The kinetics of the chromous-catalysed dissociation of the monoazidopenta-aquochromium(III) complex

Abstract

The chromous-catalysed dissociation, i.e. aquation, of CrN₃²⁺ has been studied in aqueous perchloric acid solutions, µ= 2·0M. Allowances have to be made for the uncatalysed reaction, the relatively rapid reaction between chromous and azide, and the subsequent dissociation of Cr(H₂O)₅NH₃³⁺ and the dimer which are formed in this reaction. The rate constants obtained have a hydrogen-ion dependence of the form k′=a+b[H⁺]^–1. At 40°a= 5·2 (±0·15)× 10^–4 l. mole^–1 sec.^–1 and b= 3·4 (±0·8)× 10^–5 sec.^–1. The constant b is believed to correspond to an activated complex [CrOHCrN₃]³⁺ and a to an activated complex [CrOHCrN₃H]⁴⁺ rather than [CrH₂OCrN₃]⁴⁺, although the latter may make some small contribution. For reaction path b, azide ions are about forty-times less effective than chloride.

The equilibrium constant K for the reaction, CrN₃²⁺+ H⁺⇌ Cr³⁺+ HN₃, is > 10 at 40° and µ= 2·0M.