Oxidation-reduction s. Redox

Oxidation-reduction s. Redox... N-Oxide radicals (s. a. N-Oxyls) reactions, review 22, 136 suppl. 27... 

Quinones are an interesting and valuable class of compounds because of their oxidation-reduction, or redox, properties. They can be easily reduced to hydroquinones (g-dihydroxybenzenes) by reagents such as NaBH4 and SnCl2/ and hydroquinones can be easily reoxidized back to quinones by Fremy s salt. 

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. 

In many important chemical reactions, electrons are transferred from atom to atom. We are surrounded by these reactions, commonly called oxidation-reduction (or redox) reactions, inside and out. Let s consider a typical new millennium family, sitting around the dining room table after the dishes have been cleared. 

Iron-sulfur proteins are found in a variety of organisms, bacteria, plants, and animals, and serve as electron transfer agent.s via one-electron oxidation-reduction step [redox potential --0.43V in chloroplasts to 1-0.35 V in... 

In precipitation reactions cations and anions come togetiier to form an insoluble ionic compound. In neutralization reactions H ions and OH ions come togetii-er to form H2O molecules. Now lef s consider a third imporfanf kind of reaction in which electrons are transferred between reactants. Such reactions are called oxidation-reduction, or redox, reactions. 

In Chapter 5 we saw that, in terms of the Br0nsted-Lowry theory, acid-base reactions involve proton transfer. Another large and important group of chemical reactions, particularly in aqueous solutions, involves electron transfer these are referred to as oxidation-reduction (or redox) reactions. Redox reactions are involved (1) in photosynthesis, which releases oxygen into the Earth s atmosphere (2) in the combustion of fuels, which is responsible for rising concentrations of atmospheric carbon dioxide (3) in the formation of acid precipitation and (4) in many chemical reactions in Earth sediments. 

Equation 9.6 illustrates why this is an oxidation-reduction, or redox, reaction. Redox reactions are electron-transfer reactions Electrons are actually or effectively transferred from one species to another. In Equation 9.6, a neutral iron atom becomes an iron(II) ion with a 2+ charge. It does this by losing two electrons Fe(s) Fe (aq) -l- 2 e . Where do the electrons go One goes to each of two hydrogen ions so they become neutral hydrogen atoms, and the atoms combine to form a diatomic molecule 2 H+(aq) -i-2e H + H H2(g). Thus, the electrons are literally transferred from one species, iron atoms, to another species, hydrogen ions. The iron atoms, which lost the electrons, are said to have been oxidized, and hydrogen ions, the receivers of electrons, have been reduced. 

It is also often taken for granted that many of the Earth s subsystems are exposed to free oxygen (O2), leading to a range of one-way reactions of reduced materials (such as organic carbon or metal sulfides) to an oxidized form. As pointed out many times in earlier chapters, the oxidation-reduction status of the planet is the consequence of the dynamic interactions of biogeochemical cycles. As is the case with the acid-base balances, there is considerable sensitivity to perturbations of "redox" conditions, sometimes dramatically as in the case of bodies of water that suddenly become anaerobic because of eutrophication. Another extreme... 

Equilibrium considerations other than those of binding are those of oxidation/reduction potentials to which we drew attention in Section 1.14 considering the elements in the sea. Inside cells certain oxidation/reductions also equilibrate rapidly, especially those of transition metal ions with thiols and -S-S- bonds, while most non-metal oxidation/reduction changes between C/H/N/O compounds are slow and kinetically controlled (see Chapter 2). In the case of fast redox reactions oxidation/reduction potentials are fixed constants. 

The tarnish on silver, Ag2S, can be removed by boiling the silverware in slightly salty water (to improve the water s conductivity) in an aluminum pan. The reaction is an oxidation-reduction reaction that occurs spontaneously, similar to the redox reaction occurring in a voltaic cell. The Ag in Ag2S is reduced back to silver, while the A1 in the pan is oxidized to Al3+. 

Oxidation-reduction (redox). These reactions involve transfer of electrons or change in oxidation number. A decrease in the number of H atoms bonded to C and an increase in the number of bonds to other atoms such as C, O. N, Cl. Br. F. and S signals oxidation. 

In the case of two flavoenzyme oxidase systems (glucose oxidase (18) and thiamine oxidase s where both oxidation-reduction potential and semiquinone quantitation values are available, semiquinone formation is viewed to be kinetically rather than thermodynamically stabilized. The respective one-electron redox couples (PFl/PFl- and PFI7PFIH2) are similar in value (from essential equality to a 50 mV differential) which would predict only very low levels of semiquinone (32% when both couples are identical) at equilibrium. However, near quantitative yields (90%) of semiquinone are observed either by photochemical reduction or by titration with dithionite which demonstrates a kinetic barrier for the reduction of the semiquinone to the hydroquinone form. The addition of a low potential one-electron oxidoreductant such as methyl viologen generally acts to circumvent this kinetic barrier and facilitate the rapid reduction of the semiquinone to the hydroquinone form. 

However, from a chemical viewpoint, dQ can also be expressed in terms of the electrons transferred at the electrodes for each increment di of cell reaction. For this purpose, it is convenient to write the overall redox cell reaction as separate oxidation/reduction halfreactions, expressing the loss or gain of z electrons at each electrode in the balanced cell reaction (i.e., involving z equivalents of charge transferred in oxidization and reduction steps). It is also convenient to quantify total charge in molar units (i.e., Avogadro s number NA of electrons) as expressed by the Faraday constant T,... 

A reaction in which the oxidation number of an element is increased. Examples (1, 2, 3) 2 Mg(s) + 02(g) - 2 MgO(s) (2, 3) Mg(s) - Mg2+(s) + 2e". oxidation number The effective charge on an atom in a compound, calculated according to a set of rules (Toolbox K.l). An increase in oxidation number corresponds to oxidation, and a decrease, to reduction, oxidation-reduction reaction See redox reaction. oxidation state The actual condition of a species with a specified oxidation number. 

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