THE EFFECT OF TEMPERATURE, CATALYSIS, AND DIELECTRIC CONSTANT IN THE KINETICS OF THE OXIDATION-REDUCTION REACTIONS OF [N-(2-HYDROXY-ETHYL)ETHYLENEDIAMINE-N’,N’,N’,-TRIACETATO COBALT (11)] BY Cu2+ CATION.

ABSTRACT

The effects of temperature, dielectric constant and catalysis in the kinetics of the oxidation –reduction reactions (involving electron transfer) of N-(2-hydroxy-ethyl) ethylenediammine- N’,N’,N’-Triacetatocobalt (II) by Cu²⁺ cation were determined.  The dielectric constant was decreased from 63.05 to 43.18 and it was found that the rates of the reaction did not show any appreciable change. This seems to mean that the change in the dielectric constant of the medium had no effect on the rates of reaction in this [Co^IIHEDTAH₂O]^– and Cu²⁺ systems. At constant concentration of all the reactants, the effect of added ions on the rates of reaction was investigated by varying the concentration of acetate ion (CH₃COO^–) from  30×10^-3 – 130×10^-3 mol dm^-3 and noting the rates of the reactions. The same was repeated for magnesium ion (Mg²⁺).  For this system, the rates of reaction were found unaffected by the presence of either Mg²⁺ or CH₃COO^–The temperature dependence of rates on this reaction was investigated at 35⁰C, 40⁰C, 50⁰C, 55⁰C and 60⁰C respectively. It was found that increase in temperature increases the rates of reaction. The plot of logk_obs versus the reciprocal of the square of temperature is linear, hence the activation parameters were evaluated.

TABLE OF CONTENTS

Title page –      –      –      –     –      –     –     –    –    –     –     –   –     –    –  –  –       –        –   –     –     –     –          ii

Certification –     –    –     –     –     –     –     –   –    –    –     –    –     –   –  –    –         –     –     –     –    –    –         iii

Dedication –      –     –      –     –     –     –    –   –    –     –    –    –   –    –   –      –       –    –      –     –    –     –        iv

Acknowledgement-     –     –    –    –    –    –    –     –   –    –    –    –    –    –    –      –      –     –    –     –     –         v

Abstract  –      –     –    –    –     –    –    –    –    –    –    –    –    –    –    –     –     –     –     –     –     –     –    –         vi

Table of contents –      –     –     –     –    –     –     –     –     –     –     –     –       –     –     –     –     –    –     –        vii

List of tables –      –     –       –     –    –     –     –     –     –    –     –     –     –       –       –    –      –    –    –     –      ix

List of figures –    –     –         –     –    –    –     –     –      –   –     –     –     –       –        –    –      –    –    –    –     x

