Principally a galvanic cell is the same as a voltaic cell. Constructionally there may be some difference but both types of cell act based on the same principle. Because of that we sometimes refer to a galvanic cell as a voltaic cell.
Construction of a Galvanic Cell
Before discussing further, let us take two buckets. Then we fill up one bucket with dilute zinc sulfate and another with dilute copper sulfate. After that, we immerse one solid zinc rod in the zinc sulfate solution. On the other hand, we immerse one copper rod into the copper sulfate solution. Then we place one salt bridge in between copper sulfate and zinc sulfate solutions. The salt bridge contains the concentrated nonreactive solution of sodium sulfate. Also, it may also contain sodium chloride or any other similar type of salt.
Working Principle of Galvanic Battery Cell
We will focus on the actual principle of a galvanic cell. But before that, let us recall the values of the standard reduction potential of zinc and copper.
The standard reduction potential of copper is 0.34V.
The same for zinc is -0.76V.
The standard reduction potential of copper is more than that of zinc. So, copper has the more tendency of undergoing in reduction reaction than that of zinc. Conversely zinc metal has more tendency of oxidation than that of copper. Thus the zinc atoms will lose two electrons each on the surface of the zinc rod. Therefore those atoms become Zn2+ ions.
Most of us know that metallic atoms stay in the solid form in the metal body. Whereas metallic ions always dissolved in aqua solution.
Hence after donating two electrons, each zinc ions comes out in the dilute zinc sulfate solution.
Cause of Electric Current in a Galvanic Cell
The electrons released in the zinc rod will travel through the external circuit connected between zinc and copper rods. On reaching in the copper sulfate solutions through copper rod these electrons neutralize the Cu2+ ions of the solution. Hence these ions will become copper atoms. Then these deposit on the copper rod.
The Function of Salt Bridge in a Galvanic Cell
Obviously, we will understand the function of the salt bridge in a galvanic cell. But first, we imagine that there is no such salt bridge in the galvanic cell. Since copper undergoes in the reduction reaction, the copper rod is the cathode. On the other hand, since zinc participates in oxidation reaction the zinc rod is the anode of the galvanic cell. Due to the oxidation reaction at anode, the number of Zn2+ ions rises in the solution. Hence, the positive charge of the zinc sulfate solution increases. At the same time, the volume of the zinc rod decreases accordingly.
Due to the reduction reaction, the number of Cu2+ ions decreases in the copper sulfate solutions. Hence, the volume of the copper rod increases accordingly. The negativity of the copper sulfate solution also increases. This is due to the decrease of Cu2+ ions in respect of SO42- ions.
Because of these charge distributions, the cell will not be able to continue its current in the external circuit. Therefore redox reaction stops. But when we place the salt bridge in between these two sulfate solutions the cell continues to supply current. This is because the Na+ ions dissolve inner the copper sulfate solution form the salt to neutralize it. On the other hand, the SO4– ions of the salt come out in the zinc sulfate solution. Hence it makes up the deficiency of negative SO42- in respect of positive Zn2+. Therefore, the zinc sulfate solution also becomes neutral. Hence, the current will continue to flow till the circuit remains closed and the zinc remains in the anode.
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