathrm{cc/mole-sec}{\text{\hspace{0.17em}(2000}}^{\circ }\mathrm{K}\mathrm{).}$$