The nature of the movement of molecules in the solid state. Big encyclopedia of oil and gas
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The nature of the thermal motion of molecules in liquids is more complex than in solids. According to a simplified model, the thermal motions of liquid molecules represent irregular oscillations about certain centers. The kinetic energy of vibrations of individual molecules at some moments may be sufficient to overcome intermolecular bonds. Then these molecules get the opportunity to jump into the environment of other molecules, thereby changing the center of oscillation. Thus, for some time /, called the time of settled life, each molecule is in an ordered system with several nearest molecules. Having made a jump, the liquid molecule finds itself among new molecules, arranged in a different way. Therefore, only short-range order in the arrangement of molecules is observed in a liquid.
Given the conditions on the earth's surface, only some substances can be found naturally in all three states, such as water. Most substances occur in a certain state. Individual molecules are blocked and remain in place, unable to move. Although the atoms and molecules of solids are in motion, the motion is limited by vibrational energy, and the individual molecules are fixed in place and vibrate side by side. As temperature increases solid body the number of vibrations increases, but the solid retains its shape and volume as the molecules are locked in place and do not interact with each other.
The nature of the thermal motion of molecules in a liquid differs significantly from the thermal motion of gas molecules. Due to the randomness of thermal motion, the velocities and amplitudes of oscillations of neighboring molecules are different, and from time to time neighboring molecules diverge from each other so much that individual molecules jump over a distance of the order of d, get stuck in new equilibrium positions and begin to oscillate around them. With increasing temperature, the average energy of thermal motion increases, and with it the amplitude of oscillations and the frequency of jumps of molecules from one equilibrium position to neighboring ones.
To see an example of this, click the following animation, which shows the molecular structure of ice crystals. Although molecules in liquids can move and collide with each other, they remain relatively close, like solids. Typically in liquids, intermolecular forces hold molecules together, which then break apart. As the temperature of the liquid increases, the number of movements of individual molecules increases. As a result, liquids can "circulate" to take the shape of their container, but cannot be compressed easily because the molecules are already very close together.
The nature of the thermal motion of molecules depends on the nature of the interaction of molecules and changes when a substance passes from.
Glass transition is a fast process of changing the nature of the thermal motion of polymer molecules in the amorphous state, which takes place in a narrow temperature range, depending on the rate of deformation at which it is observed. Occurs without changes in the volume of the polymer and without a thermal effect, but with a change in the coefficient of thermal expansion and specific heat.
Therefore, liquids are indefinite form, but a certain amount. In the following animation example, we see that liquid water is made up of molecules that can circulate freely, but nevertheless remain close to each other. Thus, gas molecules interact little, sometimes colliding. In the gaseous state, molecules move quickly and circulate freely in any direction, spreading over long distances. As the temperature increases, the number of movements of individual molecules increases.
Gases expand to fill their containers and have a low density. Because the individual molecules are widely separated and can circulate freely in the gaseous state, gases are easily compressed and can be indefinitely shaped. Plasmas are formed under conditions of extremely high energy, so large that the molecules separate and only free atoms exist. Even more amazing is that the plasma has so much energy that the outer electrons are strongly separated from the individual atoms, forming a highly charged and energetic ion gas.
It lies in the fact that the nature of the thermal motion of PD molecules is closer to the vibrational motions of atoms of crystal lattices and liquid molecules than to the free motion of particles in rarefied gases.
Reader B: Earlier you pointed out that the nature of the thermal motion of molecules depends on the intermolecular interaction and changes during the transition from one state of aggregation to another.
Because atoms in plasmas exist as charged ions, plasmas behave differently from gases and form the fourth state of matter. Plasma can be perceived simply by looking up; The high energy conditions that exist in stars, such as the sun, push individual atoms towards a plasma state.
As we have seen, an increase in energy results in more molecular motion. Conversely, decreasing energy results in less molecular motion. As a result, the prediction of the molecular kinetic theory is that if the energy of matter decreases, we will reach a point where all molecular motion stops. The temperature at which molecular motion stops is called absolute zero and is calculated as -15 degrees Celsius. Although scientists have cooled matter to near absolute zero, they have never been able to reach that temperature.
Such a division of the continuous spectrum of scattered light is dictated by the very nature of the thermal motion of molecules in liquids.
