2.2 Galvanic Cells

The potential difference between the two electrodes of a galvanic cell is called the cell potential and is measured in volts. The cell potential is the difference between the electrode potentials (reduction potentials) of the cathode and anode. It is called the cell electromotive force (emf) of the cell when no current is drawn through the cell. It is now an accepted convention that we keep the anode on the left and the cathode on the right while representing the galvanic cell.

A galvanic cell is generally represented by putting a vertical line between metal and electrolyte solution and putting a double vertical line between the two electrolytes connected by a salt bridge. Under this convention the emf of the cell is positive and is given by the potential of the half- cell on the right hand side minus the potential of the half-cell on the left hand side i.e., E cell = E right – E left This is illustrated by the following example: Cell reaction: Cu(s) + 2Ag + (aq) ¾® Cu + (aq) + Ag(s) ( . ) Half-cell reactions: Cathode ( reduction ): 2Ag + (aq) + 2e – ® 2Ag(s) ( . ) Anode ( oxidation ): Cu(s) ® Cu + (aq) + 2e – ( .

) It can be seen that the sum of ( . ) and ( . ) leads to overall reaction ( . ) in the cell and that silver electrode acts as a cathode and copper electrode acts as an anode.

The cell can be represented as: Cu(s)|Cu + (aq)||Ag + (aq)|Ag(s) and we have E cell = E right – E left = E Ag + ú Ag – E Cu + ú Cu ( . ) The potential of individual half-cell cannot be measured. We can measure only the difference between the two half-cell potentials that gives the emf of the cell. If we arbitrarily choose the potential of one electrode (half-cell) then that of the other can be determined with respect to this.

According to convention, a half-cell called standard hydrogen electrode (Fig. . ) represented by Pt(s) ú H (g) ú H + (aq), is assigned a zero potential at all temperatures corresponding to the reaction H + (aq) + e – ® H (g) The standard hydrogen electrode consists of a platinum electrode coated with platinum black. The electrode is dipped in an acidic solution and pure hydrogen gas is bubbled through it.

The concentration of both the reduced and oxidised forms of hydrogen is maintained at unity (Fig. . ). This implies that the pressure of hydrogen gas is one bar and the concentration of hydrogen ion in the solution is one molar.

. . Measurement of Electrode Potential Fig. .

: Standard Hydrogen Electrode (SHE). At K the emf of the cell, standard hydrogen electrode çç second half-cell constructed by taking standard hydrogen electrode as anode (reference half-cell) and the other half-cell as cathode, gives the reduction potential of the other half-cell. If the concentrations of the oxidised and the reduced forms of the species in the right hand half-cell are unity, then the cell potential is equal to standard electrode potential, E o R of the given half-cell. E o = E o