What is electrolysis? Anode and cathode. Physico-chemical process
For a long time people could not get many pure substances in a free form. Such, for example, as:
- hydrogen peroxide;
- chlororganic and others.
They were obtained either with a high content of impurities,from which it was impossible to get rid of, or did not synthesize at all. But the connections are very important for use in industry and everyday life. But with the discovery of such a process as electrolysis, the task of a huge scale was solved. Today it is used not only for synthesis, but also for many other processes.
What is electrolysis? How it happens, from what stages is formed, what is the main advantage of this method, let's try to understand the course of the article.
What is electrolysis?
To answer this question, one should first turn to terminology and understand some of the basic physicochemical concepts.
- A direct current is a directed flow of electrons emanating from any source of electricity.
- Electrolyte is a substance whose solution is capable of conducting electric current.
- Electrodes are plates of certain materials connected to each other, which pass electricity through themselves (anode and cathode).
- Oxidation-reduction reaction isThe process at which the degree of oxidation of the participants changes. That is, some ions oxidize and increase the value of the degree of oxidation, while others, on the contrary, are reduced, lowering it.
Having explained all these terms, you can answer the questionabout what is electrolysis. This oxidation-reduction process consists of passing a direct current through the electrolyte solution and ending with the release of different products on the electrodes.
The simplest installation, which can be called an electrolytic cell, includes only a few components:
- two glasses with electrolyte;
- current source;
- two electrodes connected to each other.
In the industry it uses much more complex automated designs that make it possible to produce large quantities of products - electrolytic baths.
The process of electrolysis is quite complex,obeys several theoretical laws and proceeds according to established procedures and rules. To correctly predict its outcome, it is necessary to clearly understand all the patterns and possible variants of passage.
Theoretical basis of the process
The most important fundamental canons on whichthe laws of Michael Faraday - the famous scientist-physicist, known for his work in the field of the study of electric current and all accompanying processes.
In total, there are two such rules, each of which describes the essence of the processes that occur during electrolysis.
The First Law
The first Faraday law, whose formula is written as m = kI * Δt, reads as follows.
The mass of the substance 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 a substance, I -current strength, Δt - time during which it was skipped. 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 passes through the electrolyte.
The second rule of electrolysis
The second law of Faraday, whose formula -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 valence of the substance.
The above formula is the result of the derivationfrom all united. It reflects the essence of the second law of electrolysis. M is the molar mass of the compound, I is the strength of the current passed through the entire process, Δt is the time of the entire electrolysis, F is the Faraday constant, and n is the electrons that participated in the process. Their number is equal to the charge of the ion that participated in the process.
Faraday's laws help to understand what iselectrolysis, and also to calculate the possible yield of the product by mass, to predict the required result and to influence the course of the process. They constitute the theoretical basis of the transformations under consideration.
The concept of the anode and its types
Very important in electrolysis haveelectrodes. The whole process depends on the material from which they are made, on their specific properties and character. Therefore, we consider in more detail each of them.
The anode is a plus, or a positive electrode. That is, one that joins the "+" pole of the power source. Accordingly, to it from a solution of electrolyte negative ions or anions will move. They will oxidize here, acquiring a higher degree of oxidation.
Therefore, it is possible to depict a small scheme thatwill help remember the anode processes: anode "plus" - anions - oxidation. There are two main types of this electrode, depending on which, this or that product will be produced.
- Insoluble, or inert anode. An electrode is used for this type, which serves only for the transfer of electrons and oxidation processes, but 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 metals in pure form, 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 this type of electrodes are made: nickel, copper, cadmium, lead, tin, zinc and others. The use of such anodes is necessary for the processes of electrorefining metals, electroplating, applying protective coatings against corrosion and so on.
The essence of all processes occurring on a positiveelectrode is reduced to the fact that the most electronegative ions with respect to the potential are discharged. Ivot why it is done anions of anoxic acids and hydroxide ion, and then water, when it comes to the solution. Oxygen-containing anions in the aqueous solution of the electrolyte generally do not discharge at the anode, since water does this faster, releasing oxygen.
Cathode and its characteristics
The cathode is a negatively charged electrode (foraccount of the accumulation of electrons on it with the transmission of electric current). That is why positive ions-cations that undergo restoration-move toward it, that is, they lower the oxidation state.
Here, for memorizing, the following scheme is also appropriate: cathode "minus" -cation-restoration. The material for the cathode can be:
- stainless steel;
- aluminum and others.
It is on this electrode thatthe restoration of metals to pure substances, which is one of the main ways of obtaining them in industry. It is also possible to transfer electrons from the anode to the cathode, and if the first is soluble, then its ions are restored to the negative electrode. Here, the hydrogen cations are reduced to the gas H2. Therefore, the cathode is one of the most important parts in the overall scheme of the process of electrolysis of substances.
Electrolysis of melts
From the point of view of chemistry, the process under considerationhas its own equation. With the help of it you can draw the whole scheme on paper and anticipate the result. Most importantly, what should be paid attention to - the presence or absence of an aqueous medium and the type of anode (soluble or not).
If you need to receive the following products: alkaline and alkaline earth metals, alkalis, aluminum, beryllium, gases from oxygen-containing anions, then there can be no question of electrolysis of the electrolyte solution. Only the melt, because otherwise the required connections will not work. That's why often the industry synthesizes the listed substances, using their anhydrous dry salts and hydroxides.
In general, the electrolysis equation of the melt looks quite simple and standard. For example, if you consider and write it for potassium iodide, the form will be as follows:
KI = K++ I-
Cathode (K) "-": K++ 1e = K0
Anode (A) "+": 2I-- 2e = I20
The result of the process: KI = K + I2.
Similarly, electrolysis of any metal will be recorded, regardless of the value of its electrode potential.
Electrolysis of aqueous solution
If we are talking about solutions of electrolytes, the outcomeprocess will be completely different. After all, water becomes an active participant. It is also capable of dissociating into ions and discharging at the electrodes. Therefore, in such cases, the electrode potential of the ions is of great importance. The lower its negative value, the greater the probability of faster oxidation or reduction.
The electrolysis of the aqueous solution is subject to several rules that must be remembered.
- Anode processes: Only anions of anoxic acids (except hydrofluoric) are discharged. If the ion is oxygen-containing or fluoride-ion, then oxygen will be oxidized.
- Cathodic processes: metals in the electrochemical series of stresses (up to aluminum inclusive) on the cathode are not restored due to high chemical activity. This makes water with the release of hydrogen. Metals from aluminum to hydrogen are reduced simultaneously with water to simple substances. The same ones that stand after hydrogen in a series of stresses (low-active), easily undergo restoration to simple substances.
If you follow these rules, you can depict any electrolysis and calculate the yield of the product. In the case of a soluble anode, the circuit changes and becomes much more complicated.
Electrolysis of salts
These processes are used to produce pure metals and gases, since it is technologically simple and economically profitable. 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 pure form from any salt solution. The most commonly used copper sulfate or copper (II) sulfate - CuSO4.
Both from the melt and from the solution of this salt, it is possible to extract pure metal, which is so necessary in almost all branches of electrical engineering and metal construction.
Importance and application of the process
Electrolysis is a very important process. On its basis such necessary technical operations are based as:
- Refining of metals.
- Application of anticorrosion coatings and others.