Chemical reactions accompanied by electron transfer () are divided into two types: reactions that occur spontaneously and reactions that occur when a current passes through a solution or melt .

The electrolyte solution or melt is placed in a special container - electrolytic bath .

Electricity - this is the ordered movement of charged particles - ions, electrons, etc. under the influence of external electric field. An electric field is created in a solution or melt of an electrolyte electrodes .

Electrodes- These are, as a rule, rods made of material that conducts electric current. They are placed in a solution or melt electrolyte, and connect to electrical circuit with power supply.

In this case, the negatively charged electrode cathode- attracts positively charged ions - cations. Positively charged electrode ( anode) attracts negatively charged particles ( anions). The cathode acts as a reducing agent, and the anode acts as an oxidizing agent.

There are electrolysis with active And inert electrodes. Active (soluble) electrodes undergo chemical transformations during the electrolysis process. They are usually made from copper, nickel and other metals. Inert (insoluble) electrodes are not subject to chemical transformations. They are made from active metals, For example, platinum, or graphite .

Electrolysis of solutions

There are electrolysis solution or melt chemical substance. The solution contains additional Chemical substance — water, which can take part in redox reactions.

Cathode processes

In solution salts The cathode attracts metal cations. Metal cations can act as oxidizing agents. The oxidizing abilities of metal ions vary. To assess the redox abilities of metals, they use electro-chemical voltage series :

Each metal is characterized by the value of its electrochemical potential. The less potential , those more restorative properties metal and themes less oxidizing properties the corresponding ion of that metal. Correspond to different ions different meanings this potential. Electrochemical potential - relative value. The electrochemical potential of hydrogen is assumed to be zero.

There are also molecules near the cathode water H 2 O. Water contains an oxidizing agent - the H + ion.

During the electrolysis of salt solutions at the cathode, the following patterns are observed:

1. If the metal is in salt - active (up to Al 3+ inclusive in the voltage range ), then instead of the metal at the cathode it is reduced (discharging) hydrogen, because Hydrogen has much greater potential. The process of reduction of molecular hydrogen from water occurs, with the formation of OH - ions, the environment near the cathode is alkaline:

2H 2 O +2ē → H 2 + 2OH -

For example, during electrolysis of a solution sodium chloride At the cathode, only hydrogen will be reduced from water.

2. If the metal is in salt - medium activity (between Al 3+ and H +), then it is restored at the cathode ( discharges) And metal, And hydrogen, since the potential of such metals is comparable to the potential of hydrogen:

Me n+ + nē → Me 0

For example, during electrolysis of a solution of iron (II) sulfate at the cathode will be reduced ( discharge) and iron and hydrogen:

Fe 2+ + 2ē → Fe 0

2H + 2 O +2ē → H 2 0 + 2OH —

3. If the metal is in salt - inactive (after hydrogen in the series of standard electrochemical metals) , then the ion of such a metal is more strong oxidizing agent than the hydrogen ion, and is reduced only at the cathode metal:

Me n+ + nē → Me 0

For example, during electrolysis of a solutioncopper(II) sulfateCopper will be reduced at the cathode:

Cu 2+ + 2ē → Cu 0

4. If the cathode gets hydrogen cations H+ , then they are reduced to molecular hydrogen:

2H + + 2ē → H 2 0

Anodic processes

The positively charged anode attracts anions and water molecules. The anode is an oxidizing agent. The reducing agents are either anions of the acidic residue or water molecules (due to oxygen in the oxidation state -2: H2O-2).

During electrolysis of salt solutions at the anode The following patterns are observed:

1. If the anode gets oxygen-free acid residue , then it is oxidized to a free state (to oxidation state 0):

neMe n- – nē = neMe 0

For example: during electrolysis of a sodium chloride solution at the anode, chloride ions are oxidized:

2Cl — – 2ē = Cl 2 0

Indeed, if you remember Periodic law: as the electronegativity of a nonmetal increases, its reducing properties decrease. And oxygen is the second element with the highest electronegativity. Thus, it is easier to oxidize almost any non-metal rather than oxygen. True, there is one thing exception. You probably already guessed it. Of course it's fluoride. After all, the electronegativity of fluorine is greater than that of oxygen. Thus, During the electrolysis of fluoride solutions, it is the water molecules that will be oxidized, not the fluoride ions. :

2H 2O-2 – 4ē → O 2 0 + 4H +

2. If the anode gets oxygen-containing acid residue, or fluoride ion , then water undergoes oxidation with the release of molecular oxygen:

2H 2O-2 – 4ē → O 2 0 + 4H +

3. If the anode gets hydroxide ionthen it oxidizes and release occurs molecular oxygen:

4 O-2H –– 4ē → O 2 0 + 2H 2 O

4. During electrolysis of solutions salts of carboxylic acids subject to oxidation carbon atom of the carboxyl group,carbon dioxide and the corresponding alkane are released.

For example, during electrolysis of solutions acetates carbon dioxide and ethane are released:

2CH 3 C +3 OO – – 2ē → 2C +4 O 2 + CH 3 -CH 3

Total electrolysis processes

Let's consider the electrolysis of solutions of various salts.

