Study of the reaction Li + H2O over the temperature range 850–1000 K by time-resolved laser-induced fluorescence of Li(22PJ–22S1/2)

Abstract

The first direct study of the reaction between lithium atoms and water vapour is presented. Lithium atoms were produced by the pulsed photolysis of LiOH vapour in the presence of an excess of H₂O and N₂ both gas. The Li-atom concentration was monitored by laser-induced fluorescence of the metal atoms at λ= 670.7 nm using a pulsed nitrogen-pumped dye laser and boxcar integration of the fluorescence signal. Absolute second-order rate constants were obtained at T= 850, 900, 925, 950, 973 and 1000 K. A fit of these data to the Arrhenius form yields (2σ errors)k(T)=(5.6^{+ 4.3}_–2.4)× 10^–10 exp [–(7970 ± 544)/T] cm³ molecule^–1 s^–1.

The activation energy of 66.3 ± 4.5 kJ mol^–1 may be compared to estimates for the standard enthalpy change (between 850 and 1000 K) of this reaction which rage from 62 to 73 kJ mol^–1. To be compatible with our experimental data, and within the range of values derived from studies in flames, D°₀(Li—OH) probably lies in the range 437–441 kJ mol^–1.

The reverse reaction between LiOH and H is important in both the chemistry of Li in flames (T > 1500 K) and in describing the atmospheric chemistry of Li in the mesosphere (T= 200 K). Estimates for this rate constant were obtained in both temperature regimes through detailed balancing.