According to the molecular kinetic theory (MKT), all substances consist of the smallest particles - molecules. Molecules are in constant motion and interact with each other.

MKT is substantiated by numerous experiments and a huge number of physical phenomena. Let's look at its three main points.

All substances are made up of particles.

1) All substances consist of the smallest particles: molecules, atoms, ions, etc., separated by gaps.

Molecule- the smallest stable particle of a substance that retains its basic chemical properties.

The molecules that make up a given substance are exactly the same; different substances are made up of different molecules. In nature, there is an extremely large number of different molecules.

Molecules are made up of smaller particles called atoms.

atoms- the smallest particles of a chemical element that retain its chemical properties.

The number of different atoms is relatively small and equal to the number chemical elements(116) and their isotopes (about 1500).

Atoms are very complex formations, but the classical MKT uses the model of atoms in the form of solid indivisible particles of a spherical shape.

The presence of gaps between molecules follows, for example, from experiments on the displacement of various liquids: the volume of a mixture is always less than the sum of the volumes of mixed liquids. The phenomena of permeability, compressibility and solubility of substances also indicate that they are not continuous, but consist of individual particles separated by intervals.

By using modern methods studies (electron and probe microscopes) managed to obtain images of molecules.

*Law of multiple ratios

The existence of molecules is brilliantly confirmed by the law of multiple ratios. It says: "when different compounds (substances) are formed from two elements, the masses of one of the elements in different compounds are related as integers, i.e. they are in multiple ratios." For example, nitrogen and oxygen give five compounds: N 2 O, N 2 O 2, N 2 O 3, N 2 O 4, N 2 O 5. In them, with the same amount of nitrogen, oxygen enters into a compound in quantities that are in multiple ratios of 1:2:3:4:5. The law of multiple ratios is easy to explain. Every substance is made up of identical molecules having the corresponding atomic composition. Since all molecules of a given substance are the same, the ratio of the weight quantities of simple elements that make up the whole body is the same as in a single molecule, and, therefore, is a multiple of atomic weights, which is confirmed by experience.

Mass of molecules

Determine the mass of the molecule in the usual way, i.e. weighing, of course, is impossible. She's too small for that. Currently, there are many methods for determining the masses of molecules, in particular, using a mass spectrograph, the masses m 0 of all atoms of the periodic table.

So, for the carbon isotope \(~^(12)_6C\) m 0 \u003d 1.995 10 -26 kg. Since the masses of atoms and molecules are extremely small, in calculations, not absolute, but relative mass values ​​are usually used, obtained by comparing the masses of atoms and molecules with the atomic mass unit, which is chosen as \(~\dfrac(1)(12)\) part of the mass of an atom of the carbon isotope \(~^(12)_6C\):

1 amu = 1/12 m 0C = 1.660 10 -27 kg.

Relative molecular(or atomic) weight M r is a value showing how many times the mass of a molecule (or atom) is greater than the atomic mass unit:

\(~M_r = \dfrac(m_0)(\dfrac(1)(12) \cdot m_(0C)) . \qquad (1)\)

Relative molecular (atomic) mass is a dimensionless quantity.

The relative atomic masses of all chemical elements are indicated in the periodic table. So, for hydrogen it is 1.008, for helium - 4.0026. In calculations, the relative atomic mass is rounded to the nearest whole number. For example, hydrogen has up to 1, helium has up to 4.

The relative molecular weight of a given substance is equal to the sum of the relative atomic masses of the elements that make up the molecule of this substance. It is calculated using the periodic table and the chemical formula of the substance.

Yes, for water. H 2 O relative molecular weight is M r = 1 2 + 16 = 18.

The amount of substance. Avogadro constant

The amount of matter contained in a body is determined by the number of molecules (or atoms) in that body. Since the number of molecules in macroscopic bodies is very large, to determine the amount of matter in the body, the number of molecules in it is compared with the number of atoms in 0.012 kg of the carbon isotope \(~^(12)_6C\).

