Temperature dependence of the rate constant for the reaction H + OH in liquid water up to 200 °C

Abstract

The temperature dependence of the rate constant, k₁, for the reaction H + OH → H₂O (1) has been determined in water up to 200 °C by pulse radiolysis of 10^–2 mol dm^–3 HClO₄ solutions. Values of k₁ were derived using the computer code FACSIMILE to simulate the measured absorbance vs. time profile of OH at 250 nm. Required values of k₂ and k₃ for the simultaneously occurring reactions H + H → H₂(2) and OH + OH → H₂O₂(3) were taken from the literature. Although k₂ has the temperature dependence of a diffusion-controlled reaction, a statistical spin factor of 0.25 must be included for k₂ to be described by the Smoluchowski equation. Conversely, a statistical spin factor of 1 is required to account for the values of k₁ and k₃. The temperature dependences of k₁ and k₃ are similar to one another but significantly less than that for k₂, consistent with the chemical reaction step becoming dominant at elevated temperatures in the reactions (1) and (3). These results are interpreted qualitatively in terms of transition-state theory.