Reactions of positive halogen ions in flames and their rate coefficients
Abstract
Halogens, when added to flames, usually form negative ions. In addition, iodine produces I+, I+2, IO+ and I2+, whereas bromine gives Br+ and Br+2, but chlorine forms no positive ions whatsoever. The dominant of these species are the monatomic ions I+ and Br+, which are evidently produced in the reaction zone of the flame, partly by charge transfer from primary ions, such as CHO+, which are generated during the combustion of a hydrocarbon. Otherwise, the sequences I + O → IO++ e–, IO++ H → I++ OH and, to a lesser extent, I + I → I+2+ e–, I+2+ H → I++ HI can be important. Only the origin of I2+ remains a puzzle.
The disappearance of I+ from the burnt gases of flames from 1820 to 2650 K has been studied in detail and proves to be by three routes involving: I++ e–+ M → I + M, (III), I++ I–→ I + I, (IV), I++ H2O = I+ . H2O and (– XV), I+ . H2O + H → H3O++ I. (– XIV)
The three-body recombination coefficient of reaction (III) is large, being 1.6 ± 0.8 × 10–23T–1 cm6 molecule–2 S–1, in line with other measurements for metal ions recombining with electrons in flames. The two-body ion–ion recombination step (IV) is, however, surprisingly rapid, having k4= 4.9 ± 2.0 × 10–7T–1/2 cm3 ion–1 S–1. The rate coefficient for the last of these steps is also large, being k–14= 1.1 × 10–8 with iodine and 5.9 × 10–9 cm3 molecule–1 S–1 for the analogous reaction with Br, both being correct to within a factor of 3. Finally, we conclude that the first hydration energies of I+ and Br+ are, respectively, 25 ± 12 and 45 ± 20 kJ mol–1.