Amount of substance ν - a value equal to the ratio of the number of molecules (atoms) N in a given body to the number of atoms N A in 0.012 kg of carbon isotope \(~^(12)_6C\):

\(~\nu = \dfrac(N)(N_A) . \qquad (2)\)

In SI, the unit of quantity of a substance is the mole. 1 mol- the amount of a substance that contains the same number of structural elements (atoms, molecules, ions) as there are atoms in 0.012 kg of the carbon isotope \(~^(12)_6C\).

The number of particles in one mole of a substance is called constant Avogadro.

\(~N_A = \dfrac(0.012)(m_(0C))= \dfrac(0.012)(1.995 \cdot 10^(-26))\) = 6.02 10 23 mol -1 . (3)

Thus, 1 mole of any substance contains the same number of particles - N A particles. Since the mass m 0 particles different substances different, then the mass N A particles y various substances different.

The mass of a substance taken in an amount of 1 mol is called molar mass M:

\(~M = m_0 N_A . \qquad (4)\)

The SI unit of molar mass is the kilogram per mole (kg/mol).

between molar mass Μ and relative molecular weight M r there is the following relation:

\(~M = M_r \cdot 10^(-3) .\)

So, the molecular weight of carbon dioxide is 44, the molar mass is 44 10 -3 kg / mol.

Knowing the mass of a substance and its molar mass M, you can find the number of moles (amount of substance) in the body\[~\nu = \dfrac(m)(M)\].

Then from formula (2) the number of particles in the body

\(~N = \nu N_A = \dfrac(m)(M) N_A .\)

Knowing the molar mass and Avogadro's constant, we can calculate the mass of one molecule:

\(~m_0 = \dfrac(M)(N_A) = \dfrac(m)(N) .\)

Molecule sizes

The size of a molecule is a conditional value. It is valued like this. Between the molecules, along with the forces of attraction, repulsive forces also act, so the molecules can only approach each other up to a certain distance. d(Fig. 1).

The distance of the closest approach of the centers of two molecules is called effective diameter molecules d(in this case, it is assumed that the molecules have a spherical shape).

The sizes of molecules of various substances are not the same, but they are all about 10 -10 m, i.e. very small.

see also

1. Kikoin A.K. Mass and Quantity of Substance, or About One “Mistake” of Newton // Kvant. - 1984. - No. 10. - S. 26-27
2. Kikoin A.K. A simple method for determining the size of molecules // Kvant. - 1983. - No. 9. - C.29-30

Molecules move randomly

2) Molecules are in continuous random (thermal) motion.

The type of thermal motion (translational, vibrational, rotational) of molecules depends on the nature of their interaction and changes during the transition of a substance from one state of aggregation to another. The intensity of thermal motion also depends on body temperature.

Here are some of the proofs of the random (chaotic) movement of molecules: a) the desire of a gas to occupy the entire volume provided to it; b) diffusion; c) Brownian motion.

Diffusion

Diffusion- spontaneous mutual penetration of molecules of adjoining substances, leading to the equalization of the concentration of the substance throughout the volume. During diffusion, the molecules of adjoining bodies, being in continuous motion, penetrate into the intermolecular gaps of each other and are distributed between them.

Diffusion manifests itself in all bodies - in gases, liquids, solids, but to varying degrees.

Diffusion in gases can be detected if, for example, a vessel with an odorous gas is opened indoors. After a while, the gas will spread throughout the room.

Diffusion in liquids is much slower than in gases. For example, if you first pour a layer of copper sulfate solution into a glass, and then very carefully add a layer of water and leave the glass in a room with a constant temperature, then after a while the sharp boundary between the copper sulfate solution and water will disappear, and after a few days the liquids will mix.

Diffusion in solids occurs even more slowly than in liquids (from several hours to several years). It can be observed only in well-polished bodies, when the distances between the surfaces of the polished bodies are close to the intermolecular distance (10 -8 cm). In this case, the diffusion rate increases with increasing temperature and pressure.

