Predicting Redox Reactions

A chemical species may be difficult to characterize, and its derivatives obtained by reduction or by oxidation may be easier to identify than the species itself. It may also turn out that a species endowed with oxidizing or reducing properties must be quantitatively determined. In such a case, it is logical to predict its redox titration. Therefore, we must necessarily know the direction and quantitative character of the predicted redox reactions. 

Predicting redox reactions is not necessarily an easy task. It involves a thorough examination of the reaction s experimental conditions. In particular, we must examine the influence of such factors as the medium s acidity and the complexation and precipitation of the redox species. 

The majority of this chapter is devoted to thermodynamic prediction. The last section mentions some kinetic considerations about redox reactions. 

Why does a strong oxidizing agent become a weak reducing agent when it gains an electron  

10 Given a table of the relative strengths of oxidizing and reducing agents, and information from which an electron-transfer reaction equation between two species in the table may be written, write the equation and predict the direction in which the reaction will be favored. 

In Section 9.5 you wrote net ionic equations for possible single-replacement redox reactions. You used the activity series given in Table 9.2—reprinted here in the margin—to predict whether or not the reaction would occur. In Table 9.2, the elements are listed in order of decreasing reactivity the most active element is at the top. Any element in the activity series will react with and replace the dissolved ion of any element beneath it in the series. 

Most metals in Table 9.2 are all included in the longer list of reducing agents in the right-hand column of Table 19.2. Moreover, their order, in terms of reactivity, is exactly the same in the two tables. (The order appears to be inverted because the reactivity increases as you go down the column in Table 19.2, just the opposite of Table 9.2. The most active metal in Table 19.2 is at the bottom.) Now you can understand that the arrangement of the activity series is determined by the ease with which atoms of the element release electrons and function as reducing agents. 

Different classification systems can be used for different purposes. As you categorize your knowledge about chemistry, you need to reorganize your thinking when new information leads to a more efficient system. You will also find that there are strengths and weaknesses of various classification systems, and some may work better in one context while others work better in another context. 

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In this chapter, you learned about solutions and how to use molarity to express the concentration of solutions. You also learned about electrolytes and nonelectrolytes. Using a set of solubility rules allows you to predict whether or not precipitation will occur if two solutions are mixed. You examined the properties of acids and bases and the neutralization reactions that occur between them. You then learned about redox reactions and how to use an activity table to predict redox reactions. You learned about writing net ionic equations. Finally, you learned how to use the technique of titrations to determine the concentration of an acid or base solution. 

Strengths of Oxidizing Agents and Reducing Agents Predicting Redox Reactions Redox Reactions and Acid-Base Reactions Compared... 

For the preceding reasons, we must often take into account the medium s acidity when predicting redox reactions. One way to do that is to introduce the concept of normal potential or, synonymously, that of apparent standard potential. Consider, for example, the oxidation of ferrous ions by permanganate ions according to the... 

Finally, when a half-redox reaction is accompanied by a complexation reaction, it is also useful to define an apparent standard potential or normal potential in order to predict redox reactions quickly. For example, in the case of the couple AuCl4 /Au(s), Nernst s equation can be written as... 

Some graphical means permit us to predict redox reactions according to the experimental conditions. They principally allow the prediction of these reactions according to the concentrations of the different species that participate in the studied equilibrium. Quite generally, such predictions are less accurate than those based on the calculations described in the previous chapter. However, they are very useful because they immediately provide an overview of the evolution of the processes and, in particular, of the redox phenomena as a function of some experimental parameters, specifically of the pH. 

Predicting redox reactions with the aid of graphical means involves the knowledge of the predominance areas of the redox couples that participate in the equilibria. 

Predicting Redox Reactions by Graphical Means volt-equivalents... 

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