Unit 2

Unit E ext > . e Current Cathode +ve Anode –ve Fig. . Functioning of Daniell when external voltage E ext opposing the cell potential is applied. We had studied the construction and functioning of Daniell cell (Fig. . ). This cell converts the chemical energy liberated during the redox reaction Zn(s) + Cu + (aq) ® Zn + (aq) + Cu(s) ( . ) to electrical energy and has an electrical potential equal to . V when concentration of Zn + and Cu + ions is unity ( mol dm – ) * . Such a device is called a galvanic or a voltaic cell. If an external opposite potential is applied in the galvanic cell [Fig. . (a)] and increased slowly, we find that the reaction continues to take place till the opposing voltage reaches the value . V [Fig. . (b)] when, the reaction stops altogether and no current flows through the cell. Any further increase in the external potential again starts the reaction but in the opposite direction [Fig. . (c)]. It now functions as an electrolytic cell , a device for using electrical energy to carry non-spontaneous chemical reactions. Both types of cells are quite important and we shall study some of their salient features in the following pages. * Strictly speaking activity should be used instead of concentration. It is directly proportional to concentration. In dilute solutions, it is equal to concentration. You will study more about it in higher classes.