Oxidation Numbers and Redox Reactions

GOAL 5 Given the formula of an element, molecule, or ion, assign an oxidation number to each element in the formula. 

6 Describe and explain oxidation and reduction in terms of change in oxidation numbers. 

The redox reactions that we have discussed up to this point have been relatively simple ones involving only two reactants. With Equations 19.3 and 19.5 through 19.7, we can see at a glance which species has gained and which has lost electrons. Some oxidation-reduction reactions are not so readily analyzed. Consider, for example, a reaction that is sometimes used in the general chemistry laboratory to prepare chlorine gas from hydrochloric acid  

It is by no means obvious which species are gaining and losing electrons in these reactions. 

The oxidation number of combined hydrogen is +1, except as a monatomic hydride ion, H . 

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Oxidation Numbers and Redox Reactions Oxidizing Agents (Oxidizers) and Reducing Agents (Reducers)... 

The oxidation number of copper is +2 in both of the complex ions. There is no change in oxidation number and this reaction, an example of ligand replacement (Chapter 13), is therefore not a redox reaction. 

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. 

As mentioned previously, a large number of redox reactions involving macrocyclic ligand complexes have resulted in discrete changes in the unsaturation pattern of a variety of macrocyclic systems. Chemical, electrochemical, and catalytic reactions have been widely used to change the level of unsaturation in such systems. Although the mechanisms of the majority of such transformations are not well understood, it is clear that the reactions tend to proceed via prior change in the oxidation state of the central metal ion. 

Metal carbonyls are subject to autocomplex formation in the presence of strong donor molecules 94 98>. Besides the cation which is coordinated by donor molecules, polynuclear anions are formed the latter can be degradated at higher temperatures. It may be noted that in this process of autocomplex formation changes in the oxidation numbers and thus redox reactions are involved ... 

Both carbon and chlorine undergo changes in oxidation number, so the reaction is a redox reaction. 

Step 3 If the reaction is a redox reaction, identify the element(s) that undergo an increase in oxidation number and the element(s) that undergo a decrease in oxidation number. 

For (a) a reduction reaction has occurred because the oxidation number of the product is lower than that of the initial redox state (Bi is converted to Br ), while (b) and (c) are both oxidation reactions because the oxidation number increases during reaction. 

Although an extended form of (2.147) is sometimes used (Eqn. 5.37) the simple one can account for a surprisingly large number of redox reactions. When the oxidizing power of ox, and 0x2 are comparable, / 1, and... 

Only in a limited number of instances will the value of k and its associated parameters be useful in diagnosing mechanism. When the redox rate is faster than substitution within either reactant, we can be fairly certain that an outer-sphere mechanism holds. This is the case with Fe + and RuCP+ oxidation of V(II) and with rapid electron transfer between inert partners. On the other hand, when the activation parameters for substitution and redox reactions of one of the reactants are similar, an inner-sphere redox reaction, controlled by replacement, is highly likely. This appears to be the case with the oxidation by a number of Co(III) complexes of V(II), confirmed in some instanees by the appearance of the requisite V(III) complex, e.g. 

The lower oxidation states are stabilized by soft ligands e.g. CO (Prob. 3). The aquated vanadium ions represent an interesting series of oxidation states. They are all stable with respect to disproportionation and labile towards substitution. They undergo a number of redox reactions with one another, all of which have been studied kinetically. Many of the reactions are [H ]-dependent. There has been recent interest in the biological aspects of vanadium since the discovery that vanadate can mimic phosphate and act as a potent inhibitor (Prob. 4). 

Please visit http //elsevierdirect.eom/companions/9780120885305 for brief tutorials on assignment of oxidation numbers and methods for balancing redox reactions. Both skills are required for mastery of the remainder of the materials presented in this chapter. 

Fe3+ is the oxidizing agent because it takes an electron from V2+. V2+ is the reducing agent because it gives an electron to Fe3+. Fe3+ is reduced, and V2+ is oxidized as the reaction proceeds from left to right. Appendix D reviews oxidation numbers and balancing of redox equations. 

Oxidation numbers and their changes can be used to identify the reaction of sodium with chlorine to form NaCl as a redox reaction. 

Oxidation Numbers in Organic Chemical Compounds and Redox Reactions... 

Once you have a firm grasp on the rules for oxidation numbers and solving problems involving oxidation numbers, you can use oxidation numbers to determine the substance that undergoes a reduction and an oxidation in a redox reaction. You can tell that a substance has been reduced if it has gained electrons (electrons are a reactant). A substance that has been oxidized has lost electrons (electrons are a product). After the substances that have changed oxidation states in a redox reaction are identified, you then write separate half reactions. Half reactions are two separate reactions that show the oxidation and reduction reactions separately. An example follows. 

Substance A is reduced two oxidation numbers and substance B is oxidized six oxidation numbers in their redox reaction. 

Recall from Chapter 4 that an oxidation-reduction (redox) reaction involves a transfer of electrons from the reducing agent to the oxidizing agent, and that oxidation involves a loss of electrons (an increase in oxidation number) and reduction involves a gain of electrons (a decrease in oxidation number). 

Phosphorus atoms and carbon atoms change their oxidation numbers, so the reaction is redox. Each phosphorus atom changes its oxidation number from -1-5 to zero, so the phosphorus atoms in Ca3(P04)2 are reduced, and Ca3(P04)2 is the oxidizing agent. Each carbon atom changes its oxidation number from zero to +2, so the carbon atoms are oxidized, and carbon is the reducing agent. 

A major drawback connected with the symproportionation reaction is that the number of redox reactions that can take place is limited. A more recent development, which shows obvious similarities to the traditional molten salt route, but whieh avoids the limitations of symproportionations, is the solid-state technique developed by Beck in which a volatile, high-valent transition-metal chloride acts both as halide acceptor and as oxidizing agent. The synthesis of Tes " by oxidation of tellurium with WCl6 according to (4) is representative. ... 

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