Galvanic cell is also known as voltaic cell. The major difference between an electrolytic cell and a galvanic cell is that the reaction in a galvanic cell is spontaneous, and the reaction produces electric current. The batteries that we use in TV remotes and flash lights are galvanic cells. Galvanic cells convert the stored chemical energy into electrical energy for usage.
A galvanic cell has two half-cells. Each half-cell consists of a metal electrode immersed in a solution containing the same ions. The two half-cells are connected by a wire as shown in Figure 12-2. As we mentioned earlier, the galvanic cell produces electric current. Thus the voltage developed can be measured by setting a voltmeter along the connecting wire, as seen in the figure.
Here, we will look at a cell setup which uses zinc and copper as the electrodes. In addition to the electrodes, the two containers which hold the appropriate solutions and the connecting wire, there is a salt bridge which connects the two solutions. The salt bridge is usually dipped into the solutions of the two half-cells. It contains a gel in which an electrolyte is present. The electrolyte present in the salt bridge will neutralize the buildup of ionic charge in the cells; a buildup which will otherwise slow down and stop the reaction from proceeding.
In the zinc half-cell, a zinc electrode is immersed in zinc sulfate solution. In the copper half-cell, a copper electrode is immersed in copper sulfate solution. The two electrodes are connected by a wire through which there will be flow of electrons resulting from the reaction. The half-reactions are shown below:
The process of oxidation occurs at the anode and the process of reduction occurs at the cathode. So the first half-reaction (oxidation half-reaction) occurs at the anode, and second half-reaction (reduction half-reaction) occurs at the cathode. The overall reaction can be obtained by adding the two half reactions. Here, the zinc electrode is the anode, and the copper electrode is the cathode. In a galvanic cell, the anode is the negative electrode and the cathode is the positive electrode. As far as electron flow is concerned, the flow is always from the anode to the cathode.
Let's take a look at Figure 12-2 again. Notice the irregular edges of the electrodes. Why is that so? The reason is simple. As the reaction proceeds, zinc is stripped away from the zinc electrode, and thus it becomes thinner and thinner until the reaction stops (when it is at equilibrium). On the other hand, the copper electrode becomes thicker and thicker due to the deposition of metal copper on the copper electrode.
The half-reactions are often represented by the notation shown below. By convention, the oxidation reaction is written on the left of the symbol denoting the salt bridge, and the reduction reaction is written on the right side of the salt bridge symbol.
Note: See Table 12-2 at the end of this chapter for electrolytic versus galvanic cell comparison.