Spontaneous change voltaic cells

We have already seen that a positive value of is associated with a negative value for the free-energy change and, thus, with a spontaneous process. We also know that for any spontaneous process, AG is a measure of the maximum useful work, Wmax/ that can be extracted from the process AG = u max (Section 19.5) Because AG = —iiFE, the maximum useful electrical work obtainable from a voltaic cell is... 

Figure 21.24 The tin-copper reaction as the basis of a voltaic and an electrolytic cell. A,The spontaneous reaction between Sn and Cu generates 0.48 V in a voltaic cell. B, If more than 0.48 V is supplied, the nonspontaneous (reverse) reaction between Cu and Sn occurs. Note the changes in electrode charge and direction of electron flow.

Let s explore the relationship between voltaic and electrolytic cells by returning briefly to the cell shown in Figure 19-4. When the cell functions spontaneously, electrons flow from the zinc to the copper and the overall chemical change in the voltaic cell is... 

The electrochemical cells of Figures 19-3 and 19-4 produce elecfricity as a resulf of spontaneous chemical reactions as such, they are called voltaic, or galvanic, cells. In Section 19-7 we will consider electrolytic cells—electrochemical cells in which electricity is used to accomplish a nonspontaneous chemical change. 

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. 

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