For example, electrolysis of solution copper sulfate. At the cathode copper ions are reduced:

Cathode (–): Cu 2+ + 2ē → Cu 0

At the anode molecules are oxidized water:

Anode (+): 2H 2O-2 – 4ē → O 2 + 4H +

Sulfate ions do not participate in the process. We will write them in the final equation with hydrogen ions in the form of sulfuric acid:

2 Cu 2+ SO 4+ 2H 2 O-2→ 2Cu 0 + 2H 2 SO 4 + O 2 0

Electrolysis of solution sodium chloride looks like that:

At the cathode is being restored hydrogen:

Cathode (–):

At the anode oxidize chloride ions:

Anode (+): 2Cl – – 2ē → Cl 2 0

Sodium ions do not participate in the electrolysis process. We write them with hydroxide anions in the overall solution electrolysis equation sodium chloride:

2H + 2 O +2NaCl – → H 2 0 + 2NaOH + Cl 2 0

Next example potassium carbonate.

At the cathode is being restored hydrogen from water:

Cathode (–): 2H + 2 O +2ē → H 2 0 + 2OH –

At the anode oxidize water molecules to molecular oxygen:

Anode (+): 2H 2O-2 – 4ē → O 2 0 + 4H +

Thus, potassium ions and carbonate ions do not participate in the process. Electrolysis of water occurs:

2H2+O-2 → 2H 2 0 + O 2 0

Another example: electrolysis of aqueous solution copper(II) chloride.

At the cathode is being restored copper:

Cathode (–): Cu 2+ + 2ē → Cu 0

At the anode oxidize chloride ions to molecular chlorine:

Anode (+): 2Cl – – 2ē → Cl 2 0

Thus, when electrolysis of potassium carbonate solution Electrolysis of water occurs:

Cu 2+ Cl2– → Cu 0 + Cl 2 0

A few more examples: electrolysis of sodium hydroxide solution.

At the cathode is being restored hydrogen from water:

Cathode (–): 2H + 2 O +2ē → H 2 0 + 2OH –

At the anode oxidize hydroxide ions to molecular oxygen:

Anode (+): 4O-2H –– 4ē → O 2 0 + 2H 2 O

Thus, when electrolysis of sodium hydroxide solution water decomposes; sodium cations do not participate in the process:

2H2+O-2 → 2H 2 0 + O 2 0

Electrolysis of melts

During electrolysis of the melt, the anions of acid residues are oxidized at the anode, and metal cations are reduced at the cathode. There are no water molecules in the system.

For example: melt electrolysis sodium chloride. At the cathode sodium cations are reduced:

Cathode (–): Na + + ē → Na 0

At the anode anions are oxidized chlorine:

Anode (+): 2Cl – – 2ē → Cl 2 0

melt sodium chloride:

2Na+Cl – → 2Na 0 + Cl 2 0

Another example: melt electrolysis sodium hydroxide. At the cathode sodium cations are reduced:

Cathode (–): Na + + ē → Na 0

At the anode oxidize hydroxide ions:

Anode (+): 4OH – – 4ē → O 2 0+ 2H 2 O

Summary equation of electrolysis sodium hydroxide melt:

4Na+OH – → 4Na 0 + O 2 0 + 2H 2 O

Many metals are produced industrially by electrolysis of melts.

For example , aluminum obtained by electrolysis of solution aluminum oxide in melted cryolite. Cryolite– Na 3 melts at a lower temperature (1100 o C) than aluminum oxide (2050 o C). And aluminum oxide dissolves perfectly in molten cryolite.

In cryolite solution, aluminum oxide dissociates into ions:

Al 2 O 3 = Al 3+ + AlO 3 3-

At the cathode aluminum cations are reduced:

Cathode (–): Al 3+ + 3ē → Al 0

At the anode oxidize aluminate ions:

Anode (+): 4AlO 3 3 – – 12ē → 2Al 2 O 3 + 3O 2 0

The general equation for the electrolysis of a solution of aluminum oxide in molten cryolite:

2Al 2 O 3 = 4Al 0 + 3O 2 0

In industry, graphite rods are used as electrodes in the electrolysis of aluminum oxide. In this case, the electrodes are partially oxidized (burnt) in the released oxygen:

C0+ O 2 0 = C +4 O 2 -2

Electrolysis with soluble electrodes

If the electrode material is made of the same metal that is present in the solution in the form of a salt, or of a more active metal, then discharge at the anode not water molecules or anions, but particles of the metal itself are oxidized as part of the electrode.

For example, consider the electrolysis of a solution of copper (II) sulfate with copper electrodes.

At the cathode ions are discharged copper from solution:

Cathode (–): Cu 2+ + 2ē → Cu 0

At the anode copper particles are oxidized from electrode :

Anode (+): Cu 0 – 2ē → Cu 2+

For electrolysis, i.e. implementation of electrochemical processes by passing direct current from an external source. An electrolyzer consists of a housing (bath), two or more electrodes (cathodes and anodes), sometimes separated by a diaphragm, and filled with electrolyte. According to the method in the electrical circuit, the electrolyzer is divided into mono- and bipolar. A monopolar electrolyzer consists of one electrolytic cell with electrodes of the same polarity, each of which can consist of several elements connected in parallel to the current circuit.

A bipolar electrolyzer has a large number of cells (up to 100-160), connected in series to the current circuit, and each, with the exception of the two extreme ones, works with one side as, and the other as. For the manufacture of anodes, carbon-graphite, Pb and its Ti, etc. are used. For cathodes, it is used in most electrolyzers. To regulate the processes of mass and heat transfer in the electrolyzer, stirrers or an electrolyte flow, built-in or remote heat exchangers are used. One of the important characteristics of an electrolyzer is dissipation, which depends on the design of the electrolyzer and the composition of the electrolyte. Modern large electrolysers have a high load: monopolar up to 400-500 kA, bipolar - equivalent to 1600 kA.. Encyclopedic dictionary of metallurgy. - M.: Intermet Engineering. 2000 .

