Redox reactions elements

In combination redox reactions, elements combine to form a compound, or a compound and an element combine. 

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. 

Balancing a redox reaction is often more challenging than balancing other types of reactions, because we must balance both electrons and elements. Perhaps the simplest way to balance a redox reaction is by the half-reaction method, which consists of the following steps. 

Type B (redox) reactions are more complex. Sulfide in this reaction is converted into some other oxidation state of sulfur. For example, sulfides can be converted to a zero oxidation state of elemental sulfur by oxygen ... 

Metallic elements taking part in redox reactions, such as zinc in the reaction above, commonly act as reducing agents they are oxidized to cations such as Zn2+. Other reducing agents include hydrogen gas, which can be oxidized to H+ ions ... 

The concept of oxidation number is used to simplify the electron bookkeeping in redox reactions. For a monatomic ion (e.g., Na+, S2 ), the oxidation number is, quite simply, the charge of the ion (+1, —2). In a molecule or polyatomic ion, the oxidation number of an element is a pseudo-charge obtained in a rather arbitrary way, assigning bonding electrons to the atom with the greater attraction for electrons. 

K.25 The following redox reactions are important in the refining of certain elements. Balance the equations and in each case, write the name of the source compound of the element (in bold face) and the oxidation state in that compound of the element that is being extracted ... 

Redox reactions that have a positive Gibbs free energy of reaction are not spontaneous, but an electric current can be used to make them take place. For example, there is no common spontaneous chemical reaction in which fluorine is a product, and so the element cannot be isolated by any common chemical reaction. It was not until 1886 that the French chemist Henri Moissan found a way to force the... 

The last chapter in this introductory part covers the basic physical chemistry that is required for using the rest of the book. The main ideas of this chapter relate to basic thermodynamics and kinetics. The thermodynamic conditions determine whether a reaction will occur spontaneously, and if so whether the reaction releases energy and how much of the products are produced compared to the amount of reactants once the system reaches thermodynamic equilibrium. Kinetics, on the other hand, determine how fast a reaction occurs if it is thermodynamically favorable. In the natural environment, we have systems for which reactions would be thermodynamically favorable, but the kinetics are so slow that the system remains in a state of perpetual disequilibrium. A good example of one such system is our atmosphere, as is also covered later in Chapter 7. As part of the presentation of thermodynamics, a section on oxidation-reduction (redox) is included in this chapter. This is meant primarily as preparation for Chapter 16, but it is important to keep this material in mind for the rest of the book as well, since redox reactions are responsible for many of the elemental transitions in biogeochemical cycles. 

The elemental reaction used to describe a redox reaction is the half reaction, usually written as a reduction, as in the following case for the reduction of oxygen atoms in O2 (oxidation state 0) to H2O (oxidation state —2). The half-cell potential, E°, is given in volts after the reaction ... 

Thermodynamically, virtually all metals in the elemental form are unstable with respect to redox reactions in environments where they are exposed to air and water, i.e., virtually all environments where they are used. Those metals least likely to oxidize (corrode) were long ago given the distinguished title "noble metals." Efforts to prevent metals from corroding, and the cost of repairing and replacing metal structures that have done so, runs into the billions of dollars annually. Thus, one characteristic feature of the society s use of metals is that the metals are continuously, albeit slowly, "degrading" to a less useful form from the moment they are put into use. 

Abstract Inorganic polysulfide anions and the related radical anions S play an important role in the redox reactions of elemental sulfur and therefore also in the geobio chemical sulfur cycle. This chapter describes the preparation of the solid polysulfides with up to eight sulfur atoms and univalent cations, as well as their solid state structures, vibrational spectra and their behavior in aqueous and non-aqueous solutions. In addition, the highly colored and reactive radical anions S with n = 2, 3, and 6 are discussed, some of which exist in equilibrium with the corresponding diamagnetic dianions. 

The sulfur-rich oxides S 0 and S 02 belong to the group of so-called lower oxides of sulfur named after the low oxidation state of the sulfur atom(s) compared to the best known oxide SO2 in which the sulfur is in the oxidation state +4. Sulfur monoxide SO is also a member of this class but is not subject of this review. The blue-green material of composition S2O3 described in the older literature has long been shown to be a mixture of salts with the cations S4 and Ss and polysulfate anions rather than a sulfur oxide [1,2]. Reliable reviews on the complex chemistry of the lower sulfur oxides have been published before [1, 3-6]. The present review deals with those sulfur oxides which contain at least one sulfur-sulfur bond and not more than two oxygen atoms. These species are important intermediates in a number of redox reactions of elemental sulfur and other sulfur compounds. 

