electrochemical elements. Galvanic cells

When metal zinc is placed in a solution of copper sulfate, a redox reaction occurs:

Zn (t) + Cu 2+ → Zn 2+ + Cu (t)

Both half-reactions (reduction and oxidation) occur simultaneously at the point of contact of zinc with the solution. Zinc donates two electrons to the copper cation, oxidizing in the process.

If you do the opposite and place metallic copper in a solution of zinc sulfate, then nothing will happen. Be aware of the activity of metals! Zinc is more active than copper - it donates electrons more easily.

In the example discussed above, both half-reactions occurred at the same site. What happens if we separate the reduction and oxidation half-reactions? In this case, the electrons will pass from the reducing agent to the oxidizing agent through an external circuit, which will serve as a conductor of electric current. Yes, yes - a directed flow of electrons is nothing but an electric current.

The device for converting the energy of chemical reactions into electrical energy is called galvanic cells, or, in simple terms, - electric batteries.

A copper plate (negative electrode - anode) is immersed in a container with copper sulfate.

Zinc plate (positive electrode - cathode) - in a solution of zinc sulfate.

The plates are interconnected by a metal conductor. But in order for an electric current to appear in the circuit, it is necessary to connect the containers with a salt bridge (a tube filled with concentrated saline). The salt bridge allows ions to move from one container to another, while the solutions remain electrically neutral. What's going on with the system?

Zinc is oxidized: zinc atoms turn into ions and go into solution. The released electrons move along the external circuit to the copper electrode, where copper ions are reduced. The electrons coming here combine with the copper ions leaving the solution. In this case, copper atoms are formed, which are released in the form of a metal. The salt bridge cations move into the copper electrode vessel to replace the spent copper ions. The salt bridge anions move into the zinc electrode vessel, helping to maintain an electrically neutral solution with the resulting zinc cations.

The potential difference (voltage) in such a system will be the greater, the farther the metals are from each other in the activity series.

2. Dry element

Household electric batteries use a dry cell consisting of:

zinc case (anode);
located inside the body of a graphite rod (cathode).

The rod is surrounded by a layer of manganese oxide and carbon black, and a layer of ammonium chloride and zinc chloride is used as an electrolyte. As a result, the following reactions occur:

oxidation reaction: Zn (t) → Zn 2+ + e -
recovery reaction: 2MnO 2 (t) + 2NH 4 + + 2e - → Mn 2 O 3 (t) + 2NH 3 (solution) + H 2 O (l)

The alkaline dry cell uses alkaline potassium hydroxide as the electrolyte instead of the acidic ammonium chloride, which increases the service life of the cell, because the body does not corrode so quickly.

The main disadvantage of galvanic cells is the fact that electricity is produced until one of the reagents runs out.

3. Batteries

Batteries eliminate the main drawback of dry cells - a short service life, since they can be recharged, and therefore, their operating time increases many times over and amounts to several years.

An ordinary lead-acid battery consists of six elements (cans) connected in series. Each bank gives a voltage of 2V, and their sum = 12V.

Lead is used as an anode. The cathode is lead dioxide (PbO 2). The electrodes are immersed in a solution of sulfuric acid (H 2 SO 4). When the circuit is closed in the battery, the following reactions occur:

On the anode: Pb (t) + H 2 SO 4 (p-p) → PbSO 4 (t) + 2H + + 2e -

On the cathode: 2e - + 2H + + PbO2 (t) + H 2 SO 4 (p-p) → PbSO 4 (t) + 2H 2 O (l)

General: Pb (t) + PbO 2 (t) + 2H 2 SO 4 (p-p) → 2PbSO 4 (t) + 2H 2 O (l)

The battery (when the car is in good condition) serves only to start the engine. At the time of starting, a rather significant current flows in the circuit (tens of amperes), therefore, the battery charge is consumed very quickly (in a few minutes). After the engine is started, all the power to the car is taken over by the alternator. While the engine is running, the generator recharges the battery: the initial redox reactions proceed in the opposite direction:

2PbSO 4 (t) + 2H 2 O (l) → Pb (t) + PbO 2 (t) + 2H 2 SO 4 (p-p)

As a result, lead and lead dioxide are reduced.

