The electrolysis of water It is the decomposition of water into its elemental components through the application of an electric current. When proceeding, hydrogen and molecular oxygen, Htwo metwo. These two surfaces are better known by the name of electrodes..
Theoretically, the volume of Htwo formed must be twice the volume of Otwo. Why? Because the water molecule has an H / O ratio equal to 2, that is, two H for each oxygen. This relationship is directly verified with its chemical formula, HtwoO. However, many experimental factors influence the volumes obtained.
If the electrolysis is carried out inside tubes submerged in water (upper image), the lower water column corresponds to hydrogen, since there is a greater amount of gas exerting pressure on the surface of the liquid. The bubbles surround the electrodes and end up rising after overcoming the vapor pressure of the water.
Note that the tubes are separated from each other in such a way that there is low gas migration from one electrode to the other. At low scales, this does not represent an imminent risk; but on industrial scales, the gaseous mixture of Htwo metwo it is highly dangerous and explosive.
For this reason, the electrochemical cells where the electrolysis of water is carried out are very expensive; They need a design and elements that ensure that gases never mix, a cost-effective power supply, high concentrations of electrolytes, special electrodes (electrocatalysts), and mechanisms to store Htwo produced.
Electrocatalysts represent friction and at the same time wings for profitability of electrolysis of water. Some consist of noble metal oxides, such as platinum and iridium, the prices of which are very high. It is at this point especially where researchers join forces to design efficient, stable and cheap electrodes..
The reason for these efforts is to accelerate the formation of Otwo, which occurs at lower speeds compared to Htwo. This slowing down by the electrode where the O is formedtwo brings as a general consequence the application of a potential much greater than necessary (overpotential); what is the same, to a lower performance and higher expenses.
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The electrolysis of water involves many complex aspects. However, in general terms, its basis rests on a simple global reaction:
2HtwoO (l) => 2Htwo(g) + Otwo(g)
As can be seen from the equation, two water molecules are involved: one ordinarily must be reduced, or gain electrons, while the other must oxidize or lose electrons..
The Htwo It is a product of the reduction of water, because the gain of electrons promotes that the H protons+ can bind covalently, and the oxygen is transformed into OH-. Therefore, the Htwo occurs at the cathode, which is the electrode where reduction occurs.
While the Otwo comes from the oxidation of water, due to which it loses the electrons that allow it to bind to hydrogen, and consequently releases protons H+. The Otwo it is produced at the anode, the electrode where oxidation occurs; and unlike the other electrode, the pH around the anode is acidic and not basic.
This can be summarized with the following chemical equations for half-cell reactions:
2HtwoO + 2e- => Htwo + 2OH- (Cathode, basic)
2HtwoO => Otwo + 4H+ + 4e- (Anode, acid)
However, water cannot lose any more electrons (4e-) of which the other water molecule wins at the cathode (2e-); therefore, the first equation must be multiplied by 2, and then subtracted with the second equation to obtain the net equation:
2 (2HtwoO + 2e- => Htwo + 2OH-)
2HtwoO => Otwo + 4H+ + 4e-
6HtwoO => 2Htwo + ORtwo + 4H+ + 4OH-
But 4H+ and 4OH- form 4HtwoOr, so these remove four of the six molecules of HtwoOr leaving two; and the result is the global reaction just raised.
Half-cell reactions change with pH values, techniques, and also have associated reduction or oxidation potentials, which determine how much current needs to be supplied for the electrolysis of water to proceed spontaneously..
A Hoffman voltameter is shown in the image above. The cylinders are filled with water and the selected electrolytes through the middle nozzle. The role of these electrolytes is to increase the conductivity of the water, because under normal conditions there are very few H ions3OR+ and OH- products of its self ionization.
The two electrodes are usually platinum, although in the image they were replaced by carbon electrodes. Both are connected to a battery, with which a potential difference (ΔV) is applied that promotes the oxidation of water (formation of Otwo).
The electrons travel the entire circuit until they reach the other electrode, where the water wins them over and becomes Htwo and OH-. At this point the anode and cathode are already defined, which can be differentiated by the height of the water columns; the one with the lowest height corresponds to the cathode, where the H is formedtwo.
At the top of the cylinders, there are keys that allow the gases generated to be released. The presence of H can be carefully checkedtwo making it react with a flame, the combustion of which produces gaseous water.
Water electrolysis techniques vary depending on the amount of Htwo metwo that is proposed to generate. Both gases are very dangerous if they are mixed together, and that is why electrolytic cells involve complex designs to minimize the increase in gaseous pressures and their diffusion through the aqueous medium.
Also, the techniques vary depending on the cell, the electrolyte added to the water, and the electrodes themselves. On the other hand, some imply that the reaction is carried out at higher temperatures, reducing electricity consumption, and others use enormous pressures to maintain the Htwo stored.
