Oxidation and reduction processes

Oxidation and reduction or redox reactions involve the exchange of electrons from one chemical species to another. Normally, this is done when the two chemicals contact each other in the activated complex. Generally, the oxidation reaction involves loss of electrons and can occur in a homogeneous medium or at an electrode-electrolyte interface whereas reduction reaction involves a gain of electrons. 

The initial models developed for the redox reactions are based on transfer (loss and gain) of electrons. The process of oxidation cannot occur without a corresponding reduction reaction. Oxidation is always coupled with reduction, and the electrons that are lost by one substance must always be gained by another as matter cannot be destroyed or created. The terms lost or gained simply imply that the electrons are being transferred from one particle to another. 

At the positively charged anode, an oxidation reaction occurs, generating oxygen gas and giving electrons to the cathode to complete the circuit  

The same half-reactions can also be balanced with base as listed below. Not all half-reactions must be balanced with acid or base. To add half-reactions, they must both be balanced with either acid or base. 

When a magnesium metal is reacted with oxygen, it is thought to lose electrons (oxidize) to form Mg + ions and the element or compound that gained 

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The three-phase firing process ancient Greek potters used to create this vase utilized both oxidation and reduction processes. 

OXIDATION AND REDUCTION PROCESSES INVOLVING IODINE I0D0METRIC TITRATIONS... 

From the chemical viewpoint, the galvanic cell is a current source in which a local separation of oxidation and reduction process exists. This is explained below by the example of the Daniell element (Fig. 3). Here the galvanic cell contains copper as the positive electrode, zinc as the nega-... 

Later we will describe both oxidation and reduction processes that are in agreement with the electrochemically stimulated conformational relaxation (ESCR) model presented at the end of the chapter. In a neutral state, most of the conducting polymers are an amorphous cross-linked network (Fig. 3). The linear chains between cross-linking points have strong van der Waals intrachain and interchain interactions, giving a compact solid [Fig. 14(a)]. By oxidation of the neutral chains, electrons are extracted from the chains. At the polymer/solution interface, positive radical cations (polarons) accumulate along the polymeric chains. The same density of counter-ions accumulates on the solution side. 

Polymeric chains, and oxidation and reduction processes, a schematic, 344 Polymerization anodic, 555-556 chemical, 329... 

Excited state electron transfer also needs electronic interaction between the two partners and obeys the same rules as electron transfer between ground state molecules (Marcus equation and related quantum mechanical elaborations [ 14]), taking into account that the excited state energy can be used, to a first approximation, as an extra free energy contribution for the occurrence of both oxidation and reduction processes [8]. 

The photoelectrochemical properties of 283 colloids prepared by chemical solution growth [193] have been demonstrated by carrying out oxidation and reduction processes under visible light irradiation. Charged stabilizers such as Nation were found to provide an effective microenvironment for controlling charge transfer between the semiconductor colloid and the redox relay. 

Reaction offac(S)-[ v(iicl)n] with VC13 gave [V Ir(aet)3 2]3+ according to Reaction Scheme 21 389 The structure of (218)(C104)3 confirms the linear-type structure. Cyclic voltammetric studies of (218) show irreversible oxidation and reduction processes. 

Cyclic voltammetry is an excellent tool to explore electrochemical reactions and to extract thermodynamic as well as kinetic information. Cyclic voltammetric data of complexes in solution show waves corresponding to successive oxidation and reduction processes. In the localized orbital approximation of ruthenium(II) polypyridyl complexes, these processes are viewed as MC and LC, respectively. Electrochemical and luminescence data are useful for calculating excited state redox potentials of sensitizers, an important piece of information from the point of view of determining whether charge injection into Ti02 is favorable. 

A polarographic study of 4,4-diacyl triafulvenes290 showed that both oxidative and reductive processes may occur, reduction being somewhat favored over oxidation due to mesomeric effects of the acyl grouping [one-electron reduction —1.2 to —1.3 V (475) one-electron oxidation +1.6 to +1.75 V (476)]. 

Cysteine sulfhydryls and cystine disulfides may undergo a variety of reactions, including alkylation to form stable thioether derivatives, acylation to form relatively unstable thioesters, and a number of oxidation and reduction processes (Figure 1.10). Derivatization of the side chain sulfhydryl of cysteine is one of the most important reactions of modification and conjugation techniques for proteins. 

