Equilibrium and kinetics of bromine chloride hydrolysis

Aqueous-phase halogen reactions play an important role in tropospheric ozon e depletion that is observed during Arctic sunrise where bromine chloride i s a key intermediate. The temperature dependencies of BrCl(aq) equilibratio n with BrCl2-, HOBr(aq), Br-2(aq), Cl-2(aq), HOCl(aq), Br-, and other speci es (Br-3(-); Br2Cl-, Cl-3(-), OBr-, and OCl-) are determined as a function of Cl- concentration from pH 0 to pH 7. Values for K-1 (=[BrCl2-]/([BrCl(aq )][Cl-])) at mu = 1.0 M are 3.8 M-1 at 25.0 degreesC, 4.7 M-1 at 10.0 degre esC, and 5.5 M-1 at 0.0 degreesC, with DeltaH(1)degrees = -9.9 kJ mol(-1) a nd DeltaS(1)degrees = -22 J K-1 mol(-1). BrCl(aq) hydrolysis equilibria hav e little or no temperature dependence with K-h1 (=[HOBr(aq)][Cl-1]-[H+]/[Br Cl(aq)]) = 1.3 x 10(-4) M-2 from 25.0 to 5.0 degreesC, mu = 1.0 M. When con ditions are adjusted to give a rapid partial hydrolysis of BrCl in equilibr ium with HOBr and Cl- at p[H+] 4.31, a relatively slow reaction (k(obsd) = 2.4 s(-1)) to form HOCl and Br- is observed. This takes place via BrCl reac tion with Cl- to form Cl-2; which hydrolyzes in the rate-determining step t o give HOCl; On the other hand, the rate of complete BrCl hyadolysis to for m HOBr and Cl- at p[H+] 6.4 is extremely rapid with a first-order rate cons tant of 3.0 x 10(6) s(-1) at 25.0 degreesC. The reverse reaction between HO Br, Cl-, and H+ has a rate constant of 2.3 x 10(10) M-2 S-1, so that in sea water,where [Cl-]/[Br-] = 700, the formation of BrCI is much faster than th e formation of Br-2 from HOBr, Br-, and H+. Rapid formation of BrCl(aq) and its subsequent reaction with Br- is a viable pathway to give Br-2(aq). Pho tolysis of Br-2(g) is believed to initiate the reactions associated with oz one depletion.