Kinetics of the HCN + NO2 Reaction behind Shock Waves

Article abstract of DOI:10.1021/j100212a026

The kinetics of the HCN + NO2 reaction has been studied behind incident shock waves ( T = 1329-1846 K, P = 1.7-12.6 atm) by using dilute mixtures of the reactants in argon.The rate of reaction was measured by monitoring the disappearance of NO2 absorption at 450 nm.The time-dependent emissions due to NO*, OH*, and NO2* were also followed at 237, 307.2, and 427.5 nm, respectively.The 427.5-nm emission was due primarily to the chemiluminescent recombination reaction O + NO (+M) <*> NO2* (+M) and provided a qualitative indication of oxygen-atom concentration during the reaction.The products of the reaction were determined by chemical analysis of samples heated by reflected shock waves and gas dynamically quenched.The reaction rate was independent of HCN concentration within experimental error: -d(NO2)/dt = 4.90E11 exp(-43300/RT)(Ar)^1.03(HCN)^-0.03(NO2)^0.65 mol cm^-3 s^-1.The results indicate that the reaction proceeds by a nonbranching chain process initiated by NO2 unimolecular dissociation, NO2 + M <*> NO + O + M, enabling an upper limit to be placed on the rate constant for any direct reaction between HCN and NO2.The emission measurements showed that there is a sudden increase in O atoms and electronically excited NO and OH at that point in the reaction when the NO2 is almost depleted, signaling the onset of a second stage of the reaction.Modeling calculations have been carried out to simulate the course to the reaction using elementary reactions and available rate data.The calculations suggest that the chain carriers in the HCN + NO2 reaction are O, OH, CN, and NCO, that significant amounts of O2 and HCN remain at the end of the first stage of reaction, and that the second stage begins because of NO2 depletion when the reaction O + NO2 <*> O2 + NO can no longer suppress the O atoms being generated by the reaction CN + O2 <*> NCO + O.