What is the electrolysis process. Electrolyzer

When an electric current passes through a solution or a melt of an electrolyte, dissolved substances or other substances are released on the electrodes, which are products of secondary reactions on the electrodes. This physicochemical process is called electrolysis.

The essence of electrolysis

In the electric field created by the electrodes, the ions in the conductive liquid come into orderly motion. The negative electrode is the cathode, the positive is the anode.

Negative ions, called anions (ions of the hydroxyl group and acid residues), rush to the anode, and positive ions called cations (ions of hydrogen, metals, ammonium, etc.)

The redox process takes place on the electrodes: the electrochemical reduction of particles (atoms, molecules, cations) occurs at the cathode, and the electrochemical oxidation of particles (atoms, molecules, anions) occurs at the anode. Dissociation reactions in the electrolyte are primary reactions, and reactions that take place directly on the electrodes are called secondary.

The separation of electrolysis reactions into primary and secondary ones helped Michael Faraday establish the laws of electrolysis:

Faraday's first law of electrolysis: the mass of a substance deposited on an electrode during electrolysis is directly proportional to the amount of electricity transferred to this electrode. The amount of electricity refers to the electric charge, usually measured in pendants.

Faraday's second law of electrolysis: for a given amount of electricity (electric charge), the mass of a chemical element deposited on an electrode is directly proportional to the equivalent mass of the element. The equivalent mass of a substance is its molar mass divided by an integer, depending on the chemical reaction in which the substance is involved.

m is the mass of the substance deposited on the electrode, Q is the total electric charge passed through the substance F = 96 485.33 (83) C mol − 1 is the Faraday constant, M is the molar mass of the substance (For example, the molar mass of water H2O = 18 g / mol), z is the valence number of ions of the substance (the number of electrons per ion).

Note that M / z is the equivalent mass of the deposited substance. For the first Faraday's law, M, F and z are constants, so the larger the value of Q, the larger the value of m. For Faraday's second law, Q, F, and z are constants, so the larger the value of M / z (equivalent mass), the larger the value of m.

Electrolysis is widely used today in industry and technology. For example, it is electrolysis that is one of the most effective methods for the industrial production of hydrogen, hydrogen peroxide, manganese dioxide, aluminum, sodium, magnesium, calcium and other substances. Electrolysis is used for wastewater treatment, in electroplating, in electroplating, and finally in chemical power sources. But first things first.

Thanks to electrolysis, many metals are extracted from ores and further processed. So, when ore or concentrated ore - concentrate - is treated with reagents, the metal goes into solution, then the metal is separated from the solution by means of electro-extraction. The pure metal is then precipitated at the cathode. In this way, zinc, copper, cadmium are obtained.

Metals are subjected to electrorefining to eliminate impurities and to convert the contained impurities into a form convenient for further processing. The metal to be cleaned is cast in the form of plates, and these plates are used as anodes in electrolysis.

When the current passes, the metal of the anode dissolves, passes in the form of cations into the solution, then the cations are discharged at the cathode, and form a deposit of pure metal. The impurities of the anode do not dissolve - they fall out as anode sludge, or pass into the electrolyte, from where they are continuously or periodically removed.

Consider as an example electrorefining of copper... The main component of the solution is copper sulfate - the most common and cheapest salt of this metal. The solution has a low electrical conductivity. To increase it, sulfuric acid is added to the electrolyte.

In addition, small amounts of additives are introduced into the solution, contributing to the formation of a compact metal deposit. In general, copper, nickel, lead, tin, silver, and gold are subjected to electrolytic refining.

Electrolysis is used in wastewater treatment (electrocoagulation, electroextraction and electroflotation processes). The electrochemical cleaning method is one of the most commonly used. For electrolysis, insoluble anodes are used (magnetite, lead oxide, graphite, manganese, which are deposited on a titanium base), or soluble (aluminum, iron).

This method is used to isolate toxic organic and inorganic substances from water. For example, copper pipes are descaled with a sulfuric acid solution, and industrial wastewater must then be purified by electrolysis with an insoluble anode. Copper is released at the cathode, which can be used again in the same plant.

Alkaline wastewater is purified by electrolysis to remove cyanide compounds. In order to accelerate the oxidation of cyanides, increase electrical conductivity and save electricity, an additive in the form of sodium chloride is applied to the waters.

Electrolysis is carried out with a graphite anode and a steel cathode. Cyanides are destroyed during electrochemical oxidation and chlorine is released at the anode. The efficiency of such cleaning is close to 100%.

