The Meaning of Standard Potentials

You are probably wondering what difference does it make if E° is + or - VYhat do + and -values mean The answer is that the standard potential for any battery is always a positive number. Positive values of E° mean that the reactions take place spontaneously. Galvanic cells are a source of energy—they can be made to do useful electrical work. 

Negative values of E° mean that the reactions do not take place by themselves. An external source of energy needs to do electrical work to make the reaction take place. In that case, we say the reaction is taking place in an electrolytic cell instead of a galvanic cell. The difference between a cell that does electrical work and a cell that needs electrical work to be done on it is a crucial distinction. 

We usually do not make galvanic cells using the standard hydrogen electrode. Using the two examples from above, suppose we make a galvanic cell that has the zinc half-reaction in one half-cell and the copper half-reaction in the other one. Then the standard potentials are  

The total standard potential is found by adding the two values of E° together  

Values of E° have been measured for a large number of half-reactions. Table 11.1 lists the values of E° for only a few common half-reactions. Notice that all the half-reactions are reduction half-reactions. They could just as well be oxidation half-reactions, but the common practice is to list reduction half-reactions. 

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