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:                        H_2(g)2H⁺_(aq) + 2e E° = 0.00V

Right electrode:                      Cu^{2 +} +2e Cu_(s) E° = 0.34V

Overall cell reaction:   H_2(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

Cu^{2 +}_(aq) + 2e ---> Cu_(s)                                           E⁰ = 0.34V

Overall                                Zn_(s) + Cu²⁺_(aq) --> Zn²⁺_(aq) +Cu_(s)                        

E⁰_cell = 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.