Editor-in-Chief N.P. Lyakishev:

Synonyms

    See what “Electrolyzer” is in other dictionaries: electrolyzer - electrolyzer...

    See what “Electrolyzer” is in other dictionaries: Spelling dictionary-reference book - noun, number of synonyms: 2 electrolyzer (1) electrolyzer (1) ASIS Dictionary of Synonyms. V.N. Trishin. 2013…

    Synonym dictionary Electrolyzer

    See what “Electrolyzer” is in other dictionaries: Official terminology

    Synonym dictionary- - [Ya.N.Luginsky, M.S.Fezi Zhilinskaya, Yu.S.Kabirov. English-Russian dictionary of electrical engineering and power engineering, Moscow, 1999] Topics of electrical engineering, basic concepts EN electrolyte pot ... - a prefabricated apparatus, as a rule, a press-type filter operating under pressure, consisting of bipolar electrodes compressed together by end plates and separated by insulating gaskets, with direct current passing through them... ...

    See what “Electrolyzer” is in other dictionaries: Dictionary-reference book of terms of normative and technical documentation - elektrolizeris statusas T sritis chemija apibrėžtis Elektrolizės įrenginys. atitikmenys: engl. electrolyser rus. electrolyser...

    Synonym dictionary Chemijos terminų aiškinamasis žodynas Modern Dictionary Russian language Efremova

    Mercury electrolyzer - [Ya.N.Luginsky, M.S.Fezi Zhilinskaya, Yu.S.Kabirov. English-Russian dictionary of electrical engineering and power engineering, Moscow, 1999] Topics electrical engineering, basic concepts Synonyms mercury electrolyzer EN mercury cell ... Technical Translator's Guide

    electrolyzer for producing oxygen and hydrogen- - [Ya.N.Luginsky, M.S.Fezi Zhilinskaya, Yu.S.Kabirov. English-Russian dictionary of electrical engineering and power engineering, Moscow, 1999] Topics of electrical engineering, basic concepts EN oxygen hydrogen celloxyhydrogen cell ... Technical Translator's Guide

    electrolyzer furnace with induction heating- - [Ya.N.Luginsky, M.S.Fezi Zhilinskaya, Yu.S.Kabirov. English-Russian dictionary of electrical engineering and power engineering, Moscow, 1999] Topics of electrical engineering, basic concepts EN double current furnace ... Technical Translator's Guide

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

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

FM E - molar mass equivalent substance, g/mol,

I - current strength, A;

t - electrolysis time, sec.;

F - Faraday's constant (96500 C/mol).

Faraday's second law: for a certain amount of electricity passing through a 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 mole equivalent of any substance, the same amount of electricity, equal to 96,500 C, must be passed through the solution or melt. This quantity is called Faraday constant.

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

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

F equivalent

Me - molar mass equivalent

element (substance); , g/mol

F is Faraday's constant, C/mol.


Table 4 - Electrochemical equivalents of some elements

cation Me, g/mol ε, mg Anion Me, g/mol ε, mg
Ag + Al 3+ Au3+ Ba 2+ Ca 2+ Cd 2+ Cr 3+ Cu 2+ Fe 2+ Fe 3+ H + K + Li + Mg 2+ Mn 2+ Na + Ni 2+ Pb 2+ Sn 2+ Sr 2+ Zn 2+ 107,88 8,99 65,70 58,70 20,04 56,20 17,34 31,77 27,92 18,61 1,008 39,10 6,94 12,16 27,47 22,90 29,34 103,60 59,40 43,80 32,69 1,118 0,93 0,681 0,712 0,208 0,582 0,179 0,329 0,289 0,193 0,0105 0,405 0,072 0,126 0,285 0,238 0,304 1,074 0,616 0,454 0,339 Br - BrO 3 - Cl - ClO 3 - HCOO - CH 3 COO - CN - CO 3 2- C 2 O 4 2- CrO 4 2- F - I - NO 3 - IO 3 - OH - S 2- SO 4 2 - Se 2- SiO 3 2- 79,92 127,92 35,46 83,46 45,01 59,02 26,01 30,00 44,50 58,01 19,00 126,42 174,92 62,01 17,00 16,03 48,03 39,50 38,03 0,828 1,326 0,368 0,865 0,466 0,612 0,270 0,311 0,456 0,601 0,197 1,315 1,813 0,643 0,177 0,170 0,499 0,411 0,395

Oxidation and reduction processes underlie the operation of chemical power sources such as batteries.

Batteries are galvanic cells in which reversible charging and discharging processes are possible, carried out without the addition of substances involved in their operation. .

To restore spent 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 operated as a current source.

The most common currently are lead batteries, in which the positive electrode is lead dioxide PbO 2, and the negative electrode is lead metal Pb.

A 25-30% solution of sulfuric acid is used as an electrolyte, which is why lead batteries are also called acid batteries.

The processes that occur when discharging and charging a battery can be summarized as follows: discharge

Pb 0 + Pb +4 O 2 + 4H + + 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.

Electrode processes. Electrical double layer.

There is a close connection between electrical and chemical phenomena in which mutual transformations of electrical and chemical forms of energy occur. Electrochemistry studies processes occurring at interfaces between phases capable of exchanging charged particles. Most often, one of the contacting phases is metal, the other is an electrolyte solution. The conductivity mechanism in these phases is not the same. Metal is a conductor of the first kind, the carriers of electricity are electrons. The electrical conductivity of the electrolyte solution is ensured by the movement of ions. This is a conductor of the second kind.

