Cells Fig. . : Daniell cell having electrodes of zinc and copper dipping in the solutions of their respective salts. salt bridge anode cathode current ZnSO CuSO < ext .
V e -ve +ve I= ZnSO CuSO ext . V When E ext < . V (i) Electrons flow from Zn rod to Cu rod hence current flows from Cu to Zn. (ii) Zn dissolves at anode and copper deposits at cathode.
When E ext = . V (i) No flow of electrons or current. (ii) No chemical reaction. When E ext > .
V (i) Electrons flow from Cu to Zn and current flows from Zn to Cu. (ii) Zinc is deposited at the zinc electrode and copper dissolves at copper electrode. (a) (b) (c) As mentioned earlier a galvanic cell is an electrochemical cell that converts the chemical energy of a spontaneous redox reaction into electrical energy. In this device the Gibbs energy of the spontaneous redox reaction is converted into electrical work which may be used for running a motor or other electrical gadgets like heater, fan, geyser, etc.
Daniell cell discussed earlier is one such cell in which the following redox reaction occurs. Zn(s) + Cu + (aq) ® Zn + (aq) + Cu(s) This reaction is a combination of two half reactions whose addition gives the overall cell reaction: (i) Cu + + 2e – ® Cu(s) (reduction half reaction) ( . ) (ii) Zn(s) ® Zn + + 2e – (oxidation half reaction) ( . ) These reactions occur in two different portions of the Daniell cell.
The reduction half reaction occurs on the copper electrode while the oxidation half reaction occurs on the zinc electrode. These two portions of the cell are also called half-cells or redox couples . The copper electrode may be called the reduction half cell and the zinc electrode, the oxidation half-cell. We can construct innumerable number of galvanic cells on the pattern of Daniell cell by taking combinations of different half-cells.
Each half- cell consists of a metallic electrode dipped into an electrolyte. The two half-cells are connected