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 of 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.

With the help of modern research methods (electron and probe microscopes), it was possible 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 mass 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 are different for different substances, then the mass N A particles in different substances is 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, there are also repulsive forces, so the molecules can approach each other only 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, oscillatory, 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 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 is even slower 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 the 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. By chance, a noticeably larger number of molecules will hit the particle on one side than on the other, and the particle will begin to 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 studies, intermolecular forces of interaction are inversely proportional to 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 (of 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 molecular action radius (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 the kinetic energy of the 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.

Sometimes under A.v. understand the partial pressure of water vapor. In this case, it is measured in pascals (Pa).

ABSOLUTE TEMPERATURE- temperature, measured on an absolute thermodynamic scale, independent of the properties of the thermometric substance. Counted from absolute zero. Unit A.t. in SI Kelvin (K).

ABSOLUTE ZERO- absolute temperature reference point; is 273.16 K below the temperature of the triple point of water, for which the value of 0.01 o C is accepted. the translational and rotational motion of atoms and molecules stops, but they are not at rest, but in a state of "zero" vibrations. It follows from the laws of thermodynamics that A.n. practically unattainable.

AVOGADRO LAW- one of the basic laws of ideal gases: equal volumes of different gases at the same temperature and pressure contain the same number of molecules. Opened in 1811 by the Italian. physicist A. Avogadro (1776-1856).

AVOGADRO CONSTANT(number) - the number of particles per unit amount of substance (in 1 mol): N A \u003d 6.022. 10 23 mol -1 .

AGGREGATE STATES OF SUBSTANCE- states of the same substance, differing in the nature of the thermal motion of particles. Usually there are 3 ASW: gas, liquid and solid; sometimes the plasma state is also referred to here. Substance in any A.S. exists under certain external conditions (temperature, pressure), the change of which leads to a transition from one A.S. into another.

ADIABATIC (ADIABATIC) PROCESS– a model of a thermodynamic process in which there is no heat exchange between the system under consideration and the environment. A real thermodynamic process can be considered as A. if it occurs either in a heat-insulating shell, or so quickly that heat exchange does not have time to occur.

The line depicting the equilibrium on any thermodynamic diagram adiabatic process. Equation a. for an ideal gas has the form - adiabatic exponent, and with p and with v heat capacity at constant pressure and volume, respectively.

AMORPHOUS STATE- the state of a solid in which there is no arrangement of molecules. Therefore a. the substance has isotropy, i.e. has the same physical properties in all directions, and has no definite melting point.

ANEROY- an aneroid barometer, a device for measuring atmospheric pressure, the receiving part of which is a metal box, inside which a strong vacuum is created. When changing atm. pressure, the deformation of the box changes, which, with the help of a spring associated with it and a system of levers, causes the arrow-pointer to turn.

ANISOTROPY- the dependence of the physical properties of matter on the direction (as opposed to isotropy). It is associated with the internal ordered structure of media and is found in the phenomena of elasticity, thermal and electrical conductivity, the propagation of sound and light in solids. It can also be inherent in physical space in the presence of electromagnetic, gravitational and other fields.

ATMOSPHERE PRESSURE The pressure exerted by the Earth's atmosphere on all objects in it. It is determined by the weight of the overlying column of air and is the most important quantity describing the state of the earth's atmosphere. Units A.d. in SI - Pa, mm Hg. Normal A.d. equal to 760 mm Hg. or 1013 hPa.

BAROMETER- a device for measuring atmospheric pressure. The most common deformation B., which, for example, includes B. - aneroid(1844, L. Vidi). In such a B., when the atmospheric pressure changes, the membrane sags, closing the box from which the air is evacuated, and in this case, the arrow connected to the membrane through a system of levers deflects. Action liquid B. (for example, mercury B. E. Torricelli, 1644) is based on balancing the atmospheric pressure sial with the weight of a liquid column.

SHORT ORDER- ordered arrangement of atoms or molecules within distances close to interatomic; characteristic of amorphous substances and some liquids. (cf.).

BOYLE-MARIOTTE LAW- one of the laws ideal gas: for a given mass of a given gas at constant temperature, the product of pressure and volume is a constant. Formula: pV=const. Describes an isothermal process.

One of the main physical constants, equal to the ratio of the universal gas constant R to N A .B.p. .Included in a number of important relationships of statistical physics: connects cf. kinetic energy of particles and temperature, entropy of a physical system and its thermodynamic probability.

BROWNIAN MOTION- random movement of small macroscopic particles suspended in a liquid or gas, occurring under the influence of the thermal movement of molecules. Visual confirmation of the molecular-kinetic theory. Discovered by R. Brown in 1827. Explained by A. Einstein and M. Smoluchowski in 1905. The theory was tested in the experiments of J. Perrin in 1906-11.