The difference in electrical potentials arising due to chemical reactions underlies the operation of chemical current sources - electrochemical cells and batteries.

An electrode is a section of an electrical circuit that serves for galvanic communication with an external circuit.

When an electrode is lowered into a solution, electrode processes occur.

Electrode processes include two groups of interconnected processes.

1. The appearance of an electrical potential difference, and therefore electric current as a result of the flow chemical reaction into a galvanic cell).

2. Reverse chemical processes that occur when passing El. current through the solution (electrolysis).

These two groups of processes are in many cases mutually reversible (lead battery) and are always associated with a change in the charge of atoms (ions) or atomic groups, i.e. are redox reactions.

Let us consider the mechanism of the occurrence of the electrical potential difference between a metal and a solution of its salt. From a metal immersed in a solution, some of the ions crystal lattice, possessing high energy of thermal movement, leaves it and goes into solution. This process is facilitated by the interaction of ions with solvent molecules located near the surface of the solid. At the same time, the reverse process occurs, i.e. destruction of the solvation shell of ions in solution and their inclusion in the crystal lattice of the metal.

Initially, the dissolution of the metal predominates; the cations passing into the solution carry with them a positive electrical charge. The solution becomes positively charged and the metal negatively charged. The ions of the solution, carrying an excess positive charge, and the free electrons of the metal that turn out to be uncompensated are attracted to each other and are located near the phase interface on both sides of it, forming the so-called double electric layer, within which the electric potential changes sharply. The resulting electric field complicates the dissolution of the metal and enhances the reverse process. Subsequently, a dynamic equilibrium is established, due to the mutual compensation of these processes, and a certain potential difference between the metal and the solution.

The nature of the potential change in the electric double layer allows us to distinguish dense and diffuse parts in it. The dense part of the electrical double layer (Helmholtz layer) is formed by ions located at a minimum distance from the phase interface. This layer is similar to a capacitor with metal plates. The potentials in it change linearly.

The diffuse part of the double electric layer (Guy's layer) corresponds to a capacitor, one of the plates of which seems to be blurred. This plate corresponds to ions that have moved into the depth of the solution due to their thermal movement. With distance from the phase interface, the number of excess ions quickly decreases, and the solution becomes neutral. The interfacial potential jump is the sum of jumps in the dense part of the double layer and the Ψ-potential equal to the potential jump in the Huy layer. Due to the fact that the total thickness of the electrical double layer remains insignificant, the change in potential during the transition from one phase to another is always abrupt.

Initially, the transition of ions from solution to metal predominates, then the metal is charged positively and the solution negatively. The electric potential changes abruptly within the electrical double layer, but the sign of the charge in it changes to the opposite.

Another reason for the formation of a potential difference between the phases is the adsorption of various particles on the phase interface.

There are many theories explaining the mechanism of formation of a potential jump at the solution-metal interface. The most recognized is the solvation theory of electrode potential, the foundations of which were laid by L.V. Pisarzhevsky (1912-1914). According to it, the potential jump at the solution-metal interface is caused by two processes: 1) dissociation of metal atoms into ions inside the metal; 2) solvation of metal ions located on the surface of the metal when it comes into contact with a solution containing solvent molecules.

As an example, consider a copper electrode immersed in water solution copper sulfate. The chemical potential of copper ions in a metal at a given temperature can be considered constant, while the chemical potential of copper ions in solution depends on the salt concentration. In general, these chemical potentials are not the same. Let the concentration of copper sulfate be such that the chemical potential of copper ions surrounded by a solvation shell in solution is less than the chemical potential of these ions in the metal. Then, when the metal is immersed in the solution, a driving force for the transition of copper ions from the metal crystal lattice into a solution, the implementation of which is prevented chemical bond ions with a lattice.

Dipole solvent molecules, oriented in the field of the metal surface, on the contrary, promote the release of ions from the crystal lattice. As a result, some of the copper ions will leave the neutral crystal lattice and become hydrated, and the electrode surface will be negatively charged. This charge will prevent the further transition of copper ions into the solution. An electric double layer appears and electrochemical equilibrium is established, in which the chemical potentials of the ions in the metal and in the solution will differ by the value of the potential difference of the electric double layer.

Electrochemical equilibrium is dynamic in nature, the ions forming the electric double layer are constantly renewed, however, the flow of cations from the solution into the metal is equal to the flow of cations from the metal into the solution, therefore the electrode potential retains its value under constant conditions. Its value depends on the nature of the electrode material and solvent, on the concentration of ions in the solution that make up the electric double layer, and on temperature.

Another type of equilibrium, which is established between phases containing several ions, one of which easily passes through the interphase boundary, while for others such a transition is difficult due to spatial or chemical reasons, at the boundaries of glass-solution or ion-exchange resin (ionite) - solution. This type of equilibrium is called membrane equilibrium. At the interface between two similar phases, an electric layer also forms and a corresponding potential jump occurs - the membrane potential.

Electric double layer arises as a result of redox processes and reflects the ability of the electrode material to oxidize. The easier the electrode material is oxidized, the greater the number of ions, other things being equal, leaves the crystal lattice of the electrode into the solution and the more negative its potential. Therefore, the electrode potential, measured in normal units, is called the redox potential.