Molecular scattering of light provides very valuable information about the structure and nature of the thermal motion of molecules in scattering media. Work in this area unfolded on a broad front in the 30s; they have largely contributed and continue to contribute to the solution of the problem of the liquid state of matter. Here the merits of the Soviet scientists L. I. Mandelstam, G. S. Landsberg, L. D. Landau, E. F. Gross, S. M. Rytov and their students are especially great.
The difficulty in observing matter at absolute zero temperature is that it takes light to "see" matter, and light transfers energy to matter, which raises the temperature. Despite these challenges, scientists have recently observed a fifth state of matter that only exists at temperatures very close to absolute zero.
In this strange state, all the atoms of the condensates reach the same mechanical-quantum state and can flow without any friction with each other. Several other less common states of matter have also been described or observed. Some of these states include liquid crystals, fermionic condensates, superfluids, supersolids, and the aptly named strange substance.
The theory of the liquid state at its current level, due to the complexity of the structure and nature of the thermal motion of molecules, cannot be used to describe the properties of real liquids in a fairly wide range of temperatures and pressures. Best case scenario statistical theory allows one to establish only a qualitative dependence of the equilibrium properties of liquids on the state parameters and the radial distribution function.
This occurs at very low temperatures close to absolute zero. It was first created at the University of Colorado. Many scientists consider it wrong. The nature of the condensate implies that all the particles that make it up are in the same quantum state, which is possible only if said particles are bosons. Now the Pauli exclusion principle prevents the same pair of Fermions from using the same quantum state at the same time. Therefore, the fermionic condensate should not exist.
Thus, a small change in the heat capacity of a body during melting can be considered as evidence that the nature of the thermal motion of molecules in liquids is the same as in solids, namely, the molecules oscillate around the equilibrium position.
These qualitative differences between the liquid and solid states of a substance are due to the difference in their molecular structure and in the nature of the thermal motion of molecules. When heated, a solid body under certain conditions passes into a liquid state - it melts. The liquid solidifies when the temperature drops.
Jean achieved the condensation of pairs of fermionic atoms. The sum of the spins of a pair of atoms with the same spin will always be integer. If a pair of identical fermionic atoms form a molecule, it will have an integer spin. Therefore, this molecule is a boson that can condense.
While it is true that a Cooper pair can be assimilated to a boson, this does not mean that the formation of Cooper pairs automatically implies the presence of a condensate. To obtain a condensate of Cooper pairs, it is necessary that all be grouped into the same quantum state.
As Samoilov points out, for a comprehensive consideration of the issue of solvation of ions in electrolyte solutions, one cannot be limited to determining the solvation numbers and solvation energy, but it is also necessary to investigate the changes that occur when ions are introduced, not only in the structure of the solvent, but in the nature of the thermal motion of the solvent molecules . All the above changes in the solvent are mainly due to one and the same reason - the interaction between the priests and molecules of the solvent.
The fermionic condensate behaves like a wave and not like a particle, since it remains stable for very little time. The fermion gas molecules are fermions, not bosons, since although only fermions are combined, they will complete spin to an integer and stabilize at that point.
The Pauli exclusion principle states that two fermions cannot occupy the same quantum state at the same time. This changed over time as the electrons stabilized the wave, giving it a stable shape. Jin, Markus Greiner, and Cindy Regal took it one step further and, thanks to the ultrafreezing of particles, found a new state of matter, the sixth, the fermionic gas. According to these physicists, quantum ice is made up of bosons, a class of particles that are gregarious in nature, and their statistical laws tend to favor multiple populations of the same quantum state.
In a liquid, molecules are located at small distances from each other and there are significant forces of intermolecular interaction between them. The nature of the thermal motion of molecules in a liquid differs significantly from the motion of molecules in a gas. Liquid molecules oscillate about certain equilibrium positions.
However, a fermion gas is made up entirely of fermions. They, unlike bosons, are unsociable and, by definition, none of them can ever occupy the same state of motion. A pair of identical fermions cannot occupy the same quantum state. At high temperatures the behavior of these elementary particles is almost imperceptible. However, when they cool down, they tend to seek lower energy states, and it is at this point that the antagonistic nature of bosons and fermions intensifies.
But how do ultrafrozen fermions behave? To solve the puzzle, Boulder physicists used lasers to capture a small cloud of 1,000 potassium atoms. By limiting their natural motion, they cooled the atoms down to millionths above absolute zero. Due to their arcane nature, the fermions of these atoms should repel each other, but this was not the case. By applying a magnetic field to the supercooled atoms, they met briefly in pairs and created a remarkable condensate. According to the parents of the new state, this finding could lead to a wide range of practical applications.