VACUUM- the state of a gas enclosed in a vessel, having a pressure significantly lower than atmospheric pressure. Depending on the ratio between the free path of atoms or molecules and the linear size of the vessel, ultra-high, high, medium and low vacuum are distinguished.

AIR HUMIDITY- the presence of water vapor in the air. Described by physical quantities absolute and relative AT . , which are measured hygrometers.

INTERNAL ENERGY- the energy of the body, depending only on its internal state; consists of the energy of random (thermal) motion of atoms, molecules or other particles and the energy of intra-atomic and intermolecular motions and interactions. (Cm. first law of thermodynamics). In MKT, the energy of intraatomic particles and their interactions is not taken into account.

THE SECOND LAW OF THERMODYNAMICS one of the fundamental laws thermodynamics, according to which a periodic process is impossible, the only result of which is the performance of work equivalent to the amount of heat received from the heater. Another formulation: a process is impossible, the only result of which is the transfer of energy in the form of heat from a less heated body to a hotter one. V.z.t. expresses the tendency of a system consisting of a large number of chaotically moving particles to a spontaneous transition from less probable states to more probable states. One more way of formulating the WZT: it is impossible to create a perpetual motion machine of the second kind.

GAS CONSTANT UNIVERSAL(R) - one of the main physical constants included in the equation of state (Cm.). R=(8.31441±0.00026) J/(mol K). Physical meaning: the work of expansion of one mole of an ideal gas in an isobaric process with an increase in temperature by 1 K.

GAS THERMOMETER- a device for measuring temperature, the action of which is based on the dependence of pressure or volume of gas on temperature.

one of the laws ideal gas: for a given mass of a given gas at constant pressure, the ratio of volume to absolute temperature is a constant value: (or: volume is directly proportional to absolute temperature: , where α is the temperature coefficient of pressure). Describes isobaric process.

HYGROMETER- instrument for measuring absolute or relative humidity. G. subdivided into weight (to determine the absolute humidity), condensation (to determine the dew point), hair (relative humidity), as well as G. psychrometric or psychrometers (relative humidity).

CELSIUS- off-system unit of temperature according to the International Practical Temperature Scale, where the temperature triple point water is 0.01 degrees Celsius, and the boiling point at normal atmospheric pressure is 100 degrees Celsius.

LONG ORDER- an ordered arrangement of particles (atoms or molecules) throughout the body; characteristic of crystalline substances. Wed close order.

DALTON'S LAW- one of the basic laws of an ideal gas: the pressure of a mixture of chemically non-interacting gases is equal to the sum of the partial pressures of these gases.

DEFECTS IN CRYSTALS- imperfections of the crystal structure, violations of the strict periodic arrangement of particles (atoms, molecules, ions) at the nodes of the crystal lattice. These include vacancies (point defects), dislocations (linear defects), bulk defects: cracks, pores, shells, etc. They have a significant effect on the physical properties of crystals.

DISSOCATIONS- line defects crystal lattice, violating the correct alternation of atomic planes. In two dimensions they have dimensions of the order of the size of an atom, and in the third they can pass through the entire crystal.

DISSOCIATION- the process of disintegration of molecules into simpler parts - atoms, groups of atoms or ions. It can occur with an increase in temperature (thermal D.), in an electrolyte solution (electrolytic D.), and under the action of light (photochemical D.).

LIQUID CRYSTALS- a state of matter in which structural properties are found that are intermediate between solid crystal and liquid. They are formed in substances with elongated molecules, the mutual orientation of which determines anisotropy their physical properties. They are used in engineering, biology and medicine.

LIQUID THERMOMETER- instrument for measuring temperature, the action of which is based on the thermal expansion of the liquid. Zh.t. depending on the temperature range, they are filled with mercury, ethyl alcohol, and other liquids.

LIQUID- one of aggregate states substance intermediate between solid and gaseous. J., like solid, has low compressibility, high density and at the same time. like gas characterized by variability of form (it flows easily). Liquid molecules, like particles of a solid body, perform thermal vibrations, but their equilibrium position changes from time to time, which ensures the fluidity of the liquid.

IDEAL GAS- a mental model of a gas in which the forces of interaction between particles and the sizes of these particles can be neglected. Those. particles are taken as material points, and all interaction is reduced to their absolutely elastic impacts. Rarefied gases at temperatures far from the condensation temperature are close in their properties to I.g. The equation of state is Clapeyron - Mendeleev equation.

ISOBAR- line of constant pressure, depicting on the state diagram the equilibrium isobaric process.

ISOBAR PROCESS(isobaric) - a mental model of a thermodynamic process occurring at constant pressure. For ideal gases, it is described by the law Gay-Lussac.

ISOPROCESSES are physical processes occurring at the constancy of any of the parameters describing the state of the system (see Fig. isobaric, isothermal, isochoric process).

