Electric current in liquids. Movement of charges, anions and cations. Electric current in liquids - theory, electrolysis Mechanism of current flow in liquids

« Physics - 10th grade"

What are the carriers of electric current in a vacuum?
What is the nature of their movement?

Liquids, like solids, can be dielectrics, conductors and semiconductors. Dielectrics include distilled water, conductors include solutions and melts of electrolytes: acids, alkalis and salts. Liquid semiconductors are molten selenium, molten sulfides, etc.

Electrolytic dissociation.

When electrolytes dissolve under the influence of the electric field of polar water molecules, the electrolyte molecules disintegrate into ions.

The breakdown of molecules into ions under the influence of the electric field of polar water molecules is called electrolytic dissociation.

Degree of dissociation- the proportion of molecules in a dissolved substance that have broken up into ions.

The degree of dissociation depends on temperature, solution concentration and electrical properties of the solvent.

With increasing temperature, the degree of dissociation increases and, consequently, the concentration of positively and negatively charged ions increases.

When ions of different signs meet, they can again combine into neutral molecules.

Under constant conditions, a dynamic equilibrium is established in the solution, in which the number of molecules that disintegrate into ions per second is equal to the number of pairs of ions that, at the same time, recombine into neutral molecules.

Ionic conductivity.

Charge carriers in aqueous solutions or melts of electrolytes are positively and negatively charged ions.

If a vessel with an electrolyte solution is connected to an electrical circuit, then negative ions will begin to move to the positive electrode - the anode, and positive ions - to the negative - the cathode. As a result, an electric current will flow through the circuit.

The conductivity of aqueous solutions or melts of electrolytes, which is carried out by ions, is called ionic conductivity.

Electrolysis. In ionic conduction, the passage of current is associated with the transfer of matter. At the electrodes, substances that make up the electrolytes are released. At the anode, negatively charged ions give up their extra electrons (in chemistry, this is called an oxidation reaction), and at the cathode, positive ions receive the missing electrons (a reduction reaction).

Liquids can also have electronic conductivity. Liquid metals, for example, have such conductivity.

The process of release of a substance at the electrode associated with redox reactions is called electrolysis.

What determines the mass of a substance released over a certain time? It is obvious that the mass m of the released substance is equal to the product of the mass m 0i of one ion by the number N i of ions that reached the electrode during the time Δt:

m = m 0i N i . (16.3)

The mass of the ion m 0i is equal to:

where M is the molar (or atomic) mass of the substance, and N A is Avogadro’s constant, i.e., the number of ions in one mole.

The number of ions reaching the electrode is equal to

where Δq = IΔt is the charge passed through the electrolyte during the time Δt; q 0i is the charge of the ion, which is determined by the valency n of the atom: q 0i = ne (e is the elementary charge). During the dissociation of molecules, for example KBr, consisting of monovalent atoms (n = 1), K + and Br - ions appear. The dissociation of copper sulfate molecules leads to the appearance of doubly charged Cu 2+ and SO 2- 4 ions (n ​​= 2). Substituting expressions (16.4) and (16.5) into formula (16.3) and taking into account that Δq = IΔt, a q 0i = ne, we obtain

Faraday's law.

Let us denote by k the coefficient of proportionality between the mass m of the substance and the charge Δq = IΔt passing through the electrolyte:

where F = eN A = 9.65 10 4 C/mol - Faraday's constant.

The coefficient k depends on the nature of the substance (values ​​of M and n). According to formula (16.6) we have

m = kIΔt. (16.8)

Faraday's law of electrolysis:

The mass of the substance released on the electrode during the time Δt. when an electric current passes, it is proportional to the current strength and time.

This statement, obtained theoretically, was first established experimentally by Faraday.

The quantity k in formula (16.8) is called electrochemical equivalent of this substance and is expressed in kilograms per pendant(kg/Cl).

From formula (16.8) it is clear that the coefficient k is numerically equal to the mass of the substance released on the electrodes when the ions transfer a charge equal to 1 C.

The electrochemical equivalent has a simple physical meaning. Since M/N A = m 0i and еn = q 0i, then according to formula (16.7) k = rn 0i /q 0i, i.e. k is the ratio of the mass of the ion to its charge.

By measuring the values ​​of m and Δq, it is possible to determine the electrochemical equivalents of various substances.

You can verify the validity of Faraday's law experimentally. Let's assemble the installation shown in Figure (16.25). All three electrolytic baths are filled with the same electrolyte solution, but the currents passing through them are different. Let us denote the current strengths by I1, I2, I3. Then I 1 = I 2 + I 3. By measuring the masses m 1 , m 2 , m 3 of substances released on the electrodes in different baths, one can verify that they are proportional to the corresponding current strengths I 1 , I 2 , I 3 .

Determination of electron charge.

