Lithium Hydroxide and the Dissociation Kinetics of Water Vapour

Abstract

The chemical relaxation of shock-heated water vapour in 8–10 % mixtures with argon has been studied over a temperature range of 2000–2400 K. The growth of H atom concentration was followed by introducing LiOH particles into the sample gas and observing the Li concentration profile by atomic absorption spectroscopy. By comparing H concentration profiles with computer simulations, values of the rate constant for H2O + M → H + OH + M were obtained. The results represent an activation energy of 439 ± 42 kJ mol–1(105 ± 10 kcal mol–1) and a pre-exponential factor of (1.8 ± 0.6)× 1016 cm3 mol–1 s–1 for the case M = H2O. The kinetic assumptions of the LiOH method for measuring H concentrations in flames were thereby confirmed experimentally down to relatively short times after thermal excitation. Deviations in the early stages of the experimental profiles were attributed to the reaction LiOH + M → Li + OH + M and the magnitude of the deviations indirectly suggests an activation energy for this reaction of about 377 kJ mol–1(90 kcal mol–1).