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:

• Metals;
• Alkalis;
• chlorine;
• hydrogen;
• hydrogen peroxide;
• Chlororganic and others.

They were obtained either with a high content of impurities that could not be disposed of, or not synthesized 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? As it happens, from which 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 basic physicochemical concepts.

1. The direct current is a directed flow of electrons emanating from any source of electricity.
2. Electrolyte is a substance whose solution is capable of conducting electric current.
3. Electrodes are plates of certain materials, connected to each other, which pass electricity through themselves (anode and cathode).
4. Oxidation-reduction reaction is a process in which the degree of oxidation of 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, one can answer the question about 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 sophisticated automated designs that make it possible to produce large quantities of products - electrolysis tanks.

The process of electrolysis is rather complicated, it 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 which electrolysis is held are the laws of Michael Faraday, the famous scientist-physicist, known for his work in the field of studying the 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 at 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, and 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 passes 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 valence of the substance.

The above formula is the result of a derivation from all the combined ones. 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 took part in the process.

Faraday's laws help to understand what is electrolysis, and also to calculate the possible yield of the product by mass, to predict the necessary 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

Electrodes are very important in electrolysis. 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.

Anode - plus, or 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, you can depict a small scheme that will help memorize the anode processes: anode "plus" - anions - oxidation. There are two main types of this electrode, depending on which, this or that product will be obtained.

1. 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).
2. Soluble anode. In oxidizing processes, it dissolves itself and affects the outcome of all electrolysis. The main materials from which such electrodes are made are nickel, copper, cadmium, lead, tin, zinc and others. The use of such anodes is necessary for the processes of electrorefining metals, electroforming, applying protective coatings against corrosion, and so on.

The essence of all the processes occurring on the positive electrode is reduced to the discharge of the most electronegative ions in the potential value. Ivot why this 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

A cathode is a negatively charged electrode (due to the accumulation of electrons on it by passing an 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;
• 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 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 on the negative electrode. Here, hydrogen cations are reduced to the H2 gas. 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 consideration has its own equation. With the help of it, you can draw the entire 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 it is necessary to produce the following products: alkaline and alkaline earth metals, alkalis, aluminum, beryllium, gases from oxygen-containing anions, then there can be no talk 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 equation of melt electrolysis 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 ^-

The cathode (K) "-": K ⁺ + 1e = K ⁰

Anode (A) "+": 2I ^- - 2e = I ₂ ⁰

The result of the process is: KI = K + I _2.

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 outcome of the process will be quite 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.

1. Anode processes: Only anions of anoxic acids (except hydrofluoric) are discharged. If the ion is oxygen-containing or fluoride-ion, then water will be oxidized with the release of oxygen.
2. Cathode processes: metals in the electrochemical stress range (up to and including aluminum) 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 a solution of any salt. Most often copper sulphate or copper (II) sulfate - CuSO _{4 is used} .

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:

1. Refining of metals.
2. Electroextraction.
3. Electroplating.
4. Electrosynthesis.
5. Application of anticorrosion coatings and others.