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An electrolytic cell is an electrochemical device in which electrical energy is consumed to drive a non-spontaneous oxidation reduction or redox reaction. It is the opposite of a galvanic or voltaic cell , which generates electrical energy from a spontaneous redox reaction.
Many of the non-spontaneous reactions that take place in electrolytic cells involve the breakdown of a chemical compound into its constituent elements or into simpler chemical substances. This class of electrically powered lysis or breakdown processes is called electrolysis, which is where electrolytic cells get their name.
Electrolytic cells allow electrical energy to be converted into chemical potential energy. They also form the basis of many metallurgical processes without which society as we know it today would not exist.
Electrolytic cells versus electrochemical cells
A concept related to electrolytic cells is that of electrochemical cells. There is a small division around the concept of the latter. Some authors consider that all cells in which an oxidation-reduction reaction is associated with an electric current between two electrodes represents an electrochemical cell, regardless of whether the reaction is spontaneous or not. Seen from this point of view, electrolytic cells have been a particular type of electrochemical cell.
On the other hand, another group of authors defines electrochemical cells as those in which a spontaneous oxidation reduction reaction generates an electric current. In this case, electrolytic cells would be the exact opposite of electrochemical cells.
Regardless of this dilemma, it is clear that what characterizes an electrolytic cell is that it involves a redox reaction that is not spontaneous, and therefore requires an input of energy from an external source in order to occur.
Cells, half cells and half reactions
As its name implies, every oxidation-reduction reaction involves two separate but interrelated processes, oxidation and reduction. Oxidation is the loss of electrons while reduction is the gain of them. Since in a net chemical reaction there can be no orphan electrons without an atom to live on, oxidation and reduction cannot occur without each other. However, it is not mandatory that both processes occur at the same site.
This last fact represents the raison d’être of electrochemical cells and also (or by extension), of electrolytic cells. An electrolytic cell is nothing more than an experimental device in which the oxidation and reduction processes of a redox reaction are physically separated, but which allows the flow of electrons from where oxidation occurs to where reduction occurs through a conductor. electric. The separate compartments where these half-reactions take place are called half-cells , and the specific location or surface where each half-reaction occurs is called an electrode .
Every electrochemical or electrolytic cell is defined by the characteristics of the electrodes, by the particular half-reaction that occurs in each of them and by the composition and concentration of the solutions present in each half-cell. Furthermore, the spontaneity of the oxidation-reduction reaction is determined by the so-called cell potential (represented as E cell ).
A positive cell potential implies a spontaneous reaction, while if it is negative, the reaction will not be spontaneous. Therefore, we can again define an electrolytic cell as one that has a negative cell potential, which requires electrical energy to function.
Operation of electrolytic cells
The following figure shows the components of a typical generic electrolytic cell.
As can be seen, the cell is made up of two electrodes ( the anode and the cathode ) that are submerged in an electrolyte solution (which ensures that it conducts electricity, closing the electrical circuit) and that are also connected by means of electrical conductors passing through a direct current source (the gray box that is connected to the electrical wall).
The half-reactions that occur in this generic electrolytic cell are shown on the right side of the image. As can be seen, the cell potential (that of the overall reaction) is negative, so electrons (which are also negative) do not have the tendency to flow from the anode to the cathode.
However, when the power source is turned on, it generates a potential difference that counteracts and exceeds the cell potential, which prompts the electrons to move through the conductor, causing the oxidation-reduction reaction to occur.
By definition, in an electrolytic cell, the anode is the electrode where oxidation occurs and is usually represented on the left. Instead, the cathode is where the reduction occurs and is pictured on the right, so electrons always flow from the anode to the cathode.
An easy way to remember this (in Spanish) is that “vowels go with vowels and consonants go with consonants”:
Ánode , Oxidation and left start with a vowel, so they all go together; whereas, Cathode , Reduction , and Right all start with a consonant, so they also go together.
Electrolytic Cell Uses
You could say that electrolytic cells are essential to our modern way of life. This is due, firstly, to the many essential industries that depend entirely on electrolytic processes, and secondly, to the fact that they form the basis of our ability to store electrical energy in the form of chemical potential energy. Some of the most important applications of electrolytic cells are:
Production and purification of metals
Some of the most important metals for humans, such as aluminum and copper, are produced industrially by means of electrolytic cells. They also represent one of the few ways to obtain active metals such as alkali metals (lithium, sodium and potassium) and some very important alkaline earth metals such as magnesium.
Halogen production
Halogens such as fluorine and chlorine are of great importance in the chemical industry. They are essential reagents for the production of many petroleum derivatives such as PVC and Teflon, as well as being used in countless synthetic processes for drugs that save lives every day. The main source of these halogens is the electrolysis of salts containing their ions.
Energy storage
As mentioned above, electrolytic cells are capable of storing electrical energy in the form of chemical energy. The most tangible example of this is the charging process of all rechargeable batteries. Without electrolytic cells, the lithium batteries that power the vast majority of mobile devices we use every day would not be rechargeable. The electrolysis of water is the basis for the production of gaseous hydrogen , which can be used as clean fuel in a rocket such as Blue Origin’s Blue Shephard , Jeff Bezos’ aerospace company, or as a source of electrical energy in the fuel cells of some models of electric cars.
Examples of electrolytic cells
electrolysis of water
The electrolysis of water is carried out by passing a current through a 0.1 M sulfuric acid solution. The half-reactions involved and the overall reaction are:
Electrolysis of molten sodium chloride
In molten sodium chloride the ions act as the charge carriers that conduct electricity. This is how sodium is produced at an industrial level.
References
- Halogens (nd). Reviewed in July 2021 at https://www.textoscientificos.com/quimica/inorganica/halogenos/fluor
- Electrochemical cells (sf). Reviewed July 2021 at https://courses.lumenlearning.com/boundless-chemistry/chapter/electrochemical-cells/
- Electrochemical Cells . (2020, August 14). Revised July 2021 at https://chem.libretexts.org/@go/page/41636
- http://depa.fquim.unam.mx/amyd/archivero/INTRODUCCIONALAELECTROQUIMICA_22641.pdf
- Electrochemical Cell Conventions . (2021, April 10). Revised July 2021 at https://chem.libretexts.org/@go/page/291
