Stoicheiometry and kinetics of the reaction between cerium(IV) and hydrazine in acid solution

Abstract

With excess of hydrazine, one mole of Ce^IV is required for each mole of hydrazine. The amount of Ce^IV per mole of hydrazine increases with the increase of [Ce^IV]/[N₂H₄]. In sulphuric acid solutions of low concentration the stoicheiometry is ca. 1 : 1 ([Ce^IV] : [N₂H₄)] and it increases with increase of H₂SO₄. A limiting value of 4 is obtained in ca. 10M-H₂SO₄ solutions. From the amount of ammonia produced it was possible to calculate the extents of one- and four-electron changes occurring in a particular acid solution and explain the observed stoicheiometry. A kinetic study of the reaction was also made, but only in solutions containing excess of hydrazine. The results conform to the empirical rate law (i); k₁ and k₂K were found to be 1·87 ± 0·05 l mol^–1 s^–1 and 8·5 ×–d[Ce^IV]//_dt=[Ce^IV][N₂H₅⁺](k₁+k₂K//_[H⁺])(1//_{1+K3[HSO4⁺]})(i) 10^–1± 0·05 s^–1 respectively in 3·0M-H₂SO₄ solutions at 20 °C. K is either the formation constant for the complex of Ce^IV and hydrazine or the acid dissociation constant of N₂H₅⁺. K₃ is the third successive complex formation of Ce^IV and HSO₄^–.