For a long time, people could not get many pure substances in free form. Such, for example, as:
- metals;
- alkalis;
- chlorine;
- hydrogen;
- hydrogen peroxide;
- organochlorine and others.
They were obtained either with a high content of impurities that were impossible to get rid of, or were not synthesized at all. But the compounds are very important for use in industry and everyday life. But with the discovery of a process such as electrolysis, a task of enormous scope was solved. Today it is used not only for synthesis, but also for many other processes.
What is electrolysis? How it occurs, from what stages it develops, what is the main advantage of this method, let's try to figure it out during the article.
What is electrolysis?
To answer this question, you must first turn to the terminology and understand some basic physical and chemical concepts.
- Direct current is a directed stream of electrons emanating from any source of electricity.
- An electrolyte is a substance whose solution is capable of conducting electric current.
- Electrodes - plates of certain materials, interconnected, which pass electricity through themselves (anode and cathode).
- The redox reaction is a process in which there is a change in the oxidation state of the participants. That is, some ions are oxidized and increase the value of the degree of oxidation, while others, on the contrary, are reduced, lowering it.
Having understood all these terms, one can answer the question of what electrolysis is. This is a redox process, which consists in passing a direct current through an electrolyte solution and culminating in the isolation of various products on the electrodes.
The simplest installation, which can be called an electrolyzer, includes only a few components:
- two glasses with electrolyte;
- current source;
- two electrodes interconnected.
In industry, it uses much more complex automated designs that allow to obtain large masses of products - electrolysis baths.
The electrolysis process is quite complex, obeys several theoretical laws and proceeds according to established procedures and rules. In order to correctly predict its outcome, it is necessary to clearly understand all the laws and possible options for passing.
The theoretical basis of the process
The most important fundamental canons on which electrolysis rests are the laws of Michael Faraday, a famous physicist known for his work in the study of electric current and all its accompanying processes.
There are two such rules, each of which describes the essence of the processes occurring during electrolysis.
First law
The first Faraday law, the formula of which is written as m = kI * Δt, is as follows.
The mass of matter released on the electrode is directly proportional to the electricity that passed through the electrolyte.
It can be seen from the formula that m is the mass of the substance, I is the current strength, Δt is the time during which it was passed. There is also a value of k, which is called the electrochemical equivalent of the compound. This value depends on the nature of the compound itself. Numerically, k is equal to the mass of the substance that is released on the electrode when one unit of electric charge is passed through the electrolyte.
The second rule of electrolysis
The second Faraday law, the formula of which is m = M * I * Δt / n * F, reads as follows. The electrochemical equivalent of compound (k) is directly proportional to its molar mass and inversely proportional to the valency of the substance.
The above formula is the result of the conclusion of all the combined. It reflects the essence of the second law of electrolysis. M is the molar mass of the compound, I is the current passed through the whole process, Δt is the time of all electrolysis, F is the Faraday constant, n are the electrons that participated in the process. Their number is equal to the charge of the ion that took part in the process.
Faraday's laws help to understand what electrolysis is, as well as calculate the possible yield of a product by mass, predict the desired result and affect the process. They form the theoretical basis of the transformations under consideration.
The concept of the anode and its types
Electrodes are very important in electrolysis. The whole process depends on the material from which they are made, on their specific properties and nature. Therefore, we consider in more detail each of them.
The anode is a plus, or positive electrode. That is, one that connects to the "+" pole of the power source. Accordingly, negative ions or anions will move to it from the electrolyte solution. They will be oxidized here, acquiring a higher degree of oxidation.
Therefore, you can depict a small scheme that will help to remember the anode processes: the plus anode - anions - oxidation. There are two main types of this electrode, depending on which, one or another product will be obtained.
- Insoluble, or inert anode. This type includes the electrode, which serves only for the transfer of electrons and oxidation processes, however, it itself is not consumed and does not dissolve. Such anodes are made of graphite, iridium, platinum, coal, and so on. Using such electrodes, it is possible to obtain pure metals, gases (oxygen, hydrogen, chlorine, and so on).