In addition to direct electrochemical cleaning, it can be included in the electrolysis process coagulation... Excluding the addition of salts, electrolysis is carried out with soluble aluminum or iron anodes. Then not only the contaminants on the anode are destroyed, but the anode itself is dissolved. Active dispersed compounds are formed, which coagulate (thicken) colloidal dispersed impurities.

This method is effective in treating waste water from fats, oil products, dyes, oils, radioactive substances, etc. It is called electrocoagulation.

Electroplating is the electrolytic deposition of certain metals in order to protect products from corrosion and to give them an appropriate aesthetic design (the coating is made with chrome, nickel, silver, gold, platinum, etc.). The thing is thoroughly cleaned, degreased, and used as a cathode in an electrolytic bath, into which a salt solution of the metal that needs to be coated is poured.

A plate made of the same metal is used as the anode. As a rule, a pair of anode plates are used, and the object to be electroplated is placed between them.

Electroplating - the deposition of metal on the surface of various bodies to reproduce their shape: molds for casting parts, sculptures, printing plates, etc.

Galvanic deposition of metal on the surface of an object is possible only when this surface or the entire object are conductors of electric current, therefore, it is desirable to use metals for making models or forms. Low-melting metals are most suitable for this purpose: lead, tin, solders, Wood's alloy.

These metals are soft, easy to work with locksmith tools, well engraved and cast. After building up the galvanic layer and finishing, the metal of the mold is melted from the finished product.

However, dielectric materials still present the greatest opportunities for making models. To metallize such models, you need to give their surface electrical conductivity. Success or failure ultimately depends largely on the quality of the conductive layer. This layer can be applied in one of three ways.

The most common way is graphitization, it is suitable for models made of plasticine and other materials that allow graphite to be rubbed over the surface.

The next trick is bronzing, the method is good for models of relatively complex shapes, for different materials, however, due to the thickness of the bronze layer, the transfer of small details is somewhat distorted.

And finally silvering, suitable in all cases, but especially indispensable for fragile models with a very complex shape - plants, insects, etc.

Chemical power sources

Also, electrolysis is the main process due to which the most modern chemical power sources, such as batteries and accumulators, function. There are two electrodes in contact with the electrolyte.

Lemon battery (click on the picture to enlarge)

The action of chemical current sources is based on the course of spatially separated processes with a closed external circuit: the reducing agent is oxidized at the negative anode, the resulting free electrons pass through the external circuit to the positive cathode, creating a discharge current, where they participate in the oxidant reduction reaction. Thus, the flow of negatively charged electrons through the external circuit goes from the anode to the cathode, that is, from the negative electrode to the positive one.

Electrolysis is a process by which electrical energy is converted into chemical energy. This process takes place on the electrodes under the influence of direct current. What are the products of electrolysis of melts and solutions, and what is included in the concept of "electrolysis".

Electrolysis of molten salts

Electrolysis is a redox reaction that takes place on the electrodes when direct electric current is passed through a solution or molten electrolyte.

Rice. 1. The concept of electrolysis.

The chaotic movement of ions under the action of the current becomes ordered. Anions move to the positive electrode (anode) and oxidize on it, donating electrons. Cations move to the negative pole (cathode) and are reduced on it, accepting electrons.

Electrodes can be inert (platinum or gold metal or carbon or graphite non-metal) or active. In this case, the anode dissolves during the electrolysis process (soluble anode). It is made from metals such as chromium, nickel, zinc, silver, copper, etc.

During the electrolysis of molten salts, alkalis, oxides, metal cations are discharged at the cathode with the formation of simple substances. The electrolysis of melts is an industrial method for producing metals such as sodium, potassium, calcium (electrolysis of molten salts) and aluminum (electrolysis of molten aluminum oxide Al 2 O 3 in cryolite Na 3 AlF 6, used to facilitate the transfer of oxide to the melt). For example, the electrolysis scheme of sodium chloride NaCl melt is as follows:

NaCl Na + + Cl -

Cathode(-) (Na +): Na + + e= Na 0

Anode(-) (Cl -): Cl - - e= Cl 0, 2Cl 0 = Cl 2

Summarized process:

2Na + + 2Cl- = electrolysis 2Na + 2Cl 2

2NaCl = electrolysis 2Na + Cl 2

Simultaneously with the production of the alkali metal sodium, chlorine is obtained by electrolysis of the salt.

Electrolysis of salt solutions

If salt solutions are subjected to electrolysis, then, along with the ions formed during the dissociation of the salt, water can also be oxidized or reduced on the electrodes.

There is a definite sequence for the discharge of ions at the electrodes in aqueous solutions.

