A kinetic study of the reaction of NO2 with HI over the temperature range 278 to 333 K

The rate constant, k, for the reaction, NO2 + HI → products, has been measured directly for the first time. The rate was found to be − d[NO2]/dt = k[NO2][HI] with k = (6.9 ± 1.1) × 10−19 cm3 molecule−1 s−1 at 303 K. The measurements were made under pseudo-first-order conditions with HI in excess over NO2. NO2 loss rate was monitored by FTIR in a static glass reactor. NO, H2O and I2 were observed as products and HNO2 as an intermediate. The study showed that the reaction was influenced by the conditions of the wall which had a surface loss rate constant kw = 0.13 s−1. This indicates that the measured k is an upper limit. Within a limited pressure range, the rate was independent of the total pressure in the system. From measurements made over the temperature range 278–333 K, we obtained the following expression for k = (3.6 ± 0.2) × 10−14 × exp[ − (2.7 ± 0.7) × 104/RT] cm3 molecule−1 s−1 valid between 293 and 333 K. A reaction scheme is proposed involving as the initiation step based on the kinetic and product studies, followed by a heterogeneous reaction between HNO2 and HI. A comparison of the reactions between NO2 with HX is made. This reaction is unlikely to play a major role in the removal of HI and as a source for HNO2 in the atmosphere because of its slow rate.