The elementary reaction no2 (g) → 2no (g) + o2 (g) is second order in no2 and the rate constant at 690 k is 1.27 × 101 m-1s-1. the reaction half-life at this temperature when [no2]0 = 0.45 m is s. the elementary reaction no2 (g) → 2no (g) + o2 (g) is second order in no2 and the rate constant at 690 k is 1.27 × 101 m-1s-1. the reaction half-life at this temperature when [no2]0 = 0.45 m is s. 0.17 18 5.7 0.054 0.079