Kinetics and mechanisms of the ozone/bromite and ozone/chlorite reactions.

Ozone reactions with XO2 - (X ) Cl or Br) are studied by stopped-flow spectroscopy under pseudo-first-order conditions with excess XO2 - . The O3/XO2 - reactions are first-order in [O3] and [XO2 - ], with rate constants k1 Cl ) 8.2(4) × 10 6 M -1 s -1 and k1 Br ) 8.9(3) × 10 4 M -1 s -1 at 25.0 °C and I ) 1.0 M. The proposed rate-determining step is an electron transfer from XO2 - to O3 to form XO2 and O3 - . Subsequent rapid reactions of O3 - with general acids produce O2 and OH. The OH radical reacts rapidly with XO2 - to form a second XO2 and OH - .I n the O3/ClO2 - reaction, ClO2 and ClO3 - are the final products due to competition between the OH/ClO2 - reaction to form ClO2 and the OH/ClO2 reaction to form ClO3 - . Unlike ClO2, BrO2 is not a stable product due to its rapid disproportionation to form BrO2 - and BrO3 - . However, kinetic spectra show that small but observable concentrations of BrO2 form within the dead time of the stopped-flow instrument. Bromine dioxide is a transitory intermediate, and its observed rate of decay is equal to half the rate of the O3/BrO2 - reaction. Ion chromatographic analysis shows that O3 and BrO2 - react in a 1/1 ratio to form BrO3 - as the final product. Variation of k1 X values with temperature gives ¢H q Cl ) 29(2) kJ mol -1 , ¢S q Cl ) -14.6(7) J mol -1 K -1 , ¢H q Br ) 54.9(8) kJ mol -1 , and ¢S q Br ) 34(3) J mol -1 K -1 . The positive ¢S q Br value is attributed to the loss of coordinated H2O from BrO2 - upon formation of an [O3BrO2 - ] q activated complex.