Oxidation-reduction defined

The concept of oxidation number leads directly to a working definition of the terms oxidation and reduction. Oxidation is defined as an increase in oxidation number and reduction as a decrease in oxidation number. Consider once again the reaction of zinc with a strong acid ... 

Two very important classes of chemical reactions are oxidation-reduction (redox) reactions and acid-base reactions, which are defined by molecules or ions accepting and donating electrons or protons, respectively. 

The high concentration of oxygen in the atmosphere plays a central role in the photochemistry and chemical reactivity of the atmosphere. Atmospheric oxygen also defines the oxidation reduction potential of surface waters saturated with oxygen. The presence of oxygen defines the speciation of many other aquatic species in surface waters. 

The equivalent is defined in terms of a chemical reaction. It is defined in one of two different ways, depending on whether an oxidation-reduction reaction or an acid-base reaction is under discussion. For an oxidation-reduction reaction, an equivalent is the quantity of a substance that will react with or yield 1 mol of electrons. For an acid-base reaction, an equivalent is the quantity of a substance that will react with or yield 1 mol of hydrogen ions or hydroxide ions. Note that the equivalent is defined in terms of a reaction, not merely in terms of a formula. Thus, the same mass of the same compound undergoing different reactions can correspond to different numbers of equivalents. The ability to determine the number of equivalents per mole is the key to calculations in this chapter. 

Define oxidation, reduction, oxidation number, oxidizing agent, and reducing agent. 

Oxidation is defined as the reduction of electron state by addition of oxygen or removal of electrons. Thermodynamic balance requires balance. For every oxidation, there must be a corresponding reduction. Thus, for oxidation to occur, there must be a compound capable of receiving the transferred electrons. Electron acceptor compounds can include oxygen, sulfate, Fe3+, phosphate, nitrate, C02, and certain organics. 

Reductants and oxidants are defined as electron donors and proton acceptors (Sect. 2.2.2). Because there are no free electrons, every oxidation is accompanied by a reduction and vice versa. In aqueous solutions, proton activities are defined by the pH ... 

The reduction-oxidation potential (typically expressed in volts) of a compound or molecular entity measured with an inert metallic electrode under standard conditions against a standard reference half-cell. Any oxidation-reduction reaction, or redox reaction, can be divided into two half-reactions, one in which a chemical species undergoes oxidation and one in which another chemical species undergoes reduction. In biological systems the standard redox potential is defined at pH 7.0 versus the hydrogen electrode and partial pressure of dihydrogen of 1 bar. 

The voltammetric response of an electrodeposited film of 2 in CH2CI2 with 0.1 M TBAH is shown in Figure 6 as a representative example. A well-defined, symmetrical oxidation-reduction wave is observed, which is characteristic of surface-immobilized reversible redox couples, with the expected linear relationship of peak current with potential sweep rate A formal potential value of =+0.42... 

The reaction of magnesium and oxygen is an example of an oxidation reaction. The combination of an element with oxygen was the traditional way to define an oxidation reaction. This definition of oxidation has been broadened by chemists to include reactions that do not involve oxygen. Our modern definition for oxidation is that oxidation takes place when a substance loses electrons. Anytime oxidation takes place and a substance loses one or more electrons, another substance must gain the electron(s). When a substance gains one or more electrons, the process is known as reduction. Reactions that involve the transfer of one or more electrons always involve both oxidation and reduction. These reactions are known as oxidation-reduction or redox reactions. 

Many half-reactions of interest to biochemists involve protons. As in the definition of AG °, biochemists define the standard state for oxidation-reduction reactions as pH 7 and express reduction potential as E °, the standard reduction potential at pH 7. The standard reduction potentials given in Table 13-7 and used throughout this book are values for E ° and are therefore valid only for systems at neutral pH Each value represents the potential difference when the conjugate redox pair, at 1 m concentrations and pH 7, is connected with the standard (pH 0) hydrogen electrode. Notice in Table 13-7 that when the conjugate pair 2ET/H2 at pH 7 is connected with the standard hydrogen electrode (pH 0), electrons tend to flow from the pH 7 cell to the standard (pH 0) cell the measured E ° for the 2ET/H2 pair is -0.414 V... 

