Oxidation-reduction reactions defined

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. 

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. 

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. 

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). 

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). 

In order to keep track of electron shifts in oxidation-reduction reactions, it is convenient to use the concept of oxidation number or oxidation state of various atoms involved in oxidation-reduction reactions. The oxidation number is defined as the formal charge which an atom appears to have when electrons are counted in accordance with the following rather arbitrary rules. 

One purpose of this paper is to examine the evidence that the rates of oxidation—reduction reactions are related to the conductivity of the medium separating the oxidant and reductant. This survey will then describe experiments now in progress to investigate systematically the nonadiabatic regime in oxidation—reduction reactions. First the relationship between what has loosely been referred to as the conductivity of the medium and the title term, nonadiabatic, should be defined. 

The concept of oxidation states (also called oxidation numbers) provides a way to keep track of electrons in oxidation-reduction reactions. Oxidation states are defined by a set of rules, most of which describe how to divide up the shared electrons in compounds containing covalent bonds. However, before we discuss these rules, we need to discuss the distribution of electrons in a bond. 

Equivalent mass based on oxidation-reduction reactions. For oxidation-reduction reactions, the equivalent mass is defined as the mass of substance per mole of electrons involved (Snoeyink and Jenkins, 1980). 

The work described in the foregoing sections is of a preliminary nature. Nevertheless, it offers hope that experimental scales of free hydrogen ion concentration (pcn or pmn) in seawater may be feasible. One need not know pmn or pan to derive meaningful equilibrium data, such as acid-base ratios and solubilities, provided that suitable apparent equilibrium constants are chosen (7). In these cases, the unit selected for the acidity scale disappears by cancellation. Nevertheless, the acidity of seawater is a parameter of broader impact. It plays a role, for example, in the kinetics of organic oxidation-reduction reactions and in a variety of quasi-equilibrium processes of a biological nature. The concentration of free hydrogen ions is clearly understood, and its role in these complex interactions is more clearly defined than that of a quantity whose unit purports to involve the concept of a single-ion activity. 

Standard substances in clinical chemistry include primary standards, which can be obtained sufficiently pure to be used for the preparation of solutions by weighing or by reference to other definable physical characteristics (e.g., constant-boiling hydrochloric acid). Primary standard chemicals are available for acid-base reactions, precipitation reactions, oxidation-reduction reactions, etc. (V3), and are used in these various categories of analytical determination to validate the preparation of solutions of other chemical substances which cannot be obtained in a form suitable to meet the criteria demanded for a primary standard. Following their calibration in terms of a primary standard, these other chemieals can act as secondary standards. 

Because every chemical reaction involves charge transfer (or at least partial electron shifts), the distinction between an acid-base reaction and an oxidation-reduction reaction becomes meaningless unless defined in terms of changes in conventionally assigned oxidation number.21 This point of view also has been expressed before, but still is not discussed in contemporary textbooks of general, organic, and inorganic chemistry. 

In the modern biosphere most energy comes from sunlight, but this skill had to be learned and it is likely that the first life obtained its energy from inorganic chemical reactions. There are a wide range of possible oxidation-reduction reactions which life utilizes to generate energy. These define a number of... 

On May 4, 1923, the Dutch chemical journal Receueildes Travaux Chim-iques des Pays-Bas (42 718) received a paper from Bronsted on existing concepts of acids and bases. In this paper Bronsted demonstrated how useful it was to define an acid as a proton donor and a base as a proton acceptor. In the Bronsted scheme, acid-base reactions are proton transfer reactions. Every acid is related to a conjugate base, and every base to a conjugate acid. Also in this paper he pointed out that there is an analogy between the proton transfer that is characteristic of acid-base reactions and the electron transfer that is characteristic of oxidation-reduction reactions. 

Electrochemistry is best defined as the study of the interchange of chemical and electrical energy. It is primarily concerned with two processes that involve oxidation-reduction reactions the generation of an electric current from a spontaneous chemical reaction and the opposite process, the use of a current to produce chemical change. 

In Chapter VI we defined oxidation as the loss of electrons and reduction as the gain of electrons, and we showed that oxidation-reduction reactions that involve ions can generally be made to produce an electric current. Chemical energy is thereby transformed into electrical energy, and the electromotive force of the cell is a measure of the free energy of the reaction. Conversely, we can make an electric current produce a chemical reaction. If a current is passed through an electrolyte—a conducting solution or molten salt—oxidation takes place at the anode, where electrons are withdrawn from the solution and reduction takes place simultaneously at the cathode, where the electrons enter. 

Unique characteristics of ferromanganese nodules and associated oxidation-reduction reactions have been used by soil scientists as morphological indicators to help identify hydric soils (see Chapter 3). These characteristics are termed by soil scientists as redoximorphic features however, various terms such as redox concentrations, redox depletions, and reduced matrix are synonymously used for the oxidation-reduction of iron and manganese and their respective concentrations. We prefer not to define these characteristics as redoximorphic features because oxidation-reduction reactions not only involve iron and manganese but also a range of elements that support biotic communities in the biosphere. 

Oxidation-reduction reactions or redox reactions are defined as a family of reactions in which electrons are transferred between species. The species that receives electrons is reduced and that donates electrons is oxidized. Similar to acid-base reactions, redox reactions are always a matched pair of half-reactions. An oxidation reaction cannot occur without a reduction reaction occurring simultaneously. 

In natural waters occur not one but several oxidation-reduction reactions. These reactions are associated with the presence of several elements, which are capable of changing their charge, and run in parallel. For this reason, total oxidation potential of the solution is defined by the nature and concentration of all redox-couples. Components which noticeably affect the solution s oxidation-reduction potential are called electroactive. Elements whose concentration and form of existence actually control solution s oxidation are culled potential-setting. In natural waters these are usually O, S, C, N and Fe. The medium whose oxidation potential value almost does not change with the addition of oxidizers or reducers is called redox-buffers. The redox-buffer may be associated with composition of the water itself, of its host rocks or with the effect of atmosphere. In the subsurface redox-buffers are associated, as a rule, with the content of iron, sulphur or manganese. Stably high Eh value in the surface and ground waters is caused by the inexhaustible source of in the atmosphere. 

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