KINETICS AND MECHANISMS OF CATALYSED REDUCTION OF AMINOCARBOXYLATO-COBALT(III) COMPLEXES BY L-ASCORBIC ACID AND HYDRAZINE IN AQUEOUS ACIDIC MEDIUM
KINETICS AND MECHANISMS OF CATALYSED REDUCTION OF AMINOCARBOXYLATO-COBALT(III) COMPLEXES BY L-ASCORBIC ACID AND HYDRAZINE IN AQUEOUS ACIDIC MEDIUM
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Date
2018-03
Authors
ABITI, YAKADI BALA
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Abstract
Copper(II) catalysed reduction of aminocarboxylatocobaltate(III) ion (hereafter referred to as [CoEDTA]–) and aminocarboxylatocobalt(III) complex (hereafter referred to as [Co(HEDTA)OH2]) by L-ascorbic acid (hereafter referred to as H2A) and Hydrazine (hereafter referred to as N2H4) have been spectrophotometrically studied in aqueous acidic medium. Stoichiometric studies gave a mole ratio of 2:1 for [CoEDTA]– : H2A, [CoEDTA]– : N2H4, and [Co(HEDTA)OH2] : N2H4 systems, but 1:1 for the [Co(HEDTA)OH2] : H2A system respectively. A first order with respect to the concentrations of each reactant was obtained for the [CoEDTA]– – H2A, [CoEDTA]– – N2H4, and [Co(HEDTA)OH2] – N2H4 systems. For the [Co(HEDTA)OH2] – H2A system, a first order with respect to [Co(HEDTA)OH2] and a half order with respect to [H2A] was obtained. Further kinetic studies on the effect of [H+] and [Cu2+] gave result which conforms to the following rate equations:
a = 3.47 × 10-4 s-1 , b = 4.9 s-1, c = 18.25 s-1, d = 4.47 s-1/2 and e = 1.8 × 102 s-1.
Negative salt effects were observed for [Co(EDTA)]– – H2A and [Co(EDTA)]– – N2H4 systems while neutral salt effect were obtained for [Co(HEDTA)OH2] – H2A and [Co(HEDTA)OH2]– N2H4 systems respectively. The test for the effect of added ions showed catalysis for
[Co(EDTA)]– – H2A, [Co(HEDTA)OH2] – H2A and [Co(HEDTA)OH2] – N2H4 systems, and no effect on [Co(EDTA)]– – N2H4 reaction. Results from the thermodynamic measurements indicated that ΔH‡ = 72.75 kJ mol-1 and ΔS‡ 43.65 JK–1 mol–1 for [Co(EDTA)]– – H2A system, ΔH‡ = 31.46 kJ mol-1 and ΔS‡ = –175.58 JK–1 mol–1 for [Co(EDTA)]– – N2H4 system, ΔH‡ = 85.21 kJ mol-1 and ΔS‡ = 7.02 JK–1 mol–1 for [Co(HEDTA)OH2] – H2A system, ΔH‡ = 82.89 kJ mol-1 and ΔS‡ = 17.13 JK-1 mol-1 for [Co(HEDTA)OH2] – N2H4 system respectively. The zero intercept obtained from the Michaelis – Menten plots, enhanced rate from added ions, and the absence of spectroscopically determinable intermediates led to the proposal that the [Co(EDTA)]– – H2A and [Co(HEDTA)OH2] – N2H4 reactions occurred via the outer – sphere mechanism. However, the non – zero intercept on the Michaelis – Menten plot, the presence of a spectroscopically determinable intermediate, negative values of the entropy of activation and non – catalysis by added ions led to the suggestion that the [Co(EDTA)]– – N2H4 reaction occurred via the inner –sphere mechanism. Both the outer – sphere and inner – sphere mechanisms have been proposed for the [Co(HEDTA)OH2] – H2A reaction due to non –zero intercept on the Michaelis – Menten plot, catalysis by added ions and positive values of activation entropy.
Description
A THESIS SUBMITTED TO THE SCHOOL OF POSTGRADUATE STUDIES, AHMADU BELLO UNIVERSITY, ZARIA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF MASTER OF SCIENCE DEGREE IN INORGANIC CHEMISTRY DEPARTMENT OF CHEMISTRY FACULTY OF PHYSICAL SCIENCES AHMADU BELLO UNIVERSITY, ZARIA, NIGERIA
Keywords
KINETICS,, MECHANISMS,, CATALYSED REDUCTION,, AMINOCARBOXYLATO-COBALT(III) COMPLEXES,, L-ASCORBIC ACID,, HYDRAZINE,, AQUEOUS ACIDIC MEDIUM