Kinetics of the reactions of the hydroxyl radical with molecular chlorine and bromine

Abstract

The rates of the reactions of OH with Cl₂ and Br₂: OH + Cl₂→ HOCl + Cl, ΔH^⊖₂₉₈= 4.3 kJ mol^–1(1), OH + Br₂→ HOBr + Br, ΔH^⊖₂₉₈=–38.05 kJ mol^–1(2) have been investigated over a range of temperatures between 253 and 333 K by using a discharge-flow resonance-fluorescence technique. At 293 K the rate constants for reactions (1) and (2) are k₁=(6.8 ± 1.0)× 10^–14 cm³ molecule^–1 s^–1, k₂=(3.4 ± 1.2)× 10^–11 cm³ molecule^–1 s^–1. The rate constant for the reaction of OH with Cl₂ increases with temperature, and can be fitted between 253 and 333 K by the following Arrhenius expression: k₁=(1.7^+4.4_–1.2)× 10^–12 exp –(911 ± 373)/T cm³ molecule^–1 s^–1. For the reaction of OH with Br₂, the data are consistent with a zero activation energy, although a small activation energy (not exceeding 5 kJ mol^–1) could also be supported.