1. The higher the standard electrode potential of a metal, the easier it is to recover. In other words, the more to the right the metal is in the electrochemical series of voltages, the easier its ions will be reduced at the cathode. When electrolysis of solutions of metal salts from lithium to aluminum, inclusive, water molecules are always reduced at the cathode:

2H 2 O + 2e = H 2 + 2OH-

If solutions of metal salts are subjected to electrolysis, starting from copper and to the right of copper, only metal cations are reduced at the cathode. During the electrolysis of metal salts from manganese MN to lead Pb, both metal cations and, in some cases, water can be reduced.

2. Anions of acid residues (except F-) are oxidized on the anode. If salts of oxygen-containing acids are subjected to electrolysis, then the anions of acid residues remain in solution, water is oxidized:

2H 2 O-4e = O 2 + 4H +

3. If the anode is soluble, then oxidation and dissolution of the anode itself occurs:

Example: electrolysis of an aqueous solution of sodium sulfate Na 2 SO 4:

Electrolysis processes

Electrolysis has become widespread in the metallurgy of non-ferrous metals and in a number of chemical industries. Metals such as aluminum, zinc, magnesium are obtained mainly by electrolysis. In addition, electrolysis is used for the refining (purification) of copper, nickel, lead, as well as for the production of hydrogen, oxygen, chlorine and a number of other chemicals.

The essence of electrolysis is the separation of particles of a substance from the electrolyte when a direct current flows through the electrolytic bath and their deposition on electrodes immersed in the bath (electroextraction) or in the transfer of substances from one electrode through the electrolyte to another (electrolytic refining). In both cases, the goal of the processes is to obtain the purest possible substances that are not contaminated with impurities.

Unlike metals in electrolytes (solutions of salts, acids and bases in water and in some other solvents, as well as in molten compounds), ionic conductivity is observed.

Electrolytes are second class conductors. In these solutions and melts, electrolytic dissociation takes place - decay into positively and negatively charged ions.

If electrodes connected to an electric energy source are placed in a vessel with an electrolyte - an electrolyzer, then an ion current will begin to flow in it, and positively charged ions - cations will move to the cathode (these are mainly metals and hydrogen), and negatively charged ions - anions ( chlorine, oxygen) - to the anode.

At the anode, the anions give up their charge and turn into neutral particles that settle on the electrode. At the cathode, cations take electrons from the electrode and are also neutralized, settling on it, and the gases released on the electrodes in the form of bubbles rise upward.

Rice. 1. Processes during electrolysis. Electrolysis bath circuit: 1 - bath, 2 - electrolyte, 3 - anode, 4 - cathode, 5 - power supply

The electric current in the external circuit is the movement of electrons from the anode to the cathode (Fig. 1). In this case, the solution is depleted, and to maintain the continuity of the electrolysis process it is necessary to enrich it. This is how the extraction of certain substances from the electrolyte is carried out (electroextraction).

If the anode can dissolve in the electrolyte as the latter is depleted, then its particles, dissolving in the electrolyte, acquire a positive charge and are directed to the cathode, on which they are deposited, thereby transferring material from the anode to the cathode. Since the process is carried out so that the impurities contained in the metal of the anode are not transferred to the cathode, this process is called electrolytic refining.

If the electrode is placed in a solution with ions of the same substance from which it is made, then at a certain potential between the electrode and the solution, neither dissolution of the electrode nor deposition of the substance from the solution occurs on it.

This potential is called the normal potential of the substance. If a more negative potential is applied to the electrode, then the release of a substance (cathodic process) will begin on it, but if it is more positive, then its dissolution will begin (anodic process).

The value of normal potentials depends on the concentration of ions and temperature. It is generally accepted to consider the normal potential of hydrogen as zero. Table 1 shows the normal electrode potentials of some aqueous solutions of substances at + 25 ° C.

Table 1. Normal electrode potentials at + 25 ° С

If the electrolyte contains ions of different metals, then ions with a lower negative normal potential (copper, silver, lead, nickel) are released first at the cathode; alkaline earth metals are the most difficult to isolate. In addition, there are always hydrogen ions in aqueous solutions, which will be released earlier than all metals with a negative normal potential, therefore, during the electrolysis of the latter, a significant or even most of the energy is spent on the evolution of hydrogen.

By means of special measures, it is possible to prevent the evolution of hydrogen within certain limits, however, metals with a normal potential of less than 1 V (for example, magnesium, aluminum, alkaline earth metals) cannot be obtained by electrolysis from an aqueous solution. They are obtained by the decomposition of molten salts of these metals.

