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

Abstract

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 k_w = 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.