Oxidation-reduction reactions states

In a complexation reaction, a Lewis base donates a pair of electrons to a Lewis acid. In an oxidation-reduction reaction, also known as a redox reaction, electrons are not shared, but are transferred from one reactant to another. As a result of this electron transfer, some of the elements involved in the reaction undergo a change in oxidation state. Those species experiencing an increase in their oxidation state are oxidized, while those experiencing a decrease in their oxidation state are reduced, for example, in the following redox reaction between fe + and oxalic acid, H2C2O4, iron is reduced since its oxidation state changes from -1-3 to +2. 

The abihty of iron to exist in two stable oxidation states, ie, the ferrous, Fe ", and ferric, Fe ", states in aqueous solutions, is important to the role of iron as a biocatalyst (79) (see Iron compounds). Although the cytochromes of the electron-transport chain contain porphyrins like hemoglobin and myoglobin, the iron ions therein are involved in oxidation—reduction reactions (78). Catalase is a tetramer containing four atoms of iron peroxidase is a monomer having one atom of iron. The iron in these enzymes also undergoes oxidation and reduction (80). 

The optical absorption spectra of Pu ions in aqueous solution show sharp bands in the wavelength region 400—1100 nm (Fig. 4). The maxima of some of these bands can be used to determine the concentration of Pu ions in each oxidation state (III—VI), thus quantitative deterrninations of oxidation—reduction equiUbria and kinetics are possible. A comprehensive summary of kinetic data of oxidation—reduction reactions is available (101) as are the reduction kinetics of Pu + (aq) (84). 

Oxidation—Reduction. Redox or oxidation—reduction reactions are often governed by the hard—soft base rule. For example, a metal in a low oxidation state (relatively soft) can be oxidized more easily if surrounded by hard ligands or a hard solvent. Metals tend toward hard-acid behavior on oxidation. Redox rates are often limited by substitution rates of the reactant so that direct electron transfer can occur (16). If substitution is very slow, an outer sphere or tunneling reaction may occur. One-electron transfers are normally favored over multielectron processes, especially when three or more species must aggregate prior to reaction. However, oxidative addition... 

In addition to simple dissolution, ionic dissociation and solvolysis, two further classes of reaction are of pre-eminent importance in aqueous solution chemistry, namely acid-base reactions (p. 48) and oxidation-reduction reactions. In water, the oxygen atom is in its lowest oxidation state (—2). Standard reduction potentials (p. 435) of oxygen in acid and alkaline solution are listed in Table 14.10- and shown diagramatically in the scheme opposite. It is important to remember that if or OH appear in the electrode half-reaction, then the electrode potential will change markedly with the pH. Thus for the first reaction in Table 14.10 O2 -I-4H+ -I- 4e 2H2O, although E° = 1.229 V,... 

The reactions employed in titrimetric analysis fall into four main classes. The first three of these involve no change in oxidation state as they are dependent upon the combination of ions. But the fourth class, oxidation-reduction reactions, involves a change of oxidation state or, expressed another way, a transfer of electrons. 

These results may easily be rationalized by assuming that the formation of hydroxy sulphoxides 91, 92 and 93 from hydroperoxysulphides 89 and 90 is an intramolecular oxidation-reduction reaction proceeding through a five-membered transition state 94. However, an alternative intermolecular mechanism in which the approach of the oxidant is directed by the hydroperoxy or the hydroxy function in the reductant cannot be excluded. 

The principal abiotic processes affecting americium in water is the precipitation and complex formation. In natural waters, americium solubility is limited by the formation of hydroxyl-carbonate (AmOHC03) precipitates. Solubility is unaffected by redox condition. Increased solubility at higher temperatures may be relevant in the environment of radionuclide repositories. In environmental waters, americium occurs in the +3 oxidation state oxidation-reduction reactions are not significant (Toran 1994). 

Oxidation-reduction reactions may affect the mobility of metal ions by changing the oxidation state. The environmental factors of pH and Eh (oxidation-reduction potential) strongly affect all the processes discussed above. For example, the type and number of molecular and ionic species of metals change with a change in pH (see Figures 20.5-20.7). A number of metals and nonmetals (As, Be, Cr, Cu, Fe, Ni, Se, V, Zn) are more mobile under anaerobic conditions than aerobic conditions, all other factors being equal.104 Additionally, the high salinity of deep-well injection zones increases the complexity of the equilibrium chemistry of heavy metals.106... 

The reduction of cyanocobalamin gives three possible oxidation states for the cobalt atom (Fig. 2). Electron spin resonance studies with Bi2-r reveals that this molecule is the only paramagnetic species giving a spectrum expected for a tetragonal low spin Co(II) complex. Controlled potential reduction of cyanocobalamin to Bi2-r proves that this reduction involves one electron, and further reduction of Bi2-r to B12-S requires a second single electron (16—19). At one time B12-S was considered to be a hydride of Co(III), but controlled potential coulometry experiments provided evidence against a stable hydride species (16). However, these experimental data do not exclude the possibility of a stable Co(III) hydride as the functional species in enzyme catalyzed oxidation reduction reactions. 