The only structurally characterized derivative of a trisimido organophos-phonate anion is the spirocyclic tellurium(IV) complex (19), which is obtained from the interesting redox reaction between PhPCl2 and [Li2Te(N Bu)3 ] 2 [27]. The phosphorus(V)-centered ligands are generated by imide transfer from tellurium to the phosphorus(III) atoms with concomitant reduction of one-half of the tellurium in the Te(IV) reagent to elemental tellurium [27]. 

A detailed discussion of redox reactions must wait until Chapter 19, after we explore the nature of the atom, periodic properties of the elements, and thermodynamics. For now, we focus on only a few types of redox reactions that are common and relatively simple. 

Although these reactions look similar, the redox reaction includes a pure element. Nearly all reactions involving pure elements are redox, because the oxidation number changes from zero to either a positive or a negative value. 

The first step In balancing a redox reaction is to divide the unbalanced equation into half-reactions. Identify the participants in each half-reaction by noting that each half-reaction must be balanced. That Is, each element In each half-reaction must be conserved. Consequently, any element that appears as a reactant In a half-reaction must also appear among the products. Hydrogen and oxygen frequently appear in both half-reactions, but other elements usually appear In just one of the half-reactions. Water, hydronium ions, and hydroxide ions often play roles In the overall stoichiometry of redox reactions occurring in aqueous solution. Chemists frequently omit these species in preliminary descriptions of such redox reactions. 

In terms of gross features of mechanism, a redox reaction between transition metal complexes, having adjacent stable oxidation states, generally takes place in a simple one-equivalent change. For the post-transition and actinide elements, where there is usually a difference of two between the stable oxidation states, both single two-equivalent and consecutive one-equivalent changes are possible. 

The redox reactions of the actinide elements have been the subject of a recent and authoritative review by Newton and Baker . The net activation process concept is used to interpret the experimental data. Empirical correlations shown to exist include those between the entropies of the activated complexes and their charges, and, for a set of similar reactions, between AG and AG , and and A/f . The present state of the evidence for binuclear species is discussed. 

These enzymes catalyze the addition of the elements of water to carbon-carbon double bonds (C=C), carbon-carbon triple bonds (C C), carbon-nitrogen double bonds (C=N), or carbon-nitrogen triple bonds (C N). These reactions are completely different from oxidoreductases since no redox reactions are involved. Illustrative examples include the following ... 

Fe-S complexes have important functions in today s living systems, in enzymes such as the ferredoxins and oxidoreductases, as well as in electron transport proteins. It is striking that these redox reactions mainly involve elements and compounds such as CO, H2 and N2, which were probably also components of the primeval Earth s atmosphere. Thus, the assumption of an active involvement of Fe-S clusters in a (hypothetical) Fe-S world in processes which finally led to biogenesis appears completely reasonable We now have a background to the theory of the chemoau-totrophic origin of life . 

Concentrations of trace elements in soil solution may be controlled by the solubility of certain solid phases via dissolution/(co-)precipitation or by other physicochemical and biological processes such as adsorption-desorption, complexation, and redox reactions. 

In summary, we may add that bacterial utilization of quinoline and its derivatives as a rule depends on the availability of traces of molybdate in the culture medium [363], In contrast, growth of the bacterial strains on the first intermediate of each catabolic pathway, namely, the lH-2-oxo or 1 II-4-oxo derivatives of the quinoline compound was not affected by the availability of molybdate. This observation indicated a possible role of the trace element molybdenum in the initial hydroxylation at C2. In enzymes, Mo occurs as part of the redox-active co-factor, and all the Mo-enzymes involved in N-heteroatomic compound metabolism, contain a pterin Mo co-factor. The catalyzed reaction involves the transfer of an oxygen atom to or from a substrate molecule in a two-electron redox reaction. The oxygen is supplied by the aqueous solvent. Certainly, the Mo-enzymes play an important role in the initial steps of N-containing heterocycles degradation. 

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. 

Only transformations in the longest linear sequence (LLS) are considered. The term skeleton constructions refers to C-C and C-O bond formations (notwithstanding redox reactions) that directly introduce native structural features of the bryostatins without further modification. The term other functional group manipulations refers to steps that indirectly introduce native structural elements, the interconversion of functional groups (e.g., the introduction and removal of auxiliaries) and miscellaneous transformations that do not involve skeleton construction... 

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