Kinetics And Mechanisms Of The Oxidation Of Aminocarboxylatocobaltate(ii) Complexes By Hypochlorite And Silver-catalysed Persulphate Ions In Aqueous Acidic Medium

The kinetics of the oxidation of aminocarboxylatocobaltate(II) complexes (hereafter,[Co(II)EDTA]2-and [Co(II)HEDTA(OH2)]-) by hypochlorite (ClO-) and silver-catalysedpersulphate ions (S2O82-) at 300 K; 520 nm and 299 K; 525 nm (temperature and λmax)respectively was studied in aqueous acidic mediumunder pseudo-first order conditions. For the reaction of complexes with persulphate ion, the concentrations were: I = 0.5 mol dm-3 (NaNO3), [H+] = 1.0 × 10-2 mol dm-3 (HNO3) and [Ag+] = 1.0 × 10-2mol dm-3 (AgNO3). While I = 0.5 moldm-3 and 0.2 mol dm-3(NaNO3) and [H+] = 4.0 × 10-2 mol dm-3 and 1.0 × 10-2 mol dm-3 (HNO3) are for the reaction of hypochlorite with [Co(II)EDTA]2- and [Co(II)HEDTA(OH2)]- complexes respectively. The stoichiometry obtained was 2:1 for the oxidation of complexes by persulphate ion and 1:1 for the reaction of complexes with hypochlorite ion. First order kinetics in the concentration of S2O8 2- and ClO- were observed for all the reactions except the reaction of [Co(II)HEDTA(OH2)]-with persulphate ion, where zero order with respect to concentration of S2O8 2- was obtained. The reactions of complexes with were independent of [H+ ] but dependent on [Ag+ ] while with ClOthe reaction rates were inversely dependent on [H+ ]. The reactions conform to the following rate equations: where a = 5.90 × 10-2 dm3 mol-1 s -1 , b = 5.4× 10-2 dm3 mol-1 s -1 , c = 1.23 × 10-3 dm3 mol-1 s -1 , d = 2.28 × 10-4 dm3 mol-1 s -1 . Zero salt effects for the reaction of complexes with S2O8 2- and positive salt effects with ClO-were observed. The thermodynamic parameters obtained from the temperature dependence studies showed that the activated complexes are ordered for the oxidation of [Co(II)EDTA]2- (∆H* = +28.67 kJ mol-1 , ∆S* = -170.72 JK-1mol-1 ) and [Co(II)HEDTA(OH2)]- (∆H* = +53.12 kJmol-1 , ∆S* = -113.65 JK-1mol-1 ) by persulphate ion. The free radical species were detected only in the reaction of complexes with persulphate ion. Addition of ions affected the rates for all the reactions except for the reaction of [Co(II)HEDTA(OH2)]- with persulphate ion, where both added cations and anions had no effect on the reaction rate. Moreover, Michaelis – Menten plot of 1/kobs versus 1/[oxidant] had zero intercept for all the reaction systems except for the oxidation of [Co(II)HEDTA(OH2)]- by persulphate ion. Based on the above observations,an outer-sphere mechanism is proposed as the plausible mechanism for all the reactions except for the reaction of [Co(II)HEDTA(OH2)]-with persulphate ion, where an inner-sphere mechanism is proposed.

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