Normal electrode potentials of substances indicated in table. 1, are minimal, at which the electrolysis process begins, in practice, large values ​​of the potential are required for the development of the process.

The difference between the actual potential of the electrode during electrolysis and the potential normal for it is called overvoltage. It increases energy losses during electrolysis.

On the other hand, increasing the overvoltage for hydrogen ions makes it difficult to release it at the cathode, which makes it possible to obtain by electrolysis from aqueous solutions a number of metals that are more negative than hydrogen, such as lead, tin, nickel, cobalt, chromium, and even zinc. This is achieved by conducting the process at increased current densities on the electrodes, as well as by introducing certain substances into the electrolyte.

The course of cathodic and anodic reactions during electrolysis is determined by the following two Faraday laws.

1. The mass of the substance m e released during electrolysis at the cathode or passed from the anode to the electrolyte is proportional to the amount of electricity passed through the electrolyte I τ : m e = α / τ, here a is the electrochemical equivalent of the substance, g / C.

2. The mass of the substance released during electrolysis with the same amount of electricity is directly proportional to the atomic mass of substance A and inversely proportional to its valence n: m e = A / 96480n, here 96480 is the Faraday number, C x mol -1.

Thus, the electrochemical equivalent of a substance α = A / 96480n is the mass of a substance in grams, released by a unit of the amount of electricity passing through the electrolytic bath - a coulomb (ampere-second).

For copper A = 63.54, n = 2, α = 63.54 / 96480 -2 = 0.000329 g / C, for nickel α = 0.000304 g / C, for zinc α = 0.00034 g / C

In fact, the mass of the released substance is always less than the indicated one, which is explained by a number of side processes taking place in the bath (for example, hydrogen evolution at the cathode), current leaks and short circuits between the electrodes.

The ratio of the mass of the actually released substance to its mass, which should have been released according to Faraday's law, is called the current yield of the substance η1.

Therefore, for a real process m e = η1 NS ( A / 96480n) x It

Naturally, always η1

The current efficiency significantly depends on the current density at the electrode. With an increase in the current density at the electrode, the current efficiency increases and the efficiency of the process increases.

Voltage U el, which must be supplied to the electrolyzer, consists of: decomposition voltage Ep (potential difference of the anodic and cathodic reactions), the sum of the anodic and cathodic overvoltages, the voltage drop in the electrolyte Ep, the voltage drop in the electrolyte U e = IR ep (R ep is the electrolyte resistance ), voltage drop in tires, contacts, electrodes U c = I (R w + R to + R e). We get: U el = Ep + Ep + U e + U c.

The power consumed during electrolysis is equal to: Rel = IU el = I (Ep + Ep + U e + U s)

Of this power, only the first component is spent on carrying out reactions, the rest are heat losses of the process. Only in the electrolysis of molten salts, part of the heat released in the electrolyte IU e is used useful, since it is spent on melting the salts loaded into the electrolyzer.

The efficiency of the electrolysis bath can be estimated by the mass of the substance in grams, released per 1 J of consumed electricity. This value is called the energy yield of a substance. It can be found by the expression q e = (αη1) / U el100, here α is the electrochemical equivalent of the substance, g / C, η1 is the current efficiency, U email- electrolytic cell voltage, V.

Electrolysis

The processes occurring during electrolysis are opposite to the processes occurring during the operation of a galvanic cell. If, during the operation of a galvanic cell, the energy of a spontaneously proceeding redox reaction is converted into electrical energy, then during electrolysis the chemical reaction occurs due to the energy of an electric current.

Electrolysis is a redox process that occurs on the electrodes when an electric current passes through a solution or molten electrolyte.

Electrolysis is carried out in electrolyzers, the main components of which are two electrodes immersed in an ionic conductor (electrolyte) and connected to the terminals of a direct current source.

The electrode connected to the negative pole of the current source is called cathode, and with positive - anode.

When voltage is applied, reduction processes occur at the cathode, and oxidation processes at the anode.

Anodes are insoluble (from coal, graphite, platinum and iridium) and soluble (from copper, silver, zinc, cadmium and nickel). The soluble anode undergoes oxidation, i.e. sends electrons to the external circuit.

The electrolysis of the melt proceeds according to the following scheme:

1.anions formed during the melting of the electrolyte in the order of increasing their electrode potentials (j 0)

2. cations are reduced at the cathode in decreasing order of their j 0.

