| Abstract
| - Aqueous-phase halogen reactions play an important rolein tropospheric ozone depletion that is observed during Arcticsunrise where bromine chloride is a key intermediate.The temperature dependencies of BrCl(aq) equilibrationwith BrCl2-, HOBr(aq), Br2(aq), Cl2(aq), HOCl(aq), Br-, andother species (Br3-, Br2Cl-, Cl3-, OBr-, and OCl-) aredetermined as a function of Cl- concentration from pH 0to pH 7. Values for K1 (=[BrCl2-]/([BrCl(aq)][Cl-])) at μ = 1.0M are 3.8 M-1 at 25.0 °C, 4.7 M-1 at 10.0 °C, and 5.5M-1 at 0.0 °C, with ΔH1° = −9.9 kJ mol-1 and ΔS1° =−22 J K-1 mol-1. BrCl(aq) hydrolysis equilibria have littleor no temperature dependence with Kh1 (=[HOBr(aq)][Cl-][H+]/[BrCl(aq)]) = 1.3 × 10-4 M2 from 25.0 to 5.0 °C, μ =1.0 M. When conditions are adjusted to give a rapid partialhydrolysis of BrCl in equilibrium with HOBr and Cl- atp[H+] 4.31, a relatively slow reaction (kobsd = 2.4 s-1) toform HOCl and Br- is observed. This takes place via BrClreaction with Cl- to form Cl2, which hydrolyzes in the rate-determining step to give HOCl. On the other hand, the rateof complete BrCl hydrolysis to form HOBr and Cl- atp[H+] 6.4 is extremely rapid with a first-order rate constantof 3.0 × 106 s-1 at 25.0 °C. The reverse reaction betweenHOBr, Cl-, and H+ has a rate constant of 2.3 × 1010M-2 s-1, so that in seawater, where [Cl-]/[Br-] = 700,the formation of BrCl is much faster than the formation ofBr2 from HOBr, Br-, and H+. Rapid formation of BrCl(aq)and its subsequent reaction with Br- is a viable pathway togive Br2(aq). Photolysis of Br2(g) is believed to initiatethe reactions associated with ozone depletion.
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