The anode and cathode are the types of electrodes found in electrochemical cells. These are devices capable of producing electrical energy through a chemical reaction. The most used electrochemical cells are batteries.
There are two types of electrochemical cells, electrolytic cells and galvanic or voltaic cells. In electrolytic cells, the chemical reaction that produces energy does not happen spontaneously, but the electrical current is transformed into a chemical oxidation-reduction reaction.
The galvanic cell is made up of two half cells. These are connected by two elements, a metallic conductor and a salt bridge.
The electrical conductor, as its name implies, conducts electricity because it has very little resistance to the movement of electrical charge. The best conductors are usually metal.
The salt bridge is a tube that connects the two half-cells, while maintaining their electrical contact, and without letting the components of each cell come together. Each half-cell of the galvanic cell contains an electrode and an electrolyte.
When the chemical reaction takes place, one of the half cells loses electrons towards its electrode, through the oxidation process; while the other gains electrons for its electrode, through the reduction process.
Oxidation processes occur at the anode, and reduction processes at the cathode
Anode
The name of the anode comes from the Greek ανά (aná): upwards, and οδός (odós): way. Faraday was the one who coined this term in the 19th century.
The best definition of anode is the electrode that loses electrons in an oxidation reaction. It is normally linked to the positive pole of the transit of electric current, but this is not always the case.
Although in batteries the anode is the positive pole, in LED lights it is the opposite, with the anode being the negative pole.
Normally the direction of electric current is defined, appreciating it as a direction of free charges, but if the conductor is not metallic, the positive charges that are produced are transferred to the external conductor.
This movement implies that we have positive and negative charges that move in opposite directions, so it is said that the direction of the current is the path of the positive charges of the cations that are in the anode towards the negative charge of the anodes found on the cathode.
In galvanic cells, having a metallic conductor, the current generated in the reaction does follow the path from the positive to the negative pole.
But in electrolytic cells, as they do not have a metallic conductor, but rather an electrolyte, ions with a positive and negative charge can be found that move in opposite directions.
Thermionic anodes receive most of the electrons that come from the cathode, heat the anode, and have to find a way to dissipate it. This heat is generated in the voltage that occurs between the electrons.
Special anodes
There is a special type of anode, such as those found inside X-rays. In these tubes, the energy produced by electrons, in addition to producing X-rays, generates great energy that heats up the anode.
This heat is produced at the different voltage between the two electrodes, which exerts pressure on the electrons. When electrons move in the electric current, they impact against the anode, transmitting their heat to it.
Cathode
The cathode is the negatively charged electrode, which undergoes a reduction reaction in the chemical reaction, where its oxidation state is reduced when it receives electrons.
As with the anode, it was Faraday who suggested the term cathode, which comes from the Greek κατά: 'downwards', and ὁδός: 'way'. To this electrode, the negative charge was attributed over time.
This approach turned out to be false, since depending on the device it is in, it has one load or another.
This relationship with the negative pole, as with the anode, arises from the assumption that current flows from the positive pole to the negative pole. This arises within a galvanic cell.
Inside electrolytic cells, the energy transfer medium, not being in a metal but in an electrolyte, negative and positive ions can coexist that move in opposite directions. But by convention, the current is said to go from the anode to the cathode.
Special cathodes
One type of specific cathodes are thermionic cathodes. In these, the cathode emits electrons due to the effect of heat.
In thermionic valves, the cathode can heat itself by circulating a heating current in a filament attached to it.
Equilibrium reaction
If we take a galvanic cell, which is the most common electrochemical cell, we can formulate the equilibrium reaction that is generated.
Each half cell that makes up the galvanic cell has a characteristic voltage known as the reduction potential. Within each half-cell, an oxidation reaction occurs between the different ions.
When this reaction reaches equilibrium, the cell cannot provide any more tension. At this time, the oxidation that is taking place in the half-cell at that moment will have a positive value the closer it is to equilibrium. The potential of the reaction will be greater the more equilibrium is reached.
When the anode is in equilibrium, it begins to lose electrons that pass through the conductor to the cathode.
The reduction reaction is taking place at the cathode, the further it is from equilibrium, the more potential the reaction will have when it occurs and takes the electrons that come from the anode.
References
- HUHEEY, James E., et al. Inorganic chemistry: principles of structure and reactivity. Pearson Education India, 2006.
- SIENKO, Michell J.; ROBERT, A. Chemistry: principles and properties. New York, US: McGraw-Hill, 1966.
- BRADY, James E. General chemistry: principles and structure. Wiley, 1990.
- PETRUCCI, Ralph H., et al. General chemistry. Inter-American Educational Fund, 1977.
- MASTERTON, William L.; HURLEY, Cecile N. Chemistry: principles and reactions. Cengage Learning, 2015.
- BABOR, Joseph A.; BABOR, JoseJoseph A.; AZNÁREZ, José Ibarz. Modern General Chemistry: An Introduction to Physical Chemistry and Higher Descriptive Chemistry (Inorganic, Organic, and Biochemistry). Marin,, 1979.
- CHARLOT, Gaston; TRÉMILLON, Bernard; BADOZ-LAMBLING, J. Electrochemical reactions. Toray-Masson, 1969.