Galvanic cells

A galvanic cell or voltaic cell, named after the scientists Luigi Galvani and Alessandro Volta, respectively, is an electrochemical cell in which an electric current is created from spontaneous Oxidation-Reduction reactions. A common apparatus normally comprises of two different metals, each immersed in separate beakers holding their respective metal ions in solution that are interconnected by a salt bridge or separated by a porous membrane.

Volta was the inventor of the voltaic pile, the first electrical battery. In general use, the phrase “battery” has evolved to include a single galvanic cell, whereas a battery actually consists of several cells.

Working of Galvanic Cells

The functioning of a galvanic cell is pretty straightforward. It involves a chemical reaction that makes the electric energy obtainable as the end result. During a redox reaction, a galvanic cell uses the energy transfer between electrons to transform chemical energy into electric energy.

Galvanic cell uses the ability to split the flow of electrons in the process of oxidization and reduction, creating a half reaction and connecting each with a wire so that a path may be formed for the flow of electrons through such wire. This flow of electrons is basically termed a current. Such current may be made to flow through a wire to complete a circuit and acquire its output in any device such as a television or a watch.

A galvanic cell may be created out of any two metals. These two metals may form the anode and the cathode if allowed in contact with one other. This combination permits the galvanic corrosion of that metal which is more anodic. A connecting circuit is necessary to enable this corrosion to take place.

Setup of a Galvanic Cell

In order to make a galvanic cell, one would have to go through the following setup. The cell would ideally comprise two electrodes. One of these electrodes, the cathode, should be a positively charged electrode while the other, shall be the anode, the negatively charged electrode.

These two electrodes shall form the two key components of the galvanic cell. The chemical reaction relating to reduction shall take place at the cathode while the oxidation half-reaction shall take place at the anode. As has previously been established, any two metals may be utilized to create the chemical reaction.

Example of Galvanic Cell

Let us consider an example where the two metals involved in the chemical reaction are zinc and copper. As the chemical reaction takes place, Zinc would eventually lose two electrons. This will be taken up by copper to form elemental copper. Since these two metals would be put in two distinct containers and would be interconnected by a conducting wire, an electric current would be produced, which would transfer all electrons from one metal to another.

At the same time, the two metals must be immersed in a salt solution, like, Zinc sulphate and Copper sulphate in this example. In this situation, the two solutions are not mixed together directly but can be joined through a bridge or a medium. This medium is not only responsible for the transfer of ions but also making sure that the two solutions do not  mix with each other. Such bridges aids in completing the circuit for carrying the electric charge and also makes sure that the solutions in the containers containing the metals stay neutral and do not mix with one other. As long as the salt bridge does not interfere with the redox reaction, under which oxidisation and reduction are taking place, it does not matter which salt bridge is being utilized in the chemical reaction.

Important Terms

Phase boundaries: It refers to the two metals which function as cathode and anode.

Salt bridge: The connecting bridge or medium that permits a redox reaction to take place.

Oxidation and reduction: The chemical reactions that enable the electric current to form and move through a galvanic cell.

 Anode : The anode is the electrode where oxidation (loss of electrons) takes place (metal-A electrode); in a galvanic cell, it is the negative electrode, since when oxidation occurs, electrons are left behind on the electrode.

Cathode : The cathode (metal-B electrode) is the electrode where reduction (gain of electrons) occurs; it is the positive electrode in a galvanic cell, since ions are reduced by taking electrons from the electrode and plate out (while in electrolysis, the cathode is the negative terminal and attracts positive ions from the solution). The phrase ‘the cathode draws cations’ is correct in both circumstances.

Galvanic Corrosion 

The electrochemical erosion of metals is called Galvanic corrosion. Corrosion occurs when two different metals come in contact with one other in the presence of an electrolyte, such as salt water. This creates a galvanic cell, with hydrogen gas forming on the more noble (less active) metal. The resulting electrochemical potential then creates an electric current that electrolytically dissolves the less noble material. A concentration cell can be formed if the same metal is exposed to two distinct concentrations of electrolyte.

Conclusion

Galvanic Cells enable us to harness the electron flow in a redox reaction to conduct beneficial work. Such cells find frequent use as batteries, pH meters, and as fuel cells. The setup of the cell, requires that the oxidation and reduction half-reactions are connected by a wire and by a salt bridge or porous disk. Electrons will pass through that wire generating an electrical current. The salt bridge or porous disk permits the passage of ions in solution to ensure charge neutrality in each half-cell. Instead of illustrating the setup of every cell like , chemists have invented a shortcut line notation mentioned in Line Notation. The direction of the current in a cell is governed by the standard reduction potential of each half-cell. For a reaction to be spontaneous, the total cell potential must be positive . Therefore, the half-reaction with the higher reduction potential will be a reduction and the other half-reaction will be an oxidation. The electrode in the oxidation half-reaction is termed the anode. The electrode in the reduction half-reaction is called the cathode.