Structure of matter. Models of the structure of gas, liquid and solid. Model of the structure of solids. Change of aggregative states of matter. Crystalline bodies. Properties of single crystals. Amorphous bodies

Structure of gases, liquids and solids.

Basic principles of molecular kinetic theory:

All substances are made up of molecules, and molecules are made up of atoms,

atoms and molecules are in constant motion,

There are forces of attraction and repulsion between molecules.

IN gases molecules move chaotically, the distances between molecules are large, molecular forces are small, the gas occupies the entire volume provided to it.

IN liquids molecules are arranged in an orderly manner only at short distances, and at large distances the order (symmetry) of the arrangement is violated - “short-range order”. The forces of molecular attraction keep molecules close together. The movement of molecules is “jumping” from one stable position to another (usually within one layer. This movement explains the fluidity of a liquid. A liquid has no shape, but has volume.

Solids are substances that retain their shape, divided into crystalline and amorphous. Crystalline solids bodies have a crystal lattice, in the nodes of which there may be ions, molecules or atoms. They oscillate relative to stable equilibrium positions.. Crystal lattices have a regular structure throughout the entire volume - “long-range order” of arrangement.

Amorphous bodies retain their shape, but do not have a crystal lattice and, as a result, do not have a pronounced melting point. They are called frozen liquids, since they, like liquids, have a “short-range” order of molecular arrangement.

Molecular interaction forces

All molecules of a substance interact with each other through forces of attraction and repulsion. Evidence of the interaction of molecules: the phenomenon of wetting, resistance to compression and tension, low compressibility of solids and gases, etc. The reason for the interaction of molecules is the electromagnetic interactions of charged particles in a substance. How to explain this? 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 the electrons, so the atom as a whole is electrically neutral. A molecule consisting of one or more atoms is also electrically neutral. Let's consider the interaction between molecules using the example of two stationary molecules. Gravitational and electromagnetic forces can exist between bodies in nature. Since the masses of molecules are extremely small, negligible forces of gravitational interaction between molecules can be ignored. At very large distances there is also no electromagnetic interaction between molecules. But, as the distance between molecules decreases, the molecules begin to orient themselves in such a way 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 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 acted upon by the sum of the forces of attraction and repulsion. At large distances, the force of attraction predominates (at a distance of 2-3 diameters of the molecule, attraction is maximum), at short distances the force of repulsion prevails. There is a distance between molecules at which the attractive forces become equal to the repulsive forces. 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 a stable equilibrium position, the potential energy of the 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 the molecules. In addition, all 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 relationship between the minimum potential energy of interaction of molecules and the reserve of kinetic energy of thermal motion of molecules.

Structure and properties of solid, liquid and gaseous bodies

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

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

Due to the large forces of attraction, molecules practically cannot change their position in matter, this explains the invariability 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 - 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 between molecules is comparable to the kinetic energy of molecules. Weak particle interaction. The thermal motion of molecules in a liquid is expressed by vibrations around a stable equilibrium position 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 and the ability to change its shape.

In liquids, molecules are quite firmly bound to each other by forces of attraction, 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. When the distance between molecules decreases (compression of the liquid), the repulsive forces increase sharply, so liquids are incompressible. In terms of their structure and the nature of thermal movement, 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 between molecules is less than the kinetic energy of molecules. Particles of matter practically do not interact. Gases are characterized by complete disorder in the arrangement and movement of molecules.

The distance between gas molecules is many times greater than the size of the molecules. Small attractive forces cannot keep molecules close to each other, so gases can expand without limit. Gases are easily compressed under the influence of external pressure, because the distances between molecules are large, and the interaction forces are negligible. The gas pressure on the walls of the vessel is created by the impacts of moving gas molecules.

All objects and things that surround us every day are composed of various substances. At the same time, we are accustomed to consider only something solid as objects and things - for example, a table, chair, cup, pen, book, and so on.

Three states of matter

But we don’t consider water from the tap or steam coming from hot tea to be objects and things. But all this is also part of the physical world, it’s just that liquids and gases are in a different state of matter. So, There are three states of matter: solid, liquid and gaseous. And any substance can be in each of these states in turn. If we take an ice cube out of the freezer and heat it, it will melt and turn into water. If we leave the burner on, the water will heat up to 100 degrees Celsius and soon turn into steam. Thus, we observed the same substance, that is, the same set of molecules, in turn in different states of matter. But if the molecules remain the same, what then changes? Why is ice hard and retains its shape, water easily takes the shape of a cup, and steam completely scatters in different directions? It's all about the molecular structure.