It is impossible to experimentally measure the potential of any point, but you can measure its value relative to some point, i.e. potential difference. When measuring redox potentials, a standard hydrogen electrode (SHE) is used as a reference point. The SHE potential is formed on the basis of oxidation-reduction processes similar to those on a copper electrode, but its potential was taken as zero. The electrode potential is considered positive if the electrode is more positively charged than the HVE, and negative if it is more negatively charged than the standard hydrogen electrode.

The redox abilities of materials are compared using standard electrode potentials.

The standard electrode potential is the potential difference between a given electrode and a standard hydrogen electrode, provided that the activities of all substances participating in the reaction are equal to unity.

All electrodes can be arranged in a row according to their standard electrode potentials. When recording electrodes, the ion-electrode sequence is used. The electrode reaction is written as a reduction reaction, i.e. addition of electrons.

The value of the standard electrode potential characterizes the tendency of the electrode reaction to proceed in the direction of ion reduction. The lower the electrode reaction is located, the greater the tendency for this to happen. that the oxidized form will gain electrons and go into the reduced form. And vice versa, the higher the electrode reaction is in the table, the greater the tendency of the reduced form to give up electrons and go into the oxidized form. For example, the active metals sodium and potassium have very large negative standard electrode potentials and. hence exhibit a strong tendency to lose electrons.

The reduced form of any element or ion with an activity of one will reduce the oxidized form of the element or ion having a less negative standard electrode potential.

Consider a system consisting of two electrodes, for example zinc and copper. Each of them is immersed in a solution of its own salt, and the solutions are connected by an electrolytic key. The key provides electrical contact between solutions, but does not allow ions from one part of the cell to pass to another.

Each of the metals in such a system will release into the solution the amount of ions that corresponds to its equilibrium with the solution. However, the equilibrium potentials of these metals are not the same. Zinc has a greater ability to release ions into solution than copper and will therefore acquire a higher negative charge. It contains more excess electrons than copper. If the electrodes are now connected with a wire (external circuit), then excess electrons will flow through the external circuit from zinc to copper and thereby upset the equilibrium of the double layer on both electrodes. On the zinc electrode the charge will decrease and some of the ions will leave the electrode again, and on the copper electrode the electrodes will become more than the equilibrium charge and therefore some of the ions from the solution will be discharged on the electrode. Again there will be a difference in the charges of the electrodes. Once again, electrons will transfer through the external circuit from zinc to copper. This again initiates the transition of ions, and so on until the entire zinc electrode has dissolved.

Thus, a spontaneous process occurs in which the zinc electrode dissolves, and copper ions are discharged on the copper electrode and metallic copper is released. The transfer of electrons along a wire from a zinc plate to a copper plate creates an electric current. It can be used to carry out various processes. such a device is called a galvanic cell.

A galvanic cell is any device that makes it possible to produce electric current by carrying out a particular chemical reaction.

The difference in electrode potential of a galvanic cell depends on the conditions under which it is determined. The greatest potential difference of a galvanic cell is called electromotive force. (EMF).

The basis of any galvanic chain is a redox reaction, carried out in such a way that on one of the electrodes (negative) oxidation occurs, in this case the dissolution of zinc, and on the other (positive) reduction occurs, i.e. release of copper.

The Daniel-Jacobi electrochemical cell consists of copper and zinc plates immersed in sulfuric acid solutions of their salts. These solutions are separated by a porous partition that prevents the solutions from mixing. Such a system is a current source. The positive pole is a copper plate, the negative pole is a zinc plate. When this element operates, the zinc plate dissolves, and copper is deposited from the solution on the copper plate.

In solution, copper and zinc exist as ions. An oxidation reaction occurs on the zinc electrode:

As a result, zinc goes into solution in the form of cations, and the remaining electrons give it a negative charge. On the copper electrode, a reduction reaction of copper ions takes place, approaching the copper plate from the solution and depositing on it:

As a result of this reaction, a certain number of free electrons of the copper plate are consumed and it acquires a positive charge. Total reaction:

Zn + Cu 2+ = Zn 2+ + Cu or Zn + CuSO 4 = ZnSO 4 + Cu

This reaction can occur spontaneously under normal conditions. But then the processes of oxidation and reduction are combined, and the movement of electrons occurs over a short path and no electric current occurs. This is an example of a galvanic cell that operates due to the unequal chemical nature of the electrodes.

Concentration galvanic cells consist of electrodes of the same nature, but the concentration of solutions is different. For example, an element containing two silver plates immersed in solutions of silver nitrate of unequal concentration. The plate immersed in a less concentrated solution is the negative pole, and the other is the positive pole. When the element operates, the negative plate dissolves, and silver is deposited on the positive plate.

Negative electrode:

Positive electrode:

Electromotive force of an electrochemical element.

Conventional notation of a galvanic cell:

(-) Zn | ZnSO 4 ||CuSO 4 | Cu(+)

Vertical bars indicate phase interfaces. In the case of a positive EMF, it is placed on the left, and positive on the right. At each interphase boundary there is a potential jump. The electrode together with the solution in which it is immersed is a half-cell. The EMF of a galvanic cell is equal to the potential difference of the half-cells: from the potential of the right half-cell, the potential of the left half-cell is subtracted. With this recording, the EMF of the circuit will always be positive.

A concentration galvanic cell consisting of two silver electrodes immersed in a solution of silver nitrate of different concentrations:

(-)Ag | AgNO 3 (c1) || AgNO 3 (c2) | Ag(+)

If from 1< с 2 , то левый электрод посылает в раствор ионы серебра и заряжается отрицательно. На правом электроде ионы серебра разряжаются, сообщая электроду положительный заряд.