Diffusion plays an important role in nature and technology. In nature, thanks to diffusion, for example, plants are nourished from the soil. The human and animal body absorbs nutrients through the walls of the digestive tract. In technology, with the help of diffusion, for example, the surface layer of metal products is saturated with carbon (cementation), etc.

• A type of diffusion is osmosis- penetration of liquids and solutions through a porous semi-permeable partition.

Brownian motion

Brownian motion was discovered in 1827 by the English botanist R. Brown, theoretical substantiation from the point of view of MKT was given in 1905 by A. Einstein and M. Smoluchowski.

Brownian motion- this is a random movement of the smallest solid particles "suspended" in liquids (gases).

"Suspended" particles are particles whose substance density is comparable to the density of the medium in which they are located. Such particles are in equilibrium, and the slightest external influence on it leads to their movement.

Brownian motion is characterized by the following:

The causes of Brownian motion are:

1. thermal chaotic motion of the molecules of the medium in which the Brownian particle is located;
2. the absence of full compensation for the impacts of the molecules of the medium on this particle from different sides, since the movement of the molecules is random.

Moving liquid molecules, when colliding with any solid particles, transfer them a certain amount of motion. Accidentally, on one side, the particle will hit noticeably more molecules than with the other, and the particle will move.

• If the particle is large enough, then the number of molecules attacking it from all sides is extremely large, their impacts are compensated at any given moment, and such a particle practically remains motionless.

see also

1. Bronstein M.P. How the atom was weighed // Kvant. - 1970. - No. 2. - S. 26-35

Particles interact

3) Particles in a substance are connected to each other by forces of molecular interaction - attraction and repulsion.

Attractive and repulsive forces act simultaneously between the molecules of a substance. These forces are largely dependent on the distances between molecules. According to experimental and theoretical research intermolecular forces of interaction are inversely proportional n th degree of distance between molecules:

\(~F_r \sim \pm \dfrac(1)(r^n),\)

where for the forces of attraction n= 7, and for the repulsive forces n= 9 ÷ 15. Thus, the repulsion force changes more with distance.

There are both attractive and repulsive forces between molecules. There is some distance r 0 between molecules, on which the repulsive forces are equal in absolute value to the forces of attraction. This distance corresponds to the stable equilibrium position of the molecules.

With increasing distance r between molecules, both the attractive and repulsive forces decrease, with the repulsive forces decreasing faster and becoming less than the attractive forces. The resultant force (attraction and repulsion) tends to bring the molecules closer to their original state. But starting from some distance r m , the interaction of molecules becomes so small that it can be neglected. longest distance r m , on which the molecules still interact, is called radius molecular action (r m ~ 1.57 10 -9 m).

As the distance decreases r between molecules, both the attractive and repulsive forces increase, and the repulsive forces increase faster and become larger than the attractive forces. The resultant force now tends to push the molecules away from each other.

Evidence of the force interaction of molecules:

a) deformation of bodies under the influence of force;

b) preservation of the form by solid bodies (attractive forces);

c) the presence of gaps between molecules (repulsive forces).

*Projection chart of interaction forces

The interaction of two molecules can be described using the plot of the projection of the resultant F r forces of attraction and repulsion of molecules from a distance r between their centers. Let's direct the axis r from a molecule 2 , the center of which coincides with the origin of coordinates, to the distance from it r 1 center of the molecule 2 (Fig. 3, a).

The difference in the structure of gases, liquids and solids

In various aggregate states of a substance, the distance between its molecules is different. Hence the difference in the force interaction of molecules and the essential difference in the nature of the motion of the molecules of gases, liquids and solids.

AT gases the distances between molecules are several times greater than the dimensions of the molecules themselves. As a result, the forces of interaction between gas molecules are small and kinetic energy thermal motion of molecules far exceeds the potential energy of their interaction. Each molecule moves freely from other molecules at enormous speeds (hundreds of meters per second), changing direction and velocity modulus when colliding with other molecules. Free path length λ gas molecules depends on the pressure and temperature of the gas. Under normal conditions λ ~ 10 -7 m.

