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: N2H4+M→ lim k1N2H4*,N2H4*→ lim k22NH2,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 NH2+NH2→ lim k3NH3+NH,NH+NH→ lim k4aN2H2*→ lim k4bN2+H2.At the lowest temperatures the reaction NH2+N2H4→N2H3+NH3 appears to become important. From the experimental concentrations, a value of k3=2.5×1013 cc/mole−sec (2000∘K)is obtained. An order-of-magnitude estimate gives k4∼1014 cc/mole−sec (2000∘K).