Report on the topic:

Electricity

in liquids

(electrolytes)

Electrolysis

Faraday's laws

elementary electric charge

pupils 8 th class « B »

L oginova M arias BUT ndreevny

Moscow 2003

School No. 91

Introduction

A lot of things in our life are connected with the electrical conductivity of solutions of salts in water (electrolytes). From the first heartbeat (“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 poisonous vapors, aluminum is obtained by electrolysis from bauxite melted at a huge temperature - the “winged” metal for airplanes and cans for Fanta. Everything around - from a chrome-plated radiator grill of a foreign car to a silver-plated earring in the ear - has ever encountered a solution or molten salt, and therefore an electric current in liquids. No wonder this phenomenon is studied by a whole science - electrochemistry. But now we are more interested physical foundations this phenomenon.

electric current in solution. electrolytes

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

The ordered movement of charged particles is called electric current.

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

then we will make sure that the ammeter needle deviates - current flows through the solution! What are the charged particles in 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 dissolves in water. When dissolved in water, the CuSO 4 molecule decomposes (dissociates) into two differently charged ions - Cu 2+ and SO 4 2-. Simplistically, the ongoing processes can be reflected the following formula:

CuSO 4 ÞCu 2+ +SO 4 2-

Conduct electric current solutions of salts, alkalis, acids.

Substances whose solutions conduct electricity are called electrolytes.

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

Substances whose solutions do not conduct electricity are called non-electrolytes.

Electrolytic dissociation

The process by which an electrolyte breaks 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 electrolyte with water and believed that free ions were present in solutions. In contrast to him, 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, chemical interaction solute with water, which leads to the formation of hydrates, and then they dissociate into ions. They believed that in solutions there are not free, not "bare" ions, but hydrated ones, that is, "dressed in a fur coat" of water molecules. Therefore, 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 decay into hydrated ions

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

- Strong electrolytes- those that, upon dissolution, almost completely dissociate.

Their value of the degree of dissociation tends to unity.

- Weak electrolytes- those that, when dissolved, almost do not dissociate. 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

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

cathode and anode. Cations and anions

But what happens to the ions under the influence of an electric current?

Let's go back to our device:

In solution, CuSO 4 dissociated into ions - Cu 2+ and SO 4 2-. positively charged ion Cu2+ (cation) 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 through the current solution, then it is easy to notice a red-red 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, one can accurately determine the mass of the deposited metal. 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 factor, also called electrochemical equivalent .

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

there is a charge.

So, the mass of the substance released at the electrode is proportional to the charge, or the amount of electricity that has 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

And what is the electrochemical equivalent and what does it depend on? This question was also answered by Michael Faraday.

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. During the electrolysis of a salt of another metal, 1 pendant releases a different amount of this 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:

Substance	K in mg/k
Ag (silver)
H (hydrogen)

From the table we see that the electrochemical equivalents of various substances are significantly different from each other. On what properties of a substance does the value of its electrochemical equivalent depend? The answer to this question is 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 - valency

A - atomic weight

- is called the chemical equivalent of this substance

- coefficient of proportionality, 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, then we find that it is equal to 1.037´10 -5 g/k.

Combining the first and second Faraday's 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 kg/g.

A mole and the 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 - mole. To measure one mole of a substance, you need to take as many grams of it, what is the relative molecular mass his.

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, it has been established that 1 mole of any substance is always contains the same number of molecules .

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

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

From the formula it follows that if q=F, then . This means that when a charge equal to 96400 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 its 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:

Application of electrolysis

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

good example is the electrolytic purification (refining) of copper. Copper obtained directly from the ore is cast in the form of plates and placed as an anode in a CuSO 4 solution. By selecting the voltage on the electrodes of the bath (0.20-0.25V), it is possible to ensure that only metallic copper is released on the cathode. In this case, foreign impurities either go into solution (without precipitation at the cathode) or fall to the bottom of the bath in the form of a precipitate (“anode sludge”). The cations of the anode substance combine with the SO 4 2- anion, and only metallic copper is released on the cathode at this voltage. The anode, as it were, "dissolves". Such purification allows achieving a purity of 99.99% (“four nines”). Similarly (refining) purify and precious metals(gold Au, silver Ag).