If the electrode does not exchange ions with the solution, then its symbol is enclosed in brackets. For example, a platinum electrode saturated with hydrogen, immersed in a solution of hydrochloric acid(hydrogen electrode), denote:

The EMF of any galvanic element is equal to the difference in its electrode potentials. The emf of a galvanic cell composed of two different electrodes, but with the same concentration (activity) of their salts, is equal to the difference in the standard potentials of these elements.

The work of a galvanic cell is measured by electromotive force. EMF is the greatest potential difference of a galvanic cell. It consists of contact potential - between two metals (zinc and copper), electrode - between an electrode and an electrolyte solution (zinc and zinc sulfate, copper and copper sulfate), diffusion - between two solutions (zinc sulfate and copper sulfate).

Contact potential occurs at the boundary of contact of conductors running from the zinc to the copper electrode. The reason for its appearance is associated with the transition of electrons from one metal to another. Often this potential is insignificant and does not have a significant effect on the emf of the galvanic cell.

Electrode potentials that arise at the interface of a metal with an electrolyte. Their occurrence is due to the transition of ions from the metal to the solution and from the solution to the metal. The electrode potential is determined by the potential difference between the electrode and the electrolyte solution in contact with it.

Mnemonic rule: oxidation at the anode (words begin with vowels), reduction at the cathode.

Diffusion potential occurs at the interface between different electrolytes or one electrolyte of different concentrations (activities). Due to the different mobility of cations or anions, they diffuse from a more concentrated to a less concentrated solution. As you move away from the interface between two electrolyte solutions, the concentration of ions equalizes due to their thermal movement, while diffusion potential decreases and then disappears.

The diffusion potential is small and does not exceed hundredths of a volt.

Hence the EMF is equal to:

E= E Cu - E Zn + E k + E d

If we neglect the diffusion and contact potentials, which are very small, then the formula looks like this:

EMF = E Cu - E Zn

EMF is a quantitative characteristic of the operation of a galvanic cell - it shows how completely the process of converting chemical energy into electrical energy is carried out.

The emf of a galvanic cell can be calculated using the Nernst equation:

The value is constant at a given temperature.

Temperature, C
0,0542 0,0578 0,0591 0,0621

E 0 is the standard emf of a galvanic cell, z is the number of elementary charges that participate in the reaction, a is the activity of reactants and reaction products under given conditions.

The expression for the EMF for the concentration element does not include the value of the standard EMF, since both electrodes are the same, and therefore their standard potentials are also the same.

Hydrogen electrodes of the 1st and 2nd kind.

Basic electrode potential equation.

The chemical reaction occurring in a galvanic cell can be divided into two conjugate ones that take place on half-cells:

ν 1 A 1 - ze = ν 3 A 3

ν 2 A 2 + ν 4 A 4

Then the expressions for the potentials of individual electrodes can be:

A reaction of the type M z + + zе = M usually occurs on metal electrodes. For this, taking into account the fact that the activity of a solid at a given temperature is constant and equal to unity, we obtain the equation for the potential of the electrode, reversible with respect to the metal cation:

The term “reversible with respect to the metal cation” means that the electrode potential is formed by the M + cations.

If anions participate in the potential-determining process according to the reaction A + zе = A z - , we obtain an expression for the electrode potential, reversible relative to the anion:

The general equation looks like this:

It shows that the potential of the electrode depends on its nature (characterized by the standard potential), temperature and activity of the ions in the solution.

To experimentally determine the electrode potential (φ), it is necessary to construct a galvanic cell containing the electrode under study and measure its emf. The galvanic cell, in addition to the electrode under study, must contain a reference electrode, the potential of which is precisely known and well reproducible.

Irreversible galvanic cells involve irreversible processes, so that it is impossible to reverse the direction of a chemical reaction in them by changing the applied external voltage by an infinitesimal amount. The electrodes that form such elements are called irreversible.

Reversible electrodes are divided into indicator and reference electrodes.

Indicator (working) electrodes are those whose potential unambiguously changes with changes in the concentration of the ions being detected.

Reference electrodes are those electrodes whose potential is precisely known, accurately reproducible and does not depend on the concentration of the ions being determined, i.e. is constant during measurements.

Based on their properties and design, reversible electrodes are divided into the following groups:

1. Electrodes of the first kind.

2. Electrodes of the second kind.

3. Redox electrodes

4. Ion selective (membrane) electrodes.

Electrodes of the first kind. Electrodes with an active solid or gaseous phase, reversible either only with respect to cations or only with respect to anions.

These include metal electrodes that are reversible with respect to cations, metalloid electrodes that are reversible with respect to anions, and gas electrodes that are reversible with respect to either cations or anions.

Metal electrodes that are reversible with respect to cations are electrodes in which the metal is immersed in a solution of a highly soluble salt of this metal. The cation reversible electrode potentials become more positive with increasing solution concentration.

Electrodes of the second kind- complex multiphase electrodes, formally reversible with respect to both cations and anions. They consist of a metal, a sparingly soluble salt of this metal, and a second compound that is highly soluble and contains the same anion as the sparingly soluble salt.

Representatives are silver chloride and calomel electrodes. They are widely used as reference electrodes.

Silver chloride is a silver wire coated with a layer of silver chloride, immersed in a saturated solution of potassium chloride, located in a vessel with a microslit for contact with the test solution. It is reversible with respect to chloride ions.

Calomel electrode (Pt)Hg 0 |Hg 2 Cl 2 |KCl It is a mixture of mercury and calomel placed in a vessel, into the bottom of which platinum is soldered, welded to a copper conductor.

