Electrode Reactions and Cell E.m.f

Electrode Reactions and Cell E.m.f


The standard procedure is to write the left-hand side electrode as an oxidation reaction and the right-hand side electrode as a reduction reaction. The two half-cell reactions are added to give the overall cell reaction and the overall cell e.m.f.
For the cell given above, the individual electrode reactions and the overall cell reaction are:
Left electrode:                        H2(g)2H+(aq) + 2e E° = 0.00V
Right electrode:                      Cu2 + +2e Cu(s) E° = 0.34V
Overall cell reaction:   H2(g) + Cu2+ -> 2H+(aq) + Cu(s) E° cell= 0.34V

Positive e.m.f of the cell indicates that the reaction has the tendency to occur from left to right, i.e. in the direction indicated. Conversely a negative em.f indicates the reaction occurs from right to left. In the case of positive e.m.f the electrons flow from left to right in the outer-circuit and positive electricity passes from left to right inside the cell.

By combining two half-cells whose standard electrode potentials are known, the e.m.f. of the cell can be calculated neglecting the H2 electrode. For example consider the Daniel cell which is made up of the zinc half-electrode and the copper half electrode as written below:
Zn(s)/ZnSO4 (1M)//CuSO4 (1M)/Cu(s)
The individual electrode reactions and the overall cell reaction are:
Zn(s) ---> Zn(aq)2+ + 2e-                                           E0= 0.76V
Cu2 +(aq) + 2e ---> Cu(s)                                           E0 = 0.34V
Overall                                Zn(s) + Cu2+(aq) --> Zn2+(aq) +Cu(s)                        
E0cell = 1.10V

In this set-up the electrons flow from left to right in the outer circuit. Hence positive current moves from left to right in the cell itself. By convention a potential difference corresponding to an external flow of electrons from left-hand electrode to the right hand-electrode is said to be a positive potential difference.


The purpose of the salt-bridge is to complete the circuit by connecting two half-cells. The ions move from one half-cell to the other half-cell through the salt-bridge. Ions from one half-cell move through the salt-bridge into the other half-cell to replace the ions used up at the electrodes and hence maintain the electrical neutrality. Saturated solutions of KCI, KNO3 and NH4N03 are usually employed as salt-bridges.
Another function of the salt-bridge is to minimize the liquid-junction potential in concentration electrochemical cells with liquid junctions.


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