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

Electroplating

Electroplating - the field of applied electrochemistry, which 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 is divided into electroplating and electroplating .

Through electrolysis, it is possible to cover metal objects with a layer of another metal. This process is called electroplating. Of particular technical importance are coatings with metals that are difficult to oxidize, 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 a salt of the metal with which they want to cover the object. For a more uniform coating, it is useful to use two plates as an anode, placing an object between them.

Also, by means of electrolysis, it is possible not only to cover objects with a layer of one or another metal, but also to make their relief metal copies (for example, coins, medals). This process was invented by a Russian physicist and electrical engineer, a member of Russian Academy Sciences Boris Semenovich Jacobi (1801-1874) in the forties of the XIX century and is called electroplating . To make a relief copy of an object, an impression is first made of some plastic material, such as wax. This impression 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 the printing industry in the manufacture of printing forms.

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

Obtaining oxide protective films on metals (anodizing);

Electrochemical surface treatment of a metal product (polishing);

Electrochemical coloring 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 (approaching distilled water in its properties);

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

List of used literature:

1. Gurevich A. E. “Physics. Electromagnetic Phenomena. Grade 8, Moscow, Drofa Publishing House. 1999

2. Gabrielyan O. S. “Chemistry. Grade 8, Moscow, Drofa Publishing House. 1997

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

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

5. A. N. Remizov "Course of Physics, Electronics and Cybernetics for Medical Institutes". Moscow, " graduate School» 1982 year.

Liquids, like solid bodies, can be conductors, semiconductors and dielectrics. In this lesson, we will focus on liquid conductors. And not about liquids with electronic conductivity (molten metals), but about liquid conductors of the second kind (solutions and melts of salts, acids, bases). The type of conductivity of such conductors is ionic.

Definition. Conductors of the second kind are those conductors in which chemical processes occur when current flows.

For a better understanding of the process of current conduction in liquids, the following experiment can be presented: Two electrodes connected to a current source were placed in a bath of water, a light bulb can be taken as a current indicator in the circuit. If you close such a circuit, the lamp will not burn, which means there is no current, which means that there is a break in the circuit, and the water itself does not conduct current. But if you put a certain amount of salt in the bathroom and repeat the circuit, the light will turn on. This means that free charge carriers began to move in the bath between the cathode and the anode, this case ions (Fig. 1).

Rice. 1. Scheme of experience

Conductivity of electrolytes

Where do the free charges come from in the second case? As mentioned in one of the previous lessons, some dielectrics are polar. Water has just the same polar molecules (Fig. 2).

Rice. 2. Polarity of the water molecule

When salt is added to water, water molecules are oriented in such a way that their negative poles are near sodium, positive - near chlorine. As a result of interactions between charges, water molecules break salt molecules into pairs of opposite ions. The sodium ion has a positive charge, the chlorine ion has a negative charge (Fig. 3). It is these ions that will move between the electrodes under the action of electric field.

Rice. 3. Scheme of formation of free ions

When sodium ions approach the cathode, it receives its missing electrons, while chloride ions give up theirs when they reach the anode.

Electrolysis

Since the flow of current in liquids is associated with the transfer of matter, with such a current, the process of electrolysis takes place.

Definition. Electrolysis is a process associated with redox reactions in which a substance is released at the electrodes.

Substances that, as a result of such splitting, provide ionic conductivity are called electrolytes. This name was proposed by the English physicist Michael Faraday (Fig. 4).

Electrolysis makes it possible to obtain substances in a sufficiently pure form from solutions, therefore it is used to obtain rare materials, such as sodium, calcium ... in its pure form. This is what is known as electrolytic metallurgy.