Formula (16.6) for the mass of the substance released on the electrode can be used to determine the charge of the electron. From this formula it follows that the modulus of the electron charge is equal to:

Knowing the mass m of the released substance during the passage of charge IΔt, the molar mass M, the valence of n atoms and Avogadro's constant N A, we can find the value of the modulus of the electron charge. It turns out to be equal to e = 1.6 10 -19 C.

It was in this way that the value of the elementary electric charge was first obtained in 1874.

Application of electrolysis. Electrolysis is widely used in technology for various purposes. Electrolytically cover the surface of one metal with a thin layer of another ( nickel plating, chrome plating, gold plating and so on.). This durable coating protects the surface from corrosion. If you ensure good peeling of the electrolytic coating from the surface on which the metal is deposited (this is achieved, for example, by applying graphite to the surface), then you can get a copy from the relief surface.

The process of obtaining peelable coatings - electrotype- was developed by the Russian scientist B. S. Jacobi (1801-1874), who in 1836 used this method to make hollow figures for St. Isaac's Cathedral in St. Petersburg.

Previously, in the printing industry, copies from a relief surface (stereotypes) were obtained from matrices (an imprint of a type on a plastic material), for which a thick layer of iron or other substance was deposited on the matrices. This made it possible to reproduce the set in the required number of copies.

Using electrolysis, metals are purified from impurities. Thus, the crude copper obtained from the ore is cast into the form of thick sheets, which are then placed in a bath as anodes. During electrolysis, the copper of the anode dissolves, impurities containing valuable and rare metals fall to the bottom, and pure copper settles on the cathode.

Using electrolysis, aluminum is obtained from molten bauxite. It was this method of producing aluminum that made it cheap and, along with iron, the most common in technology and everyday life.

Using electrolysis, electronic circuit boards are obtained, which serve as the basis for all electronic products. A thin copper plate is glued onto the dielectric, onto which a complex pattern of connecting wires is painted with special paint. Then the plate is placed in an electrolyte, where the areas of the copper layer that are not covered by paint are etched. After this, the paint is washed off, and the details of the microcircuit appear on the board.

Absolutely everyone knows that liquids can conduct electrical energy well. And it is also a well-known fact that all conductors according to their type are divided into several subgroups. We propose to consider in our article how electric current is carried out in liquids, metals and other semiconductors, as well as the laws of electrolysis and its types.

Electrolysis theory

To make it easier to understand what we are talking about, we suggest starting with theory; electricity, if we consider electric charge as a kind of liquid, has become known for more than 200 years. Charges consist of individual electrons, but those are so small that any large charge behaves like a continuous flow of liquid.

Like solid bodies, liquid conductors can be of three types:

• semiconductors (selenium, sulfides and others);
• dielectrics (alkaline solutions, salts and acids);
• conductors (say, in plasma).

The process by which electrolytes dissolve and ions disintegrate under the influence of an electric molar field is called dissociation. In turn, the proportion of molecules that have decayed into ions, or decayed ions in the solute, depends entirely on the physical properties and temperature in various conductors and melts. It is important to remember that ions can recombine or come back together. If the conditions do not change, then the number of decayed and combined ions will be equally proportional.

Ions conduct energy in electrolytes because they can be both positively and negatively charged particles. When the liquid (or more precisely, the vessel with the liquid is connected to the power supply), particles will begin to move towards opposite charges (positive ions will begin to be attracted to the cathodes, and negative ions to the anodes). In this case, energy is transported directly by ions, so conductivity of this type is called ionic.

During this type of conduction, current is carried by ions, and substances that are components of electrolytes are released at the electrodes. If we think from a chemical point of view, then oxidation and reduction occur. Thus, electric current in gases and liquids is transported using electrolysis.

Laws of physics and current in liquids

Electricity in our homes and equipment, as a rule, is not transmitted in metal wires. In a metal, electrons can move from atom to atom, and thus carry a negative charge.

As liquids, they are carried in the form of electrical voltage, known as voltage, in units of volts, named after the Italian scientist Alessandro Volta.

Video: Electric current in liquids: complete theory

Also, electric current flows from high voltage to low voltage and is measured in units known as amperes, named after Andre-Marie Ampere. And according to the theory and formula, if you increase the voltage, then its strength will also increase proportionally. This relationship is known as Ohm's law. As an example, the virtual ampere characteristic is below.

Ohm's Law (with additional details regarding the length and thickness of the wire) is typically one of the first things taught in physics classes, many students and teachers therefore treat electric current in gases and liquids as a fundamental law in physics.

In order to see the movement of charges with your own eyes, you need to prepare a flask with salt water, flat rectangular electrodes and power sources; you will also need an ammeter installation, with the help of which energy will be conducted from the power supply to the electrodes.

The plates that act as conductors must be lowered into the liquid and the voltage turned on. After this, the chaotic movement of particles will begin, but just like after the emergence of a magnetic field between conductors, this process will be ordered.

