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Nigerian Journal of Basic and Applied Sciences

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

A.D. Onu, S.O. Idris, Y Ahmed

Abstract


The kinetics of oxidation of Ethylenediammine-N,N,N’,N’-tetraacetatocobaltate(II) complex  ([Co(II)EDTA2-]) 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 λmax = 520 nm. Stoichiometric study showed 2:1 (Reductant: Oxidant) mole ratio. The reaction exhibited first order with respect to the [Co(II)EDTA2-] and [ S2O82-] 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)EDTA2-][s2082-]

 

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




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