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

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 kobs, 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 kobs. 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+hr))h0aw}[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 =–log10hR and Ho=–log10ho. 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.