CHAPTER ONE: INTRODUCTION

1:1     Electron Transfer ———————————————————————————–

1.2     Classes of Electron Transfer  ———————————————————————

1.2.1 Inner sphere electron transfer ———————————————————————

1.2.2 Outer sphere electron transfer ———————————————————————

1.3     Mechanism of electron transfer reactions  ——————————————————

1.3.1 Inner sphere mechanism —————————————————————————–

1.3.2 Outer sphere mechanism —————————————————————————–

1.4      Applications of Electron Transfer —————————————————————–

1.5      Chemistry of cobalt  ———————————————————————————

1.5.1   Use of cobalt  ——————————————————————————————

1.5.2    Structure of [Co¹¹HEDTAH₂O] ——————————————————————–

1.6       Chemistry of Transition Metals

1.7       Aims and Objectives

1.8        Justification

CHAPTER TWO: LITERATURE REVIEW

2.1     Dielectric constant

2.1.1   Dielectric properties

2.2    Microscopic concept of polarization

2.3    Effect of variation of dielectric constant of a medium

2.4   Catalysis

2.4.1  General characteristics of catalysed reaction

2.4.2   Types of catalysis

2.4.3   Catalytic poisoning

2.4.4   Autocatalysis

2.4.5   Examples of catalytic process

2.5    Effect of Temperature on reaction velocity

2.6   The bioinorganic chemistry of copper

CHAPTER THREE: MATERIALS AND METHODS

3.1  Materials

3.1.1 Chemicals

3.1.2 Apparatus/Equipment

3.2  Methods

3.2.1  Preparations of the complex [Co^IIHEDTAH₂O]

3.2.2  Preparation of 0.1m of perchloric acid

3.2.3  Preparation of standard solution of sodium perchlorate

3.2.4  Preparation of the standard solution of copper (II) teraoxosulphate (VI) salt

3.3     Determination of the λmax (510nm)

CHAPTER FOUR: RESULTS AND DISCUSSION

4.1  Determination of the rate constant of the reaction (k_obs )

4.2  The effect of dielectric constant

4.3   The effect of added ions

4.4   Temperature dependence of rates of reaction.

CHAPTER FIVE: CONCLUSION AND RECOMMENDATION

5.1    Conclusion

5.1    Recommendation

LIST OF TABLES

• Oxidation state and stereochemistry of cobalt.

2.1   The relative permittivity of some selected substances at 20^oC

2.2   Effect of the variation of dielectric constant of the medium on reaction rates.

2.3   Effect of the dielectric constant (D).

2.4   Application of catalytic processes

2.5   Pseudo-first order rate constant for the reaction of  [Co^IIHEDTA(H₂O)] and [Cu^II] at [Co^IIHEDTA(H₂O)]

= 1×10^-4 (mol dm^-3), T=29±1⁰C and λ_max = 510nm

4.1   Calculation of the dielectric constant for Cu²⁺ in the oxidation of [Co^IIHEDTA(H₂O)]- by Cu²⁺

4.2   Dependence of rate on dielectric constant (D) for oxidation of [Co^IIHEDTA(H₂O)- by Cu²⁺] at [Cu²⁺] =

2.0 x 10^-3 mol dm^-3, [Co^IIHEDTA(H₂O)]= 1.0×10^-4 mol dm^-3, [H⁺]= 5.0×10^-3 mol dm^-3, I=0.05 mol

dm^-3, (NaClO₄) λmax = 510nm, T= 29±1⁰C.

4.3   Dependence of rate constant on added ions for the oxidation-reduction reaction of [Co^IIHEDTA(H₂O)]=

1.0×10^-4 mol dm^-3, [Cu²⁺] = 2.0×10^-3 mol dm^-3, [H⁺] = 5.0×10^-3 mol dm^-3, I= 0.05 mol dm^-3,

(NaClO₄),λmax

4.4   Temperature dependence of rate constants for the oxidation-reduction reactin of [Co^IIHEDTA(H₂O)]^– and

Cu²⁺, at [Co^IIHEDTA(H₂O)]=1.0×10^-4 mol dm^-3, [Cu²⁺] = 2.0×10^-3 mol dm^-3, [H⁺] = 5.0×10^-3 mol

dm^-3  I= 0.05 mol dm^-3, (NaClO₄),λmax=510nm,

4.5    Activation parameter for the reaction of [Co^IIHEDTA(H₂O)]^-1 and Cu²⁺

4.6    Table for graph of temperature 1/T(k^-1) versus logk_obs

4.7     The activation parameter for the reactions of [Co^IIHEDTA(H₂O)]- and Cu²⁺.

LIST OF FIGURES

1.1 Shows SCN^– as a good bridging ligand.

1.2.  Shows a situation where both the reactants and products are liable

1.3. Examples of the applications/developments in electron transfer reactions.

1.4-1.5  structures of CO^III HEDTA(H₂O)

2.1: Effect of temperature on reaction velocity

2.2:  variation of log k with 1/T for the decomposition of N₂O₅

4.1   Graph of log(A_t– A ͚ ) versus time

4.2   Graph of temperature 1/T(k^-1) versus logk_obs.

CHAPTER ONE

1.    INTRODUCTION

 1.0    BACKGROUND OF THE STUDY

       1.1     ELECTRON TRANSFER

Electron transfer (ET) occurs when an electron moves from an atom or a chemical species ( e.g. a molecule) to another atom or chemical species.  Electron transfer is a mechanistic description of the thermodynamic concept of redox, wherein the oxidation states of both reaction partners change.

