Rate and Product Measurements for the Reactions of OH with Cl2, Br2, and BrCl at 298 K. Trend Interpretations

Article abstract of DOI:10.1021/j150669a045

The reaction rate constants for OH + Cl2, Br2, and BrCl are determined to be 6.70*10^-14, 5.28*10^-11, and 1.49*10^-12 cm³ molecule^-1 s^-1, respectively.Rate measurements are carried out at 298 K in He by using a discharge flow apparatus.Two OH sources are used: H + NO2, and F + H2O.OH decays are monitored by resonance fluorescence at 309 nm; Cl and Br are measured simultaneously with OH by resolved resonance fluorescence at 134.7 and 157.7 nm, respectively.Measurement of OH + BrCl requires corrections for Cl2 and Br2 which exist in equilibrum with BrCl.Atom yields from the title reactions are compared to the reference reactions H + Cl2 and Br2 and analyzed by a computer model of the reaction kinetics.The products in each case are found to be the halogen atoms (Cl and Br), and not the halogen oxide radicals (ClO and BrO).Using the observed, forward reaction rate constants and the equilibrum constants derived from thermodynamic data, we calculate the rate constants of the reverse reactions, Cl + HOCl, Br + HOBr, and Cl + HOBr, to be 2.5*10^-12, 2.4*10^-17, and 3.7*10^-14 cm³ molecule^-1 s^-1, respectively.The reactivity of OH toward the three halogen molecules is related to the ionization potentials of the halogens, lower ionization potentials promoting faster reactions.In addition, a comparison of the results obtained here with the rate constant measurements of the radical-halogen reactions indicates that a radical's reactivity correlates with its electron affinity.On the basis if these trends, the activation energies of the title reactions are estimated to be 2.1, 0.0, and 3.4 kcal mol^-1, respectively.The dependence of the rate constant on the ionization potential and electron affinity of the reactants may serve as a guide for estimating the rates of similar radical-molecule reactions.