In Unit 33, we have studied the use of redox reactions in electrochemical cell to produce electrical energy. Let us see what is the effect of opposing potential on the reversible electrochemical cell. To understand this, consider a Zn-CuSO4 cell; having standard emf= 1.1 V. The cell is reversible with respect to its net cell reaction,
Let us now consider an arrangement in which an opposing potential is applied to Zn-CuSO4 cell by coupling it to another cell (E ext) through rheostat and galvanometer as shown in Fig. 34.1(a).
In the beginning, the external opposing potential is less than 1.1 V. In this case the normal reaction occurs in Zn-CuSO4 cell and net current flows from copper electrode to zinc electrode. The opposing potential is now increased slowly and gradually. It is observed that normal reaction continues to occur till the opposing potential reaches a value of 1.1 V [Fig. 34.1(b)]. At this stage the cell reaction stops and no current flows. Further increase in the opposing potential (beyond 1.1 V) again causes the chemical reaction to occur. But now the reaction occurs in the reverse direction forcing Zn2+ ion to get reduced and Cu atoms to get oxiclised Fig. 34.1 (c).
The flow of current in this case occurs from Zn-electrode to Cu-electrode. This process is non-spontaneous and involves the use of electrical energy to carry out redox reaction. In this situation, the Zn-CuSO4 cell starts acting as electrolytic cell. This phenomenon is also called recharging of the electrochemical cell. Let us now study different aspects of electrolysis and electrolytic cells.