Kinetic Studies of Hydroxyl Radicals in Shock Waves. I. The Decomposition of Water between 2400° and 3200°K

Abstract

The decomposition of water vapor has been studied at temperatures between 2400° and 3200°K generated in plane shock waves by following changes in the concentration of OH with time. Specific radiation absorbimetry permits these measurements to be made with microsecond resolution. The source of OH radiation used was a flash lamp containing water vapor. Measurements were made on the gas behind reflected shock waves in argon at initial pressures near 50 mm Hg and containing the reactant, H2O vapor, in amounts of the order of 1 mole %. The shock tube and associated electronic and optical equipment are described. Problems of purity, chemical analysis, and reduction of data are discussed. Absorption by OH was calibrated with the equilibrium mixtures resulting from the decomposition of H2O and from the reaction of H2 and O2. The rate of formation of OH from H2O is proportional to the H2O concentration and nearly independent of the argon pressure. The effective activation energy is about 50 kcal/mole. Addition of 2% O2 to the mixture approximately doubles the rate of OH formation and preserves the 50 kcal activation energy. Addition of 2% H2 decreases the rate and increases the activation energy to about 80 kcal/mole. Possible reaction mechanisms are considered. The initial rate of appearance of OH cannot be interpreted in terms of the simple dissociation of H2O into H and OH, and it seems necessary to consider a relatively low-energy path involving H2O2 or the HO2 radical as an intermediate.