As soon as the ions begin to exchange charges and combine, the anodes will become cathodes, and the cathodes will become anodes. But here you need to take into account electrical resistance. Of course, the theoretical curve plays an important role, but the main influence is the temperature and the level of dissociation (depending on which carriers are chosen), and whether alternating or direct current is chosen. Completing this experimental study, you can notice that a thin layer of salt has formed on solid bodies (metal plates).

Electrolysis and vacuum

Electric current in vacuum and liquids is a rather complex issue. The fact is that in such media there are completely no charges in the bodies, which means that it is a dielectric. In other words, our goal is to create conditions so that the electron atom can begin its movement.

To do this, you need to use a modular device, conductors and metal plates, and then proceed as in the method above.

Applications of Electrolysis

This process is applied in almost all areas of life. Even the most basic work sometimes requires the intervention of electric current in liquids, say,

Using this simple process, solid bodies are coated with a thin layer of any metal, for example, nickel or chrome plating. This is one of the possible ways to combat corrosion processes. Similar technologies are used in the manufacture of transformers, meters and other electrical devices.

We hope that our rationale has answered all the questions that arise when studying the phenomenon of electric current in liquids. If you need better answers, we recommend visiting the electricians forum, where they will be happy to advise you for free.

In terms of their electrical properties, liquids are very diverse. Molten metals, like metals in the solid state, have high electrical conductivity associated with a high concentration of free electrons.

Many liquids, such as pure water, alcohol, kerosene, are good dielectrics because their molecules are electrically neutral and there are no free charge carriers.

Electrolytes. A special class of liquids consists of so-called electrolytes, which include aqueous solutions of inorganic acids, salts and bases, melts of ionic crystals, etc. Electrolytes are characterized by the presence of high concentrations of ions, which make it possible for the passage of electric current. These ions arise during melting and dissolution, when, under the influence of the electric fields of solvent molecules, the molecules of the solute decompose into separate positively and negatively charged ions. This process is called electrolytic dissociation.

Electrolytic dissociation. The degree of dissociation a of a given substance, i.e., the proportion of solute molecules that have broken up into ions, depends on temperature, solution concentration and dielectric constant of the solvent. As the temperature increases, the degree of dissociation increases. Ions of opposite signs can recombine, combining again into neutral molecules. Under constant external conditions, a dynamic equilibrium is established in the solution, in which the processes of recombination and dissociation compensate each other.

Qualitatively, the dependence of the degree of dissociation a on the concentration of the dissolved substance can be established using the following simple arguments. If a unit volume contains molecules of a dissolved substance, then some of them are dissociated, and the rest are not dissociated. The number of elementary acts of dissociation per unit volume of solution is proportional to the number of unsplit molecules and is therefore equal to where A is a coefficient depending on the nature of the electrolyte and temperature. The number of recombination events is proportional to the number of collisions of unlike ions, i.e., proportional to the number of both those and other ions. Therefore, it is equal to where B is a coefficient that is constant for a given substance at a certain temperature.

In a state of dynamic equilibrium

The ratio does not depend on the concentration. It can be seen that the lower the concentration of the solution, the closer it is to unity: in very dilute solutions, almost all the molecules of the solute are dissociated.

The higher the dielectric constant of the solvent, the more the ionic bonds in the solute molecules are weakened and, therefore, the greater the degree of dissociation. Thus, hydrochloric acid produces an electrolyte with high electrical conductivity when dissolved in water, while its solution in ethyl ether conducts electricity very poorly.

Unusual electrolytes. There are also very unusual electrolytes. For example, the electrolyte is glass, which is a highly supercooled liquid with enormous viscosity. When heated, glass softens and its viscosity decreases greatly. The sodium ions present in the glass become noticeably mobile, and the passage of electric current becomes possible, although at ordinary temperatures glass is a good insulator.

Rice. 106. Demonstration of the electrical conductivity of glass when heated

A clear demonstration of this can be seen in the experiment, the diagram of which is shown in Fig. 106. A glass rod is connected to a lighting network through a rheostat. While the rod is cold, the current in the circuit is negligible due to the high resistance of the glass. If the stick is heated with a gas burner to a temperature of 300-400 °C, then its resistance will drop to several tens of ohms and the filament of the light bulb L will become hot. Now you can short-circuit the light bulb with key K. In this case, the resistance of the circuit will decrease and the current will increase. Under such conditions, the stick will be effectively heated by electric current and glow until it glows brightly, even if the burner is removed.

Ionic conductivity. The passage of electric current in an electrolyte is described by Ohm's law

Electric current in the electrolyte occurs at an arbitrarily low applied voltage.

The charge carriers in the electrolyte are positively and negatively charged ions. The mechanism of electrical conductivity of electrolytes is in many ways similar to the mechanism of electrical conductivity of gases described above. The main differences are due to the fact that in gases the resistance to the movement of charge carriers is mainly due to their collisions with neutral atoms. In electrolytes, the mobility of ions is due to internal friction - viscosity - as they move in the solvent.