Faraday's laws

In the first work on electrolysis in 1833, Faraday presented his two laws of electrolysis. In the first one, it was about the mass of the substance released on the electrodes:

Faraday's first law states that this mass is proportional to the charge passed through the electrolyte:

Here the role of the coefficient of proportionality is played by the quantity - the electrochemical equivalent. This is a tabular value that is unique for each electrolyte and is its main characteristic. Dimension of the electrochemical equivalent:

The physical meaning of the electrochemical equivalent is the mass released on the electrode when the amount of electricity in 1 C passes through the electrolyte.

If you recall the formulas from the topic of direct current:

Then we can represent Faraday's first law in the form:

Faraday's second law directly concerns the measurement of the electrochemical equivalent through other constants for a particular electrolyte:

Here: - molar mass electrolyte; - elementary charge; - electrolyte valency; is Avogadro's number.

The value is called the chemical equivalent of the electrolyte. That is, in order to know the electrochemical equivalent, it is enough to know the chemical equivalent, the remaining components of the formula are world constants.

Based on Faraday's second law, the first law can be represented as:

Faraday proposed the terminology of these ions on the basis of the electrode to which they move. Positive ions are called cations because they move towards the negatively charged cathode, negative charges are called anions as they move towards the anode.

The above action of water to break a molecule into two ions is called electrolytic dissociation.

In addition to solutions, melts can also be conductors of the second kind. In this case, the presence of free ions is achieved by the fact that very active molecular movements and vibrations begin at a high temperature, as a result of which the molecules break down into ions.

Practical application of electrolysis

First practical use electrolysis occurred in 1838 by the Russian scientist Jacobi. With the help of electrolysis, he received an impression of figures for St. Isaac's Cathedral. This application of electrolysis is called electroplating. Another area of ​​application is electroplating - covering one metal with another (chrome plating, nickel plating, gilding, etc., Fig. 5)

Gendenstein L.E., Dick Yu.I. Physics grade 10. - M.: Ileksa, 2005.

Myakishev G.Ya., Sinyakov A.Z., Slobodskov B.A. Physics. Electrodynamics. - M.: 2010.

1. Fatyf.narod.ru ().
2. ChemiK ().
3. Ens.tpu.ru ().

Homework

1. What are electrolytes?
2. What are the two fundamental different types liquids in which an electric current can flow?
3. What are the possible mechanisms for the formation of free charge carriers?
4. *Why is the mass released on the electrode proportional to the charge?

Electric current in gases

Charge carriers: electrons, positive ions, negative ions.

Charge carriers arise in the gas as a result of ionization: due to irradiation of the gas, or collisions of heated gas particles with each other.

Ionization by electron impact.

A_(fields)=eEl

e=1.6\cdot 10^(19)Cl ;

E - field direction;

l is the mean free path between two successive collisions of an electron with gas atoms.

A_(fields)=eEl\geq W - ionization condition

W is the ionization energy, i.e. the energy required to pull an electron out of an atom

The number of electrons increases in geometric progression, resulting in an electron avalanche, and hence a discharge in the gas.

Electric current in liquid

Liquids, like solids, can be dielectrics, conductors, and semiconductors. Dielectrics include distilled water, conductors include electrolyte solutions: acids, alkalis, salts and metal melts. Liquid semiconductors are molten selenium, sulfide melts.

Electrolytic dissociation

When electrolytes are dissolved under the influence of the electric field of polar water molecules, electrolyte molecules decompose into ions. For example, CuSO_(4)\rightarrow Cu^(2+)+SO^(2-)_(4).

Along with dissociation, there is a reverse process - recombination , i.e. association of ions of opposite signs into neutral molecules.

The carriers of electricity in electrolyte solutions are ions. This conduction is called ionic .

Electrolysis

If electrodes are placed in a bath with an electrolyte solution and a current is turned on, then negative ions will move to the positive electrode, and positive ions to the negative one.

At the anode (positive electrode), negatively charged ions donate extra electrons (oxidative reaction), and at the cathode (negative electrode), positive ions receive the missing electrons (reduction reaction).

