Radiation chemistry of aqueous solutions of hydrazine at elevated temperatures. Part 1.—Oxygen-free solutions

Abstract

Rate constants for the reactions of e_aq^– and ˙OH with N₂H₄ and N₂H₅⁺ in liquid water up to 200 °C have been measured by pulse radiolysis. Linear Arrhenius plots for the reactions of e_aq^– gave k(20 °C)= 10⁶ and 1.6 × 10⁸ dm³ mol^–1 s^–1, and E_a= 13.5 and 18.2 kJ mol^–1, respectively. H is the product of the reaction with N₂H₅⁺. Non-linear Arrhenius behaviour was observed for the reactions of ˙OH with k(20 °C)= 4.5 × 10⁹ and 8.2 × 10⁷ dm³ mol^–1 s^–1, respectively. The pK_a of N₂H₅⁺ decreases linearly with temperature from 8.1 at 20 °C to 4.2 at 200 °C. The products of the ˙OH reactions are ˙N₂H₃ and ˙N₂H₄⁺, respectively, and the pK_a of ˙N₂H₄⁺ also decreases with increasing temperature. The self-reaction of ˙N₂H₃ shows the same temperature dependence as that of ˙OH with k(20 °C)= 2 × 10⁹ dm³ mol^–1 s^–1. The product of this reaction is tetrazene. Up to 200 °C the data are consistent with successive eliminations of NH₃ to form triazene and then N₂. The pH-dependent kinetics of these processes indicate that the decomposition of N₃H₃ is acid- and base-catalysed over the whole temperature range.