The hydrazine flame

Abstract

Experimental data on the hydrazine flame speed are pressure-independent at low pressures. Thus, a second-order reaction mechanism is assumed to be controlling. Kinetics are given for a chain mechanism in which the NH 2 radical is formed by a pseudo-unimolecular process, tending to first order at high pressures, but second order at low pressures. The consequent set of flame equations can be solved using the chemical steady-state hypothesis. In this instance, however, the solution to the flame equation leaves both the flame speed and the overall chemical rate constant undetermined for the hydrazine decomposition. By inserting the experimental flame speed, a value for this rate constant is obtained. It is also possible to calculate the second-order rate constant for the formation of activated N 2H 4 radicals. The rate k a⋍3 × 10 18 exp(−60 000/RT) cm 3/mole sec. The unusually high value for this constant suggests energy transfer to the NN bond from many vibrational modes of N 2H 4 . Re-examination of earlier data of Szwarc on the formation of NH 2 radicals treated as a first-order process indicates that a second-order process may be implied from the data and, also, a value for k a⋍10 19 exp(−60 000/RT) cm 3/mole sec may be deduced. Investigation of the validity of the chemical steady state for free radicals in the hydrazine flame is made. It is determined that the flame may only be roughly approximated by the chemical steady state.