ISOTHERM- line of constant temperature, depicting the equilibrium state diagram isothermal process.

ISOTHERMAL PROCESS is a model of a thermodynamic process occurring at a constant temperature. For example, boiling of a chemically homogeneous liquid, melting of a chemically homogeneous crystal at constant external pressure. For ideal gases, it is described Boyle-Mariotte law. Wed isobaric, isochoric, adiabatic process.

ISOTROPY, isotropy - the same physical properties in all directions. It is associated with the absence of an ordered internal structure of media and is inherent in gases, liquids (except liquid crystals) and amorphous bodies. Wed anisotropy.

ISOCHORE- line of constant volume, depicting an equilibrium isochoric process on the state diagram.

ISOCHORIC PROCESS, isochoric process - a thermodynamic process occurring at a constant volume of the system. For ideal gases, it is described Charles law.

EVAPORATION- the process of vaporization from the free surface of a liquid at a temperature below the boiling point. I. from the surface of solids is called sublimation. (cf. boiling, vaporization).

CALORIMETER- a device for determining various calorimetric quantities: heat capacity, heat of combustion, heat of vaporization etc.

CAPILLARY- a narrow vessel with a characteristic cross-sectional size of less than 1 mm.

CAPILLARY PHENOMENA- phenomena caused by the influence of forces of intermolecular interaction on the equilibrium and movement of the free surface of a liquid, the interface of immiscible liquids and the boundaries of liquids with solids. For example, raising or lowering liquid in very thin tubes () and in porous media.

CARNO CYCLE- a mental model of a reversible circular process, consisting of two isothermal and two adiabatic processes. During isothermal expansion (heater temperature T n) the working fluid (ideal gas) is given the amount of heat Q n, and under isothermal compression (refrigerator temperature T x) - the amount of heat removed Q x. Efficiency C.c. does not depend on the nature of the working fluid and is equal to .

BOILING- the process of intensive vaporization not only from the free surface of the liquid, but also throughout its entire volume inside the vapor bubbles formed in this case. The temperature of K. depends on the nature of the liquid and the external pressure and is between triple point and critical temperature (see critical situation).

MAYER EQUATION- a relation that establishes a relationship between the molar heat capacities of an ideal gas at constant pressure with p and at a constant volume with V : with P = with V + R . where R - .

MAXWELL DISTRIBUTION- the law of distribution of the molecules of an ideal gas, which is in a state of thermodynamic equilibrium, according to the velocities.

MANOMETER- instrument for measuring pressure liquids and gases. Distinguish between M. for measuring absolute pressure, counted from zero, and M. for measuring excess pressure (the difference between absolute and atmospheric pressure). Distinguish liquid, piston, deformation and spring M. depending on the principle of action.

MENISCUS- the curved surface of a liquid in a narrow tube (capillary) or between closely spaced solid walls (see).

- a constant physical quantity for a given material, which is a coefficient of proportionality between mechanical stress and relative elongation in Hooke law: . M.Yu. E is equal to the mechanical stress that occurs in a deformed body when its length is doubled. The SI unit of measure is pascal.

MOLECULE- the smallest stable particle of a substance that has all chemical properties and consists of the same (simple substance) or different (complex substance) atoms united by chemical bonds. Wed atom.

MOLECULAR MASS is the mass of the molecule, expressed in atomic mass units. Wed molar mass.

MOLECULAR PHYSICS- a branch of physics that studies the physical properties of bodies, the features of the aggregate states of matter and the processes of phase transitions depending on the molecular structure of bodies, the forces of intermolecular interaction and the nature of the thermal motion of particles (atoms, ions, molecules). Cm. statistical physics, thermodynamics.

MOLAR MASS is the mass of one mole of a substance; a scalar value equal to the ratio of the mass of a body to the amount of a substance (number of moles) it contains. In SI m.m. is equal to molecular weight substance multiplied by 10 -3 and is measured in kilograms per mole (kg/mol).

MONOCRYSTALS- single crystals with a single crystal lattice. They are formed in natural conditions or artificially grown from melts, solutions, vaporous or solid phases. Wed polycrystals.

SATURATED STEAM- steam in dynamic equilibrium with a liquid or solid phase. Dynamic equilibrium is such a state in which the average number of molecules leaving the liquid (solid) is equal to the average number of vapor molecules returning to the liquid (solid) in the same time.

IRREVERSIBLE PROCESS A process that can spontaneously proceed in only one direction. All real processes are n.p. and in closed systems are accompanied by an increase entropy. Cm. , .

NORMAL CONDITIONS- standard physical conditions determined by pressure P=101325 Pa (760 mm Hg) and absolute temperature T=273.15 K.