For example, 2NaCl ® 2Na + Cl 2 K (-) 2Na + + 2e = 2Na 0

melt A (+) 2Cl - - 2e = Cl 2

When determining the products of electrolysis of aqueous solutions of electrolytes, it is necessary to take into account the possibility of participation in redox reactions of water molecules, the material of which the anode is made, the nature of ions and the conditions of electrolysis.

Table 3 - General rules for writing electrolysis equations

aqueous solutions of electrolytes

1. Electrolysis of NaCl solution (inert anode)

K (-): Na +; H 2 O

H 2 O + 2e ® H 2 + 2OH -

A (+): Cl -; H 2 O

2 Cl - - 2e ® Cl 2

2H 2 O + 2NaCl e-mail current H 2 + Cl 2 + 2NaOH

As a result, Н 2 is released at the cathode, Cl 2 at the anode, and NaOH accumulates in the cathode space of the electrolyzer.

2. Electrolysis of ZnSO 4 solution (inert anode)

K (-): Zn 2+; H 2 O

Zn 2+ + 2е ® Zn 0

2H 2 O + 2e ® H 2 + 2OH -

А (+): 2H 2 O - 4e ® O 2 + 4H +

Zn 2+ + 4H 2 O ® Zn + H 2 + O 2 + 2OH - + 4H +

After reducing the H 2 O molecules and adding SO 4 2- ions to both sides of the equation, we obtain the molecular equation of electrolysis:

ZnSO 4 + 2H 2 O e-mail current Zn + H 2 + O 2 + H 2 SO 4

3. Electrolysis of K 2 SO 4 solution (inert anode)

K (-): K +; H 2 O

H 2 O + 2e ® H 2 + 2OH -

A (+): SO 4 2-; H 2 O

2H 2 O - 4e ® O 2 + 4H +

2H 2 O + 2e e-mail current О 2 + 2H 2

those. electrolysis of potassium sulfate solution is reduced to the decomposition of water. The salt concentration in the solution increases.

4. Electrolysis of ZnSO 4 solution with zinc anode.

K (-): Zn 2+; H 2 O

Zn 2+ + 2е ® Zn 0

2H 2 O + 2e ® H 2 + 2OH -

A (+): Zn 0; H 2 O

Zn 0 -2е ® Zn 2+

Zn 0 + Zn 2+ ® Zn 2+ + Zn 0

Those. electrolysis of a ZnSO 4 solution with a zinc anode is reduced to the transfer of zinc from the anode to the cathode.

There are dependencies between the amount of substance released on the electrodes during electrolysis, the amount of electricity passed through the solution and the time of electrolysis, which are expressed by Faraday's law.

Faraday's first law: the mass of a substance released or dissolved on the electrodes is directly proportional to the amount of electricity passed through the solution:

m = ---------; where m is the mass of the substance released on the electrodes,

FM E is the molar mass of the substance equivalent, g / mol,

I - current strength, A;

t - electrolysis time, sec .;

F - Faraday constant (96500 C / mol).

Faraday's second law: for a certain amount of electricity passed through the solution, the ratio of the masses of the reacted substances is equal to the ratio of the molar masses of their chemical equivalents:

Const

ME 1 ME 2 ME 3

To isolate or dissolve 1 mol of an equivalent of any substance, it is necessary to pass through the solution or melt the same amount of electricity, equal to 96,500 Cl. This quantity is called Faraday constant.

The amount of substance released on the electrode during the passage of 1Cl of electricity is called its electrochemical equivalent (ε ).

ε = . -------, where ε - electrochemical

F equivalent

Me - molar mass equivalent

element (substance); , g / mol

F - Faraday constant, C / mol.

Table 4 - Electrochemical equivalents of some elements

Oxidation and reduction processes are at the heart of chemical current sources such as batteries.

Accumulators are galvanic cells in which reversible charging and discharging processes are possible, which are performed without the addition of substances involved in their work.

To restore the consumed chemical energy, the battery is charged by passing current from an external source. In this case, electrochemical reactions occur on the electrodes, the opposite of those that took place when the battery was operating as a current source.

The most common at present are lead-acid batteries, in which lead dioxide PbO 2 serves as a positive electrode and lead metal Pb as a negative electrode.

As an electrolyte, a 25-30% solution of sulfuric acid is used, therefore lead-acid batteries are also called acid.

The processes occurring during battery discharge and charging can be summarized as: discharge

Pb 0 + Pb +4 O 2 + 4Н + + 2SO 4 2- «2Pb 0 + 2SO 4 2- + 2H 2 O

In addition to the lead battery, alkaline batteries are used in practice: nickel-cadmium, nickel-iron.

Table 5 - Types of batteries