AT solids the forces of interaction between molecules are so great that the kinetic energy of the movement of molecules is much less than the potential energy of their interaction. Molecules perform continuous vibrations with small amplitude around a certain constant equilibrium position - a node of the crystal lattice.

The time during which the particle oscillates around one equilibrium position, - time of "sedentary life" of a particle- in solids is very large. Therefore, solids retain their shape and they do not flow under normal conditions. The time of "sedentary life" of a molecule depends on temperature. Near the melting point, it is about 10–1 – 10–3 s; at lower temperatures, it can be hours, days, months.

AT liquids the distance between molecules is much smaller than in gases, and approximately the same as in solids. Therefore, the forces of interaction between molecules are large. The molecules of a liquid, like the molecules of a solid body, oscillate around a certain equilibrium position. But the kinetic energy of particle motion is commensurate with the potential energy of their interaction, and molecules more often move to new equilibrium positions (the “sedentary life” time is 10–10 – 10–12 s). This helps to explain the fluidity of the liquid.

see also

1. Kikoin A.K. On aggregate states of matter // Kvant. - 1984. - No. 9. - S. 20-21

Literature

Aksenovich L. A. Physics in high school: Theory. Tasks. Tests: Proc. allowance for institutions providing general. environments, education / L. A. Aksenovich, N. N. Rakina, K. S. Farino; Ed. K. S. Farino. - Minsk: Adukatsia i vykhavanne, 2004. - C. 119-126.

Lesson 1

Topic: The main provisions of the molecular kinetic theory and their experimental substantiation

Goals: to acquaint students with the main provisions of the molecular kinetic theory and their experimental confirmations, with the quantities that characterize molecules (the size and mass of molecules, the amount of substance, the Avogadro constant) and methods for measuring them; develop attention, logical thinking students, to cultivate a conscientious attitude towards educational work

Lesson type: a lesson in learning new knowledge

During the classes

Organizing time

Setting the goal of the lesson

Presentation of new material

Molecular-kinetic theory originated in the 19th century. in order to explain the structure and properties of matter based on the idea that matter consists of tiny particles - molecules that are constantly moving and interacting with each other. This theory achieved particular success in explaining the properties of gases.

Molecular Kinetic Theory called the doctrine that explains the structure and properties of bodies by the movement and interaction of the particles that make up

body.

The ICT is based on three key principles:

all substances are made up of molecules;

molecules are in continuous chaotic motion;

molecules interact with each other.

The assumption about the molecular structure of the substance was confirmed only indirectly. The main provisions of the MCT of gases were in good agreement with experiment. Today, technology has reached a level at which it is possible to consider even individual atoms. It is quite easy to verify the existence of molecules and estimate their size.

Place a drop of oil on the surface of the water. The oil stain will spread over the surface of the water, but the area of ​​the oil film cannot exceed a certain value. It is natural to assume that the maximum area of ​​the film corresponds to an oil layer one molecule thick.

It is quite simple to make sure that the molecules are moving: if you drop a drop of perfume at one end of the room, then in a few seconds this smell will spread throughout the room. In the air around us, molecules move at the speed of artillery shells - hundreds of meters per second. Amazing property the movement of molecules is that it never stops. In this, the movement of molecules differs significantly from the movement of objects surrounding us: after all, mechanical movement inevitably stops due to friction.

AT early XIX in. The English botanist Brown, observing pollen particles suspended in water through a microscope, noticed that these particles were in an “eternal dance”. The reason for the so-called "Brownian motion" was understood only 56 years after its discovery: individual impacts of liquid molecules on a particle do not compensate each other if this particle is small enough. Since then, Brownian motion has been regarded as a clear experimental confirmation of the motion of molecules.

