Kinetics of oxidation of ethylenediammine-N,N,N’,N’-tetraacetatocobaltate(II)complex by silver-activated persulphate ion in aqueous acidic medium

Abstract

The kinetics of oxidation of Ethylenediammine-N,N,N’,N’-tetraacetatocobaltate(II) complex  ([Co(II)EDTA^2-]) by silver-activated persulphate ion in a ueous nitric acid medium was studied at I = 0.5 mol dm^-3(NaNO₃), [H⁺] = 1 × 10^-2 mol dm^-3, [Ag⁺] = 1 × 10^-2 mol dm^-3, T = 299 ± 1 K and λ_max = 520 nm. Stoichiometric study showed 2:1 (Reductant: Oxidant) mole ratio. The reaction exhibited first order with respect to the [Co(II)EDTA^2-] and [ S₂O₈^2-] and the rate constant, (5.90 × 10^-2 dm³mol^-1s^-1) was independent of the acid concentration, but varied linearly with catalyst concentration, [Ag⁺]. The overall rate law is represented as:

^{d[Co(II)EDTA2-]}/_{dt = 5.90 x 10^-2 [Ag⁺][Co(II)EDTA^2-][s208^2-]}

The reaction displayed zero salt effect suggesting an activated complex composed of either a neutralcharged, neutral-neutral reactant species or ion-pair. The reaction was catalysed by addition of formate ion, HCOO- and the Michaelis-Menten’s type plot gave zero intercept indicating the absence of intermediate complex. Also, the activation enthalpy  ( △H^# = +39.27 kJ mol^-1) and entropy ( △H^# = -136.73 J K^-1 mol^-1) were determined. The entropy of activation shows a more ordered activated complex relative to the reactants species. Evaluating the experimental data, an outer-sphere mechanistic pathway via ion pair activated complex is proposed for the reaction.

Keywords: Kinetics, Oxidation, Aminocarboxylactocobaltate(II), Persulphate ion