Among all the techniques, the following three can be mentioned:
Electrolysis is carried out with basic solutions of the alkali metals (KOH or NaOH). With this technique the reactions occur:
4HtwoO (l) + 4e- => 2Htwo(g) + 4OH-(ac)
4OH-(ac) => Otwo(g) + 2HtwoO (l) + 4e-
As can be seen, both at the cathode and at the anode, water has a basic pH; and in addition, the OH- migrate to the anode where they oxidize to Otwo.
This technique uses a solid polymer that serves as a permeable membrane for H+, but impervious to gases. This ensures greater safety during electrolysis..
The half-cell reactions for this case are:
4H+(ac) + 4e- => 2Htwo(g)
2HtwoO (l) => Otwo(g) + 4H+(ac) + 4e-
H ions+ migrate from the anode to the cathode, where they are reduced to become Htwo.
Very different from other techniques, this one uses oxides as electrolytes, which at high temperatures (600-900ºC) function as a means of transporting the anion Otwo-.
The reactions are:
2HtwoO (g) + 4e- => 2Htwo(g) + 2Otwo-
2Otwo- => Otwo(g) + 4e-
Note that this time they are the oxide anions, Otwo-, those that travel to the anode.
The electrolysis of water produces Htwo (g) and Otwo (g). Approximately 5% of the hydrogen gas produced in the world is made through the electrolysis of water.
The Htwo it is a by-product of the electrolysis of aqueous NaCl solutions. The presence of salt facilitates electrolysis by increasing the electrical conductivity of the water.
The overall reaction that takes place is:
2NaCl + 2HtwoO => Cltwo + Htwo + 2NaOH
To understand the enormous importance of this reaction, some of the uses of gaseous products will be mentioned; Because at the end of the day, these are the ones that drive the development of new methods to achieve electrolysis of water in a more efficient and green way.
Of all of them, the most desired is to function as cells that energetically replace the use of burning fossil fuels..
-Hydrogen produced in electrolysis can be used in the chemical industry acting in addiction reactions, in hydrogenation processes or as a reducing agent in reduction processes.
-Likewise, it is essential in some actions of commercial importance, such as: the production of hydrochloric acid, hydrogen peroxide, hydroxylamines, etc. It is involved in the synthesis of ammonia through a catalytic reaction with nitrogen.
-In combination with oxygen, it produces flames with a high caloric content, with temperatures ranging between 3,000 and 3,500 K. These temperatures can be used for cutting and welding in the metal industry, for growth of synthetic crystals, production of quartz, etc..
-Water treatment: excessively high nitrate content in water can be reduced by its elimination in bioreactors, in which bacteria use hydrogen as an energy source
-Hydrogen is involved in the synthesis of plastics, polyester and nylon. In addition, it is part of the production of glass, increasing combustion during baking..
-Reacts with the oxides and chloride of many metals, including: silver, copper, lead, bismuth, and mercury to produce pure metals.
-And additionally, it is used as a fuel in chromatographic analysis with a flame detector..
The electrolysis of sodium chloride solutions is used to purify swimming pool water. During electrolysis, hydrogen is produced at the cathode and chlorine (Cltwo) at the anode. Electrolysis is referred to in this case as a salt chlorinator.
Chlorine dissolves in water to form hypochlorous acid and sodium hypochlorite. Hypochlorous acid and sodium hypochlorite sterilize water.
The electrolysis of water is also used to generate oxygen on the International Space Station, which serves to maintain an atmosphere of oxygen in the station..
Hydrogen can be used in a fuel cell, a method to store energy, and use the water generated in the cell for consumption by astronauts.
Water electrolysis experiments have been carried out at laboratory scales with Hoffman voltmeters, or another assembly that allows to contain all the necessary elements of an electrochemical cell..
Of all the possible assemblies and equipment, the simplest may be a large transparent water container, which will serve as a cell. In addition to this, any metal or electrically conductive surface must also be on hand to function as electrodes; one for the cathode, and the other for the anode.
For this purpose even pencils with sharp graphite tips at both ends can be useful. And finally, a small battery and some cables that connect it to the improvised electrodes..
If it is not carried out in a transparent container, the formation of gaseous bubbles would not be appreciated.
Although the electrolysis of water is a subject that contains many intriguing and encouraging aspects for those looking for alternative energy sources, the home experiment can be boring for children and other bystanders..
Therefore, sufficient voltage can be applied to generate the formation of Htwo metwo alternating certain variables and noting the changes.
The first of these is the variation of the pH of the water, using either vinegar to acidify the water, or NatwoCO3 to basify it slightly. A change must occur in the number of bubbles observed.
Additionally, the same experiment could be repeated with hot and cold water. In this way, the effect of temperature on the reaction would then be considered..
Finally, to make the data collection a little less colorless, a very dilute solution of purple cabbage juice can be used. This juice is an acid-base indicator of natural origin.
Adding it to the container with the inserted electrodes, it will be noticed that at the anode the water will turn pink (acid), while at the cathode, the color will be yellow (basic).
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