The kinetics and mechanism of the hyponitrous acid–nitrous acid reaction at 0 °C

Abstract

The kinetics of the reaction between hyponitrous acid and nitrous acid have been examined, mainly at 0 °C, over the acidity range 0.01 to 11.6 mol dme^–3 perchloric acid. Values of k_obs, the measured first-order rate constant, increase dramatically with [HClO₄] from about 4 mol dme^–3 HClO₄, and then decrease again as [HClO₄] increases beyond about 7 mol dm^–3. The maximum value of k_obs. at 0 °C occurs just above 7 mol dm^–3 perchloric acid and is greater than 2.5 × 10^–2s^–1 at 0.005 mol dm^–3 nitrous acid. The reaction may be described by the two-term rate law: rate ={k₂′+k₃**(K/(K+h_r))h₀a_w}[HNO₂][H₂N₂O₂], where k₂′= 4.62 × 10^–2 dm³ mol^–1 s^–1 and k₃**= 5.41 × 10^–3 dm6 mol^–2 s^–1 at 0 °C, and acidity functions, HR =–log₁₀h_R and H_o=–log₁₀h_o. The fall in rate at high acidity is attributed to the conversion of nitrous acid to the nitrosonium ion, suggesting that molecular nitrous acid is a kinetically active species. Added bromide ion at 5.56 mol dm^–3 perchloric acid results in lowered rates due to the conversion of nitrous acid to nitrosyl bromide.