4. Electroplating

The essence of electrolytic cells is the implementation of chemical reactions at the expense of electricity - reduction at the cathode and oxidation at the anode.

The redox reaction that occurs on the electrodes when an electric current passes through an electrolytic cell is called electrolysis:

Water electrolysis: 2H 2 O (g) → 2H 2 (g) + O 2 (g)

Electrolytic cells are used to produce electroplating. In this case, one metal is applied in a thin layer on the surface of another metal.

The source of electricity in electroplating is an external current source. The bar of gold is the source of gold ions, which are restored on the surface of the medal.

Coatings applied by electrolysis are uniform in thickness and durable. As a result, the product outwardly does not differ in any way from the "clean" version, and at a price it is much cheaper.

Double electric layer, mechanism of occurrence and structure.

electrochemical elements. Electromotive force. Thermodynamics of a galvanic cell. EMF measurement.

When an electric current passes through the electrolyte, electrochemical reactions occur on the surface of the electrodes. The flow of electrochemical reactions can be generated by an external current source. The reverse phenomenon is also possible: electrochemical reactions occurring on two electrodes immersed in an electrolyte generate an electric current, and the reactions occur only when the circuit is closed (with the passage of current).

Electrochemical (or galvanic) element called a device for generating electric current due to electrochemical reactions. The simplest electrochemical cell consists of two metal electrodes (conductors of the first kind) immersed in an electrolyte (conductor of the second kind) and interconnected by a metal contact. Several electrochemical cells connected in series form electrochemical circuit .

The most important quantitative characteristic of an electrochemical element is the electromotive force(EMF, E), which is equal to the potential difference correctly open element (one in which conductors of the first kind from the same material are attached to the end electrodes of the element).

If, when an electric current passes in different directions, the same reaction occurs on the surface of the electrode, but in opposite directions, then such electrodes, as well as the element or circuit made up of them, are called reversible . The EMF of reversible elements is their thermodynamic property, i.e. depends only on T, P, the nature of the substances that make up the electrodes and solutions, and the concentration of these solutions. An example of a reversible element - Daniel-Jacobi element :

(-) Cu çZn çZnSO 4 ççCuSO 4 çCu (+)

in which each electrode is reversible. During the operation of the element, the following reactions take place: Zn ® Zn 2+ + 2 e, Cu 2+ + 2 e® Cu. When an infinitely small current is passed from an external source, reverse reactions occur on the electrodes.

An example of an irreversible element - Volta element :

(-) Zn ç H 2 SO 4 çCu (+)

During the operation of the element, reactions occur: Zn ® Zn 2+ + 2 e, 2H + + 2 e® H 2 . When current is passed from an external source, the electrode reactions will be: 2H + + 2 e® H 2 , Cu ® Cu 2+ + 2 e .

The EMF of an electrochemical element is a positive value, because it corresponds to a certain spontaneous process that produces positive work. The reverse process, which cannot proceed independently, would be answered by a negative emf. When compiling a chain of electrochemical elements, the process in one of the elements can be directed so that it is accompanied by the expenditure of work from the outside (non-spontaneous process), using for this the work of another element of the circuit in which a spontaneous process takes place. The total EMF of any circuit is equal to the algebraic sum of positive and negative values. Therefore, it is very important when writing a circuit diagram to take into account the signs of the EMF, using the accepted rules.

EMF of an electrochemical circuit is considered positive if, when the circuit is written, the right electrode is positively charged relative to the left one (during the circuit operation, cations pass in solution from the electrode written on the left towards the electrode written on the right, and electrons move in the same direction in the external circuit). Example.