Thermal motion is the chaotic movement of molecules, atoms and ions in gases, solids and liquids. The nature of the thermal motion of molecules, atoms and ions depends on the aggregate state of the substance and is determined by the forces of intermolecular interaction.
Devices and accessories: wire conductor made of the investigated metal, measuring device, electrical measuring instruments.
For example, the fermionic gas offers a new line of research into superconductivity, the phenomenon in which electricity flows without resistance. There may be several states that can be called the seventh state of matter. These states only occur under extreme conditions in space, or only occur during the Big Bang theory of explosions.
In highly symmetrical matter. In weakly symmetrical matter. In the plasma of gluon quarks. These are procedures that serve to separate the components of a mixture, whether solid, liquid or gas mixture. The main methods for separating mixtures are decantation, filtration, centrifugation, fractional dissolution, etc. there are also other methods such as flotation, screening, levigation, ventilation, magnetic separation, crystallization, fractional liquefaction, fractional evaporation, chromatography and solvent extraction.
The movement of molecules of gases, liquids and solids
According to the molecular kinetic theory, one of the founders of which is the great Russian scientist M.V. Lomonosov, All substances are made up of tiny particles - molecules that are in continuous motion and interacting with each other.
A molecule is the smallest particle of a substance that has its own chemical properties. molecules various substances have different atomic composition.
Knowing that the system is a mixture, one or more methods of separating mixtures can be used to isolate two or more components of the mixture. Methods for separating a mixture are called immediate analysis without changing the nature of the substances. And for each type of mixture, there are several different ways separation. Below are the most common ways to separate a mix.
A method used to separate heterogeneous solid-liquid and liquid-liquid mixtures. Examples: cloudy water, water and oil. If we leave the bucket of cloudy water alone for a while, we will notice that the clay will settle, that is, it will go to the bottom of the bucket, this is due to being larger compared to water. Thus, water can be easily removed from the bucket.
In the nature of the movement of molecules of gases, liquids and solids, there is much in common, there are also significant differences.
Common features molecular motion:
a) average speed the more molecules, the higher the temperature of the substance;
b) the speeds of various molecules of a given substance are distributed in such a way that the number of molecules with a particular speed is the greater, the closer this speed is to the most probable speed of movement of the molecules of a given substance at a given temperature.
Water can be removed and liquids can be separated from the mixture. It can be said that most of the things that are present in nature are some kind of mixture. Atmospheric air, rocks, plant leaves and even hair are examples of mixtures, each mixture has different physical properties. In addition, each component of the mixture has unique physical and chemical properties. That is, the fact that the mixture of components does not mean a change in their basic chemical structures. When changing the main structure of a component, chemical reaction rather than just mixing. Thus, the three states of matter are solid, liquid and gaseous.
A significant difference in the nature of the movement of the molecules of gases, liquids and solids is explained by the difference in the force interaction of their molecules, associated with the difference in the average distances between the molecules.
In gases, the average distances between molecules are many times greater than the size of the molecules themselves. As a result, the interaction forces between gas molecules are small and the molecules move throughout the entire vessel in which the gas is located, almost independently of each other, changing the direction and magnitude of the velocity in collisions with other molecules and with the walls of the vessel. The path of a gas molecule is a broken line similar to the trajectory of Brownian motion.
It behaves like a solid body. When trying to classify the various states of matter, it is usually said that there are three states: the gaseous state, solid state and solid state. The difference between these different states has to do with the arrangement of the atoms that make up matter and the mixing of the molecules that make it up.
Since these molecules are very far apart, we understand that gases are light: weighing a gas is first of all weighing the void between molecules, and that gases take up all the space they have: molecules are so numerous that they are not sensitive to gravity.
The mean free path of gas molecules, i.e. the average path length of molecules between two successive collisions depends on the pressure and temperature of the gas. At normal temperature and pressure, the free path is about 10 -5 cm. Gas molecules collide with each other or with the walls of the vessel about 1010 times per second, changing the direction of their movement. This explains the fact that the rate of diffusion of gases is small compared to the rate of translational movement of gas molecules, which under normal conditions is approximately 1.5 times greater than the speed of sound in a given gas and is equal to 500 m/s.
