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-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.
Description
A THESIS 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,, HYPOCHLORITE,, SILVER-CATALYSED PERSULPHATE IONS,, AQUEOUS ACIDIC MEDIUM