Usage: concentration current sources. The essence of the invention: the electrochemical cell contains a dielectric housing with an inner diameter of 5 mm, filled with an electrolyte - 1.5 normal solution (NH 4) 2 SO 4 , located vertically and provided with thermal insulation, and indifferent electrodes in the electrolyte at a distance of 85 cm. Inside the housing can be a platinum-based catalyst is located. 1 s. p. f-ly, 2 ill.

The invention relates to the electrical industry and can be used in primary chemical current sources. Known electrochemical cell, including an anode of alkali metal, an inert electrically conductive cathode and an electrolyte based on an aqueous solution of alkali metal hydroxide. The disadvantage of this element is its non-renewability and environmental pollution. Known hermetic chemical current source containing bipolar electrodes separated by a separator, electrolyte, housing, sealing ring and cover. The disadvantage of this element is its non-renewability and the complexity of the design of the cover. The closest in design is a hermetic chemical current source containing bipolar indifferent electrodes separated by a separator, an electrolyte, a housing, a sealing ring and a cover. The disadvantage of this chemical element is its finite service life, due to the destruction of its main elements, mainly electrolyte, which occurs in the process its operation. The aim of the invention is to eliminate this drawback, namely the creation of a renewable chemical source of electric current. This goal is achieved by the fact that in an electrochemical source of electric current (element) containing bipolar indifferent electrodes and a liquid electrolyte, placed in a housing, the latter is made of a dielectric material in the form of a long tube, at the ends of which electrodes are fixed, installed in a working position strictly vertically. In order to obtain an environmentally friendly chemical current source, a plate of platinized asbestos is strengthened in the upper part of the body, which acts as a catalyst in chemical reactions taking place in the electrolyte environment. A comparative analysis with the prototype shows that the proposed device is distinguished by the execution of the body in the form of a long tube, at the ends of which electrodes are fixed from an indifferent (inert to ongoing chemical reactions) material, and in the working position the body (tube) must be fixed strictly vertically. Usually, in well-known electrochemical cells, one of the electrodes (usually a "cathode") is a case, the material of which participates in chemical reactions (see Dasoyan M.A. et al. Production of electric batteries M. 1965 or Fuel cells. Edited by G.D. Inga, from I.A. M. 1963). Thus, the proposed device meets the criterion of the invention "novelty". Comparison of the proposed technical solution and other similar solutions shows that the spatial orientation of the cell body is strictly vertical is not inherent in any of the known chemical current sources, which, as a rule, can work in any position, if only the conditions were met under which it would be prevented electrolyte leakage. In the proposed solution, the requirement for a vertical arrangement of the body (tube) is decisive, since in a different position the operation of the source is less efficient, and with a horizontal position of the tube, the current source will not work. This allows us to conclude that the technical solution meets the criterion of "significant differences". Figure 1 shows an electrochemical element, a longitudinal section; in fig. 2 is a longitudinal section of a sealed electrochemical cell. The electrochemical element consists of a housing 1 made in the form of a long tube. The material of the tube must be any dielectric, such as glass. The cross section of the tube can be any geometric figure, circle, square, etc. this element is non-defining. The ends of the tube 1 are covered with plugs 2, which can be made of the same material as the body 1, but other material can be used, such as rubber. The plugs are fixed electrodes 3, made of indefinite material, such as graphite. An electrolyte solution 4 is poured into the body 1 of the