It is convenient to divide organic chemical reactions between acid-base and oxidation-reduction reactions as in inorganic chemistry. In acid-base reactions the oxidation states of carbon do not change, e.g. in hydrolysis, where reaction is, for example,... 

CH3CO.OR + H20 - CH3COOH + ROH In oxidation-reduction reactions the oxidation states of carbon change, for example, CH3COOH + H2 - CH3CHO + h2o... 

Three classes of polynuclear complexes containing metal-metal bonds possess emissive excited states that undergo oxidation-reduction reactions in solution the prototypes are Re2Cl T(dlt d lt),... 

Pt2(P205H2) - (d8-d8), and Mo6Clft ( )6. Two- electron oxidations of Re2Cl and Pt2(P205H2)it have been achieved by one-electron acceptor quenching of the excited complexes in the presence of Cl, followed by one-electron oxidation of the Cl -trapped mixed-valence species. Two-electron photochemical oxidation-reduction reactions also could occur by excited-state atom transfer pathways, and some encouraging preliminary observations along those lines are reported. 

A (a) This is a metathesis or double displacement reaction. Elements do not change oxidation states during this reaction. It is not an oxidation-reduction reaction. 

In well-aerated soil, it is expected that all species will be in their highest oxidation states. However, this does not happen for reasons elucidated in previous chapters. In well-aerated soil, both ferrous and ferric iron can exist along with elemental iron.3 Zinc, copper, and especially manganese can apparently exist in a mixture of oxidation states simultaneously in soil. Add to this a multitude of organic species that are also capable of oxidation-reduction reactions and the result is truly a complex voltammetric system [12,13],... 

Chemically, nonmetals are usually the opposite of metals. The nonmetallic nature will increase towards the top of any column and toward the right in any row on the periodic table. Most nonmetal oxides are acid anhydrides. When added to water, they will form acids. A few nonmetals oxides, most notably CO and NO, do not react. Nonmetal oxides that do not react are neutral oxides. The reaction of a nonmetal oxide with water is not an oxidation-reduction reaction. The acid that forms will have the nonmetal in the same oxidation state as in the reacting oxide. The main exception to this is N02, which undergoes an oxidation-reduction (disproportionation) reaction to produce HN03 and NO. When a nonmetal can form more than one oxide, the higher the oxidation number of the nonmetal, the stronger the acid it forms. 

Since many of the transformations undergone by metabolites involve changes in oxidation state, it is understandable that cofactors have been developed to act as electron acceptors/ donors. One of the most important is that based on NAD/NADP. NAD+ can accept what is essentially two electrons and a proton (a hydride ion) from a substrate such as ethanol in a reaction catalysed by alcohol dehydrogenase, to give the oxidized product, acetaldehyde and the reduced cofactor NADH plus a proton (Figure 5.2). Whereas redox reactions on metal centres usually involve only electron transfers, many oxidation/reduction reactions in intermediary metabolism, as in the case above, involve not only electron transfer but... 

It is also sometimes simply referred to as the gram-equivalent . However, GEW has two distinct definitions for neutralization as well as as oxidation-reduction reactions as stated below ... 

Self-propagating reduction reactions Several oxide reduction solid-state reactions have been known for a long time. Starting materials are considered which react together highly exothermically. Once the reaction is initiated, enough heat is produced for very high temperature to be attained, and complete reaction occurs rapidly. The so-called thermite process corresponds to the reactions ... 

We have been investigating the oxidation-reduction reactions of the binuclear iron site in the protein matrix (27-32). The methemerythrin form contains both irons in the +3 oxidation state and can be reduced in two steps (by dithionite ion (27, 31), reduced methylviologen, and photochemically using a riboflavin/EDTA mixture (28)) to the deoxy form in which both irons are in the +2 oxidation state. The intermediate (semi-met), in which one iron is +3 and the other iron +2, has been... 

A primary goal of chemical separation processes in the nuclear industry is to recover actinide isotopes contained in mixtures of fission products. To separate the actinide cations, advantage can be taken of their general chemical properties [18]. The different oxidation states of the actinide ions lead to ions of charges from +1 (e.g., NpOj) to +4 (e.g., Pu" " ) (see Fig. 12.1), which allows the design of processes based on oxidation reduction reactions. In the Purex process, for example, uranium is separated from plutonium by reducing extractable Pu(IV) to nonextractable Pu(III). Under these conditions, U(VI) (as U02 ) and also U(IV) (as if present, remain in the... 

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