The Galvanic Cell:
I have learned that, when a Zn rod is dipped into a copper (II) sulfate solution, zinc atoms are oxidized to zinc ions and copper (II) ions are reduced to copper metal, which deposits on the zinc rod. Following reaction occurs:
In this reaction, electrons flow directly from the zinc rod to Cu+2
ions in solution. However, if the electrons transfer from the zinc rod to the copper ions in solution could be directed through an external circuit, the spontaneous redox reaction could be used to generated electric current. But when a zinc rod is dipped in zinc sulfate solution in one container is connected by a copper wire the copper metal and the copper rod dipped in copper (II) sulfate solution in a separate container, no current flows through the external circuit, no current flows through the external circuit. However, when the two solutions are connected with a tube (salt bridge) filled with a solution of an electrolyte such as KCl, KNO3, or Na2SO4, Current flows through the external circuits.
The salt bridge allows the movement of ions to form one solution to the other without mixing of the two solutions and maintains electrical neutrality in each half-cell.
In one half cell oxidation takes place and is called oxidation half-cell or anode. Whereas in the other half-cell reduction takes place is called a reduction half-cell or cathode. The reaction taking place in the oxidation half-cell is called oxidation half-reaction and the reaction taking place in reduction half-cell is called reduction half-reaction.
Zn has a greater tendency to lose electrons than Cu. Therefore, the Zn electrode acquires negative charges relative to the Cu electrode. The electrons flow from the Zn electrode through the external circuit to the Cu electrode. The following half cell reactions occur at the two electrodes.
An electrochemical cell in which spontaneous redox reaction produces an electric current is known as a galvanic or voltaic cell.
The electrode at which oxidation occurs is called the anode. Whereas, the electrode at which reduction occurs is called cathode.