cell, for example, a 1.5 normal solution (NH 4) 2 SO 4 . Housing 1, in order to exclude the occurrence of convection currents, is covered with another layer of heat insulator 5 (glass wool, polystyrene, etc.). The top plug 2 has a drainage hole 6 (figure 1) for the exit of gaseous hydrogen and oxygen (reaction products of the electrochemical decomposition of water). To electrically close the circuit, conductors 7 are used, which connect an ammeter 8 to the element through a switch 9. A plate 10 (Fig. 2), made of platinized asbestos and serving as a catalyst, is fixed in the hermetic electrochemical element in the upper part of the housing 1 (Fig. 2). The electrochemical element works as follows. The (NH 4) 2 SO 4 molecule in an aqueous solution dissociates into positive NH 4 + ions and negative SO 4 - - ions, the nature of the distribution of which along the height of the tube under the influence of the gravitational field of the earth differs significantly from each other. The diameter of the hydrated NH4+ ion is about 3.2 3.2, and the molecular weight is 18 c.u. The diameter of the hydrated ion SO 4 - - is about 4.4 4.4, and the molecular weight is 96 c.u. These differences cause the heavier SO 4 - - ions to increase their concentration towards the bottom of the tube, while the NH 4 + ions are more evenly distributed along the height of the tube. The whole process of redistribution of ions in a solution with an inner tube diameter of 0.5 cm and a height of 85 cm at a temperature of 20 ° C takes about a day. As a result, a potential difference of about 0.05 V appears between the surface and bottom layers of the solution. The power of such an electrochemical element is about 10 -8 W. When the electrical circuit is closed using switch 9, an electric current will flow through the circuit electrolyte 4, electrodes 3, conductors 7 and ammeter 8, which will lead to decomposition of electrolyte 4. In the case of a solution of (NH 4) 2 SO 4, water decomposes and releases on the hydrogen electrode, and on the lower oxygen. These two gases diffuse through the electrolyte solution 4 and exit through the hole 6 in the upper part of the housing into the atmosphere. With continuous operation of this element, decomposition of 1 ml of water occurs after 310 6 hours. Adding water to the solution brings the element to its original state. The combination of hydrogen with oxygen to form water can occur naturally or in the presence of catalysts, and this indicates the renewability of the element. In this design, it is possible to use a solution of any electrolyte in any concentration, including mixtures of electrolytes. The differences will be only in the magnitude of the potential difference, current strength and decomposition products of electrolysis. An increase in power can be achieved by connecting several elements into a battery, increasing the height of the cases, or their diameters with a simultaneous increase in the contact area of electrodes 3 with electrolyte 4. Natural electrolyte solutions, such as sea water and salt lake water, can be used as the most powerful current sources. Due to the large depths and practically unlimited mass of the electrolyte, using electrodes of a large area, it is possible to obtain electricity in quantities sufficient for its industrial use. The hermetic electrochemical cell uses a plate of platinized asbestos 10 (figure 2), which is a catalyst on which hydrogen combines with oxygen to form water. The resulting water falls into the solution and is uniformly distributed in it by means of diffusion, while the cycle of water transformations closes, the element becomes renewable in the full sense of the word. In this element, you can use a solution of any electrolyte, during the decomposition of which hydrogen and oxygen are released, these are almost all oxygen-containing acids, alkali metal salts of these acids and alkali. In the case of working with other compounds and isolating other electrolysis products, it is possible to use other catalysts that return the electrolysis products to their original state. The main advantage of this element is its absolute environmental friendliness, ease of manufacture and durability.