If the molecules were not attracted to each other, there would be neither liquids nor solids - they would simply crumble into separate molecules. On the other hand, if the molecules were only attracted, they would turn into extremely dense clots, and the gas molecules, hitting the walls of the vessel, would stick to them. The interaction of molecules is electrical in nature. Although molecules are generally electrically neutral, the distribution of positive and negative electric charges they are such that at large distances (compared to the size of the molecules themselves) the molecules attract, and at short distances they repel. Try to break a steel or nylon thread with a diameter of 1 mm 2. It is unlikely that this will succeed, even if you make every effort, and in fact the efforts of your body are opposed by the forces of attraction of molecules in a small section of the thread.

Gas parameters associated with the individual characteristics of its constituent molecules are called microscopic parameters(mass of molecules, their speed, concentration).

The parameters that characterize the state of macroscopic bodies are called macroscopic parameters (volume, pressure, temperature).

The main task of the MKT is establish a relationship between the microscopic and macroscopic parameters of a substance, based on this, find the equation of state of a given substance.

For example, knowing the masses of molecules, their average velocities and concentrations, one can find the volume, pressure and temperature of a given mass of gas, as well as determine the gas pressure through its volume and temperature.

Usually, the construction of any theory is based on the method of models, which consists in the fact that instead of a real physical object or phenomenon, its simplified model is considered. The MKT of gases uses the ideal gas model.

From the point of view of molecular concepts, gases consist of atoms and molecules, the distances between which are much greater than their sizes. As a result, the forces of interaction between gas molecules are practically absent. Interaction between them actually occurs only during their collisions.

Since the interaction of molecules of an ideal gas is reduced to only short-term collisions and the sizes of molecules do not affect the pressure and temperature of the gas, we can assume that

The ideal gas is this is a gas model that neglects the size of molecules and their interaction; the molecules of such a gas are in free random motion, sometimes colliding with other molecules or the walls of the vessel in which they are located.

Real rarefied gases behave like an ideal gas.

An approximate estimate of the size of molecules can be obtained from experiments conducted by the German physicist Roentgen and the English physicist Rayleigh. A drop of oil spreads on the surface of the water, forming a thin film with a thickness of only one molecule. It is easy to determine the thickness of this layer and thus estimate the size of the oil molecule. Currently, there are a number of methods to determine the size of molecules and atoms. For example, the linear dimensions of oxygen molecules are 3 10 -10 m, water - about 2.6 10 -10 m. Thus, the characteristic length in the world of molecules is 10 -10 m. If a water molecule is increased to the size of an apple, then the apple itself will become the diameter of the globe.

In the last century, the Italian scientist Avogadro discovered amazing fact: if two different gases occupy vessels of the same volume at the same temperatures and pressures, then each vessel contains the same number of molecules. Note that the masses of gases in this case can vary greatly: for example, if there is hydrogen in one vessel and oxygen in the other, then the mass of oxygen is 16 times the mass of hydrogen.

It means. That some, and quite important, properties of a body are determined by the number of molecules in this body: the number of molecules turns out to be even more significant than the mass.

The physical quantity that determines the number of molecules in a given body is called amount of matter and is denoted. The unit of quantity of a substance is mol.

Since the masses of individual molecules differ from each other, the same amounts of different substances have different masses.

1 mol - is the amount of a substance that contains as many molecules as there are carbon atoms in 0.012 kg of carbon.

The masses of individual molecules are very small. Therefore, it is convenient to use not absolute, but relative mass values ​​in calculations. By international agreement, the masses of all atoms and molecules are compared with 1/12 of the mass of a carbon atom. The main reason for this choice is that carbon is included in a large number of different chemical compounds.

Relative molecular (or atomic) mass of substance M is the ratio of the mass of a molecule (or atom)m 0 given substance to 1 / 12 masses of a carbon atom:

M G =

m r - mass of a molecule of a given substance;

m a (C) is the mass of the carbon atom 12 C.

For example, the relative atomic weight of carbon is 12, of a water pipe is 1. The relative molecular weight of a water pipe is 2, since a hydrogen molecule consists of two atoms.