Molecular structure of solids such that the molecules are located very close to each other (the distance between the molecules is much less than the size of the molecules themselves), and it is very difficult to move the molecules in this arrangement. Therefore, solids retain volume and keep their shape. Molecular structure of liquid characterized by the fact that the distance between the molecules is approximately equal to the size of the molecules themselves, that is, the molecules are no longer as close as in solids. This means that they are easier to move relative to each other (which is why liquids take on different shapes so easily), but the attractive force of the molecules is still sufficient to prevent the molecules from flying apart and maintaining their volume. And here molecular structure of gas, on the contrary, does not allow the gas to either maintain volume or maintain shape. The reason is that the distance between gas molecules is much greater than the size of the molecules themselves, and even the slightest force can destroy this shaky system.

The reason for the transition of a substance to another state

Now let’s find out what is the reason for the transition of a substance from one state to another. For example, why does ice become water when heated? The answer is simple: The thermal energy of the burner is converted into the internal energy of ice molecules. Having received this energy, the ice molecules begin to vibrate faster and faster and, in the end, become out of control of neighboring molecules. If we turn off the heating device, then the water will remain water, but if we leave it on, then the water will turn into steam for a reason already known there.

Due to the fact that solids retain volume and shape, they are the ones we associate with the world around us. But if we look closely, we will find that gases and liquids also occupy an important part of the physical world. For example, the air around us consists of a mixture of gases, the main of which, nitrogen, can also be a liquid - but for this it must be cooled to a temperature of almost minus 200 degrees Celsius. But the main element of an ordinary paw - a tungsten filament - can be melted, that is, turned into liquid, on the contrary, only at a temperature of 3422 degrees Celsius.

In the previous two paragraphs, we examined the structure and properties of solids - crystalline and amorphous. Let us now move on to the study of the structure and properties of liquids.

A characteristic feature of the liquid is fluidity– the ability to change shape in a short time under the influence of even small forces. Thanks to this, liquids flow in streams, flow in streams, and take the shape of the vessel into which they are poured.

The ability to change shape is expressed differently in different liquids. Take a look at the picture. Under the influence of approximately equal gravity, honey takes longer to change its shape than water. Therefore, they say that these substances have unequal viscosity: honey has more than water. This is explained by the unequally complex structure of the molecules of water and honey. Water is made up of molecules that resemble lumpy balls, while honey is made up of molecules that look like tree branches. Therefore, as honey moves, the “branches” of its molecules engage with each other, giving it greater viscosity than water.

Important: changing shape, the liquid retains its volume. Let's consider the experiment (see figure). The liquid in the beaker has the shape of a cylinder and a volume of 300 ml. After pouring into the bowl, the liquid took on a flat shape, but retained the same volume: 300 ml. This is explained by the attraction and repulsion of its particles: on average, they continue to remain at the same distances from each other.

One more A common property of all liquids is their submission to Pascal's law. In grade 7, we learned that it describes the property of liquids and gases to transfer the pressure exerted on them in all directions (see § 4-c). Now note that less viscous liquids do this quickly, while viscous ones take a long time.

The structure of liquids. In molecular kinetic theory it is believed that in liquids, as in amorphous bodies, there is no strict order in the arrangement of particles, that is, they are not equally dense. The gaps have different sizes, including such that another particle can fit there. This allows them to jump from “densely populated” places to freer ones. Jumping of each liquid particle occurs very often: several billion times per second.

If some external force (for example, gravity) acts on the liquid, the movement and jumping of particles will occur mainly in the direction of its action (down). This will cause the liquid to take the form of an elongated drop or flowing stream (see figure). So, The fluidity of liquids is explained by the jumps of their particles from one stable position to another.

Jumps of liquid particles occur frequently, but much more often their particles, as in solids, oscillate in one place, continuously interacting with each other. Therefore, even a small compression of a liquid leads to a sharp “hardening” of the interaction of particles, which means a sharp increase in the pressure of the liquid on the walls of the vessel in which it is compressed. This explains the transmission of pressure by liquids, that is, Pascal’s law, and, at the same time, the property of liquids to resist compression, that is, to maintain volume.