Definition. The process of release of substances on the electrodes associated with redox reactions is called electrolysis.

Faraday's laws

I. The mass of the substance that is released on the electrode is directly proportional to the charge that has flowed through the electrolyte:

m=kq

k is the electrochemical equivalent of a substance.

q=I\Delta t , then

m=kI\Delta t

k=\frac(1)(F)\frac(\mu)(n)

\frac(\mu)(n) - chemical equivalent of a substance;

\mu - molar mass;

n - valency

The electrochemical equivalents of substances are proportional to the chemical equivalents.

F - Faraday's constant;

Everyone is familiar with the definition of electric current. It is represented as a directed motion of charged particles. Such a movement in various environments has fundamental differences. As a basic example of this phenomenon, one can imagine the flow and propagation of electric current in liquids. Such phenomena are characterized by different properties and are seriously different from the ordered movement of charged particles, which occurs under normal conditions not under the influence of various liquids.

Figure 1. Electric current in liquids. Author24 - online exchange of student papers

Formation of electric current in liquids

Despite the fact that the process of conduction of electric current is carried out by means of metal devices (conductors), the current in liquids depends on the movement of charged ions that have acquired or lost such atoms and molecules for some specific reason. An indicator of such a movement is a change in the properties of a certain substance, where the ions pass. Thus, it is necessary to rely on the basic definition of electric current in order to form a specific concept of the formation of current in various liquids. It is determined that the decomposition of negatively charged ions contributes to the movement to the region of the current source with positive values. Positively charged ions in such processes will move in the opposite direction - to a negative current source.

Liquid conductors are divided into three main types:

• semiconductors;
• dielectrics;
• conductors.

Definition 1

Electrolytic dissociation is the process of decomposition of molecules of a certain solution into negative and positive charged ions.

It can be established that an electric current in liquids can occur after a change in the composition and chemical property liquids used. This completely contradicts the theory of the propagation of electric current in other ways when using a conventional metal conductor.

Faraday's experiments and electrolysis

The flow of electric current in liquids is a product of the movement of charged ions. The problems associated with the emergence and propagation of electric current in liquids led to the study of the famous scientist Michael Faraday. With the help of numerous practical studies, he was able to find evidence that the mass of a substance released during electrolysis depends on the amount of time and electricity. In this case, the time during which the experiments were carried out is important.

The scientist was also able to find out that in the process of electrolysis, when a certain amount of a substance is released, the same amount is needed. electric charges. This quantity was accurately established and fixed in a constant value, which was called the Faraday number.

In liquids, electric current has different propagation conditions. It interacts with water molecules. They significantly impede all movement of ions, which was not observed in experiments using a conventional metal conductor. It follows from this that the generation of current during electrolytic reactions will not be so large. However, as the temperature of the solution increases, the conductivity gradually increases. This means that the voltage of the electric current is increasing. Also in the process of electrolysis, it has been observed that the probability of a particular molecule decomposing into negative or positive ion charges increases due to the large number of molecules of the substance or solvent used. When the solution is saturated with ions in excess of a certain norm, the reverse process occurs. The conductivity of the solution begins to decrease again.

Currently, the electrolysis process has found its application in many fields and fields of science and in production. Industrial enterprises use it in the production or processing of metal. Electrochemical reactions are involved in:

• salt electrolysis;
• electroplating;
• surface polishing;
• other redox processes.

Electric current in vacuum and liquids

The propagation of electric current in liquids and other media is a rather complex process that has its own characteristics, features and properties. The fact is that in such media there are completely no charges in the bodies, therefore they are usually called dielectrics. The main goal of the research was to create conditions under which atoms and molecules could begin to move and the process of generating an electric current began. For this, it is customary to use special mechanisms or devices. The main element of such modular devices are conductors in the form of metal plates.

To determine the main parameters of the current, it is necessary to use known theories and formulas. The most common is Ohm's law. It acts as a universal ampere characteristic, where the principle of current-voltage dependence is implemented. Recall that voltage is measured in units of amperes.