As the temperature increases, the conductivity of electrolytes, in contrast to metals, increases. This is due to the fact that with increasing temperature the degree of dissociation increases and the viscosity decreases.

Unlike electronic conductivity, characteristic of metals and semiconductors, where the passage of electric current is not accompanied by any change in the chemical composition of the substance, ionic conductivity is associated with the transfer of substance

and the release of substances included in the electrolytes on the electrodes. This process is called electrolysis.

Electrolysis. When a substance is released on the electrode, the concentration of the corresponding ions in the electrolyte region adjacent to the electrode decreases. Thus, the dynamic balance between dissociation and recombination is disrupted here: it is here that the decomposition of the substance occurs as a result of electrolysis.

Electrolysis was first observed during the decomposition of water by current from a voltaic column. A few years later, the famous chemist G. Davy discovered sodium, isolating it by electrolysis from caustic soda. The quantitative laws of electrolysis were experimentally established by M. Faraday. They can be easily substantiated based on the mechanism of the phenomenon of electrolysis.

Faraday's laws. Each ion has an electrical charge that is a multiple of the elementary charge e. In other words, the charge of the ion is equal to , where is an integer equal to the valence of the corresponding chemical element or compound. Suppose that when a current passes through the electrode, ions are released. Their charge in absolute value is equal to Positive ions reach the cathode and their charge is neutralized by electrons flowing to the cathode through the wires from the current source. Negative ions approach the anode and the same number of electrons goes through the wires to the current source. In this case, a charge passes through a closed electrical circuit

Let us denote by the mass of the substance released on one of the electrodes, and by the mass of the ion (atom or molecule). It is obvious that, therefore, Multiplying the numerator and denominator of this fraction by Avogadro’s constant we get

where is the atomic or molar mass, Faraday’s constant, determined by the expression

From (4) it is clear that Faraday’s constant has the meaning of “one mole of electricity,” i.e., it is the total electric charge of one mole of elementary charges:

Formula (3) contains both Faraday's laws. It says that the mass of the substance released during electrolysis is proportional to the charge passed through the circuit (Faraday’s first law):

The coefficient is called the electrochemical equivalent of a given substance and is expressed in

kilograms per coulomb It has the meaning of the reciprocal of the specific charge of the ion.

The electrochemical equivalent of k is proportional to the chemical equivalent of the substance (Faraday's second law).

Faraday's laws and elementary charge. Since the concept of the atomic nature of electricity did not yet exist in Faraday's time, the experimental discovery of the laws of electrolysis was far from trivial. On the contrary, it was Faraday's laws that essentially served as the first experimental proof of the validity of these ideas.

The experimental measurement of Faraday's constant made it possible for the first time to obtain a numerical estimate of the value of the elementary charge long before direct measurements of the elementary electric charge in Millikan's experiments with oil drops. It is remarkable that the idea of ​​the atomic structure of electricity received unequivocal experimental confirmation in electrolysis experiments performed in the 30s of the 19th century, when even the idea of ​​​​the atomic structure of matter was not yet shared by all scientists. In a famous speech given to the Royal Society and dedicated to the memory of Faraday, Helmholtz commented on this circumstance in this way:

“If we admit the existence of atoms of chemical elements, then we cannot avoid the further conclusion that electricity, both positive and negative, is divided into certain elementary quantities, which behave like atoms of electricity.”

Chemical current sources. If a metal, such as zinc, is immersed in water, then a certain amount of positive zinc ions, under the influence of polar water molecules, will begin to move from the surface layer of the metal’s crystal lattice into the water. As a result, the zinc will be charged negatively and the water positively. A thin layer called an electrical double layer forms at the interface between metal and water; there is a strong electric field in it, the intensity of which is directed from the water to the metal. This field prevents the further transition of zinc ions into water, and as a result, a dynamic equilibrium arises in which the average number of ions coming from the metal into the water is equal to the number of ions returning from the water to the metal.

Dynamic equilibrium will also be established if the metal is immersed in an aqueous solution of a salt of the same metal, for example, zinc in a solution of zinc sulfate. In the solution, the salt dissociates into ions. The resulting zinc ions are no different from the zinc ions that entered the solution from the electrode. An increase in the concentration of zinc ions in the electrolyte facilitates the transition of these ions into the metal from solution and makes it more difficult

transition from metal to solution. Therefore, in a solution of zinc sulfate, the immersed zinc electrode, although charged negatively, is weaker than in pure water.

When a metal is immersed in a solution, the metal does not always become negatively charged. For example, if a copper electrode is immersed in a solution of copper sulfate, then ions will begin to precipitate from the solution on the electrode, charging it positively. The field strength in the electric double layer in this case is directed from copper to the solution.

Thus, when a metal is immersed in water or an aqueous solution containing ions of the same metal, a potential difference arises between them at the interface between the metal and the solution. The sign and magnitude of this potential difference depends on the type of metal (copper, zinc, etc., on the concentration of ions in the solution and is almost independent of temperature and pressure.