Note that a liquid retaining its volume is a conditional representation. This means that in comparison with gases, which are easy to compress even with the strength of a child’s hand (for example, in a balloon), liquids can be considered incompressible. However, at a depth of 10 km in the World Ocean, water is under such high pressure that every kilogram of water reduces its volume by 5% - from 1 liter to 950 ml. Using greater pressure, liquids can be compressed even more.

1. Model of the structure of solids. Change of aggregative states of matter. Crystalline bodies. Properties of single crystals. Amorphous bodies.

A solid is a state of aggregation of a substance, characterized by constancy of shape and the nature of the movement of atoms, which perform small vibrations around equilibrium positions.

In the absence of external influences, a solid body retains its shape and volume.

This is explained by the fact that the attraction between atoms (or molecules) is greater than that of liquids (and especially gases). It is sufficient to keep the atoms near their equilibrium positions.

The molecules or atoms of most solids, such as ice, salt, diamond, and metals, are arranged in a certain order. Such solids are called crystalline . Although the particles of these bodies are in motion, these movements represent oscillations around certain points (equilibrium positions). The particles cannot move far from these points, so the solid retains its shape and volume.

In addition, unlike liquids, the equilibrium points of atoms or ions of a solid body, being connected, are located at the vertices of a regular spatial lattice, which is called crystalline.

The equilibrium positions relative to which thermal vibrations of particles occur are called nodes of the crystal lattice.

Monocrystal- a solid body whose particles form a single crystal lattice (single crystal).

One of the main properties of single crystals, which distinguishes them from liquids and gases, is anisotropy their physical properties. Under anisotropy refers to the dependence of physical properties on direction in a crystal . Anisotropic are mechanical properties (for example, it is known that mica is easy to exfoliate in one direction and very difficult in a perpendicular direction), electrical properties (the electrical conductivity of many crystals depends on the direction), optical properties (the phenomenon of birefringence, and dichroism - anisotropy of absorption; so, for example, a single crystal of tourmaline is “colored” in different colors - green and brown, depending on which side you look at it from).

Polycrystal- a solid consisting of randomly oriented single crystals. Most of the solids we deal with in everyday life are polycrystalline - salt, sugar, various metal products. The random orientation of the fused microcrystals of which they consist leads to the disappearance of the anisotropy of properties.

Crystalline bodies have a certain melting point.

Amorphous bodies. In addition to crystalline bodies, amorphous bodies are also classified as solids. Amorphous means “shapeless” in Greek.

Amorphous bodies- these are solid bodies that are characterized by a disordered arrangement of particles in space.

In these bodies, molecules (or atoms) vibrate around randomly located points and, like liquid molecules, have a certain settled life time. But, unlike liquids, this time is very long.

Amorphous bodies include glass, amber, various other resins, and plastics. Although at room temperature these bodies retain their shape, but as the temperature rises they gradually soften and begin to flow like liquids: Amorphous bodies do not have a certain temperature or melting point.

In this they differ from crystalline bodies, which, with increasing temperature, do not gradually, but abruptly, transform into a liquid state (at a very specific temperature - melting point).

All amorphous bodies isotropic, i.e., they have the same physical properties in different directions. When impacted, they behave like solid bodies - they split, and if exposed for a very long time, they flow.

Currently, there are many substances in an amorphous state obtained artificially, for example, amorphous and glassy semiconductors, magnetic materials and even metals.

2. Dispersion of light. Types of spectra. Spectrograph and spectroscope. Spectral analysis. Types of electromagnetic radiation and their application in railway transport.

A ray of white light passing through a triangular prism is not only deflected, but also decomposed into component colored rays.
This phenomenon was discovered by Isaac Newton through a series of experiments.

Structure of gases, liquids and solids. Features of the structure of solutions. The concept of a “reactive field”
The theory of the structure of liquids: comparison with the structure of gases and solids Structure (structure) of liquids. The structure of liquids is currently the subject of close study by physical chemists. For research in this direction, the most modern methods are used, including spectral (IR, NMR, light scattering of various wavelengths), X-ray scattering, quantum mechanical and statistical calculation methods, etc. The theory of liquids is much less developed than that of gases, since the properties of liquids depend on the geometry and polarity of mutually closely located molecules. In addition, the lack of a specific structure of liquids makes their formal description difficult - in most textbooks much less space is devoted to liquids than to gases and crystalline solids. What are the features of each of the three aggregate states of matter: solid, liquid and gas. (table)
1) Solid: the body retains volume and shape
2) Liquids retain volume, but easily change shape.
3) Gas has neither shape nor volume.