REVERSIBLE PROCESS– a process model for which a reverse process is possible, successively repeating all intermediate states of the process under consideration. Reversible is only equilibrium process. Example - . Wed .

RELATIVE HUMIDITY- a physical quantity equal to the ratio of the density (elasticity) of water vapor contained in the air to the density (elasticity) of saturated vapor at the same temperature. Expressed as a percentage. Wed absolute humidity.

STEAM- a substance in a gaseous state under conditions where, by compression, it is possible to achieve equilibrium with the same substance in a liquid or solid state, i.e. at temperatures and pressures below critical (see critical situation). At low pressures and high temperatures, the properties of steam approach those of ideal gas.

STATUS PARAMETER, a thermodynamic parameter is a physical quantity that serves in thermodynamics to describe the state of a system. For example, pressure, temperature, internal energy, entropy, etc. P.s. are interrelated, so the equilibrium state of the system can be uniquely determined by a limited number of parameters (see Fig. equation of state).

STEAMING The process by which a substance changes from a liquid or solid state to a gaseous state. It goes on in a closed volume until it forms saturated steam. There are two types of P.: evaporation and boiling.

PARTIAL PRESSURE- the pressure of the gas that is part of the gas mixture, which it would have, occupying the entire volume of the mixture alone and being at the temperature of the mixture. Cm. .

PASCAL'S LAW- the basic Law hydrostatics: pressure produced by external forces on the surface of a liquid or gas is transmitted equally in all directions.

THE FIRST LAW OF THERMODYNAMICS one of the fundamental laws thermodynamics, which is the law of conservation of energy for a thermodynamic system: the amount of heat Q, reported to the system, is spent on changing the internal energy of the system ∆ U and making the system work A system against outside forces. Formula: Q=ΔU+A syst. On the use of P.z.t. the operation of heat engines is based. It can be formulated in another way: the change in the internal energy of the system ∆ U equal to the sum of the amount of heat transferred to the system Q and the work of external forces on the system A ext. Formula: ∆U=Q+A external. In these formulas A ext. = - A syst.

MELTING- the process of transition of a substance from a crystalline state to a liquid state. Occurs with the absorption of a certain amount of heat at the melting point, depending on the nature of the substance and pressure. Cm. melting heat.

PLASMA- an ionized gas in which the concentrations of positive and negative charges are almost the same. Formed at electrical discharge in gases, when the gas is heated to a temperature sufficient for thermal ionization. The vast majority of matter in the Universe is in the plasma state: stars, galactic nebulae and the interstellar medium.

PLASTIC- the property of solids under the action of external forces to change, without collapsing, their shape and dimensions and retain the residual (plastic) deformations. Depends on the type of liquid and temperature. May be altered by surfactants (e.g. soap).

SURFACE TENSION- a phenomenon expressed in the desire of a liquid to reduce its surface area. It is due to intermolecular interaction and is caused by the formation of a surface layer of molecules whose energy is greater than the energy of the molecules inside a given liquid at the same temperature.

The content of the article

MOLECULAR-KINETIC THEORY- a branch of molecular physics that studies the properties of a substance based on ideas about their molecular structure and certain laws of interaction between atoms (molecules) that make up a substance. It is believed that the particles of matter are in continuous, random motion and this movement is perceived as heat.

Until the 19th century A very popular basis for the theory of heat was the theory of caloric or some liquid substance flowing from one body to another. The heating of the bodies was explained by an increase, and the cooling - by a decrease in the caloric contained inside them. The concept of atoms for a long time seemed unnecessary for the theory of heat, but many scientists even then intuitively associated heat with the movement of molecules. So, in particular, the Russian scientist M.V. Lomonosov thought. A lot of time passed before the molecular-kinetic theory finally won in the minds of scientists and became an inalienable property of physics.

Many phenomena in gases, liquids and solids find a simple and convincing explanation within the framework of molecular kinetic theory. So pressure, exerted by the gas on the walls of the vessel in which it is enclosed, is considered as the total result of numerous collisions of rapidly moving molecules with the wall, as a result of which they transfer their momentum to the wall. (Recall that it is the change in momentum per unit time that, according to the laws of mechanics, leads to the appearance of a force, and the force per unit wall surface is pressure). The kinetic energy of the movement of particles, averaged over their huge number, determines what is commonly called temperature substances.

The origins of the atomistic idea, i.e. the idea that all bodies in nature consist of the smallest indivisible particles-atoms, goes back to the ancient Greek philosophers - Leucippus and Democritus. More than two thousand years ago, Democritus wrote: “... atoms are countless in size and multitude, but they rush in the universe, circling in a whirlwind, and thus everything complex is born: fire, water, air, earth.” A decisive contribution to the development of molecular kinetic theory was made in the second half of the 19th century. works of remarkable scientists J.K. The statistical approach was generalized (in relation to any state of matter) at the beginning of the 20th century. in the writings of the American scientist J. Gibbs, who is considered one of the founders of statistical mechanics or statistical physics. Finally, in the first decades of the 20th century physicists realized that the behavior of atoms and molecules obeys the laws of not classical, but quantum mechanics. This gave a powerful impetus to the development of statistical physics and made it possible to describe a number of physical phenomena that previously could not be explained within the framework of the usual concepts of classical mechanics.