- Soluble anode. In oxidative processes, it dissolves itself and affects the outcome of all electrolysis. The main materials from which electrodes of this type are made: nickel, copper, cadmium, lead, tin, zinc and others. The use of such anodes is necessary for the processes of metal electrorefining, electroplating, the application of protective coatings against corrosion, and so on.
The essence of all the processes taking place on the positive electrode is that the most electronegative ions in terms of potential are discharged. And this is why anions of oxygen-free acids and hydroxide-ion do this, and then water, if we are talking about a solution. Oxygen-containing anions in an aqueous electrolyte solution generally do not discharge at the anode, since water does this faster, releasing oxygen.
Cathode and its characteristic
A cathode is a negatively charged electrode (due to the accumulation of electrons on it when passing an electric current). That is why positively charged ions move towards it - cations that undergo reduction, that is, lower the degree of oxidation.
Here, the scheme is also relevant for memorization: the minus cathode - cation - reduction. As the material for the cathode can serve:
- stainless steel;
- copper;
- carbon;
- brass;
- iron;
- aluminum and others.
It is on this electrode that metals are reduced to pure substances, which is one of the main ways to obtain them in industry. It is also possible the transition of electrons from the anode to the cathode, and if the first is soluble, then its ions are restored on the negative electrode. Here, hydrogen cations are reduced to H 2 gas. Therefore, the cathode is one of the most important parts in the general scheme of the process of electrolysis of substances.
Electrolysis of melts
From the point of view of chemistry, the process under consideration has its own equation. With it, you can depict the entire scheme on paper and predict the result. The most important thing to pay attention to is the presence or absence of an aqueous medium and the type of anode (soluble or not).
If it is necessary to obtain the following products: alkali and alkaline earth metals, alkalis, aluminum, beryllium, gases from oxygen-containing anions, then we cannot talk about the electrolysis of an electrolyte solution. Only melt, because otherwise the required compounds will not work. That is why often these substances are synthesized in industry using their dry anhydrous salts and hydroxides.
In general, the equation of electrolysis of the melt looks quite simple and standard. For example, if you consider and write it for potassium iodide, the view will be as follows:
KI = K + + I -
Cathode (K) "-": K + + 1e = K 0
Anode (A) "+": 2I - - 2e = I 2 0
The result of the process: KI = K + I 2.
In the same way, the electrolysis of any metal will be recorded, regardless of the value of its electrode potential.
Electrolysis of an aqueous solution
If we are talking about electrolyte solutions, then the outcome of the process will be completely different. After all, water becomes an active participant. It can also dissociate into ions and discharge at the electrodes. Therefore, in such cases, the electrode potential of ions is important. The lower its negative value, the greater the likelihood of faster oxidation or reduction.
The electrolysis of an aqueous solution is subject to several rules that should be remembered.
- Anodic processes: only anions of oxygen-free acids are discharged (except hydrogen fluoride). If the ion is oxygen-containing or fluoride-ion, then water will be oxidized with the release of oxygen.
- Cathode processes: metals in the electrochemical series of voltages (up to and including aluminum) at the cathode are not reduced due to the high chemical activity. This makes water release hydrogen. Metals from aluminum to hydrogen are reduced simultaneously with water to simple substances. Those that stand in a series of voltages after hydrogen (inactive) are easily reduced to simple substances.
If you follow these rules, you can display any electrolysis and calculate the yield of the product. In the case of a soluble anode, the circuit changes and becomes much more complex.
Electrolysis of salts
These processes are used to produce pure metals and gases, as it is technologically simple and cost-effective. In addition, the products come out with a high degree of purity, which is important.
For example, electrolysis of copper allows you to quickly get it in its pure form from a solution of any salt. Most often, copper sulfate or copper sulfate (II) - CuSO 4 is used .
Pure metal, which is so necessary in almost all branches of electrical engineering and metal construction, can be extracted both from the melt and from the solution of this salt.
The meaning and application of the process
Electrolysis is a very important process. Based on it are based such necessary technical operations as:
- Refining of metals.
- Electroextraction.
- Electroplating.
- Electrosynthesis
- Drawing anti-corrosion coatings and others.