These states of the same substance differ not in the sort of molecules (it is the same), but in how the molecules are located and move.
1) In gases, the distance between molecules is much greater than the size of the molecules themselves
2) The molecules of the liquid do not disperse over long distances and the liquid under normal conditions retains its volume.
3) Particles of solids are arranged in a certain order. Each particle moves around a certain point in the crystal lattice, like a clock pendulum, that is, it oscillates.
When the temperature decreases, liquids solidify, and when they rise above the boiling point, they turn into a gaseous state. This fact alone indicates that liquids occupy an intermediate position between gases and solids, differing from both. However, the liquid has similarities with each of these states.
There is a temperature at which the boundary between gas and liquid completely disappears. This is the so-called critical point. For each gas there is a known temperature above which it cannot be liquid at any pressure; at this critical temperature the boundary (meniscus) between the liquid and its saturated vapor disappears. The existence of a critical temperature (“absolute boiling point”) was established by D.I. Mendeleev in 1860. The second property that unites liquids and gases is isotropy. That is, at first glance it can be assumed that liquids are closer to gases than to crystals. Just like gases, liquids are isotropic, i.e. their properties are the same in all directions. Crystals, on the contrary, are anisotropic: the refractive index, compressibility, strength and many other properties of crystals in different directions turn out to be different. Solid crystalline substances have an ordered structure with repeating elements, which allows them to be studied by X-ray diffraction (X-ray diffraction method, used since 1912).