The convenience of choosing a mole as a unit for measuring the amount of a substance is due to the fact that the mass of one mole of a substance in grams is numerically equal to its relative molecular weight.

Masa m body is proportional to the amount of matter contained in this body. Therefore, the ratio characterizes the substance of which it is composed uh that body: the "heavier" the molecules of the substance, the greater this ratio.

The ratio of the mass of a substance m to the amount of matter calledmolar mass and is denoted by M:

M =

If we take =1 in this formula, we get that molar mass substance is numerically equal to the mass of one mole of this substance. For example, the mass of hydrogen is

2
= 2 10 -3
.

1
- unit of measure of molar mass in SI.

Mass of matter m = M .

The number N of molecules contained in the body is directly proportional to the number

the substance contained in that body.

The proportionality factor is a constant value and is calledconstant Avogadro N A

Whence it follows that the Avogadro constant is numerically equal to the number of molecules in 1 mole.

Main results.

Questions for students:

Prove that all bodies are made up of tiny particles.

Give facts showing the divisibility of substances.

What is the phenomenon of diffusion?

What is the essence of Brownian motion?

What facts prove that attractive and repulsive forces act between the molecules of solid and liquid bodies?

What is the relative atomic mass of oxygen? water molecules? Molecules of carbon dioxide?

4. Homework:

Molecular Kinetic Theory (MKT)- This is a branch of physics that studies the properties of various states of matter, based on the concept of the existence of molecules and atoms as the smallest particles of matter. MKT is based on three main points:

1 . All substances are made up of tiny particles: molecules, atoms or ions.
2 . These particles are in continuous chaotic motion, the speed of which determines the temperature of the substance.
3 . Between the particles there are forces of attraction and repulsion, the nature of which depends on the distance between them, i.e. particles interact with each other.

The main provisions of the MKT are confirmed by many experimental facts.

The existence of molecules, atoms and ions has been experimentally proven, the molecules have been sufficiently studied and photographed using electron microscopes.

The ability of gases to expand indefinitely and occupy the entire volume provided to them is explained by the continuous chaotic movement of molecules.

The elasticity of gases, solids and liquids, the ability of liquids to wet some solids, the processes of coloring, gluing, maintaining the shape of solids, and much more indicate the existence of forces of attraction and repulsion between molecules.

Phenomenon diffusion- the ability of the molecules of one substance to penetrate into the gaps between the molecules of another - also confirms the basic provisions of the MKT. The phenomenon of diffusion explains, for example, the spread of odors, the mixing of dissimilar liquids, the process of dissolving solids in liquids, the welding of metals by melting them or by pressure. A confirmation of the continuous chaotic motion of molecules is also Brownian motion- continuous chaotic movement of microscopic particles insoluble in liquid. The movement of Brownian particles is explained by the chaotic movement of fluid particles that collide with microscopic particles and set them in motion. It has been experimentally proved that the speed of Brownian particles depends on the temperature of the liquid. The theory of Brownian motion was developed by A. Einstein.

Any substance consists of particles, so the amount of substance ν considered to be proportional to the number of particles contained in the body. The unit of quantity of a substance is the mole. The ratio of the number of molecules of a substance to the amount of a substance is called constant Avogadro: , N A \u003d 6.02 ∙ 10 23 mol -1.

The Avogadro constant shows how many atoms and molecules are contained in one mole of a substance.

Molar mass- the mass of one mole of a substance, equal to the ratio of the mass of the substance to the amount of the substance:. The molar mass is expressed in kg/mol. Knowing the molar mass, you can calculate the mass of one molecule: .