Numerous biological processes involve electron transfer reactions. These processes include oxygen binding, photosynthesis, respiration, and detoxication. Additionally, the process of energy transfer can be formalized as two-electron exchange (two concurrent electron transfer events in opposite directions) in case of small distances between the transferring molecules.

1.2           CLASSES OF ELECTION TRANSFER

There are several classes of electron transfer, defined by the state of the two redox centers and their connectivity.

1.2.1       Inner sphere electron transfer

In inner sphere electron transfer, the two redox centers are covalently linked during the electron transfer (Burgees, 1978). This bridge can be permanent, in which case the electron transfer event is termed intermolecular electron transfer. More commonly, however, the covalent linkage is transitory, forming just prior to the electron transfer and then disconnecting following the electron transfer event. In such cases, the electron transfer is termed intermolecular electron transfer. A famous example of an inner sphere electron transfer that proceeds by a transitory bridged intermediate is the reduction of [CoCl(NH₃)₅]²⁺ by [Cr(H₂O)₆]²⁺ (Taub and Meyer, 1954). In this case the chloride ligands is the bridging ligands that covalently connects the redox partners.

1.2.2     

 Outer sphere electron transfer

In outer-space reactions, the participating redox centers are not linked by any bridge during the electron transfer event. Instead, the electron “hops” through space from the reducing center to the acceptor. Outer sphere electron transfer can occur between different chemical species or between identical chemical species that differ only in their oxidation sate. The later process is termed self-exchange. As an example, self-exchange describes the degenerate reaction between permanganate and its one-electron reduced relative, manganese:

[MnO₄]^– + [Mn*O₄]₂^–                  [MnO₄]₂^–  +  [Mn*O₄]^–  (Lavallee, et al; 1973)         (1.1)

In general, if electron transfer is faster than ligands substitution, the reaction will follow the outer-sphere electron transfer. Often occurs when one/both reactants are inert or if there is no suitable bridging ligands.

1.3        MECHANISM OF ELECTRON TRANSFER REACTIONS

In a redox process, the oxidizing and reducing centers can react with or without a change in their coordination spheres. In some reactions, the electron transfer can only be accomplished by the transfer of ligands from reducing agent to the oxidizing agent.

There are two stoichiometric mechanism: the inner sphere mechanism involves a ligands transfer, and transient shared ligands, while the outer sphere mechanism includes the simple electron transfers, without the presence of shared ligands.

1.3.1      Inner sphere mechanism

The reduction of the non-liable Co complex by the aqueous Cr complex produces a reduced Co complex and an oxidized CrCl complex. The chloride ligands has been transferred between the metal centers as proven by the fact that addiction of ³⁶Cl^– to the solution results in no incorporation of ³⁶ Cl^– into the Cr complex (Wilkins, 1991).

The reaction is faster than reactions which remove Cl^– from Co¹¹¹ or introduces Cl^– to Cr₃⁺ _(aq), and hence the Cl^– ion must have moved directly from the coordination sphere of one complex to the other during the reaction.

N.B The intermediate has a bridging Cl^–  ligand.

The Cl^– ion is a good bridging ligand as it has more than one pair of electrons, and so can form bonds to each of the metal centers simultaneously. Other good bridging ligands include SCN^–, N₂, N₃^– and CN^– (Wilkins, 1991).

Fig 1.1 Shows SCN^– as a good bridging ligand (Wilkins, 1991).

1.3.2     The Outer Sphere Mechanism

When both the species in the redox reaction have non-liable coordination spheres, no ligands substitution can take place on the very short time scale of the redox reaction. The electron transfer must proceed by a mechanism involving transfer between the two complex ions in outer-sphere contact.

If the redox reaction is faster than the ligands substitution, then the reaction has an outer-sphere mechanism.

When the reaction involves ligands transfer from an initially non-liable reactant to a non-liable product, there is no difficulty in assigning the inner-sphere mechanism.