What do liquids and gases have in common?
A) Isotropy. The properties of liquids, like gases, are the same in all directions, i.e. are isotropic, unlike crystals, which are anisotropic.
B) Liquids, like gases, do not have a specific shape and take the shape of a container (low viscosity and high fluidity).
Molecules of both liquids and gases move fairly freely, colliding with each other. Previously, it was believed that within the volume occupied by a liquid, any distance exceeding the sum of their radii was considered equally probable, i.e. the tendency towards an ordered arrangement of molecules was denied. Thus, liquids and gases were to a certain extent opposed to crystals.
As research progressed, an increasing number of facts indicated the presence of similarities between the structure of liquids and solids. For example, the values ​​of heat capacities and compressibility coefficients, especially near the melting point, practically coincide with each other, while these values ​​for liquid and gas differ sharply.
Already from this example we can conclude that the picture of thermal motion in liquids at a temperature close to the solidification temperature resembles thermal motion in solids, and not in gases. Along with this, one can note such significant differences between the gaseous and liquid states of matter. In gases, molecules are distributed throughout space completely chaotically, i.e. the latter is considered an example of structureless education. The liquid still has a certain structure. This is experimentally confirmed by X-ray diffraction, which shows at least one clear maximum. The structure of a liquid is the way its molecules are distributed in space. The table illustrates the similarities and differences between gas and liquid states.
Gas phase Liquid phase
1. The distance between molecules l is usually (for low pressures) much larger than the radius of the molecule r: l  r ; Almost the entire volume V occupied by gas is free volume. In the liquid phase, on the contrary, l 2. The average kinetic energy of particles, equal to 3/2kT, is greater than the potential energy U of their intermolecular interaction. The potential energy of interaction of molecules is greater than the average kinetic energy of their movement: U3/2 kT
3. Particles collide during their translational motion, the collision frequency factor depends on the mass of the particles, their size and temperature. Each particle undergoes oscillatory motion in a cage created by the molecules surrounding it. The vibration amplitude a depends on the free volume, a  (Vf/ L)1/3
4. Diffusion of particles occurs as a result of their translational motion, diffusion coefficient D  0.1 - 1 cm2/s (p  105 Pa) and depends on gas pressure
(D  p-1) Diffusion occurs as a result of a particle jumping from one cell to another with activation energy ED,
D  e-ED/RT in non-viscous liquids
D  0.3 - 3 cm2/day.
5. The particle rotates freely, the rotation frequency r is determined only by the moments of inertia of the particle and temperature, the rotation frequency r T1/2 The rotation is inhibited by the walls of the cell, the rotation of the particle is accompanied by overcoming the potential barrier Er, which depends on the forces of intermolecular interaction, vr  e- Er/RT
However, the liquid state is close to the solid state in a number of important indicators (quasicrystallinity). The accumulation of experimental facts indicated that liquids and crystals have much in common. Physicochemical studies of individual liquids have shown that almost all of them possess some elements of a crystalline structure.
Firstly, intermolecular distances in a liquid are close to those in a solid. This is proven by the fact that when the latter melts, the volume of the substance changes slightly (usually it increases by no more than 10%). Secondly, the energy of intermolecular interaction in a liquid and in a solid differs slightly. This follows from the fact that the heat of fusion is much less than the heat of evaporation. For example, for water Hpl = 6 kJ/mol, and Hsp = 45 kJ/mol; for benzene Hpl = 11 kJ/mol, and Hsp = 48 kJ/mol.
Thirdly, the heat capacity of a substance changes very little during melting, i.e. it is close for both of these states. It follows that the nature of the motion of particles in a liquid is close to that in a solid. Fourthly, a liquid, like a solid, can withstand large tensile forces without breaking.
The difference between a liquid and a solid is fluidity: a solid retains its shape, a liquid easily changes it even under the influence of a small force. These properties arise from such structural features of the liquid as strong intermolecular interaction, short-range order in the arrangement of molecules and the ability of molecules to change their position relatively quickly. When a liquid is heated from the freezing point to the boiling point, its properties gradually change; with heating, its similarities with a gas gradually increase.
Each of us can easily recall many substances that he considers liquids. However, it is not so easy to give an exact definition of this state of matter, since liquids have such physical properties that in some respects they resemble solids and in others they resemble gases. The similarities between liquids and solids are most pronounced in glassy materials. Their transition from solid to liquid with increasing temperature occurs gradually, and not as a pronounced melting point, they simply become softer and softer, so it is impossible to indicate in which temperature range they should be called solids and in which liquids. We can only say that the viscosity of a glassy substance in a liquid state is less than in a solid state. Solid glasses are therefore often called supercooled liquids. Apparently, the most characteristic property of liquids, which distinguishes them from solids, is low viscosity, i.e. high turnover. Thanks to it, they take the shape of the vessel into which they are poured. At the molecular level, high fluidity means relatively greater freedom of fluid particles. In this respect, liquids resemble gases, although the forces of intermolecular interaction between liquids are greater, the molecules are located closer together and are more limited in their movement.
This can be approached differently - from the point of view of the idea of ​​long-range and short-range order. Long-range order exists in crystalline solids, the atoms of which are arranged in a strictly ordered manner, forming three-dimensional structures that can be obtained by repeating the unit cell many times. There is no long-range order in liquids and glass. This, however, does not mean that they are not ordered at all. The number of nearest neighbors for all atoms is almost the same, but the arrangement of atoms as they move away from any selected position becomes more and more chaotic. Thus, order exists only at short distances, hence the name: short-range order. 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 detecting any molecule at a distance r from the given one chosen as a reference point. This function can be found experimentally by studying the diffraction of x-rays or neutrons, and with the advent of high-speed computers, it began to be calculated by computer simulation, based on existing data on the nature of the forces acting between molecules, or on assumptions about these forces, as well as on Newton's laws of mechanics . By comparing radial distribution functions obtained theoretically and experimentally, it is possible to verify the correctness of assumptions about the nature of intermolecular forces.
In organic substances, the molecules of which have an elongated shape, in one temperature range or another, regions of the liquid phase with long-range orientational order are sometimes found, which manifests itself in a tendency to parallel alignment of the long axes of the molecules. In this case, orientational ordering can be accompanied by coordination ordering of the centers of molecules. Liquid phases of this type are usually called liquid crystals. The liquid crystalline state is intermediate between crystalline and liquid. Liquid crystals possess both fluidity and anisotropy (optical, electrical, magnetic). Sometimes this state is called mesomorphic (mesophase) - due to the absence of long-range order. The upper limit of existence is the clearing temperature (isotropic liquid). Thermotropic (mesogenic) FAs exist above a certain temperature. Typical ones are cyanobiphenyls. Lyotropic - when dissolved, for example, aqueous solutions of soaps, polypeptides, lipids, DNA.
The study of liquid crystals (mesophase - melting in two stages - cloudy melt, then transparent, transition from the crystalline phase to the liquid through an intermediate form with anisotropic optical properties) is important for technology purposes - liquid crystal display.
Molecules in a gas move chaotically (randomly). In gases, the distance between atoms or molecules is on average many times greater than the size of the molecules themselves. Molecules in gas move at high speeds (hundreds of m/s). When they collide, they bounce off each other like absolutely elastic balls, changing the magnitude and direction of the velocities. At large distances between molecules, the attractive forces are small and are not able to hold gas molecules near each other. Therefore, gases can expand without limit. Gases are easily compressed, the average distance between molecules decreases, but still remains larger than their size. Gases retain neither shape nor volume; their volume and shape coincide with the volume and shape of the vessel they fill. Numerous impacts of molecules on the walls of the vessel create gas pressure.
The molecules of the liquid are located almost close to each other. Therefore, liquids are very difficult to compress and retain their volume. Molecules of a liquid vibrate around an equilibrium position. From time to time, a molecule makes transitions from one stationary state to another, usually in the direction of the action of an external force. The time of the settled state of a molecule is short and decreases with increasing temperature, and the time of transition of the molecule to a new settled state is even shorter. Therefore, liquids are fluid, do not retain their shape and take the shape of the vessel into which they are poured.

