Nonmetal redox reactions

By far the most important redox reaction relative to chemical stability is the reaction between an oxidizable material and oxygen from air. The particle size and any droplets have a large effect on the combustion properties. Some substances react so rapidly in air that ignition occurs spontaneously. These so called pyrophoric compounds (white phosphorus, alkali metals, metal hydrides, some metal catalysts, and fully alkylated metals and nonmetals) must be stored in the absence of air. 

We ll see in later chapters that redox reactions are common for almost every element in the periodic table except for the noble gas elements of group 8A. In general, metals act as reducing agents, and reactive nonmetals such as O2 and the halogens act as oxidizing agents. 

Electrolysis reactions use direct current to prodnce redox reactions. Production of very active elemental metals and very active elemental nonmetals is often done using electrolysis. The conditions under which an electrolysis is carried ont often make a great deal of difference as to which prodncts are obtained. (Section 17.4)... 

I really like to call my interest the study of oxidation-reduction (redox) reactions, and insofar as such reactions between metal ions are concerned, there was little prior work when I began my own. Werner had encountered redox reactions in the coirrse of his research and, in fact, made good use of them in the preparative procedures he developed. I m not sure that he thought much about the fundamental difference between them and the reactions he was primarily interested in, which were substitution reactions of metal complexes (coordination complexes). Many of the basic ideas underlying redox reactions were developed in the study of nonmetal chemistry. For example, my mentor at Berkeley became an expert in the redox chemistry of halogenates, after working with Professor Luther in Germany in the first part of this century. 

Almost all of the reactions that the practicing inorganic chemist observes in the laboratory take place in solution. Although water is the best-known solvent, it is not the only one of importance to the chemist. The organic chemist often uses nonpolar solvents such as carbon tetrachloride and benzene to dissolve nonpolar compounds. These are also of interest to the inorganic chemist and, in addition, polar solvents such as liquid ammonia, sulfuric acid, glacial acetic acid, sulfur dioxide, and various nonmetal halides have been studied extensively. The study of solution chemistry is intimately connected with acid-base theory, and the separation of this material into a separate chapter is merely a matter of convenience. For example, nonaqueous solvents are often interpreted in terms of the solvent system concept, the formation of solvates involve acid-base interactions, and even redox reactions may be included within the Usanovich definition of acid-base reactions. 

In an electrolytic cell, an external energy source makes a nonspontaneous redox reaction (AG > 0) occur. In electrolysis of a molten binary ionic compound (salt), the cation is reduced to the metal and the anion is oxidized to the nonmetal. For an aqueous salt solution, the products depend on whether water or one of the ions of the salt requires less energy to be reduced or oxidized. 

Reactions Between Nonmetals Although we can identify reactions between metals and nonmetals as redox reactions, it is more difficult to decide whether a given reaction between nonmetals is a redox reaction. In fact, many of fhe mosf significanf redox reactions involve only nonmetals. For example, combustion reactions such as methane burning in oxygen are oxidation-reduction reactions. 

Figure 19.3 The metal potassium and the nonmetal chlorine undergo a redox reaction to form potassium chloride. 

Oxidation states are used for two purposes to determine if a redox reaction has occurred, and to balance the redox equations. In Chap. 3, oxidation was defined as a loss of electrons from the valence shell of the metals, and reduction was defined as the gain of electrons by the valence shell of a nonmetal. The reaction of sodium with chlorine was written as two separate processes, to emphasize the loss of electrons by sodium with the simultaneous gain of electrons by chlorine, as... 

Reduction is the gain of electrons, and in the conversion of chlorine to chloride ions (Cl ) in this reaction, each chlorine atom has been reduced by the gain of a single electron. Reactions in which one reactant is oxidized and another is reduced are known as oxidation-reduction reactions, usually referred to as redox reactions. For example, every reaction in which a metallic element combines with a nonmetallic element is a redox reaction in which the metal is oxidized and the nonmetal is reduced. 

The reaction of Cu with Og can be recognized as a redox reaction because it is the addition of oxygen to a reactant and also because it is the combination of a metal and a nonmetal. Since the product is an ionic compound, the electron gain and loss is determined by the charges on the ions. The oxide CuO must be composed of Cu " and ions. Therefore, the copper metal has been oxidized by the loss of two electrons from each atom, and the oxygen has been reduced by the gain of two electrons by each atom. 