The masses of molecules are very small, for example, the mass of a water molecule: m=29.9∙10 -27 kg, so it is convenient to use not absolute values ​​of the masses, but relative ones. The relative atomic masses of all chemical elements are indicated in the periodic table. By physical methods it was possible to determine the masses of some atoms in absolute units. This is how the atomic mass unit (a.m.u.) appeared, equal to 1/12 of the mass of carbon atoms: 1 amu =1, 66∙10 -2 7 .
Molar mass is related to relative molecular weight Mr. Relative molecular weight- this is a value equal to the ratio of the mass of a molecule of a given substance to 1/12 of the mass of a carbon atom. If known chemical formula substance, then using the periodic table its relative mass can be determined.

The molecular-kinetic theory of the structure of matter is based on three statements:

• matter is made up of particles;
• particles move randomly;
• particles interact with each other.

Each assertion is rigorously proven by experiments.

The volume V of the oil layer is equal to the product of its surface area S and the thickness d of the layer, i.e. V=S*d/ Therefore, the size of an olive oil molecule is equal to:

The diameter of a water molecule is approximately 3 10 cm . Assuming that each water molecule in densely packed molecules occupies a volume of approximately 3*10 8 cm 3 , you can find the number of molecules in a drop by dividing the volume of the drop 1 cm 3 per volume per molecule:

MASS OF MOLECULES. QUANTITY OF SUBSTANCE.

The mass of atoms and molecules differ significantly. What quantities are convenient to characterize them? How to determine the number of atoms in any macroscopic body?

A new quantity appears - the amount of matter.

The mass of a water molecule. The masses of individual molecules and atoms are very small. For example, in 1 g water contains 3.7 * 10 22 molecules. Therefore, the mass of one water molecule (H 2 O) is:

Since the masses of molecules are very small, it is convenient to use in calculations not the absolute values ​​of the masses, but relative ones. By international agreement, the masses of all atoms and molecules are compared with the masses of a carbon atom (the so-called carbon scale of atomic masses).

The relative molecular (or atomic) mass of the substance M r . call the ratio of the mass of a molecule (or atom) m 0 of a given substance to the mass of a carbon atom m os:

Amount of substance the most natural would be to measure it by the number of molecules or atoms in the body. But the number of molecules in any macroscopic body is so great that the calculations do not use the absolute number of molecules, but the relative one.

AT international system units, the amount of a substance is expressed in moles.

One mole is the amount of a substance that contains as many molecules or atoms as there are atoms in 0.012 kg of carbon.

This means that 1 mole of any substance contains the same number of atoms or molecules. This number of atoms is N A and is called the Avogadro constant in honor of the Italian scientist (nineteenth century).

N A - Avogadro constant.

To determine the Avogadro constant, you need to find the mass of one carbon atom. A rough estimate of the mass can be made in the same way as was done above for the mass of the water molecule (the most accurate methods are based on the deflection of ion beams by an electromagnetic field).

Explanation of Brownian motion.

Brownian motion can be explained only on the basis of molecular-kinetic theory. The reason for the Brownian motion of a particle is that the impacts of liquid molecules on the particle do not cancel each other out. When molecules move randomly, the impulses they transmit to a Brownian particle, for example, from the left and right, are not the same, therefore the resulting force of pressure of liquid molecules on a Brownian particle is nonzero, which causes a change in its motion.

gases are easily compressed, thus reducing the average distance

between molecules, but the molecules do not squeeze each other. The volume of the vessel is tens of thousands of times greater than the volume of

him molecules. Gases are easily compressed this reduces the average distance between the molecules, but the molecules do not squeeze each other.

Molecules with huge speeds - hundreds of meters per second - move in space. Colliding, they bounce off each other in different directions like billiard balls. Weak forces of attraction of gas molecules are not able to keep them near each other. That's why gases can expand indefinitely. They retain neither shape nor volume. Numerous impacts of molecules on the walls of the vessel create gas pressure.

We are surrounded by various things. We can see that they are either solids or liquids or gases. There are a lot of questions about everything that surrounds us. Gives answers to many questions molecular kinetic theory.

Molecular-kinetic theory is a set of views used to describe the observed and measured properties of a substance based on the study of the properties of atoms and molecules of a given substance, their interaction and movement.

