The effect of isotopic substitution on the decomposition flame of hydrazine

Abstract

The mode of decomposition of hydrazine in the low-pressure flame has been investigated by a comparison of flame speeds for the N2D4 system with those of the previously investigated N2H4 system. Measurements were made using a closed-vessel technique, whereby flame speeds relative to the burned gas were determined by schlieren photography. For pure N2D4, flame speeds were measured at an initial temperature of 62 C in the pressure range 20–70 torr. In this region flame speed is virtually pressure independent, indicating an overall reaction order of 2. The comparison was extended by measuring flame speeds for binary mixtures containing inert diluents. Increasing amounts of two diluents, argon and nitrogen, were added to a fixed partial pressure (50.2 torr) of N2D4, and flame speed was measured over the entire flammability range. The effect of the added diluents is to decrease the flame speed with increasing partial pressure of diluent, and to extinguish the flame when the concentration of diluent exceeds ca. 50%. The effective activation energy was estimated from the dependence of flame speed on temperature for diluted mixtures. It is estimated to be 35 ± 2 kcal mole−1, based on calculated values of adiabatic flame temperature and thermal conductivity, assuming a stoichiometry identical to that for N2H4 decomposition. A large isotope effect of flame speed is apparent when the present results are compared directly with those of a previous investigation of the N2H4 flame, obtained in the same apparatus and under the same initial conditions. The speed of the hydrazine decomposition flame is halved when deuterium is substituted for hydrogen. This decrease in flame speed is explained in terms of transport properties, equilibrium thermodynamics, and kinetic factors, Isotopic changes in thermal conductivity contribute a factor of 1.1 to the total effect: isotopic changes in adiabatic flame temperature contribute a factor of 1.3–1.5; and kinetic isotope effects (kH/kD) contribute a factor of 1.5. The results have been applied to test the validity of two possible modes of hydrazine decomposition: by a chain reaction and by a unimolecular reaction in the falloff region. Both mechanisms show inconsistencies with the present measurements, and it is impossible to eliminate unambiguously either possibility. The present evidence, however, favors the chain reaction.