Two electrodes of different metals immersed in an electrolyte form a galvanic cell. For example, in a Volta cell, the zinc and copper electrodes are immersed in an aqueous solution of sulfuric acid. At first, the solution contains neither zinc ions nor copper ions. However, later these ions enter the solution from the electrodes and dynamic equilibrium is established. As long as the electrodes are not connected to each other by wire, the potential of the electrolyte is the same at all points, and the potentials of the electrodes differ from the potential of the electrolyte due to the double layers formed at their interface with the electrolyte. In this case, the electrode potential of zinc is equal to -0.763 V, and of copper. The electromotive force of the Volt element, consisting of these potential jumps, will be equal to

Current in a circuit with a galvanic element. If the electrodes of a galvanic cell are connected with a wire, then electrons through this wire will move from the negative electrode (zinc) to the positive electrode (copper), which upsets the dynamic balance between the electrodes and the electrolyte in which they are immersed. Zinc ions will begin to move from the electrode into the solution, so as to maintain the electrical double layer in the same state with a constant potential jump between the electrode and the electrolyte. Similarly, with a copper electrode, copper ions will begin to move out of solution and precipitate on the electrode. In this case, a deficiency of ions is formed near the negative electrode, and an excess of such ions is formed near the positive electrode. The total number of ions in the solution will not change.

As a result of the described processes, an electric current will be maintained in a closed circuit, which is created in the connecting wire by the movement of electrons, and in the electrolyte by ions. When an electric current passes, the zinc electrode gradually dissolves and copper is deposited on the positive (copper)

electrode. The ion concentration increases at the zinc electrode and decreases at the copper electrode.

Potential in a circuit with a galvanic element. The described pattern of the passage of electric current in a non-uniform closed circuit containing a chemical element corresponds to the potential distribution along the circuit, shown schematically in Fig. 107. In the external circuit, i.e., in the wire connecting the electrodes, the potential smoothly decreases from the value at the positive (copper) electrode A to the value at the negative (zinc) electrode B in accordance with Ohm’s law for a homogeneous conductor. In the internal circuit, that is, in the electrolyte between the electrodes, the potential gradually decreases from a value near the zinc electrode to a value near the copper electrode. If in the external circuit the current flows from the copper electrode to the zinc electrode, then inside the electrolyte it flows from the zinc to the copper. Potential jumps in electrical double layers are created as a result of the action of external (in this case chemical) forces. The movement of electric charges in double layers due to external forces occurs opposite to the direction of action of the electric forces.

Rice. 107. Potential distribution along a chain containing a chemical element

The inclined sections of the potential change in Fig. 107 corresponds to the electrical resistance of the external and internal sections of the closed circuit. The total potential drop along these sections is equal to the sum of the potential jumps in the double layers, i.e., the electromotive force of the element.

The passage of electric current in a galvanic cell is complicated by by-products released on the electrodes and the appearance of a concentration difference in the electrolyte. These phenomena are referred to as electrolytic polarization. For example, in Volta elements, when the circuit is closed, positive ions move to the copper electrode and are deposited on it. As a result, after some time the copper electrode is replaced by a hydrogen one. Since the electrode potential of hydrogen is 0.337 V lower than the electrode potential of copper, the emf of the element decreases by approximately the same amount. In addition, hydrogen released on the copper electrode increases the internal resistance of the element.

To reduce the harmful effects of hydrogen, depolarizers are used - various oxidizing agents. For example, in the most commonly used element Leclanche (“dry” batteries)

The positive electrode is a graphite rod surrounded by a compressed mass of manganese peroxide and graphite.

Batteries. A practically important type of galvanic cells are batteries, for which, after discharging, a reverse charging process is possible with the conversion of electrical energy into chemical energy. Substances consumed during the production of electric current are restored inside the battery through electrolysis.

It can be seen that when charging the battery, the concentration of sulfuric acid increases, which leads to an increase in the density of the electrolyte.

Thus, during the charging process, a sharp asymmetry of the electrodes is created: one becomes lead, the other becomes lead peroxide. A charged battery is a galvanic cell that can serve as a source of current.

When electrical energy consumers are connected to the battery, an electric current will flow through the circuit, the direction of which is opposite to the charging current. Chemical reactions go in the opposite direction and the battery returns to its original state. Both electrodes will be covered with a layer of salt, and the concentration of sulfuric acid will return to its original value.

For a charged battery, the EMF is approximately 2.2 V. When discharging, it drops to 1.85 V. Further discharging is not recommended, since the formation of lead sulfate becomes irreversible and the battery deteriorates.

The maximum charge that a battery can deliver when discharged is called its capacity. Battery capacity usually

measured in ampere hours. The larger the surface of the plates, the larger it is.