Molecular-kinetic theory of gases.

Each molecule flying towards the wall, when colliding with it, transfers its momentum to the wall. Since the velocity of the molecule during elastic collision with the wall varies from the value v before - v, the value of the transmitted impulse is 2 mv. Force acting on the wall surface D S in time D t, is determined by the value of the total momentum transmitted by all molecules reaching the wall during this period of time, i.e. F= 2mv n c D S/D t, where n c defined by expression (1). For pressure value p = F/D S in this case we find: p= (1/3)nmv 2.

To obtain the final result, it is possible to abandon the assumption of the same speed of molecules by separating independent groups of molecules, each of which has its own approximately equal speed. Then the average pressure is found by averaging the square of the velocity over all groups of molecules, or

This expression can also be represented as

It is convenient to give this formula a different form by multiplying the numerator and denominator under the square root sign by the Avogadro number

N a= 6.023 10 23 .

Here M = mN A- atomic or molecular weight, value R = kN A\u003d 8.318 10 7 erg is called the gas constant.

The average velocity of molecules in a gas, even at moderate temperatures, turns out to be very high. So, for hydrogen molecules (H 2) at room temperature ( T= 293K) this speed is about 1900 m/s, for nitrogen molecules in air it is about 500 m/s. The speed of sound in air under the same conditions is 340 m/s.

Given that n = N/V, where V is the volume occupied by the gas, N is the total number of molecules in this volume, it is easy to obtain consequences from (5) in the form of known gas laws. For this, the total number of molecules is represented as N = vN A, where v is the number of gas moles, and equation (5) takes the form

(8) pV = vRT,

which is called the Clapeyron-Mendeleev equation.

Given that T= const gas pressure varies inversely with the volume it occupies (Boyle-Mariotte law).

In a closed vessel of fixed volume V= const pressure changes in direct proportion to the change in the absolute temperature of the gas T. If a gas is under conditions where its pressure remains constant p= const, but the temperature changes (such conditions can be realized, for example, if gas is placed in a cylinder closed by a movable piston), then the volume occupied by the gas will change in proportion to the change in its temperature (Gay-Lussac's law).

Let there be a mixture of gases in the vessel, i.e. there are several different kinds of molecules. In this case, the magnitude of the momentum transferred to the wall by molecules of each type does not depend on the presence of molecules of other types. Hence it follows that the pressure of a mixture of ideal gases is equal to the sum of the partial pressures that each gas would create separately if it occupied the entire volume. This is another of the gas laws - the famous Dalton's law.

Mean free path of molecules . One of the first who, back in the 1850s, gave reasonable estimates of the average thermal velocity of the molecules of various gases was the Austrian physicist Clausius. The unusually large values ​​of these velocities obtained by him immediately aroused objections. If the velocities of molecules are indeed so great, then the smell of any odorous substance should almost instantly spread from one end of a closed room to the other. In fact, the spread of odor is very slow and is now known to be carried out by a process called diffusion in the gas. Clausius, and later others, were able to convincingly explain this and other transport processes in a gas (such as thermal conductivity and viscosity) using the concept of mean free path molecules , those. the average distance traveled by a molecule from one collision to the next.

Each molecule in a gas experiences a very large number of collisions with other molecules. In the interval between collisions, the molecules move almost in a straight line, experiencing sharp changes in speed only at the moment of the collision itself. Naturally, the lengths of straight sections along the path of a molecule can be different, so it makes sense to speak only of a certain mean free path of molecules.

For time D t the molecule goes through a complex zigzag path equal to v D t. There are as many kinks in the trajectory as there are collisions. Let be Z means the number of collisions that a molecule experiences per unit time. The mean free path is then equal to the ratio of the path length N 2, for example, a» 2.0 10 –10 m. Table 1 shows the values ​​of l 0 calculated by formula (10) in µm (1 µm = 10 –6 m) for some gases under normal conditions ( p= 1 atm, T=273K). These values ​​turn out to be approximately 100–300 times greater than the intrinsic diameter of the molecules.

Any substance is considered by physics as a collection of the smallest particles: atoms, molecules and ions. All these particles are in continuous chaotic motion and interact with each other through elastic collisions.

