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).