Applications of electrolysis. Electrolysis is used in metallurgy. The most common electrolytic production of aluminum and pure copper. Using electrolysis, it is possible to create thin layers of some substances on the surface of others in order to obtain decorative and protective coatings (nickel plating, chrome plating). The process of producing peelable coatings (electroplasty) was developed by Russian scientist B. S. Jacobi, who used it to make hollow sculptures decorating St. Isaac's Cathedral in St. Petersburg.

What is the difference between the physical mechanism of electrical conductivity in metals and electrolytes?

Explain why the degree of dissociation of a given substance depends on the dielectric constant of the solvent.

Explain why in highly dilute electrolyte solutions almost all solute molecules are dissociated.

Explain how the mechanism of electrical conductivity of electrolytes is similar to the mechanism of electrical conductivity of gases. Why, under constant external conditions, is the electric current proportional to the applied voltage?

What role does the law of conservation of electric charge play in deriving the law of electrolysis (3)?

Explain the relationship between the electrochemical equivalent of a substance and the specific charge of its ions.

How can one experimentally determine the ratio of electrochemical equivalents of different substances if there are several electrolytic baths, but there are no instruments for measuring current?

How can the phenomenon of electrolysis be used to create an electricity meter in a DC network?

Why can Faraday's laws be considered as experimental proof of the ideas about the atomic nature of electricity?

What processes occur when metal electrodes are immersed in water and in an electrolyte containing ions of these metals?

Describe the processes occurring in the electrolyte near the electrodes of a galvanic cell during the passage of current.

Why do positive ions inside a voltaic cell move from the negative (zinc) electrode to the positive (copper) electrode? How does a potential distribution occur in a circuit that causes the ions to move in this way?

Why can the degree of charge of an acid battery be checked using a hydrometer, i.e. a device for measuring the density of a liquid?

How do processes in batteries fundamentally differ from processes in “dry” batteries?

What part of the electrical energy expended in the process of charging the battery c can be used when discharging it, if during the charging process the voltage was maintained at its terminals

Report on the topic:

Electricity

in liquids

(electrolytes)

Electrolysis

Faraday's laws

Elementary electric charge

Students 8 th class « B »

L Oginova M arias A ndreevny

Moscow 2003

School No. 91

Introduction

A lot in our lives is connected with the electrical conductivity of solutions of salts in water (electrolytes). From the first beat of the heart (“living” electricity in the human body, which is 80% water) to cars on the street, players and mobile phones (an integral part of these devices are “batteries” - electrochemical batteries and various batteries - from lead-acid in cars to lithium polymer in the most expensive mobile phones). In huge vats smoking with toxic fumes, aluminum is produced by electrolysis from bauxite melted at high temperatures - the “winged” metal for airplanes and cans for Fanta. Everything around – from the chrome-plated radiator grill of a foreign car to the silver-plated earring in the ear – has at some time encountered a solution or molten salts, and, consequently, an electric current in liquids. It’s not for nothing that this phenomenon is studied by an entire science – electrochemistry. But now we are more interested in the physical basis of this phenomenon.

Electric current in solution. Electrolytes

From 8th grade physics lessons we know that charge in conductors (metals) is carried by negatively charged electrons.

The ordered movement of charged particles is called electric current.

But if we assemble a device (with graphite electrodes):

then we will make sure that the ammeter needle deflects - current flows through the solution! What charged particles are there in the solution?

Back in 1877, the Swedish scientist Svante Arrhenius, studying the electrical conductivity of solutions of various substances, came to the conclusion that it is caused by ions that are formed when salt is dissolved in water. When dissolved in water, the CuSO 4 molecule breaks up (dissociates) into two differently charged ions - Cu 2+ and SO 4 2-. Simplified processes can be reflected by the following formula:

CuSO 4 ÞCu 2+ +SO 4 2-

Solutions of salts, alkalis, and acids conduct electric current.

Substances whose solutions conduct electric current are called electrolytes.

Solutions of sugar, alcohol, glucose and some other substances do not conduct electricity.

Substances whose solutions do not conduct electric current are called nonelectrolytes.

Electrolytic dissociation

The process of electrolyte breaking down into ions is called electrolytic dissociation.

S. Arrhenius, who adhered to the physical theory of solutions, did not take into account the interaction of the electrolyte with water and believed that there were free ions in solutions. In contrast, Russian chemists I.A. Kablukov and V.A. Kistyakovsky applied the chemical theory of D.I. Mendeleev to explain electrolytic dissociation and proved that when an electrolyte is dissolved, a chemical interaction of the dissolved substance with water occurs, which leads to the formation of hydrates, and then they dissociate into ions. They believed that solutions contained not free, not “naked” ions, but hydrated ones, that is, “dressed in a coat” of water molecules. Consequently, the dissociation of electrolyte molecules occurs in the following sequence:

a) orientation of water molecules around the poles of an electrolyte molecule

b) hydration of the electrolyte molecule

c) its ionization

d) its decomposition into hydrated ions

In relation to the degree of electrolytic dissociation, electrolytes are divided into strong and weak.

