Cell potential- E.m.f. of cell

Electromotive Force

The electrochemical cell consists of two half cells. The electrodes in these half cells have different electrode potentials. When the circuit is completed the loss of electrons occurs at the electrode having lower reduction potential whereas the gain of electrons occurs at the electrode with higher reduction potential. 

The difference in the electrode potentials of the two electrodes of the cell is termed as electromotive force (abbreviated as EMF) or cell voltage (Ecell).

The EMF of the cell at the standard state conditions is called standard EMF. The Emf of the cell or cell potential can be calculated from the values of electrode potentials of the two half-cells constitut­ing the cell

Ø   When oxidation potential of anode and reduction poten­tial of cathode are taken into account:

                 ECello = Oxidation potential of anode + Reduction potential of cathode
                 ECello  = Eoxo (anode) + Eredo (cathode)
Ø   When reduction potentials of both electrodes are taken into account:
                    ECello = Reduction potential of cathode - Reduction potential of anode
                    ECello = ECathodeo - EAnode 
Consider the following example: If standard potential for the following reactions is given:

A2+ + 2e A     E0 = +2.5 V

B2+ + 2e B     E0 = – 1.0V

C2+ + 2e C     E0 = +1.5 V

We can deduce from the above values that A2+ has the maximum tendency to get reduced as it has the most positive reduction potential and the lowest tendency to get reduced is shown by B2+ in the above three cases.

In an electrochemical cell the electrode with higher electrode potential (Eº) acts as cathode while that with lower electrode potential will act as anode.

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