In liquids, the distances between molecules are much smaller than in gases. The forces of interaction of each molecule with its neighbors are sufficiently large, as a result of which the molecules of the liquid oscillate around some average equilibrium positions. At the same time, since the average kinetic energy of liquid molecules is comparable to their interaction energy, molecules with a random excess of kinetic energy overcome the interaction of neighboring particles and change the center of oscillation. Practically oscillating particles of a liquid at very short time intervals (~10 -8 s) jump in space.
Thus, a liquid consists of many microscopic regions in which there is some order in the arrangement of nearby particles, which changes with time and space, i.e. not repeated in the entire volume of the liquid. Such a structure is said to have short-range order .
In solids, the distances between molecules are even smaller, as a result of which the forces of interaction of each molecule with its neighbors are so great that the molecule performs only small oscillations around a certain constant equilibrium position - a node. In a crystalline body, a certain definite mutual arrangement of nodes is distinguished, which is called crystal lattice. The nature of the crystal lattice is determined by the nature of the intermolecular interactions of a given substance.
The foregoing applies to an ideal crystalline solid. In real crystals, there are various violations of the order that occur during the crystallization of a substance.
Along with crystals, there are also amorphous solids in nature, in which, similarly to liquids, atoms vibrate around randomly located nodes. However, the movement of particles of an amorphous body from one center of oscillation to another occurs at such long intervals of time that practically amorphous bodies are solid bodies.
Thermal conductivity
Thermal conductivity is the transfer of heat that occurs in the presence of a temperature gradient and is due to the thermal motion of particles. Figure 1a shows a straight body
coal-shaped with bases 1 and 2 located normal to the axis x. Let the body temperature be a function of one coordinate T = T(x), wherein dT/dx < 0 (температура убывает в положительном направлении оси X). Then, through any section of the body normal to the chosen axis, heat is transferred, which is described by the Fourier law (1820)
where ∆ Q- the amount of heat transferred through the area with a cross section S in time Δ t, c- coefficient of thermal conductivity, depending on the properties of the substance. The minus sign in (1) indicates that the heat transfer is directed towards the temperature decrease (opposite to the temperature gradient dT/dx). If the body is homogeneous and the process is steady, then the temperature drop along the axis X linear: dT/dx=const(Fig. 1b).
Expression (1) allows you to find the density heat flow(heat flow through unit area per unit time):
It follows from the latter that
The thermal conductivity coefficient is numerically equal to the amount of heat transferred through a unit surface area per unit time at a unit temperature gradient. .
When determining the thermal conductivity of gases and liquids, it is necessary to carefully exclude other types of heat transfer - convection (moving the hotter parts of the medium up and lowering the colder ones) and heat transfer by radiation (radiant heat transfer).
The thermal conductivity of a substance depends on its state. Table I shows the values of the thermal conductivity of some substances.
Table I
For liquids (if we exclude liquid metals), the thermal conductivity coefficient is on average less than that of solids, and greater than that of gases. The thermal conductivity of gases and metals increases with increasing temperature, while liquids, as a rule, decrease.
For gases, molecular kinetic theory makes it possible to establish that the thermal conductivity coefficient is equal to
where is the mean free path of molecules,
Average speed of their movement, r - density, c V is the isochoric specific heat capacity.
Mechanism of thermal conductivity of gases, liquids and solids
The randomness of the thermal motion of gas molecules, continuous collisions between them lead to constant mixing of particles and a change in their velocities and energies. AT gas thermal conductivity takes place when there is a temperature difference in it caused by some external causes. Gas molecules in different places of its volume have different average kinetic energies. Therefore, during the chaotic thermal motion of molecules, directed energy transfer . Molecules that have fallen from the heated parts of the gas into colder parts give off an excess of their energy to the surrounding particles. On the contrary, slowly moving molecules, getting from cold parts to hotter parts, increase their energy due to collisions with molecules with high speeds.
Thermal conductivity in liquids as in gases, occurs in the presence of a temperature gradient. However, if in gases energy is transferred during collisions of particles making translational motions, then in liquids energy is transferred during collisions of oscillating particles. Particles with a higher energy oscillate with a larger amplitude and, when colliding with other particles, shake them, as it were, transferring energy to them. Such a mechanism of energy transfer, just like the mechanism operating in gases, does not ensure its rapid transfer and therefore the thermal conductivity of liquids is very low, although it exceeds the thermal conductivity of gases by several times. The exception is liquid metals, whose thermal conductivity coefficients are close to those of solid metals. This is explained by the fact that in liquid metals, heat is transferred not only along with the transfer of vibrations from one particle to another, but also with the help of mobile electrically charged particles - electrons that are present in metals, but absent in other liquids.