Atomic theory - the basis of molecular kinetic theory

Democritus

Molecular kinetic theory originated in ancient Greece about 2500 years ago. Its foundation is considered atomic hypothesis , sponsored by ancient Greek philosopher Leucippus and his student Ancient Greek scholar Democritus from the city of Abdera.

Leucippus

Leucippus and Democritus assumed that all material things consist of indivisible smallest particles, which are called atoms (from Greekἄτομος - indivisible). And the space between the atoms is filled with emptiness. All atoms have a size and shape, and are able to move. The proponents of this theory in the Middle Ages were Giordano Bruno, Galileo, Isaac Beckman and other scientists. The foundations of the molecular kinetic theory were laid in the work "Hydrodynamics", published in 1738. Its author was a Swiss physicist, mechanic and mathematician Daniel Bernoulli.

Basic Provisions of Molecular Kinetic Theory

Mikhail Vasilievich Lomonosov

The closest thing to modern physics was the theory of the atomic structure of matter, which was developed in the 18th century by the great Russian scientist Mikhail Vasilievich Lomonosov. He argued that all substances are made up of molecules which he called corpuscles . And corpuscles, in turn, consist of atoms . Lomonosov's theory was called corpuscular .

But as it turned out, the atom is divided. It consists of a positively charged nucleus and negative electrons. In general, it is electrically neutral.

Modern science calls atom the smallest part of a chemical element, which is the carrier of its basic properties. Connected by interatomic bonds, atoms form molecules. A molecule can contain one or more atoms of the same or different chemical elements.

All bodies are made up of a huge number of particles: atoms, molecules and ions. These particles are constantly and randomly moving. Their movement does not have any definite direction and is called thermal motion . During their motion, the particles interact with each other by absolutely elastic collisions.

We cannot observe molecules and atoms with the naked eye. But we can see the result of their actions.

Confirmation of the main provisions of the molecular kinetic theory are: diffusion , Brownian motion and change aggregate states of substances .

Diffusion

Diffusion in liquid

One of the proofs of the constant movement of molecules is the phenomenon diffusion .

In the process of movement, the molecules and atoms of one substance penetrate between the molecules and atoms of another substance in contact with it. Molecules and atoms of the second substance behave in exactly the same way. relation to the first. And after a while, the molecules of both substances are evenly distributed throughout the volume.

The process of penetration of molecules of one substance between the molecules of another is called diffusion . We encounter the phenomenon of diffusion at home every day when we drop a tea bag into a glass of boiling water. We observe how colorless boiling water changes its color. Throwing a few crystals of manganese into a test tube with water, you can see that the water turns pink. This is also diffusion.

The number of particles per unit volume is called concentration substances. During diffusion, molecules move from those parts of the substance where the concentration is higher to those parts where it is less. The movement of molecules is called diffusion flow . As a result of diffusion, the concentrations in different parts of substances are aligned.

Diffusion can be observed in gases, liquids and solids. In gases, it occurs at a faster rate than in liquids. We know how quickly smells spread in the air. The liquid in the test tube stains much more slowly if ink is dropped into it. And if we put salt crystals on the bottom of a container with water and do not mix it, then more than one day will pass before the solution becomes homogeneous.

Diffusion also occurs at the boundary of the contacting metals. But its speed in this case is very small. If you cover copper with gold, then at room temperature and atmospheric pressure, gold will penetrate copper by only a few microns in a few thousand years.

Lead from an ingot placed under a load on a gold ingot will penetrate into it only to a depth of 1 cm in 5 years.

Diffusion in metals

Diffusion rate

The diffusion rate depends on the cross-sectional area of ​​the flow, the difference in the concentrations of substances, the difference in their temperatures or charges. Through a rod with a diameter of 2 cm, heat spreads 4 times faster than through a rod with a diameter of 1 cm. The higher the temperature difference of the substances, the higher the diffusion rate. During thermal diffusion, its rate depends on thermal conductivity material, and in the case of a flow of electric charges - from electrical conductivity .

Fick's law

Adolf Fick

In 1855, the German physiologist Adolf Eugene Fick made the first quantitative description of diffusion processes:

where J - density diffusion flow of matter,

D - diffusion coefficient,

C - substance concentration.

Diffusion flux density of matterJ [cm -2 s -1 ] is proportional to the diffusion coefficientD [cm -2 s -1 ] and the concentration gradient taken with the opposite sign.

This equation is called Fick's first equation .

Diffusion, as a result of which the concentrations of substances are equalized, is called non-stationary diffusion . With such diffusion, the concentration gradient changes with time. And in case stationary diffusion this gradient remains constant.