electrochemical elements. Electromotive force. Thermodynamics of a galvanic cell. EMF measurement.

Double electric layer, mechanism of occurrence and structure.

GALVANIC ELEMENTS. EMF.

Electrochemical (or galvanic) element called a device for generating electric current due to electrochemical reactions. The simplest electrochemical cell consists of two metal electrodes (conductors of the first kind) immersed in an electrolyte (conductor of the second kind) and interconnected by a metal contact. Several electrochemical cells connected in series form electrochemical circuit .

(-) Cu çZn çZnSO 4 ççCuSO 4 çCu (+)

An example of an irreversible element - Volta element :

(-) Zn ç H 2 SO 4 çCu (+)

THERMODYNAMICS OF A GALVANIC CELL .

Let the following reaction proceed reversibly and isothermally in an electrochemical system:

n A A + n B B + ... ± nF Û n L L + n M M + ... ±

The electrical energy generated by the element is equal to the useful work A¢ of the total process. The useful work A¢ of the reversible process is maximum and at P, T = const it is equal to the decrease in the isobaric potential of the system:

DG P , T = nFE P , T

E P , T - reversible EMF of the system.

E P,T = -DG P,T / nF , E V,T = -DF V,T / nF

Thus, by measuring the EMF of the cell and its temperature coefficient, one can find the values of DG and DS for the total process occurring in the galvanic cell. This process is spontaneous, hence DG< 0.

Using the Gibbs-Helmholtz equation, you can calculate the change in the enthalpy of the process:

DH=DG-T=-nFE P+TnF

nFE P = -DH + nFT = + nFT

nFE V = -DU + nFT = + nFT

It follows from the equations that the ratio between the electrical energy reversibly generated or absorbed in an electrochemical system and the thermal effect of the reaction occurring in it depends on the sign and magnitude of the temperature coefficient of the EMF dE/ dT :

1. If adE / dT > 0 , then nFE > (DG > DH) and the system will convert into electrical energy not only the amount of heat that corresponds to the thermal effect of the reaction, but also additional heat - the warmth of Peletier Q P \u003d nFT dE/ dT borrowed from the environment. Under adiabatic conditions (under conditions of thermal insulation, when exchange with the environment is impossible), the T of the system decreases. The cooling of the system is especially noticeable if, at dE/ dT > 0 < 0 (реакция эндотермична).

2. If adE / dT < 0 , then nFE< (DG < DH) и часть теплоты реакции будет рассеиваться в виде теплоты Пелетье. В адиабатическом режиме система будет нагреваться.

3. If adE / dT = 0 , then DG = DH and nFE = - the electrical energy reversibly produced by the system is equivalent to the thermal effect of a chemical reaction. This ratio is known as principle (rule) of Thomson .

To calculate the EMF, the equations can be rewritten as:

When using equations, remember that they valid only for reversible electrochemical systems, therefore, when studying the dependence of EMF on T, it is necessary to avoid the use of electrochemical systems with liquid boundaries, because the diffusion potentials arising on them are not equilibrium.

Let's connect the EMF of the element with the equilibrium constant of the reaction taking place in the element. Chemical reaction isotherm equation:

DG=RT ln K a -RT

E=-= ln K a -

The first term on the right side of the equation for given P, T is a constant value, it can be denoted by E o. E o - standard emf of an element (electrochemical system) , i.e. EMF at all a i = 0.

E \u003d E o + ln = Eo + 2.303 lg

Thus, the EMF of an electrochemical system is a function of the activities of the participants in the electrochemical reaction. The above equations make it possible to calculate the quantities DG and K a from the experimental values of E and, conversely, calculate E knowing the thermodynamic characteristics of the chemical reaction.

EMF MEASUREMENT .

To measure the equilibrium (reversible) value of the EMF of an electrochemical element, it is necessary that the process proceeds infinitely slowly, i.e. so that the element operates at an infinitely small current strength. This condition is met in the compensation method, which is based on the fact that the element is connected in series against the external potential difference, and the latter is chosen so that there is no current in the circuit. Then the external potential difference is equal to the EMF of the circuit.

Using the compensation method, you can directly measure the value of the EMF, but this is a rather complicated operation, therefore, in laboratory practice, they prefer to compare the EMF of the element under study with the EMF of the so-called standard (normal) elements, which is carefully measured at different T. This comparative method is also compensatory.

The basic normal element is saturated Weston element .

(Scheme for measuring EMF - independently).

STRUCTURE OF THE ELECTRODE BOUNDARY - SOLUTION. DOUBLE ELECTRIC LAYER .

