Mechanism of the ``Bimolecular'' Hydrogen—Iodine Reaction

The rate of photochemical production of hydrogen iodide in a hydrogen—iodine system irradiated with 5780-Å light was measured and found to be proportional to the square of the iodine atom concentration. Absolute light intensities were determined and rate constants were calculated for the termolecular reaction H2+2I→2HI at 417.9°, 480.7°, 520.1°K. The previously obtained thermal data in the range 633°—738°K were treated considering one of the elementary reactions to be the termolecular reaction and not H2+I2→2HI; the rate constants obtained have the same Arrhenius parameters as those from the photochemical measurements. The bimolecular reaction, H2+I2→2HI, does not occur. Two mechanisms consistent with the photochemical and the thermal data are H2+2I→2HI; H2+I+M=H2I+M, H2I+I→2HI. The specific rate calculated for the termolecular reaction is k=(6.66±0.5)×107 exp (−5310±85/RT) liter2 mole−2·sec−1.