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A cell that works on non-spontaneous redox reactions is called an electrolytic cell. The electrolytic cell works only when an external voltage is applied to it. The external voltage applied should be greater than the internal voltage generated by the cell, i.e., Ecell.
An electrolytic cell works oppositely compared to an electrochemical cell. It also consists of two electrodes–an anode and a cathode. Here, the flow of electrons is in the opposite direction, i.e., from cathode to anode. Electrolytic cells are those that work based on electrolysis. An electrolytic cell undergoes electrolysis and is used for the chemical decomposition of substances.
Electrolysis is the decomposition of an electrolyte by passing an electric current through its solution. The process of electrolysis is done by an apparatus called the electrolytic cell. A cell that works on the system can be built by passing an electric current through a chemical system.
Components of an electrolytic cell
It consists of three components:
- Anode: The negative electrode
- Cathode: The positive electrode
- Electrolyte: A salt solution
Here, the convention is opposite to that of an electrochemical cell. The cathode undergoes oxidation, and the anode undergoes reduction. Both electrodes are dipped in the same electrolyte. When an external voltage is applied, the electrons start flowing from the cathode to the anode. The electrolyte is usually a salt solution. The positive ions from the solution go to the cathode, and the negative ions go to the anode.
The Process of electrolysis
The cations move to the cathode and absorb electrons from it and become neutral atoms. The anions travel to the anode, which transfers electrons to give a neutral atom. Chemical reactions occur due to anions losing electrons and cations gaining electrons at their corresponding electrodes.
For example, in the electrolysis of HCl, it gets ionised as:
HCl → H+ + Cl−
Cl– ions travel toward the anode in an electrolytic cell, while H+ ions migrate towards the cathode. The following reactions occur at the electrodes.
At cathode: H+ + e– → H
Each hydrogen ion takes an electron from the cathode to produce a hydrogen atom. The hydrogen atoms then combine to produce hydrogen gas molecules, H2.
At anode: Cl- → Cl + e−
After losing an electron by the chloride ion to the anode, a pair of chlorine atoms combine to produce chlorine gas, Cl2.
The procedure results in the breakdown of HCl into hydrogen and chlorine gases.
The overall reaction is:
2HCl → H2 + Cl2
Electrolysis can also occur in molten alkali halides.
Differences between electrolytic and galvanic cell
Galvanic cell | Electrolytic cell |
It converts chemical energy to electrical energy. | It converts electrical energy to chemical energy. |
Spontaneous | Non-spontaneous |
The salt bridge or porous barrier connects the two half-cells set up in different containers. | Both electrodes are dipped in a molten electrolyte solution in the same container. |
Anode = negative; Cathode = positive. | Anode = positive; Cathode = negative, |
The anode gets oxidised, while the cathode gets reduced. | The cathode gets oxidised, while the anode gets reduced. |
Electrons move from the anode to the cathode. | Electrons move from the cathode to the anode. |
Conditions for the operation of an electrolytic cell
Oxidation occurs at the electrode, which is strongly reducing, while reduction occurs at the electrode, which is strongly reducing.
Conditions for cell reaction to occur
- To overcome interactions at the electrode surface, overpotential or voltage excess is sometimes required. When it comes to gases, this is a more common occurrence.
- There could be several electrode reactions, which means there could be many half-reactions, leaving two or more choices for the cell reaction.
- As the reactants could be in non-standard circumstances, the voltage in the half cells could be less or more than the standard value.
- The capacity of an inert electrode to electrolyze is dependent on the reactants in the electrolyte solution. Still, an active electrode can execute the oxidation or reduction half-reaction on its own.
Uses of electrolytic cells
- Purification of substances: Several non-ferrous metals are electro refined and electron using electrolytic cells. Electrolytic cells manufacture almost all high-purity aluminium, copper, zinc, and lead used in industry. Saltwater can be purified by electrolysis.
- Electrolysis decomposes water into hydrogen and oxygen and bauxite into aluminium and some other products.
- An electrolytic cell is used for electroplating purposes. Electroplating metals like copper, silver, nickel, or chromium is done by this method. This technique uses the direct electric current (DC).
Although electrolytic cells work non-spontaneously, they are as important as electrochemical cells and provide a wide range of feasible applications on laboratory and industrial scales. Such cells can be constructed easily by the proper selection of electrodes and electrolytes.
Conclusion
Electrolytic cells, though non-spontaneous, contribute to the industry and the laboratory. Galvanic and electrolytic cells have opposite working principles, but both are significant in electrochemistry. Electrolytic cells can be used for metallurgical purposes such as extraction and purification of metals from their ores. Hence, electrolysis and the electrolytic cell can be used in industries for bulk scale production of chemicals. Depending on the choice of electrodes, a wide variety of cells can be constructed for different purposes.