Kinetic theory of liquids Developed by Ya. I. Frenkel, the kinetic theory of liquids considers a liquid as a dynamic system of particles, partly reminiscent of a crystalline state. At temperatures close to the melting point, thermal motion in a liquid is reduced mainly to harmonic vibrations of particles around certain average equilibrium positions. In contrast to the crystalline state, these equilibrium positions of molecules in a liquid are temporary in nature for each molecule. After oscillating around one equilibrium position for some time t, the molecule jumps to a new position located nearby. Such a jump occurs with the expenditure of energy U, therefore the “settled life” time t depends on temperature as follows: t = t0 eU/RT, where t0 is the period of one oscillation around the equilibrium position. For water at room temperature t » 10-10 s, t0 = 1.4 x 10-12 s, i.e. one molecule, having completed about 100 vibrations, jumps to a new position, where it continues to oscillate. From data on the scattering of X-rays and neutrons, it is possible to calculate the particle distribution density function  depending on the distance r from one particle chosen as the center. In the presence of long-range order in a crystalline solid, the function (r) has a number of clear maxima and minima. In a liquid, due to the high mobility of particles, only short-range order is maintained. This clearly follows from the X-ray diffraction patterns of liquids: the function (r) for a liquid has a clear first maximum, a blurry second one, and then (r) = const. The kinetic theory describes melting as follows. In the crystal lattice of a solid, there are always small amounts of vacancies (holes) that slowly wander around the crystal. The closer the temperature is to the melting point, the higher the concentration of “holes”, and the faster they move through the sample. At the melting point, the process of formation of “holes” acquires an avalanche-like cooperative character, the system of particles becomes dynamic, long-range order disappears, and fluidity appears. The decisive role in melting is played by the formation of free volume in the liquid, which makes the system fluid. The most important difference between a liquid and a solid crystalline body is that there is a free volume in the liquid, a significant part of which has the form of fluctuations (“holes”), the wandering of which through the liquid gives it such a characteristic quality as fluidity. The number of such “holes”, their volume and mobility depend on temperature. At low temperatures, a liquid, if it has not turned into a crystalline body, becomes an amorphous solid with very low fluidity due to a decrease in volume and mobility of “holes”. Along with the kinetic theory, the statistical theory of liquids has been successfully developing in recent decades.

