Galvanic cell is the device in which chemical energy is converted into electrical energy
A most remarkable feature of oxidation – reduction reactions is that they can be carried out with the reactants separated in space and linked only by an electrical connection. That is to say, chemical energy is converted to electrical energy.
Consider the figure below a representation of a galvanic cell which involves the reaction between metallic zinc and cupric ion:
Zn(s) + Cu2+ (aq) –> Cu(s) + Zn2+(aq)
The cell consists of two beakers, one of which contains a solution of Cu2+ and a copper rod, the other a Zn2+ solution and a zinc rod. A connection is made between the two solutions by means of a “salt bridge”, a tube containing a solution of an electrolyte, generally NH4NO3 or KCl. Flow of the solution from the salt bridge is prevented either by plugging the ends of the bridge with glass wool, or by using a salt dissolved in a gelatinous material as the bridge electrolyte.
When the two metallic rods are connected through an ammeter, a deflection is observed in ammeter which is an evidence that a chemical reaction is occurring. The zinc rod starts to dissolve, and copper is deposited on the copper rod. The solution of Zn2+ becomes more concentrated, and the solution of Cu2+ becomes more dilute.
The ammeter indicates that electrons are flowing from the Zinc rod to the copper rod. This activity is continuous as long as the electrical connection and the salt bridge are maintained, and visible amounts of reactants remain.
Now let us analyze what happens in each beaker more carefully. We note that electrons flow from the Zinc rod through the external circuit, and that Zinc ions are produced as the Zinc rod dissolves. We can summarize these observations by writing,
Zn –> Zn2+ + 2e– (at the zinc rod).
Also, we observe that electrons flow to the copper rod as cupric ions leave the solution and metallic copper is deposited. We can represent these occurrences by 2e– + Cu2+ (aq) –> Cu (at the copper rod).
In addition, we must examine the purpose of the salt bridge. Since Zinc ions are produced as electrons leave the zinc electrode, we have a process which tends to produce a net positive charge in the left beaker.
The purpose of the salt bridge is to prevent any net charge accumulation in either beaker negative ions, diffuse through the salt bridge, and enter the left beaker. At the same time, there can be a diffusion of positive ions from left to right.
If this diffusional exchange of ions did not occur, the net charge accumulating in the beakers would immediately stop the electron flow through the external circuit, and the oxidation reduction reaction would stop. Thus, while the salt bridge does not participate chemically in the cell reaction, it is necessary if the cell is to operate.