For experiments with water and salt, it is necessary to prepare a vessel with salt water. This will give a practical and visual representation of the processes that occur when an electric current is generated in liquids. Also, the installation should contain rectangular electrodes and power supplies. For full-scale preparation for experiments, you need to have an ampere installation. It will help conduct energy from the power supply to the electrodes.

Metal plates will act as conductors. They are dipped into the liquid used, and then the voltage is connected. The movement of particles begins immediately. It runs randomly. When magnetic field between the conductors, all the processes of particle movement are ordered.

The ions begin to change charges and combine. Thus cathodes become anodes and anodes become cathodes. In this process, there are also several other important factors to consider:

• dissociation level;
• temperature;
• electrical resistance;
• use of alternating or direct current.

At the end of the experiment, a layer of salt is formed on the plates.

Water as a universal solvent.. Aqueous solutions.. Electrolytic dissociation.. Electrolyte.. Weak and strong electrolytes.. Carriers of electric charges in liquids.. Positive and negative ions.. Electrolysis.. Melts.. Nature of electric current in melts..

One of the conditions for the occurrence of an electric current is the presence of free charges capable of moving under the action of an electric field. We talked about the nature of electric current in metals and.
In this lesson, we will try to figure out what particles carry electric charge in liquids and melts.

Water as a universal solvent

As we know, distilled water does not contain charge carriers and therefore does not conduct electric current, that is, it is a dielectric. However, the presence of any impurities already makes water a fairly good conductor.
Water has the phenomenal ability to dissolve almost everything in itself. chemical elements. When various substances (acids, alkalis, bases, salts, etc.) are dissolved in water, the solution becomes a conductor due to the breakdown of substance molecules into ions. This phenomenon is called electrolytic dissociation, and the solution itself is an electrolyte capable of conducting an electric current. All water basins on Earth are, to a greater or lesser extent, natural electrolytes.

The world ocean is a solution of ions of almost all elements of the periodic table.

Gastric juice, blood, lymph, all fluids in the human body are electrolytes. All animals and plants are also primarily composed of electrolytes.

According to the degree of dissociation, there are weak and strong electrolytes. Water belongs to weak electrolytes, and most inorganic acids belong to strong electrolytes. Electrolytes are also called conductors of the second kind.

Carriers of electric charges in a liquid

When dissolved in water (or other liquid) of various substances, they decompose into ions.
For example, common table salt NaCl (sodium chloride) in water separates into positive sodium ions (Na +) and negative chloride ions (Cl -). If the two poles in the resulting electrolyte are at different potentials, then the negative ions drift towards the positive pole while the positive ions drift towards the negative pole.

Thus, the electric current in a liquid consists of flows of positive and negative ions directed towards each other.

While absolutely pure water is an insulator, water containing even small impurities (natural or introduced from outside) ionized substance, is a conductor of electric current.

Electrolysis

Since the positive and negative ions of the solute drift in different directions under the influence of the electric field, the substance gradually separates into two parts.

This separation of matter into its constituent elements is called electrolysis.

Electrolytes are used in electrochemistry, in chemical current sources ( galvanic cells and batteries), in the production processes of electroplating and other technologies based on the movement of electric charges in liquids under the influence of an electric field.

melts

The dissociation of a substance is possible without the participation of water. Enough to melt the crystals chemical composition substances and get a melt. Melts of matter, like aqueous electrolytes, are conductors of the second kind, and therefore they can be called electrolytes. The electric current in melts has the same nature as the current in aqueous electrolytes - these are counter flows of positive and negative ions.

Using melts, in metallurgy, aluminum is obtained electrolytically from alumina. An electric current is passed through aluminum oxide and during electrolysis, pure aluminum accumulates at one of the electrodes (cathode). This is a very energy-intensive process, which, in terms of energy consumption, resembles the decomposition of water into hydrogen and oxygen using electric current.

In the aluminum electrolysis shop