Relative Strengths of Oxidizing and Reducing Agents

By combining many pairs of half-cells into voltaic cells, we can create a list of reduction half-reactions and arrange them in decreasing order of standard electrode potential (from most positive to most negative). Such a list, called an emf series or a table of standard electrode potentials, appears in Appendix D, with a few examples in Table 21.2 on the next page. 

CHAPTER 21 El ectrochemistry Chemical Change and Electrical Work 

All values are relative to the standard hydrogen (reference) electrode  

Writing Spontaneous Redox Reactions Appendix D can be used to write spontaneous redox reactions, which is useful for constructing voltaic cells. 

Cu (left), and Cu and Zn react spontaneously. In other words,/or a spontaneous reaction to occur, the half-reaction higher in the list proceeds at the cathode 

CHAPTER 21 Beclrachemistry Chemical Change and Seclrical Wark 

However, if we know the electrode potentials, we can write a spontaneous redox reaction even if Appendix D is not available. Let s choose a pair of halfreactions from the appendix and, without referring to their relative positions in the list, use them to write a spontaneous redox reaction  

Reverse one of the half-reactions into an oxidation step such that the difference of the electrode potentials (cathode minus anode) gives a positive Eceii-Note that when we reverse the half-reaction, we need not reverse the sign of haif-ceii because the minus sign in Equation 21.3 (Eceii = °athode anode) will do that. 

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Table of relative strengths of oxidizing and reducing agents (see table on the following page)... 

Figure 20.11 Relative strengths of oxidizing and reducing agents. 

Standard Cell Potentials 697 Relative Strengths of Oxidizing and Reducing Agents 700... 

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. 

CHECK FOR UNDERSTANDING Describe In your own words, describe the relationship between the relative strength of oxidizing and reducing agents and their placement on the activity series table (given in the chapter Chemical Equations and Reactions ). 

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