Redox electrodes.

All potential-determining electrodes form a potential based on redox processes, therefore any electrode can be called redox. But we agreed to call such electrodes, the metal of which does not take part in the redox reaction, but is only a carrier of electrons, while the oxidation-reduction process occurs between the ions in the solution.

Such electrodes are obtained by dipping an inert metal into a solution of a mixture of electrolytes containing ions of different oxidation states.

OB electrode diagram:

(Pt)|Ox, Red;Ox + ze = Red

Example of OF electrodes: (Pt)|FeCl 3 ; FeCl 2 (Pt)|SnCl 4 ; SnCl2.

Quinhydrone electrode is widely used: (Pt)|X, H 2 X, H +. It consists of a platinum plate dipped in a solution of quinhydrone. This complex compound from quinone and its reduced form hydroquinone.

The reaction occurs at the electrode:

C 6 H 4 O 2 + 2H +2e = C 6 H 4 (OH) 2

It cannot be used when analyzing alkaline media, because hydroquinone reacts with hydroxyl ions. Advantage: low error of results.

Ion exchange (ion selective) electrodes. Electrodes consisting of an ion exchanger and a solution, the potential at the interface of which arises due to the selective ion exchange process between the phases.

Glass electrode Ag|AgCl|HCl. This is a thin-walled ball made of special conductive glass (membrane) filled with a solution of hydrochloric acid 0.1 mol/l. An auxiliary silver chloride electrode is immersed in the HCl solution.

The principle of operation is based on the fact that in the glass structure, cations of potassium, sodium and lithium can exchange with cations of the solution (H +). The exchange of cations between the glass and the solution occurs in accordance with the law of distribution of the third component between the two phases. Due to the difference in the activities of cations in the solution and in the membrane, boundary potentials φ 1 and φ 2 arise on both sides of the membrane. Only a well-soaked membrane is sensitive to hydrogen ions. Therefore, glass electrodes are prepared before use. First, the glass is hydrated. Leave for several hours in water and then in a 0.1 M HCl solution.

The glass electrode has high resistance, limiting the pH measurement range (-1-12). Its potential changes over time, so it is calibrated according to standard buffer solutions. The advantage is indifference to oxidizing agents and reducing agents. By changing the composition of the glass, it is possible to obtain membranes with low selectivity to hydrogen ions and high selectivity to metal ions (sodium, potassium).

Nonequilibrium electrode processes. Electrolysis is a process in which chemical reactions occur by passing an electric current.

Electrolysis scheme:

Electrolyte ions, reaching the corresponding electrodes (cathode cations, anode anions), as a result of interaction with them, reduce their charge, turning into neutral atoms that settle on the electrode or enter into a secondary reaction. Electrolysis - OVR.

The anode (+) has fewer electrons than its material in the neutral state. Therefore, it must intensively take away electrons, i.e. oxidation occurs.

The cathode (-) has redundant electrodes compared to its material in the electrically neutral state. Therefore, it easily gives up electrons, i.e., a reduction reaction occurs.

Both of these processes are the basis of electrolysis. There is a certain relationship between the amount of electricity and the amount of substance released during electrolysis. m = Eq

Electrolysis cannot occur spontaneously. The energy required for its flow comes from an external current source.

During electrolysis, chemical reaction is carried out using the energy of an electric current. And when a galvanic cell operates, the energy of the chemical reaction spontaneously occurring in it is converted into electrical energy.

During cathodic processes, it is necessary to take into account the magnitude of the reduction potential of hydrogen ions. In a neutral environment: E n = = 0.059*(-?) = -0.41 V

Rules:

1) If the electrolyte is formed by a metal whose electrode potential is significantly higher than -0.41 V, then the metal will be released from the neutral solution at the cathode (to the right of tin).

2) In the case of electrolytes, the metal of which has a potential significantly more negative than -0.41 V, hydrogen will be released (from start to)

3) If the potential of the metal is close to -0.41 V (zinc, chromium, iron, cobalt, nickel), then both hydrogen and metal can be released)

Direction of oxidation-reduction reactions.

To determine the direction, the EMF of the reaction is calculated:

EMF = E oxide - E reduction

1) If the emf is greater than zero, then the reaction is direct.

2) If the EMF is less than zero, then the reaction is reverse.

3) If EMF = 0, then chemical equilibrium has occurred.

Intensity: If the EMF is greater than 0.1 V, then the reaction is intense.

If the EMF is less than 0.01 V, then the reaction is low-intensity

Potentiometric pH determination

There are two types of potentiometric measurement:

1. Direct potentiometric determination;

2. Methods of potentiometric titration.

1. Direct potentiometric measurement is used to determine the concentration of metal ions in aqueous solutions: calcium, magnesium, sodium, potassium, etc. It is especially widely used to determine hydrogen ions, i.e., the pH of solutions.

(pH = - log a n+)

This method requires selective electrodes.

2. Potentiametric titration also has an applied role in determining the concentration of substances in a solution, but by titrating it with a standard solution of the corresponding reagent. But the equivalence point here is determined not by a chemical indicator, but by the potential value on the indicator electrode immersed in the test solution. This method does not require specific electrodes. And various metal, silver, latin, tungsten, and graphite electrodes can be used as indicator electrodes.

This method makes it possible to determine substances in cloudy and strongly colored solutions, as well as to differentiate (separately) titrate the components of a mixture of substances in the same portion of the solution.

For any type of potentiometry, two electrodes are placed in the solution being studied.