Brownian motion

Robert Brown

This phenomenon was discovered by the Scottish botanist Robert Brown in 1827. Studying under a microscope cytoplasmic grains suspended in water isolated from pollen cells of a North American plantClarkia pulchella, he drew attention to the smallest solid grains. They trembled and moved slowly for no apparent reason. If the temperature of the liquid increased, the speed of the particles increased. The same thing happened when the particle size decreased. And if their size increased, the temperature of the liquid decreased or its viscosity increased, the movement of the particles slowed down. And these amazing "dances" of particles could be observed indefinitely. Deciding that the reason for this movement is that the particles are alive, Brown replaced the grains with small particles of coal. The result was the same.

Brownian motion

To repeat Brown's experiments, it is enough to have the most ordinary microscope. The molecular size is too small. And it is impossible to consider them with such a device. But if we color the water in a test tube with watercolor paint and then look at it through a microscope, we see tiny colored particles that move randomly. These are not molecules, but paint particles suspended in water. And they are forced to move by water molecules that hit them from all sides.

This is the behavior of all particles visible in a microscope that are suspended in liquids or gases. Their random movement, caused by the thermal motion of molecules or atoms, is called brownian motion . A Brownian particle is continuously subjected to impacts from the molecules and atoms that make up liquids and gases. And this movement does not stop.

But particles up to 5 microns (micrometers) in size can participate in Brownian motion. If their size is larger, they are immobile. The smaller the size of a Brownian particle, the faster it moves. Particles smaller than 3 microns move progressively along all complex trajectories or rotate.

Brown himself could not explain the phenomenon he discovered. And only in the 19th century, scientists found the answer to this question: the movement of Brownian particles is caused by the influence of the thermal movement of molecules and atoms on them.

Three states of matter

The molecules and atoms that make up matter are not only in motion, but also interact with each other, mutually attracting or repelling.

If the distance between the molecules is comparable to their size, then they experience attraction. If it becomes smaller, then the repulsive force begins to prevail. This explains the resistance of physical bodies to deformation (compression or tension).

If the body is compressed, then the distance between the molecules decreases, and the repulsive forces will try to return the molecules to their original state. When stretched, the deformation of the body will interfere with the forces of attraction between the molecules.

Molecules interact not only within one body. Dip a piece of cloth into the liquid. We will see that it gets wet. This is due to the fact that the molecules of a liquid are attracted to the molecules of solids more strongly than to each other.

Each physical substance, depending on temperatures and pressures, can be in three states: solid, liquid or gaseous . They're called aggregate .

In gases the distance between molecules is large. Therefore, the forces of attraction between them are so weak that they perform a chaotic and almost free movement in space. They change the direction of their movement by hitting each other or the walls of blood vessels.

in liquids molecules are closer together than in a gas. There is more attraction between them. The molecules in them no longer move freely, but oscillate randomly near the equilibrium position. But they are able to jump in the direction of the external force, changing places with each other. The result of this is fluid flow.

In solids the forces of interaction between molecules are very large due to the close distance between them. They cannot overcome the attraction of neighboring molecules, therefore they are only capable of performing oscillatory movements around the equilibrium position.

Solid bodies retain volume and shape. The liquid has no form, it always takes the form of the vessel in which it is located at the moment. But its volume remains the same. Gaseous bodies behave differently. They easily change both shape and volume, taking the form of the vessel in which they were placed, and occupying the entire volume provided to them.

However, there are also such bodies that have the structure of a liquid, have a slight fluidity, but at the same time are able to retain their shape. Such bodies are called amorphous .

Modern physics singles out the fourth aggregate state of matter - plasma .

Definition 1

Molecular Kinetic Theory- this is the doctrine of the structure and properties of matter, based on the idea of ​​the existence of atoms and molecules, as the smallest particles of chemical substances.

The main provisions of the molecular-kinetic theory of the molecule:

1. All substances can be in liquid, solid and gaseous state. They are formed from particles that are made up of atoms. Elementary molecules can have a complex structure, that is, they can contain several atoms. Molecules and atoms are electrically neutral particles that, under certain conditions, acquire an additional electrical charge and turn into positive or negative ions.
2. Atoms and molecules move continuously.
3. Particles with an electrical nature of force interact with each other.

The main provisions of the MKT and their examples have been listed above. Between particles there is a small gravitational influence.

Figure 3. one . one . The trajectory of a Brownian particle.

Definition 2

The Brownian motion of molecules and atoms confirms the existence of the main provisions of the molecular kinetic theory and substantiates it experimentally. This thermal movement of particles occurs with molecules suspended in a liquid or gas.

Experimental substantiation of the main provisions of the molecular kinetic theory

In 1827, R. Brown discovered this movement, which was due to random impacts and movements of molecules. Since the process was chaotic, the blows could not balance each other. Hence the conclusion that the speed of a Brownian particle cannot be constant, it is constantly changing, and the movement of the direction is depicted as a zigzag, shown in Figure 3. one . one .

A. Einstein spoke about Brownian motion in 1905. His theory was confirmed in the experiments of J. Perrin in 1908 - 1911.

Definition 3

Consequence from Einstein's theory: offset square< r 2 >of the Brownian particle relative to the initial position, averaged over many Brownian particles, is proportional to the observation time t .

Expression< r 2 >= D t explains the diffusion law. According to the theory, we have that D increases monotonically with increasing temperature. Random motion is visible in the presence of diffusion.

Definition 4

Diffusion- this is the definition of the phenomenon of penetration of two or more contiguous substances into each other.

This process occurs rapidly in an inhomogeneous gas. Thanks to diffusion examples with different densities, a homogeneous mixture can be obtained. When oxygen O 2 and hydrogen H 2 are in the same vessel with a partition, when it is removed, the gases begin to mix, forming a dangerous mixture. The process is possible when hydrogen is at the top and oxygen is at the bottom.

Interpenetration processes also occur in liquids, but much more slowly. If we dissolve a solid, sugar, in water, we get a homogeneous solution, which is a clear example of diffusion processes in liquids. Under real conditions, mixing in liquids and gases is masked by rapid mixing processes, for example, when convection currents occur.

Diffusion of solids is distinguished by its slow speed. If the interaction surface of metals is cleaned, then it can be seen that over a long period of time, atoms of another metal will appear in each of them.

Definition 5

Diffusion and Brownian motion are considered related phenomena.

With the interpenetration of particles of both substances, the movement is random, that is, there is a chaotic thermal movement of molecules.

The forces acting between two molecules depend on the distance between them. Molecules have both positive and negative charges. At large distances, forces of intermolecular attraction predominate, at small distances, repulsive forces prevail.

Picture 3 . 1 . 2 shows the dependence of the resulting force F and potential energy E p of the interaction between molecules on the distance between their centers. At a distance r = r 0, the interaction force vanishes. This distance is conditionally taken as the diameter of the molecule. At r = r 0 the potential energy of interaction is minimal.

Definition 6

To move two molecules apart with distance r 0 , E 0 should be reported, called binding energy or potential well depth.

Figure 3. one . 2.The power of interaction F and potential energy of interaction E p two molecules. F > 0- repulsive force F< 0 - force of gravity.

Since molecules are small in size, simple monatomic ones can be no more than 10 - 10 m. Complex ones can reach sizes hundreds of times larger.

Definition 7

The random random movement of molecules is called thermal movement.

As the temperature increases, the kinetic energy of thermal motion increases. At low temperatures, the average kinetic energy, in most cases, is less than the potential well depth E 0 . This case shows that the molecules flow into a liquid or solid with an average distance between them r 0 . If the temperature rises, then the average kinetic energy of the molecule exceeds E 0, then they fly apart and form a gaseous substance.

In solids, molecules move randomly around fixed centers, that is, equilibrium positions. In space, it can be distributed in an irregular manner (in amorphous bodies) or with the formation of ordered bulk structures (crystalline bodies).

Aggregate states of substances

The freedom of thermal motion of molecules is seen in liquids, since they do not have binding to centers, which allows movement throughout the volume. This explains its fluidity.

Definition 8

If the molecules are close, they can form ordered structures with several molecules. This phenomenon has been named close order. distant order characteristic of crystalline bodies.

The distance in gases between molecules is much larger, so the acting forces are small, and their movements go along a straight line, waiting for the next collision. The value of 10 - 8 m is the average distance between air molecules under normal conditions. Since the interaction of forces is weak, the gases expand and can fill any volume of the vessel. When their interaction tends to zero, then one speaks of the representation of an ideal gas.

Kinetic model of an ideal gas

In microns, the amount of matter is considered proportional to the number of particles.

Definition 9

mole- this is the amount of a substance containing as many particles (molecules) as there are atoms in 0, 012 to g of carbon C 12. A carbon molecule is made up of one atom. It follows that 1 mole of a substance has the same number of molecules. This number is called permanent Avogadro N A: N A \u003d 6, 02 ċ 1023 mol - 1.

Formula for determining the amount of a substance ν is written as the ratio N of the number of particles to the Avogadro constant N A: ν = N N A .

Definition 10

The mass of one mole of a substance call the molar mass M. It is fixed in the form of the formula M \u003d N A ċ m 0.

The expression of the molar mass is made in kilograms per mole (k g / mol b).

Definition 11

If the substance has one atom in its composition, then it is appropriate to speak of the atomic mass of the particle. The unit of an atom is 1 12 masses of the carbon isotope C 12, called atomic mass unit and written as ( a. eat.): 1 a. e. m. \u003d 1, 66 ċ 10 - 27 to g.

This value coincides with the mass of the proton and neutron.

Definition 12

The ratio of the mass of an atom or molecule of a given substance to 1 12 of the mass of a carbon atom is called relative weight.

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