Kinetics and Mechanism of the Reaction of Azide with Ozone in Aqueous Solution

The stoichiometry of the reaction of aqueous ozone with sodium azide was studied at pH 12 (mainly) where a yellow metastable intermediate is observed. We propose that this is hypoazidite (N₃O^- ), analogous to hypobromite, and that it plays a central role in the azide catalyzed decompostion of ozone. The yellow intermediate is unstable in acid, in which it rapidly decomposes, generating N₂ and NO₂^-. The rate of reaction was studied at pH 2.0-3.5, with the ionic strength at 0.6 M and temperature at 3-15 ° C. The intrinsic second-order rate constants were found to be k _HN3 ≤ ≈ 400 M^-1sec^-1 and k _N3- = (8.7 ± 0.5) × 10⁵ M^-1sec^-1 (3 °C, 0.6 M), both in agreement with the only other previous study. The rate constant at 25 °C was estimated using the following experimentally determined parameters: ln k_N3- (M^-1sec^-1) = (5.73 ± 0.36) × 10³/T (K) + (28.34 ± 1.27). The value of k_N3- estimated in this way is (2.5 ± 0.1) × 10⁶ M^-1sec^-1 at 25 °C and 0.6 M. The enthalpy of reaction (A H) is -48 ± 3 kJ mol^-1.