- Strong electrolytes- those that dissociate almost completely when dissolved.

Their degree of dissociation tends to unity.

- Weak electrolytes- those that almost do not dissociate when dissolved. Their degree of dissociation tends to zero.

From this we conclude that the carriers of electric charge (carriers of electric current) in electrolyte solutions are not electrons, but positively and negatively charged hydrated ions .

Temperature dependence of electrolyte resistance

As the temperature rises the dissociation process is facilitated, the mobility of ions increases and electrolyte resistance drops .

Cathode and anode. Cations and anions

What happens to ions under the influence of electric current?

Let's return to our device:

In solution, CuSO 4 dissociated into ions – Cu 2+ and SO 4 2-. Positively charged ion Cu 2+ (cation) is attracted to a negatively charged electrode – cathode, where it receives the missing electrons and is reduced to metallic copper - a simple substance. If you remove the cathode from the device after passing current through the solution, it is easy to notice a red-brown coating - this is metallic copper.

Faraday's first law

Can we find out how much copper was released? By weighing the cathode before and after the experiment, the mass of deposited metal can be accurately determined. Measurements show that the mass of the substance released on the electrodes depends on the current strength and electrolysis time:

where K is the proportionality coefficient, also called electrochemical equivalent .

Consequently, the mass of the released substance is directly proportional to the current strength and electrolysis time. But current over time (according to the formula):

there is a charge.

So, the mass of the substance released on the electrode is proportional to the charge, or the amount of electricity passed through the electrolyte.

M=K´q

This law was experimentally discovered in 1843 by the English scientist Michael Faraday and is called Faraday's first law .

Faraday's second law

What is the electrochemical equivalent and what does it depend on? Michael Faraday also answered this question.

Based on numerous experiments, he came to the conclusion that this value is characteristic of each substance. So, for example, during the electrolysis of a solution of lapis (silver nitrate AgNO 3), 1 pendant releases 1.1180 mg of silver; exactly the same amount of silver is released during electrolysis with a charge of 1 pendant of any silver salt. When electrolysis of a salt of another metal, 1 pendant releases another amount of that metal. Thus , The electrochemical equivalent of a substance is the mass of this substance released during electrolysis by 1 coulomb of electricity flowing through a solution . Here are its values ​​for some substances:

From the table we see that the electrochemical equivalents of various substances are significantly different from one another. On what properties of a substance does the value of its electrochemical equivalent depend? The answer to this question is given by Faraday's second law :

The electrochemical equivalents of various substances are proportional to their atomic weights and inversely proportional to the numbers expressing their chemical valency.

n – valence

A – atomic weight

- called the chemical equivalent of a given substance

– proportionality coefficient, which is already a universal constant, that is, it has the same value for all substances. If we measure the electrochemical equivalent in g/k, we find that it is equal to 1.037´10 -5 g/k.

Combining Faraday's first and second laws we get:

This formula has a simple physical meaning: F is numerically equal to the charge that must be passed through any electrolyte in order to release a substance on the electrodes in an amount equal to one chemical equivalent. F is called the Faraday number and it is equal to 96400 k/g.

Mole and number of molecules in it. Avogadro's number

From the 8th grade chemistry course we know that to measure the amounts of substances involved in chemical reactions, a special unit was chosen - the mole. To measure one mole of a substance, you need to take as many grams of it as its relative molecular mass.

For example, 1 mole of water (H 2 O) is equal to 18 grams (1 + 1 + 16 = 18), a mole of oxygen (O 2) is 32 grams, and a mole of iron (Fe) is 56 grams. But what is especially important for us has been established that 1 mole of any substance is always contains same number of molecules .

A mole is an amount of substance that contains 6 ´ 10 23 molecules of this substance.

In honor of the Italian scientist A. Avogadro, this number ( N) is called Avogadro's constant or Avogadro's number .

From the formula it follows that if q=F, That . This means that when a charge equal to 96,400 coulombs passes through the electrolyte, grams of any substance will be released. In other words, to release one mole of a monovalent substance, a charge must flow through the electrolyte q=F pendants. But we know that any mole of a substance contains the same number of molecules - N=6x10 23. This allows us to calculate the charge of one ion of a monovalent substance - the elementary electric charge - the charge of one (!) electron:

Applications of Electrolysis

Electrolytic method for obtaining pure metals (refining, refining). Electrolysis accompanied by dissolution of the anode

A good example is the electrolytic purification (refining) of copper. Copper obtained directly from the ore is cast into plates and placed as an anode in a CuSO 4 solution. By selecting the voltage on the electrodes of the bath (0.20-0.25 V), it is possible to ensure that only metallic copper is released at the cathode. In this case, foreign impurities either go into solution (without being released at the cathode) or fall to the bottom of the bath in the form of sediment (“anode sludge”). The cations of the anode substance combine with the SO 4 2- anion, and at this voltage only metallic copper is released at the cathode. The anode seems to “dissolve”. This purification allows us to achieve a purity of 99.99% (“four nines”). Precious metals (gold Au, silver Ag) are also purified similarly (refining).