One indicator is the second reference electrode, which serves to determine the potential that has arisen at the indicator electrode.

1. Reference electrodes (semi elements), their potential is determined by the concentration of Cl - ions. At a saturated concentration of Cl - their potential is constant

Hg (Hg 2 Cl 2 ) HCl - calomel electrode.

Ag (AgCl) KCl - silver chloride electrode (II kind).

2. Metal electrodes (type I):

Cu 0 /Cu +2 ; Hg 0 /Hg +2 ; Ag0/Ag+

3. Measuring membrane electrodes - consisting of plates (very thin) capable of ensuring interaction in solution only with certain K + ions; Na+; Hg+2; Ka +2 ; NH4+; Na - ; Cl - ; I - ; etc. The membrane electrode also includes the well-known glass electrode for determining pH. This is a glass tube at the end of which there is a thin-walled ball, inside the ball acid is poured and a platinum wire is lowered

φ = φ 0 + 0.059 pH

At = φ 0 = 0.7044, but the quinhydrone electrode can be used only up to pH - 8.5 because in a strongly alkaline environment the hydroxyl group strongly dissociates and the results are distorted. Oxidizing ions Fe + Sn +3 Ti +3 are also distorted

Antimony electrode is a metal electrode of the second kind. Here the metal Sb is coated with Sb 2 O 3 (hydroxide Sb (OH) 3, poorly soluble.

The potential of the Sb/Sb 2 O 3 electrode depends on the concentration and on the processes occurring on it:

Sb 2 0 3 + 3H 2 O = 2Sb(OH) 3

Sb(OH) 3 ↔ Sb +3 + 3OH -

φ = φ 0 + 0.059 log

But this Nernst equation for this electrode is not strictly satisfied, because reactions on the electrode are not completely reversible; they also affect the state of the electrode surface and the method of its preparation.

Advantages: This electrode is simple in design and can be prepared. Resistant to the presence of many substances. Oxidizing agents do not interfere with it.

Fundamentals of chemical thermodynamics.

The first law (beginning) of thermodynamics is a consequence of the law of conservation of energy. This law is fulfilled in all natural phenomena and is confirmed by all the experience of mankind.

Thermodynamics primarily considers two forms in which energy transformation occurs - heat and work. The first law of thermodynamics establishes the relationship between thermal energy Q and work A when changing internal energy systemsΔU.

From the constancy of the internal energy reserve of an isolated system it directly follows: in any process, the change in the internal energy of any system is equal to the difference between the amount of heat imparted to the system and the amount of work done by the system:

ΔU = Q - A, from here

This equation is the mathematical expression of the first law of thermodynamics.

In this case first law of thermodynamics is formulated: heat Q supplied to the system goes to increase the internal energy of the system ΔU and to perform external work A.

The first law (first law) of thermodynamics has several formulations, but they all express the same essence - indestructibility and equivalence of energy during mutual transitions various types her into each other.

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

The essence of electrolysis

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

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

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

The division of electrolysis reactions into primary and secondary helped Michael Faraday to 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. By quantity of electricity we mean electric charge, usually measured in coulombs.

    Faraday's second law of electrolysis: for a given amount of electricity (electric charge), mass chemical element, deposited on the 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 participates.

m is the mass of the substance deposited on the electrode, Q is the total electric charge passing through the substance F = 96,485.33(83) C mol−1 is Faraday’s 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 Faraday's first law, M, F and z are constants, so what larger value Q, the larger the value of m will be. For Faraday's second law, Q, F and z are constants, so the larger the M/z value (equivalent mass), the larger the m value will be.

Electrolysis is widely used today in industry and technology. For example, electrolysis is one of the the most effective ways industrial production of hydrogen, hydrogen peroxide, manganese dioxide, aluminum, sodium, magnesium, calcium and other substances. Electrolysis is used for cleaning Wastewater, in electroplating, in galvanoplasty, and finally in chemical current sources. But first things first.

Thanks to electrolysis, many metals are extracted from ores and subjected to further processing. So, when ore or enriched ore - concentrate - is treated with reagents, the metal goes into solution, then the metal is separated from the solution by electrical extraction. Pure metal is released at the cathode. Zinc, copper, and cadmium are obtained in this way.

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

When a current passes, the anode metal dissolves, passes into solution in the form of cations, then the cations are discharged at the cathode and form a precipitate 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.

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

In addition, small amounts of additives are introduced into the solution to help obtain a compact metal precipitate. 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 applied to a titanium base), or soluble (aluminum, iron).

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

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

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

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

This method is effective in treating wastewater from fats, petroleum products, dyes, oils, radioactive substances, etc. It is called electrocoagulation.

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

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

Electroplating - deposition of metal on a surface different bodies to reproduce their shape: molds for casting parts, sculptures, printing clichés, etc.

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

These metals are soft, easily processed with metalworking tools, and are easy to engrave and cast. After building up the galvanic layer and finishing, the metal of the mold is smelted from the finished product.

However, dielectric materials still offer the greatest opportunities for making models. To metallize such models, it is necessary to impart electrical conductivity to their surface. Success or failure ultimately depends mainly 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.

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

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

Chemical current sources

Electrolysis is also the main process through which the most modern chemical power sources, such as batteries and accumulators, operate. 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 occurrence of spatially separated processes in a closed external circuit: at the negative anode, the reducing agent is oxidized, the resulting free electrons pass through the external circuit to the positive cathode, creating a discharge current, where they participate in the reduction reaction of the oxidizing agent. 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.