The cells which produce electrical energy from chemical reactions are known as galvanic or voltaic cells. A galvanic cell in which one electrode is Zn plate in (or ion) solution and the other is Cu plate in ion) solution is called the Daniel cell.
A schematic diagram of the cell is shown below:
The two half reactions,
are made to take place simultaneously with the electron transfer occurring through an extemal conducting wire. The two solutions (i.e., ) are linked by an inverted U-shaped tube as salt bridge. The tube is filled with a solution of an electrolyte like KCl, to which gelatin or agar-agar has been added to obtain a semi solid paste. Sometimes, a porous partition is used in place of salt bridge, which separates the two solutions.
At anode, metallic zinc passes into solution as ions and liberates electrons. while at cathode, ions gain these electrons and are discharged as metallic copper. Hence net reaction may be represented as:
It is the chemical energy of this reaction which is converted into electrical energy.
It is evident from the above equation that redox reaction is used as a source of electric energy in a galvanic cell. Some other examples of galvanic cells are given below:
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