When a conductor of the first kind comes into contact with an electrolyte, a electrical double layer . As an example, consider a copper electrode immersed in a CuSO 4 solution. The chemical potential of copper ions in the metal at a given T 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 CuSO 4 be such that > . Then, when the metal is immersed in the solution, some of the Cu 2+ ions from the solution dehydrate and transfer to the metal, creating a positive charge on it. This charge will prevent the further transition of Cu 2+ ions from the solution to the metal and will lead to the formation near the electrode of a layer of SO 4 2- anions attracted to it. The so-called electrochemical equilibrium , at which the chemical potentials of ions in the metal and in solution will differ by the value of the potential difference of the resulting double electric layer (DEL):

The difference in electrical potentials and the difference in chemical potentials are compensated at electrochemical equilibrium.

Let the concentration of CuSO 4 be so low that< . В этом случае при погружении металла в раствор будет наблюдаться обратный процесс перехода ионов меди из кристаллической решетки металла в раствор и электрод окажется заряженным отрицательно. Этот заряд будет препятствовать дальнейшему переходу ионов Cu 2+ в раствор, установится новое электрохимическое равновесие.

You can choose such an electrolyte concentration at which the chemical potentials of the ions in the metal and solution are the same. Solutions of this concentration are called zero solutions . When a metal is immersed in its zero solution, DES does not appear on the electrode surface, however, in this case, the potential difference between the metal and the solution is not equal to zero.

According to Nernst, the only source of EMF in an electrochemical cell is DES on the surface of the electrodes. The potential of metals in a zero solution was defined by Nernst as the absolute zero of potentials. In the works of A.N. Frumkin, it was shown that Nernst's ideas are incorrect. It has been experimentally established that the EMF of an element composed of two different electrodes immersed in their zero solutions is very significantly different from zero (maybe more than 1 V). The potential of a metal in zero solution, called zero charge potential , cannot be regarded as the absolute zero potential.

HELMHOLTZ CONDENSED DOUBLE LAYER THEORY. The first quantitative theory of the DEL structure at the metal-solution interface was created by Helmholtz (1853). According to Helmholtz, DES can be likened to a flat capacitor, one of the plates of which coincides with the plane passing through the surface charges in the metal, the other with the plane connecting the charge centers of ions in solution attracted to the metal surface by electrostatic forces. Double layer thickness l equal to the ion radius r. According to the electrical neutrality condition, the number of ions attracted to the metal surface should be such that their charges compensate the surface charges of the metal, i.e.

The theory of a condensed double layer makes it possible to obtain DEL capacitance values consistent with experience and a physically plausible DEL thickness. However, it cannot interpret many experimental patterns: the experimentally found values of the electrokinetic potential (x-potential) and their dependence on the electrolyte concentration, the change in the sign of the metal surface charge in the presence of surfactants.

THE THEORY OF A DIFFUSE DOUBLE GOOY LAYER- CHAPMAN. The Helmholtz theory does not take into account that the properties of the DEL change with the concentration of the electrolyte, and T. Gouy (1910) and Chapman (1913) tried to relate the charge density in the DEL to the composition of the solution. They took into account that in addition to the electrostatic forces that arise between the metal and the ions, the ions are also affected by the forces of thermal molecular motion. When these two forces are applied, the ions in the solution should be distributed diffusely relative to the metal surface - with the volume charge density decreasing with distance from it.

Gouy and Chapman believed that ions can be considered as material points that do not have their own volume, but have a charge, and that their distribution in the electrode charge field obeys the Boltzmann distribution.

The Gouy-Chapman theory is better than the Helmholtz theory in agreement with the laws of electrokinetic phenomena. If we assume that starting from a certain distance l 1, the ions are no longer firmly bound to the electrode surface during the relative movement of the solid and liquid phases, then the potential corresponding to this distance can be considered the x-potential (x< j). Однако теория не объясняет изменение знака x-потенциала и перезарядку поверхности с изменением состава раствора. Кроме того, теория Гуи-Чапмана оказывается менее удовлетворительной, чем теория Гельмгольца, при использовании ее для количественных расчетов емкости ДЭС, т.к. она не учитывает собственного объема ионов, которые отождествляются с материальными точками.