If in solid body there is a temperature difference between its various parts, then, just as it happens in gases and liquids, heat is transferred from a more heated to a less heated part.
Unlike liquids and gases, convection cannot occur in a solid body, i.e. the movement of a mass of matter with heat. Therefore, heat transfer in a solid is carried out only by thermal conduction.
The mechanism of heat transfer in a solid follows from the nature of thermal motions in it. A solid body is a collection of atoms that vibrate. But these fluctuations
independent from each other. Vibrations can be transmitted (at the speed of sound) from one atom to another. In this case, a wave is formed, which transfers the energy of vibrations. Such propagation of oscillations is the transfer of heat.
Quantitatively, heat transfer in a solid body is described by expression (1). The value of the thermal conductivity coefficient c cannot be calculated in the same way as it is done for a gas - a simpler system consisting of non-interacting particles.
An approximate calculation of the thermal conductivity of a solid can be performed using quantum concepts.
Quantum theory allows us to compare certain quasiparticles propagating in a solid at the speed of sound with vibrations - phonons. Each particle is characterized by an energy equal to Planck's constant multiplied by the oscillation frequency n. The energy of a quantum of vibrations - a phonon, therefore, is equal to h n.
If we use the concept of phonons, then we can say that thermal motions in a solid are caused precisely by them, so that at absolute zero there are no phonons, and with increasing temperature their number increases, but not linearly, but according to a more complex law (at low temperatures, proportionally temperature cube).
We can now consider a solid body as a vessel containing a gas of phonons, a gas that at very high temperatures can be considered an ideal gas. As in the case of an ordinary gas, heat transfer in a phonon gas is carried out by collisions of phonons with lattice atoms, and all arguments for ideal gas are true here too. Therefore, the thermal conductivity of a solid can be expressed by exactly the same formula
where r is the density of the body, c V is its specific heat capacity, with is the speed of sound in the body, l is the mean free path of phonons.
In metals, in addition to lattice vibrations, charged particles, electrons, also participate in heat transfer, which at the same time are carriers of electric current in the metal. At high temperatures electronic part of the thermal conductivity is much larger lattice . This explains the high thermal conductivity of metals compared to non-metals, in which phonons are the only heat carriers. The coefficient of thermal conductivity of metals can be calculated by the formula:
where is the mean free path of electrons, is the mean velocity of their thermal motion.
In superconductors, in which the electric current does not encounter resistance, there is practically no electronic thermal conductivity: electrons that carry charge without resistance do not participate in heat transfer, and thermal conductivity in superconductors is purely lattice.
Wiedemann-Franz law
Metals have both high electrical conductivity and high thermal conductivity. This is explained by the fact that the carriers of current and heat in metals are the same particles - free electrons, which, when mixed in the metal, carry not only an electric charge, but also the energy of chaotic (thermal) motion inherent in them, i.e. carry out heat transfer.
In 1853, Wiedemann and Franz experimentally established a law according to which thermal conductivity ratio c to electrical conductivity s for metals at the same temperature is the same and increases in proportion to the thermodynamic temperature:
where k and e are constants (Boltzmann constant and electron charge).
Considering electrons as a monatomic gas, for the coefficient of thermal conductivity one can use the expression of the kinetic theory of gases
where n×m= r is the density of the gas.
Specific heat monatomic gas is equal to . Substituting this value into the expression for χ , we obtain
According to the classical theory of metals, their electrical conductivity
Then the relation
After replacing , we arrive at relation (5), which expresses Wiedemann-Franz law .
Substituting the values k= 1.38 10 -23 J/K and e= 1.60 10 -19 C into formula (5), we find
If, using this formula, calculate the value for all metals at T\u003d 300 K, then we get 6.7 10 -6 J Ω / s K. The Wiedemann-Franz law for most metals corresponds to experience at temperatures of 100–400 K, but at low temperatures the law is significantly violated. The discrepancies between the calculated and experimental data at low temperatures are especially great for silver, copper, and gold. There are metals (beryllium, manganese) that do not obey the Wiedemann-Franz law at all.