Electrolysis Causing Nonspontaneous Reactions to Occur

Until now, the emphasis has been on voltaic (galvanic) cells, electrochemical cells in which chemical change is used to produce electricity. Another type of electrochemical cell—the electrolytic cell—uses electricity to produce a nm-spontaneous reaction. The process in which a nonspontaneous reaction is driven by the application of electric energy is called electrolysis. 

Now suppose the same cell is connected to an external electric source of voltage greater than 1.103 V (Fig. 19-22). That is, the connection is made so that electrons are forced into the zinc electrode (now the cathode) and removed from the copper electrode (now the anode). The overall reaction in this case is the reverse of the voltaic cell reaction, and EceU is negative. 

reversing the direction of the electron flow changes the voltaic cell into an electrolytic cell. 

We can make similar calculations for other electrolyses. What actually happens, however, does not always correspond to these calculations. Four complicating factors must be considered  

A voltage significantly in excess of the calculated value, an overpotential, may be necessary to cause a particular electrode reaction to occur. Overpotentials are needed to overcome interactions at the electrode surface and are particularly common when gases are involved. For example, the overpotential for the discharge of H2(g) at a mercury cathode is approximately 1.5 V the overpotential on a platinum cathode is practically zero. 

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Electrolysis Causing Nonspontaneous Reactions to Occur—In electrolysis, a nonspontaneous chemical reaction occurs as electrons from an external source are forced to flow in a direction opposite that in which they would flow spontaneously. The electrochemical cell in which electrolysis is conducted is called an electrolytic cell. E°, values are used to establish the theoretical voltage requirements for an electrolysis. Sometimes, particularly when a gas is liberated at an... 

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