Currently, all aluminum (Al) is mined electrolytically (from bauxite melt).

Electroplating

Electroplating – a field of applied electrochemistry that deals with the processes of applying metal coatings to the surface of both metal and non-metal products when a direct electric current passes through solutions of their salts. Electroplating technology is divided into electroplating And galvanoplasty .

Electrolysis can be used to coat metal objects with a layer of another metal. This process is called electroplating. Of particular technical importance are coatings with hard-to-oxidize metals, in particular nickel and chromium plating, as well as silver and gold plating, which are often used to protect metals from corrosion. To obtain the desired coatings, the object is thoroughly cleaned, well degreased and placed as a cathode in an electrolytic bath containing the salt of the metal with which the object is desired to be coated. For a more uniform coating, it is useful to use two plates as an anode, placing the object between them.

Also, through electrolysis, you can not only coat objects with a layer of one metal or another, but also make their relief metal copies (for example, coins, medals). This process was invented by the Russian physicist and electrical engineer, member of the Russian Academy of Sciences Boris Semenovich Jacobi (1801-1874) in the forties of the 19th century and is called electroplating . To make a relief copy of an object, a cast is first made from some plastic material, such as wax. This cast is rubbed with graphite and immersed in an electrolytic bath as a cathode, where a layer of metal is deposited on it. This is used in printing in the production of printed forms.

In addition to those mentioned above, electrolysis has found application in other areas:

Obtaining oxide protective films on metals (anodizing);

Electrochemical surface treatment of a metal product (polishing);

Electrochemical painting of metals (for example, copper, brass, zinc, chromium, etc.);

Water purification is the removal of soluble impurities from it. The result is so-called soft water (its properties are similar to distilled water);

Electrochemical sharpening of cutting instruments (for example, surgical knives, razors, etc.).

List of used literature:

1. Gurevich A. E. “Physics. Electromagnetic phenomena. 8th grade" Moscow, Publishing house "Drofa". 1999

2. Gabrielyan O. S. “Chemistry. 8th grade" Moscow, Publishing house "Drofa". 1997

3. “Elementary textbook of physics edited by academician G. S. Landsberg - Volume II - electricity and magnetism.” Moscow, “Science” 1972.

4. Eric M. Rogers. "Physics for the Inquiring Mind (the methods, nature and philosophy of physical science)". "Princeton University press" 1966. Volume III - electricity and magnetism. Translation Moscow, “World” 1971.

5. A. N. Remizov “Course of physics, electronics and cybernetics for medical institutes.” Moscow, "Higher School" 1982.

Liquids, like any other substances, can be conductors, semiconductors and dielectrics. For example, distilled water will be a dielectric, and solutions and melts of electrolytes will be conductors. Semiconductors will be, for example, molten selenium or sulfide melts.

Ionic conductivity

Electrolytic dissociation is the process of decomposition of electrolyte molecules into ions under the influence of the electric field of polar water molecules. The degree of dissociation is the proportion of molecules that have broken up into ions in a dissolved substance.

The degree of dissociation will depend on various factors: temperature, solution concentration, solvent properties. As the temperature increases, the degree of dissociation will also increase.

After the molecules are separated into ions, they move randomly. In this case, two ions of different signs can recombine, that is, they can again combine into neutral molecules. In the absence of external changes in the solution, dynamic equilibrium should be established. With it, the number of molecules that broke up into ions per unit time will be equal to the number of molecules that will unite again.

Charge carriers in aqueous solutions and melts of electrolytes will be ions. If a vessel with a solution or melt is connected to a circuit, then positively charged ions will begin to move towards the cathode, and negative ones - towards the anode. As a result of this movement, an electric current will arise. This type of conductivity is called ionic conductivity.

In addition to ionic conductivity in liquids, it can also have electronic conductivity. This type of conductivity is characteristic, for example, of liquid metals. As noted above, with ionic conduction, the passage of current is associated with the transfer of matter.

Electrolysis

Substances that are part of electrolytes will settle on the electrodes. This process is called electrolysis. Electrolysis is the process of releasing a substance at an electrode associated with redox reactions.

Electrolysis has found wide application in physics and technology. Using electrolysis, the surface of one metal is coated with a thin layer of another metal. For example, chrome and nickel plating.

Using electrolysis, you can make a copy from a relief surface. To do this, it is necessary that the layer of metal that settles on the surface of the electrode can be easily removed. To achieve this, graphite is sometimes applied to the surface.

The process of obtaining such easily peelable coatings is called electroplating. This method was developed by the Russian scientist Boris Jacobi when making hollow figures for St. Isaac's Cathedral in St. Petersburg.