pKa of the hydrazinium ion and the reaction of hydrogen atoms with hydrazine in aqueous solution

Abstract

The techniques of electron pulse radiolysis and paramagnetic resonance free induction decay (FID) attenuation measurements have been used to determine Arrhenius parameters for the reaction of the hydrogen atom with N₂H₅⁺ and N₂H₄ in aqueous solution. At 41.6 °C, a scavenging rate constant of (5.8 ± 1.9)× 10⁶ dm³ mol^–1 s^–1 was directly measured for the N₂H₅⁺ reaction, with a corresponding activation energy of 61.4 ± 1.2 kJ mol^–1 over the temperature range 41.6–82.1 °C. The pK_a of the hydrazinium ion was determined by absorption spectroscopy, and over the temperature range 0–80 °C, this equilibrium was well described by pK_a=(2600 ± 30)/T–(0.771 ± 0.099). Using these values, and FID attenuation measurements in borax buffered solution over the temperature range 0.2–83.0 °C, the hydrogen atom rate constant for N₂H₄ reaction was calculated to be (6.74 ± 0.23)× 10⁷ dm³ mol^–1 s^–1(24.5 °C), with an activation energy of 16.28 ± 0.80 kJ mol^–1. The reaction rate of ˙H with N₂H₄ is consistent with a hydrogen atom abstraction, giving H₂ and ˙N₂H₃ radical as products, as in the gas phase. Based on the large activation enthalpy and entropy obtained for the reaction of hydrogen atoms with N₂H₅⁺, however, we suggest that this is more likely an addition–fragmentation process, to give ˙NH₂, NH₃ and (H⁺)_aq as products.