Thus, the Gouy-Chapman theory is best justified where the Helmholtz theory is inapplicable, and, conversely, the latter gives the best convergence with experience in cases where the former gives incorrect results. Consequently, the structure of the DES should correspond to some combination of models proposed by Helmholtz and Gouy-Chapman. Such an assumption was made by Stern (1924) in his DEL adsorption theory.

ADSORPTION STERN THEORY. Stern believed that a certain part of the ions is retained near the metal-electrolyte interface, forming a Helmholtz or condensed plate of a double layer with a thickness corresponding to the average radius of the electrolyte ions. The remaining ions included in the DEL are diffusely distributed with a gradually decreasing charge density. For the diffuse part of the DEL, Stern, like Gouy, neglected the intrinsic dimensions of the ions. In addition, Stern suggested that ions are retained in the dense part of the DEL due not only to electrostatic forces, but also to specific adsorption forces, i.e. forces of non-Coulomb origin. Therefore, in solutions containing surface-active ions, their number in the dense part of the EDL can exceed the charge of the metal surface by some value, depending on the properties of the ions and the charge of the metal. Thus, according to Stern, two DES models should be distinguished, one of which refers to solutions of surface-inactive electrolytes, the other to solutions containing specifically adsorbed ions.

The adsorption theory also preserves the equality:

Q M = q L = q 1 + q 2

The charge density on the solution side q L consists of two parts: the charge density in the Helmholtz layer q 1 and the charge density in the diffuse layer q 2 .

The Stern theory makes it possible to define the x-potential as the potential drop in the diffuse part of the DEL, where the strong bond between the metal and ions has already been lost. With this definition, the x-potential should not coincide with the Nerst potential, as is observed experimentally. Stern's theory was also able to explain the recharging of the surface of a solid body.

At an infinitesimal concentration, all charges in the solution are diffusely distributed, and the structure of the DEL is described by the Gouy-Chapman theory. On the contrary, in concentrated solutions the DEL structure approaches the model proposed by Helmholtz. In the range of medium concentrations, where x is comparable in value to RT/F, its concentration dependence can be expressed by approximate equations:

for positive values x: x = B - ln with

for negative x values: x = B¢ + ln with

Stern's theory gives a qualitatively correct picture of DES. The determination of capacitance using the Stern model is consistent with experience both in terms of capacitance values and in the nature of its dependence on the electrode potential and solution concentration. But Stern's theory is not free from shortcomings. Among them is the impossibility of a quantitative description of the capacitance curves, especially when moving away from the zero-charge potential.

FURTHER DEVELOPMENT OF THE THEORY OF DES STATION. Many attempts have been made to develop a DEL theory that is quantitatively consistent with experimental data (Rice, Frumkin et al., Bockris, Devanathan, Esin, Muller, Parsons, Ershler, and others). Graham's model (1947) received the greatest recognition. According to Graham, the DEL lining, which is in solution, does not consist of two, but of three parts. The first, counting from the surface of the metal, is called the internal Helmholtz plane; it contains only surface-active ions (the charge of the plane is q 1) or, if they are not in solution, solvent molecules (q 1 = 0); its potential, referred to the solution, is denoted by y 1 . The next one, remote from the metal surface at a distance to which ions (the centers of their charge) can approach, is called the outer Helmholtz plane; its total charge is equal to q 2 , and the potential of the plane y 2 . Behind the outer Helmholtz plane there is a diffuse layer with a potential varying from y 2 to zero and with a charge density coinciding with q 2 .

Graham's model reflects the main features and structural features of the DEL metal-electrolyte. It allows one to calculate differential capacitance curves for any concentration of a given electrolyte, if there is an experimental curve for at least one of its solutions. However, this model does not cover all aspects of the problem.