Structure of ice and water. The most important and common liquid under normal conditions is water. This is the most common molecule on Earth! It is an excellent solvent. For example, all biological fluids contain water. Water dissolves many inorganic (salts, acids, bases) and organic substances (alcohols, sugars, carboxylic acids, amines). What is the structure of this liquid? We will again have to return to the issue that we considered in the first lecture, namely, to such a specific intermolecular interaction as the hydrogen bond. Water, both in liquid and crystalline form, exhibits anomalous properties precisely because of the presence of many hydrogen bonds. What are these anomalous properties: high boiling point, high melting point and high enthalpy of vaporization. Let's look first at the graph, then at the table, and then at the diagram of a hydrogen bond between two water molecules. In fact, each water molecule coordinates 4 other water molecules around itself: two due to oxygen, as a donor of two lone electron pairs to two protonated hydrogens, and two due to protonated hydrogens, coordinated with the oxygens of other water molecules. In the previous lecture, I showed you a slide with graphs of the melting point, boiling point and enthalpy of vaporization of group VI hydrides depending on the period. These dependences have a clear anomaly for oxygen hydride. All these parameters for water are noticeably higher than those predicted from the almost linear dependence for the following hydrides of sulfur, selenium and tellurium. We explained this by the existence of a hydrogen bond between protonated hydrogen and the electron density acceptor - oxygen. Hydrogen bonding is most successfully studied using vibrational infrared spectroscopy. The free OH group has a characteristic vibrational energy that causes the O-H bond to alternately lengthen and shorten, giving rise to a characteristic band in the infrared absorption spectrum of the molecule. However, if the OH group is involved in a hydrogen bond, the hydrogen atom becomes bound by atoms on both sides and thus its vibration is “damped” and the frequency decreases. The following table shows that increasing the strength and “concentration” of the hydrogen bond leads to a decrease in the absorption frequency. In the above figure, curve 1 corresponds to the maximum of the infrared absorption spectrum of O-H groups in ice (where all H-bonds are connected); curve 2 corresponds to the maximum of the infrared absorption spectrum of O-H groups of individual H2O molecules dissolved in CCl4 (where there are no H bonds - the solution of H2O in CCl4 is too dilute); and curve 3 corresponds to the absorption spectrum of liquid water. If in liquid water there were two types of O-H groups - those that form hydrogen bonds and those that do not - and some O-H groups in water would vibrate in the same way (with the same frequency) as in ice (where they form H- bonds), and others - as in the environment of CCl4 (where they do not form H-bonds). Then the spectrum of water would have two maxima, corresponding to two states of O-H groups, their two characteristic vibration frequencies: with the frequency at which the group vibrates, it absorbs light. But the “two-maximum” picture is not observed! Instead, on curve 3 we see one, very blurred maximum, extending from the maximum of curve 1 to the maximum of curve 2. This means that all O-H groups in liquid water form hydrogen bonds - but all these bonds have a different energy, “loose” (have different energy), and in different ways. This shows that the picture in which some of the hydrogen bonds in water are broken and some are preserved is, strictly speaking, incorrect. However, it is so simple and convenient for describing the thermodynamic properties of water that it is widely used - and we will also turn to it. But we must keep in mind that it is not entirely accurate.
Thus, IR spectroscopy is a powerful method for studying hydrogen bonding, and much information about the structure of liquids and solids associated with it has been obtained using this spectral method. As a result, for liquid water the ice-like model (O.Ya. Samoilov’s model) is one of the most generally accepted. According to this model, liquid water has an ice-like tetrahedral framework disturbed by thermal motion (evidence and consequence of thermal motion - Brownian motion, which was first observed by the English botanist Robert Brown in 1827 on pollen under a microscope) (each water molecule in an ice crystal is connected by hydrogen bonds with a reduced energy compared to that in ice - “loose” hydrogen bonds) with four surrounding water molecules), the voids of this frame are partially filled with water molecules, and the water molecules located in the voids and in the nodes of the ice-like frame are energetically unequal.

Unlike water, in an ice crystal, at the nodes of the crystal lattice there are water molecules of equal energy and they can only perform vibrational movements. In such a crystal there is both short- and long-range order. In liquid water (as for a polar liquid), some elements of the crystal structure are preserved (and even in the gas phase, liquid molecules are ordered into small, unstable clusters), but there is no long-range order. Thus, the structure of a liquid differs from the structure of a gas in the presence of short-range order, but differs from the structure of a crystal in the absence of long-range order. This is most convincingly demonstrated by the study of X-ray scattering. The three neighbors of each molecule in liquid water are located in one layer and are at a greater distance from it (0.294 nm) than the fourth molecule from the adjacent layer (0.276 nm). Each water molecule in the ice-like framework forms one mirror-symmetric (strong) and three centrally symmetric (less strong) bonds. The first refers to the bonds between water molecules of a given layer and neighboring layers, the rest - to the bonds between water molecules of the same layer. Therefore, a quarter of all connections are mirror-symmetric, and three-quarters are centrally symmetric. Ideas about the tetrahedral environment of water molecules have led to the conclusion that its structure is highly delicacy and the presence of voids in it, the dimensions of which are equal to or greater than the dimensions of water molecules.