When the products and reactants are liable, it is difficult to make an unambiguous assignment of either an inner or an outer-sphere mechanism (Richardson, 1984).

  Fig 1.2: Shows a situation where both the reactants and products are labile (Richardson 1984).

 1.4 APPLICATIONS/DEVELOPMENTS IN THE ELECTRON TRANSFER REACTIONS

Electron transfer experiment since the late 1940s (Marcus, 1956)

Since the late 1940s, the field of electron transfer processes has grown enormously, both in chemistry and biology. The development of the field, experimentally and theoretically, as well as it relation to the study of other kinds of chemical reactions, represents to us an intriguing history, one in which many threads have been brought together.

Fig. 1.3. Examples of the applications/developments in electron transfer reactions. (Marcus and Siddarth, 1992).

1.5 CHEMISTRY OF COBALT

       Cobalt is a chemical element with the symbol Co and atomic number 27. Cobalt always occurs in nature in association with Ni and usually also with arsenic (Seyferth et al., 1989). The most important Co minerals are smaltite, (CoAs2),  and cobaltite (CoAsS) but the chief technical source of Co are residue called “speisses” which are obtain in the smelting of arsenical ores of Ni, Cu and Pb,

Cobalt is a hard bluish-white metal (mp 14930c, bp 31000c). it dissolve slowly in dilute mineral acids, the Co2+/Co potential being -0.2227 V, but it is relatively unreactive. While it does not combine directly,with C,P and S on heating, it is attacked by atmospheric O2 and by water vapor at elevated temperatures, giving CoO. Very reactive finely divided metal particles can be made by reduction of CoCl2 with Li naphthalenide in glyme (Beattie et al., 1996).

Table 1.1:   OXIDATION STATE AND STEREOCHEMISTRY OF COBALT

(Lippard et al., 1992)

1.5.1  USES OF COBALT

Cobalt plays an important biological role for instance;

Coenzymes B12; A vitamin known as coenzymes B12 is a known organometallic compound in nature (Crossnoe et al., 2002). It incorporates cobalt into a corrin ring structure. This compound is known to prevent anemia and also has been found to have many catalytic properties (Morales et al., 2003).  Methylcobalamin can methylate many compounds, including metals. The reactions of alkylcobalamine depends on cleverage of the

alky-cobalt bond, which can result in Co^(I) and an alkyl cation, Co^(II) and alkyl radical, or Co ^(III) and alkyl anion (Abeles, 1977). Cobalt also contains other enzymes and proteins like glutamate mutase, dioidehydrase, methionime synthetase, and dipeptidase (Frieden, 1985).

• STRUCTURES OF Co^III HEDTA(H₂O)

1.6     CHEMISTRY OF TRANSITION METALS

In chemistry, the term transition metal (or transition element) has three possible meanings:

The IUPAC definition (IUPAC, 2006) defines a transition metal as “an element whose atom has a partially filled d sub-shell, or which can give rise to cations with an incomplete d sub-shell”.

Many scientists describe a “transition metal” as any element in the d-block of the periodic table, which includes groups 3 to 12 on the periodic table (petrucci et al., 2002), (Housecroft and Sharpe, 2005). In actual practice, the f-block lanthanide and actinide series are also considered transition metals and are called “inner transition metals”.

Cotton and Wilkinson (cotton and Wilkinson, 1988) expand the brief IUPAC definition by specifying which elements are included. As well as the elements of groups 4 to 11, they add scandium and yttrium in group 3 which have a partially filled d sub-shell in the metallic state.

These last two element are included even though they do not (so far) seem to possess the catalytic properties which are so characteristic of the transition metals in general. Lanthanum and actinium in group 3 are however classified as lanthanides and actinides respectively.

English chemist Charles Bury (1890-1968) first used the word transition in this context in 1921, when he referred to a transition series of elements during the change of an inner layer of electrons (for example n=3 in the 4^th row of the periodic table) from a stable group of 8 to one of 18, or from 18 to 32. (Jensen,2003), (Bury, 1921), (Bury, 2008) These elements are now known as the d-block.

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