When an electric current passes through the solution, a flow occurs on the surface of the electrodes electrochemical reactions, which are accompanied by electrons entering or leaving the electrode. In reverse processes, electrochemical reactions occurring on the interfaces of conductors of the first and second kind, lead to the appearance of an electric current.

Electrochemical processes differ from conventional chemical reactions in a number of ways.

A chemical reaction is possible only when reacting particles collide. When they come into contact, the transfer of electrons from one particle to another becomes possible. Whether such a transition actually occurs depends on the energy of the particles and their mutual orientation. The activation energy depends on the nature of the chemical reaction, and for ionic reactions it is usually low. The electron transition path is very small, which is also a feature of a chemical reaction. Collisions of particles can occur at any point of the reaction space at different mutual positions; therefore, electronic transitions can occur in arbitrary directions, i.e. The peculiarities of the chemical process are the chaotic nature of collisions and the absence of directionality of electronic transitions. As a result, the energy effects of chemical reactions appear mainly in the form of heat (minor expansion work is also possible).

In order for the energy changes corresponding to a chemical transformation to manifest themselves in the form of electrical energy, i.e. In order for an electrochemical process to proceed, it is necessary to change the reaction conditions.

Electrical energy is always associated with the passage of electric current, i.e. flow of electrons in a certain direction. Therefore, the reaction must be carried out in such a way that the electronic transitions are not random, but are carried out in one direction, and their path must be much larger than the atomic dimensions. Therefore, in electrochemical processes, the transition of electrons from one participant to another must occur at a considerable distance, for which the spatial separation of the participants in the reaction is necessarily necessary. However, spatial separation alone is not enough, as it will simply lead to the termination of the reaction.

To carry out an electrochemical process, additional conditions are necessary: electrons must be detached from some particles and pass through one common path to others. This can be achieved by replacing the direct contact between the participants in the reaction with their contact with two metals connected to each other by a metallic conductor. In order for the electron flow to be continuous, it is also necessary to ensure the passage of electric current through the reaction space, which is usually carried out by the participants in the electrochemical reaction themselves (if they are in an ionized state) or special compounds with high ionic conductivity.

A device for generating electrical energy through electrochemical reactions is called electrochemical(or galvanic)element. The simplest electrochemical cell consists of two metal electrodes (conductors of the first kind) immersed in an electrolyte solution (conductor of the second kind).

If, when an electric current passes in different directions, the same reaction occurs on the electrode surface, but in opposite directions, then such electrodes, as well as electrochemical elements composed of them, are called reversible. An example of an invertible element is the Daniel–Jacobi element

(–) Zn | ZnSO 4 , solution || CuSO 4 , solution | Cu(+)

During the operation of such an element, electrochemical reactions occur on the electrodes:

Zn Zn 2 + + 2e

Cu 2 + + 2eCu

The overall reaction equation in the element can be represented as

Zn + Cu 2 + Zn 2 + + Cu

When an infinitely small force is passed through the element from an external source, these reactions proceed in the opposite direction.

An example irreversible element is Volta element

(–) Zn | H2SO4 | Cu(+)

During the operation of such an element, reactions occur on the electrodes:

Zn Zn 2 + + 2e

2H + + 2eH 2 ,

and the reaction in the element is represented by the equation

Zn + 2H + Zn 2+ + H 2

When current is passed from an external source, other reactions occur on the electrodes:

Cu Cu 2 + + 2e,

those. in an electrochemical cell, copper is dissolved in sulfuric acid with the release of hydrogen:

Cu + 2H +  Cu 2 + + H 2

The most important characteristic of an electrochemical cell is its electromotive force(EMF) E is the potential difference of a correctly open element, i.e. potential difference between the ends of conductors of the first kind of the same material, attached to the electrodes of a galvanic cell. In other words, EMF is the potential difference at equilibrium, when no electric current flows in the circuit. If the electrodes are closed, then an electric current will flow in the circuit, and the potential difference is voltage an electrochemical element that differs from the EMF by the magnitude of the voltage drop across the internal resistance of the element.