EMF of A Galvanic Cell

EMF OF A GALVANIC CELL

Every galvanic or voltaic cell is made up of two half-cells, the oxidation half-cell (anode) and the reduction half-cell (cathode). The potentials of these half-cells are always dif­ferent. On account of this difference in electrode potentials, the electric current moves from the electrode at higher potential to the electrode at lower potential, i.e., from cathode to anode. The direction of the flow of electrons is from anode to cathode.

                           Flow of electrons 

           Anode   <==============> Cathode

                           Flow of current

The difference in potentials of the two half-cells is known as the electromotive force (emf) of the cell or cell potential.

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. The following three methods are in use:

(i) When oxidation potential of anode and reduction poten­tial of cathode are taken into account:

            E_Cell^o = Oxidation potential of anode

                                                     + Reduction potential of cathode

                     = E_ox^o (anode) + E_red^o (cathode)

(ii) When reduction potentials of both electrodes are taken into account:

         E_Cell^o = Reduction potential of cathode - Reduction potential of anode

                 = E_Cathode^o - E_Anode^o

                 = E_right^o - E_left^o

(iii)   When oxidation potentials of both electrodes are taken into account:

       = Oxidation potential of anode - Oxidation potential of cathode

     E_Cell^o  =  E_ox^o (anode) - E_red^o (cathode)

Difference between Emf and potential difference:

The potential difference is the difference between the electrode potentials of the two electrodes of the cell under any condition while emf is the potential generated by a cell when there is zero electron flow, i.e., it draws no current. The points of difference are given below: