How are particles arranged in solids, liquids and gases? Liquids. The movement of molecules in liquids The gaseous state of matter the arrangement of molecules

The liquid occupies an intermediate position in properties and structure between gases and solid crystalline substances. Therefore, it has the properties of both gaseous and solid substances. In the molecular kinetic theory, various aggregate states of a substance are associated with a different degree of ordering of molecules. For solids, the so-called long range order in the arrangement of particles, i.e. their orderly arrangement, repeating over long distances. In liquids, the so-called short range order in the arrangement of particles, i.e. their ordered arrangement, repeating at distances, is comparable with interatomic ones. At temperatures close to the crystallization temperature, the liquid structure is close to that of a solid. At high temperatures, close to the boiling point, the structure of the liquid corresponds to the gaseous state - almost all molecules participate in chaotic thermal motion.

Liquids, like solids, have a certain volume, and like gases, they take the shape of the vessel in which they are located. Gas molecules are practically not interconnected by the forces of intermolecular interaction, and in this case the average energy of the thermal motion of gas molecules is much greater than the average potential energy due to the forces of attraction between them, so the gas molecules scatter in different directions and the gas occupies the volume provided to it. In solid and liquid bodies, the forces of attraction between molecules are already significant and keep the molecules at a certain distance from each other. In this case, the average energy of the thermal motion of molecules is less than the average potential energy due to the forces of intermolecular interaction, and it is not enough to overcome the forces of attraction between molecules, so solids and liquids have a certain volume.

The pressure in liquids increases very sharply with increasing temperature and decreasing volume. The volumetric expansion of liquids is much less than that of vapors and gases, since the forces that bind molecules in a liquid are more significant; the same remark applies to thermal expansion.

The heat capacities of liquids usually increase with temperature (albeit slightly). The C p /C V ratio is practically equal to one.

The theory of fluid has not been fully developed to date. The development of a number of problems in the study of the complex properties of a liquid belongs to Ya.I. Frenkel (1894–1952). He explained the thermal motion in a liquid by the fact that each molecule oscillates for some time around a certain equilibrium position, after which it jumps to a new position, which is at a distance of the order of the interatomic distance from the initial one. Thus, the molecules of the liquid move quite slowly throughout the mass of the liquid. With an increase in the temperature of the liquid, the frequency of oscillatory motion increases sharply, and the mobility of molecules increases.

Based on the Frenkel model, it is possible to explain some distinctive features properties of the liquid. Thus, liquids, even near the critical temperature, have a much greater viscosity than gases, and the viscosity decreases with increasing temperature (rather than increases, as in gases). This is explained by a different nature of the momentum transfer process: it is transferred by molecules that jump from one equilibrium state to another, and these jumps become much more frequent with increasing temperature. Diffusion in liquids occurs only due to molecular jumps, and it occurs much more slowly than in gases. Thermal conductivity liquids is due to the exchange of kinetic energy between particles oscillating around their equilibrium positions with different amplitudes; sharp jumps of molecules do not play a noticeable role. The mechanism of heat conduction is similar to its mechanism in gases. A characteristic feature of a liquid is its ability to have free surface(not limited by solid walls).

Molecular physics is easy!

Interaction forces of molecules

All molecules of a substance interact with each other by forces of attraction and repulsion.
Proof of the interaction of molecules: the phenomenon of wetting, resistance to compression and stretching, low compressibility of solids and gases, etc.
The reason for the interaction of molecules is the electromagnetic interactions of charged particles in matter.

How to explain it?

An atom consists of a positively charged nucleus and a negatively charged electron shell. The charge of the nucleus is equal to the total charge of all electrons, therefore, as a whole, the atom is electrically neutral.
A molecule consisting of one or more atoms is also electrically neutral.

Consider the interaction between molecules using the example of two immobile molecules.

Gravitational and electromagnetic forces can exist between bodies in nature.
Since the masses of molecules are extremely small, the negligible forces of gravitational interaction between molecules can be ignored.

At very large distances, there is no electromagnetic interaction between molecules either.

But, with a decrease in the distance between the molecules, the molecules begin to orient themselves so that their sides facing each other will have charges of different signs (in general, the molecules remain neutral), and attractive forces arise between the molecules.

With an even greater decrease in the distance between the molecules, repulsive forces arise as a result of the interaction of negatively charged electron shells of the atoms of the molecules.

As a result, the molecule is affected by the sum of the forces of attraction and repulsion. At large distances, the attractive force prevails (at a distance of 2-3 molecular diameters, attraction is maximum), at short distances, the repulsive force.

There is such a distance between molecules at which the forces of attraction become equal to the forces of repulsion. This position of the molecules is called the position of stable equilibrium.

Molecules located at a distance from each other and connected by electromagnetic forces have potential energy.
In the position of stable equilibrium, the potential energy of molecules is minimal.

In a substance, each molecule interacts simultaneously with many neighboring molecules, which also affects the value of the minimum potential energy of molecules.

In addition, all the molecules of a substance are in continuous motion, i.e. have kinetic energy.

Thus, the structure of a substance and its properties (solid, liquid and gaseous bodies) are determined by the ratio between the minimum potential energy of interaction of molecules and the kinetic energy of the thermal motion of molecules.

The structure and properties of solid, liquid and gaseous bodies

The structure of bodies is explained by the interaction of body particles and the nature of their thermal motion.

Solid

Solids have a constant shape and volume, and are practically incompressible.
The minimum potential energy of interaction of molecules is greater than the kinetic energy of molecules.
Strong interaction of particles.

The thermal motion of molecules in a solid is expressed only by oscillations of particles (atoms, molecules) around the position of stable equilibrium.

Due to the large forces of attraction, molecules practically cannot change their position in a substance, which explains the invariance of the volume and shape of solids.

Most solids have a spatially ordered arrangement of particles that form a regular crystal lattice. Particles of matter (atoms, molecules, ions) are located at the vertices - the nodes of the crystal lattice. The nodes of the crystal lattice coincide with the position of stable equilibrium of the particles.
Such solids are called crystalline.

Liquid

Liquids have a certain volume, but do not have their own shape, they take the shape of the vessel in which they are located.
The minimum potential energy of interaction of molecules is comparable to the kinetic energy of molecules.
Weak particle interaction.
The thermal motion of molecules in a liquid is expressed by oscillations around the position of stable equilibrium within the volume provided to the molecule by its neighbors

Molecules cannot move freely throughout the entire volume of a substance, but transitions of molecules to neighboring places are possible. This explains the fluidity of the liquid, the ability to change its shape.

In liquids, the molecules are quite strongly bound to each other by attractive forces, which explains the invariance of the volume of the liquid.

In a liquid, the distance between molecules is approximately equal to the diameter of the molecule. With a decrease in the distance between molecules (compressing a liquid), the repulsive forces sharply increase, so liquids are incompressible.

In terms of their structure and nature of thermal motion, liquids occupy an intermediate position between solids and gases.
Although the difference between a liquid and a gas is much greater than between a liquid and a solid. For example, during melting or crystallization, the volume of a body changes many times less than during evaporation or condensation.

Gases do not have a constant volume and occupy the entire volume of the vessel in which they are located.
The minimum potential energy of interaction of molecules is less than the kinetic energy of molecules.
Particles of matter practically do not interact.
Gases are characterized by a complete disorder in the arrangement and movement of molecules.

Molecules and atoms of a solid body are arranged in a certain order and form crystal lattice. Such solids are called crystalline. The atoms oscillate about the equilibrium position, and the attraction between them is very strong. Therefore, solid bodies under normal conditions retain volume and have their own shape.

Thermal equilibrium is the state of a thermodynamic system into which it spontaneously passes after a sufficiently long period of time under conditions of isolation from the environment.

Temperature is a physical quantity that characterizes the average kinetic energy of the particles of a macroscopic system in a state of thermodynamic equilibrium. In an equilibrium state, the temperature has the same value for all macroscopic parts of the system.

Degree Celsius(symbol: °C) is a common unit of temperature used in the International System of Units (SI) along with the kelvin.

Mercury medical thermometer

Mechanical thermometer

The degree Celsius is named after the Swedish scientist Anders Celsius, who in 1742 proposed a new scale for measuring temperature. Zero on the Celsius scale was the melting point of ice, and 100° was the boiling point of water at standard atmospheric pressure. (Initially, Celsius took the melting temperature of ice as 100 °, and the boiling point of water as 0 °. And only later did his contemporary Carl Linnaeus “turn over” this scale). This scale is linear in the range 0-100° and also continues linearly in the region below 0° and above 100°. Linearity is a major issue with accurate temperature measurements. Suffice it to mention that a classic thermometer filled with water cannot be marked for temperatures below 4 degrees Celsius, because in this range the water begins to expand again.

The original definition of the degree Celsius depended on the definition of standard atmospheric pressure, because both the boiling point of water and the melting point of ice depend on pressure. This is not very convenient for standardizing the unit of measurement. Therefore, after the adoption of the kelvin K as the basic unit of temperature, the definition of the degree Celsius was revised.

According to the modern definition, a degree Celsius is equal to one kelvin K, and the zero of the Celsius scale is set so that the temperature of the triple point of water is 0.01 °C. As a result, the Celsius and Kelvin scales are shifted by 273.15:

26)Ideal gas- a mathematical model of a gas, in which it is assumed that the potential energy of the interaction of molecules can be neglected in comparison with their kinetic energy. The forces of attraction or repulsion do not act between molecules, the collisions of particles between themselves and with the walls of the vessel are absolutely elastic, and the time of interaction between molecules is negligibly small compared to the average time between collisions.

Where k is the Boltzmann constant (the ratio of the universal gas constant R to the number of Avogadro N A), i- the number of degrees of freedom of molecules (in most problems about ideal gases, where molecules are assumed to be spheres of small radius, the physical analogue of which can be inert gases), and T is the absolute temperature.

The basic equation of the MKT connects the macroscopic parameters (pressure, volume, temperature) of a gas system with the microscopic ones (molecular mass, average speed of their movement).

Physics. Molecules. Arrangement of molecules in gaseous, liquid and solid distance.

1. 
   
   
2. In the gaseous state, the molecules are not connected to each other, they are located at a great distance from each other. Brownian motion. The gas can be compressed relatively easily.
   In a liquid, the molecules are close together, vibrating together. Almost incompressible.
   In a solid - the molecules are arranged in a strict order (in crystal lattices), there is no movement of the molecules. Compression will not succumb.
3. The structure of matter and the beginning of chemistry:
   http://samlib.ru/a/anemow_e_m/aa0.shtml
   (without registration and SMS messages, in a convenient text format: you can use Ctrl+C)
4. It is by no means possible to agree that in the solid state the molecules do not move.

   Movement of molecules in gases

   In gases, the distance between molecules and atoms is usually much larger than the size of the molecules, and the attractive forces are very small. Therefore, gases do not have their own shape and constant volume. Gases are easily compressed because the repulsive forces at large distances are also small. Gases have the property of expanding indefinitely, filling the entire volume provided to them. Gas molecules move at very high speeds, collide with each other, bounce off each other in different directions. Numerous impacts of molecules on the walls of the vessel create gas pressure.

   Movement of molecules in liquids

   In liquids, molecules not only oscillate around the equilibrium position, but also jump from one equilibrium position to the next. These jumps happen periodically. The time interval between such jumps is called the average time of settled life (or average relaxation time) and is denoted by the letter?. In other words, the relaxation time is the time of oscillation around one specific equilibrium position. At room temperature, this time is on average 10–11 s. The time of one oscillation is 10-1210-13 s.

   The time of settled life decreases with increasing temperature. The distance between liquid molecules is smaller than the size of the molecules, the particles are close to each other, and the intermolecular attraction is large. However, the arrangement of liquid molecules is not strictly ordered throughout the volume.

   Liquids, like solids, retain their volume, but do not have their own shape. Therefore, they take the form of the vessel in which they are located. A liquid has the property of fluidity. Due to this property, the liquid does not resist shape change, it compresses little, and its physical properties are the same in all directions inside the liquid (liquid isotropy). The nature of molecular motion in liquids was first established by the Soviet physicist Yakov Ilyich Frenkel (1894-1952).

   Movement of molecules in solids

   Molecules and atoms of a solid body are arranged in a certain order and form a crystal lattice. Such solids are called crystalline. The atoms oscillate about the equilibrium position, and the attraction between them is very strong. Therefore, solid bodies under normal conditions retain their volume and have their own shape.

5. In gaseous-move randomly, cut in
   In liquid-moving in line with each other
   In solid - do not move.

Kinetic energy of a molecule

In a gas, the molecules perform free (isolated from other molecules) movement, only from time to time colliding with each other or with the walls of the vessel. As long as the molecule is in free motion, it has only kinetic energy. During the collision, the molecules also have potential energy. Thus, the total energy of a gas is the sum of the kinetic and potential energies of its molecules. The rarefied the gas, the more molecules at each moment of time are in a state of free movement, having only kinetic energy. Consequently, when the gas is rarefied, the share of potential energy decreases in comparison with kinetic energy.

The average kinetic energy of a molecule in the equilibrium of an ideal gas has one very important feature: in a mixture of different gases, the average kinetic energy of a molecule for different components of the mixture is the same.

For example, air is a mixture of gases. The average energy of an air molecule for all its components under normal conditions, when air can still be considered as an ideal gas, is the same. This property of ideal gases can be proved on the basis of general statistical considerations. An important consequence follows from it: if two different gases (in different vessels) are in thermal equilibrium with each other, then the average kinetic energies of their molecules are the same.

In gases, the distance between molecules and atoms is usually much greater than the size of the molecules themselves, the interaction forces of molecules are not large. As a result, the gas does not have its own shape and constant volume. The gas is easily compressible and can expand indefinitely. Gas molecules move freely (translationally, they can rotate), only occasionally colliding with other molecules and the walls of the vessel in which the gas is located, and they move at very high speeds.

Motion of particles in solids

The structure of solids is fundamentally different from the structure of gases. In them, the intermolecular distances are small and the potential energy of the molecules is comparable to the kinetic one. Atoms (or ions, or whole molecules) cannot be called immobile, they perform random oscillatory motion around their middle positions. The higher the temperature, the greater the energy of oscillations, and hence the average amplitude of oscillations. Thermal vibrations of atoms also explain the heat capacity of solids. Let us consider in more detail the motions of particles in crystalline solids. The entire crystal as a whole is a very complex coupled oscillatory system. The deviations of the atoms from the average positions are small, and therefore we can assume that the atoms are subjected to the action of quasi-elastic forces obeying the linear Hooke's law. Such oscillatory systems are called linear.

There is a developed mathematical theory of systems subject to linear oscillations. It proves a very important theorem, the essence of which is as follows. If the system performs small (linear) interconnected oscillations, then by transforming the coordinates it can be formally reduced to a system of independent oscillators (for which the oscillation equations do not depend on each other). The system of independent oscillators behaves like an ideal gas in the sense that the atoms of the latter can also be considered independent.

It is using the idea of ​​the independence of gas atoms that we arrive at Boltzmann's law. This very important conclusion provides a simple and reliable basis for the whole theory of solids.

Boltzmann's law

The number of oscillators with given parameters (coordinates and velocities) is determined in the same way as the number of gas molecules in a given state, according to the formula:

Oscillator energy.

Boltzmann's law (1) in the theory of a solid body has no restrictions, however, formula (2) for the energy of an oscillator is taken from classical mechanics. In the theoretical consideration of solids, it is necessary to rely on quantum mechanics, which is characterized by a discrete change in the energy of an oscillator. The discreteness of the oscillator energy becomes insignificant only at sufficiently high values ​​of its energy. This means that (2) can only be used at sufficiently high temperatures. At high temperatures of a solid, close to the melting point, Boltzmann's law implies the law of uniform distribution of energy over degrees of freedom. If in gases for each degree of freedom, on average, there is an amount of energy equal to (1/2) kT, then the oscillator has one degree of freedom, in addition to kinetic, has potential energy. Therefore, one degree of freedom in a solid at a sufficiently high temperature has an energy equal to kT. Based on this law, it is not difficult to calculate the total internal energy of a solid, and after it, its heat capacity. A mole of a solid contains NA atoms, and each atom has three degrees of freedom. Therefore, the mole contains 3 NA oscillators. Mole energy of a solid body

and the molar heat capacity of a solid at sufficiently high temperatures

Experience confirms this law.

