Electromotive Force of Cell or Cell Potential Chemistry Notes
Electromotive Force (e.m.f.) of Cell or Cell Potential :
→ The half-cell of that Galvanic cell in which oxidation takes place is called anode and its potential related to solution is negative. Second half-cell in which reduction takes place, is called cathode and its potential related to soution is positive. So, a potential difference is present between both electrodes and as switch is ON-position electrons flow from negative electrode to positive electrode.
→ Here, the direction of flow of electric current is opposite to that of direction of flow of electrons. Thus, the difference in electrode potential between both electrodes of a electro-chemical cell is called electromotive force or e.m.f. or cell potential. It is measured in volt. At this time, no current flows in the cell. During writing of Galvanic cell, write anode at left side and cathode at right side.
Note The e.m.f. of cell is positive.
→ The actual difference of potential on electrons in open circuit ie, electromotive force (e.m.f.) of cell is equal to energy given by cell which is necessary for flow of unit charge in cell circuit. Hence, if work done (W) is necessary for flow of charge in a cell circuit, then
So, if 1 joule work has done for flowing of 1 coulomb charge in circuit then the cell potential or emf is 1 volt.
Potential Difference :
→ When electric current flows in closed circuit then the difference in potential between two electrodes is called potential difference. Potential difference is always lesser than cell potential because fluctuation will produce in voltmeter by flowing of electric current in small amount as circuit is established. It is the reason that potential difference can be measured by voltmeter but the cell potential is measured by potentiometer but not by voltmeter.
There are following main differences in cell potential and potential difference:
|Cell Potential||Potential Difference|
|It is that difference in potential between two electrodes when no electric current flows in a cell i. a, circuit is open.||It is that difference in potentials between two electrodes when electric current flows in a cell i. a, circuit is closed.|
|It is equal to maximum electromotive force of cell.||It is always lesser then maximum electromotive force of cell.|
|Its measurement is not, possible by voltmeter because small amount of electric current flows when circuit is closed. It can be measured by potentiometer.||It can be measured by voltmeter.|