KINETICS AND MECHANISMS OF THE OXIDATION OF AMINOCARBOXYLATOCOBALTATE(II) COMPLEXES BY HYPOCHLORITE AND SILVER-CATALYSED PERSULPHATE IONS IN AQUEOUS ACIDIC MEDIUM
KINETICS AND MECHANISMS OF THE OXIDATION OF AMINOCARBOXYLATOCOBALTATE(II) COMPLEXES BY HYPOCHLORITE AND SILVER-CATALYSED PERSULPHATE IONS IN AQUEOUS ACIDIC MEDIUM
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Date
2018-02
Authors
AHMED, Yusif
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Abstract
The kinetics of the oxidation of aminocarboxylatocobaltate(II) complexes (hereafter, [Co(II)EDTA]2- and [Co(II)HEDTA(OH2)]-) by hypochlorite (ClO-) and silver-catalysed persulphate ions (S2O82-) at 300 K; 520 nm and 299 K; 525 nm (temperature and λmax) respectively was studied in aqueous acidic medium under 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 × 10-2 mol dm-3 (AgNO3). While I = 0.5 mol dm-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 S2O82- 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 S2O82- was obtained. The reactions of complexes with 𝑆2𝑂82−were independent of [H+] but dependent on [Ag+] while with ClO- the reaction rates were inversely dependent on [H+]. The reactions conform to the following rate equations:
𝑑[𝐶𝑜(𝐼𝐼𝐼)𝐸𝐷𝑇𝐴−] 𝑑𝑡=𝑎[𝐴𝑔+][𝐶𝑜(𝐼𝐼)𝐸𝐷𝑇𝐴2−][𝑆2𝑂82−]
𝑑[𝐶𝑜(𝐼𝐼𝐼)𝐻𝐸𝐷𝑇𝐴] 𝑑𝑡=𝑏[𝐴𝑔+][𝐶𝑜(𝐼𝐼)𝐻𝐸𝐷𝑇𝐴−]
𝑑[𝐶𝑜(𝐼𝐼𝐼)𝐸𝐷𝑇𝐴−]𝑑𝑡=𝑐[𝐻+]−1[𝐶𝑜(𝐼𝐼)𝐸𝐷𝑇𝐴2−][𝐶𝑙𝑂−]
𝑑[𝐶𝑜(𝐼𝐼𝐼)𝐻𝐸𝐷𝑇𝐴]𝑑𝑡= 𝑑[𝐻+]−1[𝐶𝑜(𝐼𝐼)𝐻𝐸𝐷𝑇𝐴−][𝐶𝑙𝑂−]
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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 S2O82- 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- (ΔH* = +28.67 kJ mol-1, ΔS* = -170.72 J K-1 mol-1) and [Co(II)HEDTA(OH2)]- (ΔH* = +53.12 kJmol-1, ΔS* = -113.65 J K-1 mol-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.
Description
DISSERTATION SUBMITTED TO THE SCHOOL OF POSTGRADUATE STUDIES,
AHMADU BELLO UNIVERSITY, ZARIA
IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE AWARD OF A
MASTER OF SCIENCE DEGREE IN INORGANIC CHEMSTRY
DEPARTMENT OF CHEMISTRY
FACULTY OF PHYSICAL SCIENCES
AHMADU BELLO UNIVERSITY, ZARIA
NIGERIA
Keywords
KINETICS,, MECHANISMS,, OXIDATION,, AMINOCARBOXYLATOCOBALTATE(II),, COMPLEXES BY HYPOCHLORITE,, SILVER-CATALYSED PERSULPHATE IONS,, AQUEOUS ACIDIC,, MEDIUM,