Basic Provisions of Molecular Kinetic Theory

• All bodies are made up of particles - atoms, molecules, ions.

• All particles are in continuous chaotic thermal motion.

• Between the particles of any body there are forces of interaction - attraction and repulsion.

Thus, in the molecular-kinetic theory, the object of study is a system consisting of a large number of particles - macrosystem. To explain the behavior of such a system, the laws of mechanics are not applicable. Therefore, the main research method is statistical method studying the properties of matter.

To explain and predict phenomena, it is important to know main characteristics of molecules:

1. Dimensions

An estimate of the size of a molecule can be made as the size of a cube a containing one molecule, based on the density of solid or liquid substances and the mass of one molecule:

1. Mass of molecules

The ratio of the mass of a substance m to the number of molecules N in this substance:

1. Relative molecular weight

The ratio of the mass of a molecule (or atom) of a given substance to 1/12 of the mass of a carbon atom:

1. Amount of substance

The amount of substance is equal to the ratio of the number of particles N in the body (atoms - in the atomic substance, molecules - in the molecular) to the number of molecules in one mole of the substance NBUT:

1. Avogadro constant

The number of molecules contained in 1 mol of a substance.

1. Molar mass

The molar mass of a substance is the mass of a substance taken in an amount of 1 mole.

In the International System of Units, the molar mass of a substance is expressed as kg/mol.

1. Interaction (quantitative based on experiences)

The interaction of molecules is characterized by both attraction and repulsion at the same time: at distances r 0 repulsion dominates, at a distance r>r 0 - attraction, and it quickly decreases. On distance r 0 a system of two molecules has a minimum of potential energy (the interaction force is zero) - this is a state of stable equilibrium

The molecular kinetic theory makes it possible to understand why a substance can be in gaseous, liquid and solid states. From the point of view of the MKT, the states of aggregation differ in terms of the value of the average distance between molecules and the nature of the movement of molecules relative to each other.

The main provisions of the molecular kinetic theory have been repeatedly confirmed by various physical experiments. For example, research:

A) diffusion

B) Brownian motion

Brief summary

Molecular-kinetic theory explains the structure and properties of bodies on the basis of the movement and interaction of atoms, molecules and ions. MKT is based on three positions, which are fully confirmed experimentally and theoretically:

1) all bodies consist of particles - molecules, atoms, ions;

2) the particles are in continuous chaotic thermal motion;

3) between the particles of any body there are forces of interaction - attraction and repulsion.

The molecular structure of a substance is confirmed by the direct observation of molecules in electron microscopes, as well as the dissolution of solids in liquids, the compressibility and permeability of a substance. Thermal motion - brownian motion and diffusion. Availability intermolecular interaction strength and elasticity of solids, surface tension of liquids.

Reference outline for the lesson:

Questions for self-control in the block "Basic provisions of the molecular kinetic theory and their experimental substantiation"

1. Formulate the main provisions of the molecular-kinetic theory.
2. What observations and experiments confirm the main provisions of the molecular kinetic theory?
3. What is a molecule? atom?
4. What is called relative molecular weight? What formula expresses this concept?
5. What is the quantity of a substance? What formula expresses this concept? What is the unit of quantity of a substance?
6. What is called the Avogadro constant?
7. What is the molar mass of a substance? What formula expresses the meaning of this concept? What is the unit of molar mass?
8. What is the nature of intermolecular forces?
9. What are the properties of molecular forces?
10. How do the forces of interaction depend on the distance between them?
11. Describe the nature of the movement of molecules in gases, liquids and solids.
12. What is the nature of particle packing in gases, liquids and solids?
13. What is the average distance between molecules in gases, liquids and solids?
14. List the main properties of gases, liquids, solids.
15. What is called Brownian motion?
16. What does Brownian motion indicate?
17. What is called diffusion? Give examples of diffusion in gases, liquids and solids.
18. 18. How does the diffusion rate depend on the temperature of bodies?