Systematic design of chemical oscillators. Part 35. Kinetics and mechanism of the reaction between chlorine(III) and bromide ion

Abstract

The stoichiometry and kinetics of the reaction between chlorine(III) and bromide ion were studied spectrophotometrically at 25.0 +/- 0.5 /sup 0/C and ionic strength 1.2 M (NaClO/sub 4/). The main products are Br/sub 3//sup -/ and Cl/sup -/ when bromide ion is in excess, ClO/sub 2/ and Br/sub 2/ when chlorine(III) is in excess. With sufficient acid and excess bromide ion, the stoichiometry of the reaction is HClO/sub 2/ + 6Br/sup -/ + 3H/sup +/ ..-->.. 2Br/sub 3//sup -/ + Cl/sup -/ + 2H/sub 2/O. The rate law for this reaction is (1/2)d(Br/sub 3//sup -/)/dt= k(H/sup +/)(Br/sup -/)(Cl(III)) where k = 9.51 +/- 0.14) x 10/sup -2/ M/sup -2/ s/sup -1/. When the reaction is carried out with (Cl(III)) > (Br/sup -/), the stoichiometry is difficult to define. In the range (Cl(III)) approx. = (1.50 - 2.00) x 10/sup -3/ M, (Br/sup -/) approx. = 5.00 x 10/sup -4/ M, and (perchloric acid) approx. = 0.20 M, a clock reaction occurs, the lag time of which decreases with addition of small amounts (<10/sup -4/ M) of molecular bromine. The complex rate law for the chlorine(III)-bromide ion reaction with excess Cl(III) can be explained by a 16-step mechanism including oxidation ofmore » bromide ion to bromine by chlorine(III), reduction of bromine to bromide ion, and decomposition of chlorous acid. A reduced set of 10 reactions and associated rate and equilibrium constants successfully modeled the clock reaction by computer simulation.« less