Reinvestigation of the kinetics of reduction of bromite by hexacyanoferrate(ii) in slightly basic solution

Abstract

The reaction BrO₂⁻ + 4Fe(CN)₆⁴⁻ + 2H₂O → Br⁻ + 4Fe(CN)₆³⁻ + 4OH⁻ has been studied by spectrophotometry at 30, 40 and 50 °C and both with equivalent amounts of reactants and with an excess of bromite. It follows the kinetic expression with k_obs = 8.2 M⁻² s⁻¹ at 25 °C, ΔH^≠ = 66 kJ mol⁻¹ and ΔS^≠ = −121.7 J K⁻¹ mol⁻¹. The proposed mechanism is a two-stage process, the first, rate-determining stage being reduction of bromite to hypobromous acid, the second further reduction to bromide. Either stage consists of two bimolecular reactions. The major contribution to the energy barriers comes from electrostatic repulsion between negative ions.