Complex-formation and redox reactions have so far been the most frequently used for the determination of both metals and nonmetals (Table 3). A variety of transition metal ions have been thus determined including such common elements as iron, copper, and calcium and rare earths as well as technetium and europium. Common nonmetals such as nitrogen anions and phosphate, bromide, and sulfur anions have also been determined this way. However, the most important applications of noncatalytic reactions in inorganic analysis are the simultaneous determinations of metal ions. As can be seen in Table 4, a wide variety of binary mixtures and some ternary and even... 

Experience tells us that a pile of nails left out in the yard rusts without contact with another metal (Figure 13.6). This implies that the second half-cell in this backyard redox reaction must involve a nonmetal. In uniform corrosion, rust can cover the surface of iron or steel. The second electrode is a second region of the iron itself, located some distance away from the first spot. Ions that can conduct current help facilitate this process, so when the chloride ions of salt are present the rate of rusting is enhanced. 

Regarding H, the oxidation number for nomenclature shall be I and -I when H is connected with nonmetals and metals, respectively, except in hydride complexes when it shall be -I. While these recommendations are useful for their purpose, they have some consequences that are conceptually curious. For example, the following two reactions and others of a similar kind become redox reactions ... 

Electrolysis Electrolysis is the process in which electrical energy is used to cause a nonspontaneous redox reaction to occur. The quantitative relationship between the current supplied and the products formed is provided by Faraday. Electrolysis is the major method for producing active metals and nonmetals and many essential industrial chemicals. 

In redox reactions between a metal and a nonmetal, the metal is oxidized and the nonmetal is reduced. 

When ionic bonds form, as in Na20 or NaCl, the transfer of electrons is obvious the metal transfers one or more electrons to the nonmetal. The metal is oxidized and the nonmetal is reduced. However, redox reactions need not involve ionic bonding. In covalent bonding the transfer is oniy partiai, but the same definitions apply. Those atoms that lose electrons, even if oniy partially, are oxidized and those that gain electrons are reduced. 

The reactions of compounds of the nonmetals are less easy to classify straightforwardly under the heading of substitution reactions than are the reactions of metal complexes. Accordingly there will be some discussion of the redox reactions of these compounds, although those involving a transition metal coreactant will usually not be included. In some cases there will be an overlap with topics usually regarded to be the province of the organic chemist. 

We can use oxidation states to identify redox reactions, even between nonmetals. For example, is the following reaction between carbon and sulfur a redox reaction ... 

In the development of effective catalytic oxidation systems, there is a qualitative correlation between the desirability of the net or terminal oxidant, (OX in equation 1 and DO in equation 2) and the complexity of its chemistry and the difficulty of its use. The desirability of an oxidant is inversely proportional to its cost and directly proportional to the selectivity, rate, and stability of the associated oxidation reaction. The weight % of active oxygen, ease of deployment, and environmental friendliness of the oxidant are also key issues. Pertinent data for representative oxidants are summarized in Table I (4). The most desirable oxidant, in principle, but the one with the most complex chemistry, is O2. The radical chain or autoxidation chemistry inherent in 02-based organic oxidations, whether it is mediated by redox active transition metal ions, nonmetal species, metal oxide surfaces, or other species, is fascinatingly complex and represents nearly a field unto itself (7,75). Although initiation, termination, hydroperoxide breakdown, concentration dependent inhibition... 

Direct evidence for the formation of radical o-quinone (and sometimes p-qui-none) complexes was established in the studies quoted above. Various synthetic techniques starting from elemental metals, nonmetals, metal salts, and complexes have been developed for obtaining these coordination compounds. The peculiarities of their thin structure and physical-chemical properties were investigated. The obtained products have practical applications, in particular for medical purposes. Quinone-based metal complexes have a potential applicability as cocatalysts in a wide range of reactions involving electron exchange between substrate and catalysts. Further studies in this field and on mechanisms of electron mobility between the metal center and the o-quinone ligands are still necessary to understand the vast and complex redox chemistry of these compounds. 

Its solutions are naturally alkaline (pH 10-10.5) and these conditions promote oxidation reactions with hydrogen peroxide, especially those involving the nucleophilic attack by the perhydroxyl anion, HOO [24], With the use of an appropriate metal or nonmetal catalyst, free radical oxidation and other redox oxidation pathways are possible with this oxidant. Perborates find application in at least one oilfield chemical activity, namely as a so-called oxidative polymer breakers for spent fracturing fluids (see 17.2.1.2). 

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