Oxidation-reduction reactions electron transfer

In oxidation-reduction reactions, electron transfers (e ) are coupled with the transfer of protons (H ) to maintain a charge balance. A modification of the redox balance corresponds to a modification of the acid-base balance. The net reactions of the oxidation of C, S, and N exceed reduction reactions in these elemental cycles. A net production of ions in atmospheric precipitation is a necessary consequence. The disturbance is transferred to the terrestrial and aquatic environments, and it can impair terrestrial and aquatic ecosystems. 

Mitochondria organize electron transfer and the associated reactions leading to the adenosine triphosphate synthesis called oxidative phosphorylation. Synthesis of adenosine triphosphate is an endothermic reaction, and hence conserves the energy released during biological oxidation-reduction reactions. Electron transfer and associated reactions leading to ATP synthesis are membrane-bound phenomena. NADH and FADH2 are the reduced cofactors of NAD" " and the FAD. The oxidation of one NADH produces approximately three... 

When you place a piece of zinc metal into a solution of CuS04, you expect a chemical reaction because the more active zinc displaces the less active copper from its compound (Sec. 7.3). We learned in Chap. 13 that this is an oxidation-reduction reaction, involving transfer of electrons from the zinc to the copper. 

Photochromic compounds functioning by an oxidation-reduction mechanism (electron transfer), especially a photoreduction mechanism, are known in inorganic materials such as silver halides, which are utilized in eyewear lenses. Although the number of organic photochromic compounds operating via electron transfer is fewer than those by isomerization, heterolytic (or homolytic) cleavage, and pericyclic reactions, several classes of compounds have been reported, such as thiazines,1 viologens,2 and polycyclic quinones.3... 

Fig. 1. A schematic illustration of the mitochondrial (A) and Escherichia coli (B) respiratory chains. Respiratory enzymes perform a series of oxidation-reduction reactions by transferring electrons (dashed lines) through mobile electron carriers. Electron transfer is coupled to the pumping of protons (thick black arrows) from the A/-side (negative side) to the P-side (positive side) generating a proton gradient that ultimately drives the conversion of ADP to ATP.

The majority of such reactions entail the transfer of a functional group, such as a phosphoryl or an ammonium group, from one substrate to the other. In oxidation-reduction reactions, electrons are transferred between substrates. Multiple substrate reactions can be divided into two classes sequential displacement and double displacement. 

Oxidation-Reduction Versus Electron Transfer Reactions... 

In an oxidation/reduction reaction, electrons are transferred from one reactant to another. An example is the oxidation of iron(II) ions by cerium(IV) ions. The reaction is described by the equation... 

The nature of the ligand donor atom and the stereochemistry at the metal ion can have a profound effect on the redox potential of redox-active metal ions. What, we may ask, is the redox potential In the sense that they involve group transfer, redox reactions (more correctly oxidation—reduction reactions) are like other types of chemical reactions. Whereas, for example, in hydrolytic reactions a functional group is transferred to water, in oxidation-reduction reactions, electrons are transferred from electron donors (reductants) to electron acceptors (oxidants). Thus, in the reaction... 

Altogether, about 34% of the students used the term redox reaction or oxidation-reduction and electron transfer (22% of those from the beginner level, 66% from the advanced level) 14% use the terms precious metal or base... 

Oxidation-reduction (or electron transfer) reactions involving two metal complexes may occur by one of two mechanisms outer sphere (no direct involvement of the coordination sphere) or inner sphere (where one ligand on one complex forms a bridge to the other metal in the transition state). 

Enzymes have been named by adding the suffix -a.se to the name of the substrate or to a word or phrase describing their activity. Enzymes are classified according to reaction type. There are 6 major classes (with subclasses). Oxidoreductases catalyze oxidation-reduction reactions, the transfer of hydrogen atoms and electrons, for example dehydrogenation of lactate ... 

From the oxidation numbers below the equation, we see that the oxidation number of Zn changes from 0 to -1-2 and that of IT changes from -I-1 to 0. Thus, this is an oxidation-reduction reaction. Electrons are transferred from zinc atoms to hydrogen ions and, therefore, Zn is oxidized and IT is reduced. 

Oxidation-reduction reactions, which transfer electrons from a donor species to an acceptor species, play a central role in many biological and geological processes. Unlike strongly coupled reactions where the reactants form a structurally defined activated complex, as described by transition-state theory, the species that participate in electron transfer reactions are weakly coupled and retain their individuality. R.A. Marcus (Marcus, 1964, 1968,1985) developed the foimdation of electron transfer theory. 

Redox reaction n. An oxidation-reduction, or electron transfer, reaction. Mark JE (ed) (1996) Physical properties of polymers handbook. Springer-Verlag, New York. 

In oxidation-reduction reactions, electrons are transferred from one reactant to another. 

Oxidation—reduction, or electron-transfer, reactions, shown in the lower left of Figure 5.1, involve the oxidation or reduction of a coordinated transition metal atom or ion. The figure represents the simplest case of electron transfer—namely, one in which the coordination number remains constant. Equation (5.15) shows an example in which the hexaammineruthenium(III) ion is reduced by the reaction with the chromium(II) ion, and Equation (5.16) shows the oxidation of a hexacyanoferrate(II) complex by hexachloroiridate(IV). In neither of these complexes is the coordination number altered. 

There are six major classes (with subclasses). Oxidoreductases catalyze oxidation-reduction reactions, the transfer of hydrogen atoms and electrons, for example dehydrogenation of lactate ... 

The ready reversibility of this reaction is essential to the role that qumones play in cellular respiration the process by which an organism uses molecular oxygen to convert Its food to carbon dioxide water and energy Electrons are not transferred directly from the substrate molecule to oxygen but instead are transferred by way of an electron trans port chain involving a succession of oxidation-reduction reactions A key component of this electron transport chain is the substance known as ubiquinone or coenzyme Q... 

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. 

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... 

Nicotinamide is an essential part of two important coenzymes nicotinamide adenine dinucleotide (NAD ) and nicotinamide adenine dinucleotide phosphate (NADP ) (Figure 18.19). The reduced forms of these coenzymes are NADH and NADPH. The nieotinamide eoenzymes (also known as pyridine nucleotides) are electron carriers. They play vital roles in a variety of enzyme-catalyzed oxidation-reduction reactions. (NAD is an electron acceptor in oxidative (catabolic) pathways and NADPH is an electron donor in reductive (biosynthetic) pathways.) These reactions involve direct transfer of hydride anion either to NAD(P) or from NAD(P)H. The enzymes that facilitate such... 

Thus, Experiment 7 involved the same oxidation-reduction reaction but the electron transfer must have occurred locally between individual copper atoms (in the metal) and individual silver ions (in the solution near the metal surface). This local transfer replaces the wire middleman in the cell, which carries electrons from one beaker (where they are released by copper) to the other (where they are accepted by silver ions). 

Thus, the reaction by which a metal dissolves in an acid is conveniently discussed in terms of oxidation and reduction involving electron transfer. The reaction can be divided into half-reactions to show the electron gain (by H+ ions) and the electron loss (by metal atoms). 

Is this an oxidation-reduction reaction Historically, it surely is, for the term oxidation originally referred specifically to reactions with oxygen. Yet our electron-transfer view of oxidation-reduction reactions provides no help in deciding so. Where in reaction (76) is there any evidence of electrons being gained or lost In such a doubtful case, our oxidation number scheme provides an answer. Applying the same assumptions used in treating the HSOf-HSOi"... 

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. 

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