These solution NMR and X-ray crystallographic findings have been contradicted by X-ray structures solved by Rypniewski et al.32 The results show a reduced active site unchanged from the oxidized state and let these authors to propose a five-coordinate copper ion that exists throughout the oxidation and reduction process. In 2001 the Protein Data Bank listed 39 X-ray crystallographic and NMR solution structures for CuZnSOD, including oxidized, reduced, genetically modified, and other species with or without attached substrates or substrate mimics such as azide ion. The reader is advised to search the Protein Data Bank for additional and more up-to-date structural depositions and search the literature for further discussion of mechanism. 

Step 2 Write a half-reaction for both the oxidation and reduction processes and label oxidation and reduction. These half-reactions show only the species being oxidized (or the species being reduced) on the left side, with only the product of the oxidation (or reduction) on the right side. 

Electroanalytical techniques are an extension of classical oxidation-reduction chemistry, and indeed oxidation and reduction processes occur at the surface of or within the two electrodes, oxidation at one and reduction at the other. Electrons are consumed by the reduction process at one electrode and generated by the oxidation process at the other. The electrode at which oxidation occurs is termed the anode. The electrode at which reduction occurs is termed the cathode. The complete system, with the anode connected to the cathode via an external conductor, is often called a cell. The individual oxidation and reduction reactions are called half-reactions. The individual electrodes with their half-reactions are called half-cells. As we shall see in this chapter, the half-cells are often in separate containers (mostly to prevent contamination) and are themselves often referred to as electrodes because they are housed in portable glass or plastic tubes. In any case, there must be contact between the half-cells to facilitate ionic diffusion. This contact is called the salt bridge and may take the form of an inverted U-shaped tube filled with an electrolyte solution, as shown in Figure 14.2, or, in most cases, a small fibrous plug at the tip of the portable unit, as we will see later in this chapter. 

The situation is shown schematically in Figure 11.7, which shows that oxidation and reduction processes can be brought about when the potential values for the CB and VB straddle the potentials of the reduction and oxidation processes. 

The ability of varying the rate of the mass transport by agitating the solution (or the working electrode) constitutes the basis of hydrodynamic methods (hydrodynamics = liquids in motion), which are a further support to the study of electrode kinetics. Nevertheless we wish to cite them here simply to cover a drawback of cyclic voltammetry. In fact, cyclic voltammetry is unable to discriminate between oxidation and reduction processes, and vice-versa. 

Reduction and oxidation reactions in the subsurface environment lead to transformation of organic and inorganic contaminants. We consider chromium (Cr) as an example of an inorganic toxic chemical for which both oxidation and reduction processes may transform the valence of this element, in subsurface aqueous solutions, as a function of the local chemistry. 

The Mn- Mn separations in both planar and butterfly complexes are very similar and range between 2.80 A and 3.45 A. The magnetic properties of these complexes have been studied extensively, and showed antiferromagnetic interactions between the manganese(III) centers. Electrochemical behavior of the butterfly conmlexes is summarized in Table 9 and both oxidative and reductive processes are observed. 

The electrochemical oxidation of [ (bpy)2(NH3)Ru 2(/i-0)] releases N2. Oxidation of the ruthenium species initially gives [ (bpy)2(NH3)Ru 2(/i-0)] followed by irreversible five-electron oxidation and H+ loss. The Ru ° complexes [ (bpy)2LRu 2(/i-0)(p-02CMe)2] have been prepared as perchlorate salts for L = im, 1 - and 4-Meim. Structural data for L = 1 -Meim confirm a trans arrangement of imidazole and 0x0 ligands. The complexes exhibit reversible one-electron oxidation and reduction processes. The interaction of [ (bpy)2(H20)Ru 2(/u-0)] " with DNA results in reductive cleavage of the complex to form [Ru(bpy)2(H20)2] and the rate of reaction increases in the presence... 

The rotating disc electrode is constructed from a solid material, usually glassy carbon, platinum or gold. It is rotated at constant speed to maintain the hydrodynamic characteristics of the electrode-solution interface. The counter electrode and reference electrode are both stationary. A slow linear potential sweep is applied and the current response registered. Both oxidation and reduction processes can be examined. The curve of current response versus electrode potential is equivalent to a polarographic wave. The plateau current is proportional to substrate concentration and also depends on the rotation speed, which governs the substrate mass transport coefficient. The current-voltage response for a reversible process follows Equation 1.17. For an irreversible process this follows Equation 1.18 where the mass transfer coefficient is proportional to the square root of the disc rotation speed. 

Since the oxidation reactions require O2 while NO reduction requires CO, it is essential that the gases entering the ACC be at nearly the stoichiometric composition. This has led to the three-way catalysf in which oxidation and reduction processes are accomphshed simultaneously by maintaining the air-fuel mixture near stoichiometric. 

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