We live under a blanket of the powerful oxidant 02. By cell respiration oxygen is reduced to H20, which is a very poor reductant. Toward the other end of the scale of oxidizing strength lies the very weak oxidant H+, which some bacteria are able to convert to the strong reductant H2. The 02 -H20 and H+ - H2 couples define two biologically important oxidation-reduction (redox) systems. Lying between these two systems are a host of other pairs of metabolically important substances engaged in oxidation-reduction reactions within cells. 

ANODIC OXIDATION. Oxidation is defined not only as reaction with oxygen, but as any chemical reaction attended by removal of electrons. Therefore, when current is applied to a pair of electrodes so as to make them anode and cathode, the former can act as a continuous remover of electrons and hence bring about oxidation (while the latter will favor reduction since it supplies electrons). This anodic oxidation is utilized in industry for various purposes, One of tire earliest to be discovered (H, Kolbe. 1849) was the production of hydrocarbons from aliphatic acids, or more commonly, from their alkali salts. Many other substances may be produced, on a laboratory scale or even, in some cases, on an economically sound production scale, by anodic oxidation. The process is also widely used to impart corrosion-resistant or decorative (colored) films to metal surfaces. For example, in the anodization or Eloxal process, the protection afforded by the oxide film ordinarily present on the surface of aluminum articles is considerably increased by building up this film by anodic oxidation. 

Today, the words oxidation and reduction have taken on a much broader meaning. An oxidation is now defined as the loss of one or more electrons by a substance—element, compound, or ion—and a reduction is the gain of one or more electrons by a substance. Thus, an oxidation-reduction reaction, or redox reaction, is a process in which electrons are transferred from one substance to another. 

Before we review the methods used to determine surface acidity, we wish to define the type of acidity that should be measured. An acid is an electron-pair acceptor. In our opinion, the term acid should be limited to this definition rather than broadening the term to include oxidizing agents as well. We agree with Flockhart and Pink (10) who suggest a clear distinction be made between Lewis acid-Lewis base reactions (which involve coordinate bond formation) and oxidation-reduction reactions (which involve complete transfer of one or more electrons). 

The term photochromism can be defined as a light-driven reversible transformation between two isomers possessing different absorption spectra.111,21 The two isomers differ from one another not only in their absorption spectra, but also in their geometrical structures, oxidation/reduction potentials, refractive indices, and dielectric constants. When such photochromic chromophores are incorporated into functional molecules, such as polymers, host molecules, conductive molecular wires, or liquid crystals, the functions can be switched by photoirradiation.[3 61 Photostimulated reversible changes in refractive index can also be applied to optical waveguide switching.171 This chapter reviews applications of photochromic chromophores, especially diar-ylethene derivatives, in various photo switching molecular systems. 

This chapter mainly focuses on the reactivity of 02 and its partially reduced forms. Over the past 5 years, oxygen isotope fractionation has been applied to a number of mechanistic problems. The experimental and computational methods developed to examine the relevant oxidation/reduction reactions are initially discussed. The use of oxygen equilibrium isotope effects as structural probes of transition metal 02 adducts will then be presented followed by a discussion of density function theory (DFT) calculations, which have been vital to their interpretation. Following this, studies of kinetic isotope effects upon defined outer-sphere and inner-sphere reactions will be described in the context of an electron transfer theory framework. The final sections will concentrate on implications for the reaction mechanisms of metalloenzymes that react with 02, 02 -, and H202 in order to illustrate the generality of the competitive isotope fractionation method. 

Equivalent masses are so defined because equal numbers of equivalents of two substances react exactly with each other. This is true for neutralization because one H+ neutralizes one OH-, and for oxidation-reduction reaction because the number of electrons lost by the reducing agent equals the number gained by the oxidation agent (electrons cannot be eliminated—by the law of conservation of matter). 

Steady state and transient experiments, the substantial though fragmented literature, and new interpretations are combined in an attempt to define and understand the catalytic kinetics for crrbon monoxide oxidation over cobalt oxide (C03O4) supported on alumina. The result is a rather coherent picture of oxidation-reduction catalysis by a metal oxide. It is shown that the dynamic methods yield vastly more information than steady state studies with significantly less experimental effort. 

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