Liquids occupy an intermediate position between gases and solids. Molecules of a liquid do not diverge over long distances, and the liquid under normal conditions retains its volume. But unlike solids, molecules not only oscillate, but also jump from place to place, that is, they make free movements. When the temperature rises, liquids boil (there is a so-called boiling point) and turn into a gas. As the temperature drops, liquids crystallize and become solids. There is a point in the temperature field at which the boundary between gas (saturated vapor) and liquid disappears (critical point). The pattern of thermal motion of molecules in liquids near the solidification temperature is very similar to the behavior of molecules in solids. For example, the heat capacity coefficients are almost the same. Since the heat capacity of a substance during melting changes slightly, it can be concluded that the nature of the movement of particles in a liquid is close to the movement in a solid (at the melting temperature). When heated, the properties of the liquid gradually change, and it becomes more like a gas. In liquids, the average kinetic energy of particles is less than the potential energy of their intermolecular interaction. The energy of intermolecular interaction in liquids and solids differ insignificantly. If we compare the heat of fusion and the heat of evaporation, we will see that during the transition from one state of aggregation to another, the heat of fusion is significantly lower than the heat of vaporization. An adequate mathematical description of the structure of a liquid can only be given with the help of statistical physics. For example, if a liquid consists of identical spherical molecules, then its structure can be described by the radial distribution function g(r), which gives the probability of finding any molecule at a distance r from the given one, chosen as a reference point. Experimentally, this function can be found by studying the diffraction of X-rays or neutrons; it is possible to conduct computer simulations of this function using Newtonian mechanics.

The kinetic theory of liquid was developed by Ya.I. Frenkel. In this theory, the liquid is considered, as in the case of a solid body, as a dynamic system of harmonic oscillators. But unlike a solid body, the equilibrium position of molecules in a liquid is temporary. After oscillating around one position, the liquid molecule jumps to a new position located in the neighborhood. Such a jump occurs with the expenditure of energy. The average "settled life" time of a liquid molecule can be calculated as:

\[\left\langle t\right\rangle =t_0e^(\frac(W)(kT))\left(5\right),\]

where $t_0\ $ is the period of oscillations around one equilibrium position. The energy that a molecule must receive in order to move from one position to another is called the activation energy W, and the time the molecule is in the equilibrium position is called the “settled life” time t.

For a water molecule, for example, at room temperature, one molecule makes about 100 vibrations and jumps to a new position. The forces of attraction between the molecules of a liquid are great to preserve the volume, but the limited sedentary life of the molecules leads to the emergence of such a phenomenon as fluidity. During particle oscillations near the equilibrium position, they continuously collide with each other, therefore, even a small compression of the liquid leads to a sharp "hardening" of particle collisions. This means a sharp increase in the pressure of the liquid on the walls of the vessel in which it is compressed.

Example 1

Task: Determine the specific heat capacity of copper. Assume that the copper temperature is close to the melting point. (Molar mass of copper $\mu =63\cdot 10^(-3)\frac(kg)(mol))$

According to the Dulong and Petit law, a mole of chemically simple substances at temperatures close to the melting point has a heat capacity:

Specific heat capacity of copper:

\[C=\frac(c)(\mu )\to C=\frac(3R)(\mu )\left(1.2\right),\] \[C=\frac(3\cdot 8,31) (63\cdot 10^(-3))=0.39\ \cdot 10^3(\frac(J)(kgK))\]

Answer: The specific heat capacity of copper is $0.39\ \cdot 10^3\left(\frac(J)(kgK)\right).$

Task: Explain in a simplified way from the point of view of physics the process of dissolution of salt (NaCl) in water.

The basis of the modern theory of solutions was created by D.I. Mendeleev. He found that during dissolution, two processes occur simultaneously: physical - uniform distribution of particles of the dissolved substance throughout the volume of the solution, and chemical - interaction of the solvent with the dissolved substance. We are interested in the physical process. Salt molecules do not destroy water molecules. In this case, it would be impossible to evaporate the water. If salt molecules were attached to water molecules, we would get some new substance. And salt molecules cannot penetrate inside water molecules.

An ion-dipole bond occurs between the Na+ and Cl- ions of chlorine and polar water molecules. It turns out to be stronger than the ionic bonds in the salt molecules. As a result of this process, the bond between ions located on the surface of NaCl crystals is weakened, sodium and chlorine ions are detached from the crystal, and water molecules form so-called hydration shells around them. The separated hydrated ions under the influence of thermal motion are uniformly distributed among the solvent molecules.