Elements of the structure of liquid water. a - elementary water tetrahedron (open circles - oxygen atoms, black halves - possible positions of protons on the hydrogen bond); b - mirror-symmetric arrangement of tetrahedra; c - centrally symmetrical arrangement; d - location of oxygen centers in the structure of ordinary ice. Water is characterized by significant forces of intermolecular interaction due to hydrogen bonds, which form a spatial network. As we said in the previous lecture, a hydrogen bond is caused by the ability of a hydrogen atom connected to an electronegative element to form an additional bond with an electronegative atom of another molecule. The hydrogen bond is relatively strong and amounts to several 20-30 kilojoules per mole. In terms of strength, it occupies an intermediate place between the van der Waals energy and the energy of a typical ionic bond. In a water molecule, the energy of the H-O chemical bond is 456 kJ/mol, and the energy of the H…O hydrogen bond is 21 kJ/mol.

Hydrogen compounds
Molecular weight Temperature,  C
Freezing Boiling
H2Te 130 -51 -4
H2Se 81 -64 -42
H2S 34 -82 -61
H2O 18 0! +100!

Ice structure. Normal ice. Dotted line - H-bonds. In the openwork structure of the ice, small cavities are visible, surrounded by H2O molecules.
Thus, the structure of ice is an openwork structure of water molecules connected to each other only by hydrogen bonds. The arrangement of water molecules in the ice structure determines the presence of wide channels in the structure. As ice melts, water molecules “fall” into these channels, which explains the increase in the density of water compared to the density of ice. Ice crystals occur in the form of regular hexagonal plates, tabular formations, and intergrowths of complex shapes. The structure of normal ice is dictated by hydrogen H bonds: it is good for the geometry of these bonds (the O-H faces directly at the O), but not so good for the tight Vander Waals contact of the H2O molecules. Therefore, the structure of ice is openwork; in it, H2O molecules envelop microscopic (smaller than an H2O molecule in size) pores. The lacy structure of ice leads to two well-known effects: (1) ice is less dense than water, it floats in it; and (2) under strong pressure - for example, the blade of a skate melts the ice. Most of the hydrogen bonds that exist in ice are also preserved in liquid water. This follows from the small heat of melting of ice (80 cal/g) compared to the heat of boiling of water (600 cal/g at 0°C). One could say that in liquid water only 80/(600+80) = 12% of the H-bonds existing in ice are broken. However, this picture - that some of the hydrogen bonds in water are broken, and some are preserved - is not entirely accurate: rather, all the hydrogen bonds in water are becoming loose. This is well illustrated by the following experimental data.

Structure of solutions. From specific examples for water, let's move on to other liquids. Different liquids differ from each other in the sizes of their molecules and the nature of intermolecular interactions. Thus, in each specific liquid there is a certain pseudocrystalline structure, characterized by short-range order and, to some extent, reminiscent of the structure obtained when a liquid freezes and turns into a solid. When another substance is dissolved, i.e. When a solution is formed, the nature of intermolecular interactions changes and a new structure appears with a different arrangement of particles than in a pure solvent. This structure depends on the composition of the solution and is specific to each specific solution. The formation of liquid solutions is usually accompanied by a solvation process, i.e. alignment of solvent molecules around solute molecules due to the action of intermolecular forces. There are short-range and long-range solvation, i.e. Primary and secondary solvation shells are formed around the molecules (particles) of the dissolved substance. In the primary solvation shell, there are solvent molecules in close proximity, which move together with the solute molecules. The number of solvent molecules located in the primary solvation shell is called the solvation coordination number, which depends on both the nature of the solvent and the nature of the solute. The secondary solvation shell includes solvent molecules that are located at significantly greater distances and affect the processes occurring in the solution due to interaction with the primary solvation shell.
When considering the stability of solvates, a distinction is made between kinetic and thermodynamic stability.
In aqueous solutions, the quantitative characteristics of kinetic hydration (O.Ya. Samoilov) are the values ​​i/ and Ei=Ei-E, where i and  are the average residence time of water molecules in the equilibrium position near the i-th ion and in pure water , and Ei and E are the activation energy of exchange and the activation energy of the self-diffusion process in water. These quantities are related to each other by an approximate relationship:
i/  exp(Ei/RT) In this case,
if EI  0, i/  1 (the exchange of water molecules closest to the ion occurs less frequently (slower) than the exchange between molecules in pure water) – positive hydration
if EI  0, i/  1 (the exchange of water molecules closest to the ion occurs more often (faster) than the exchange between molecules in pure water) – negative hydration

So, for the lithium ion EI = 1.7 kJ/mol, and for the cesium ion Ei= - 1.4 kJ/mol, i.e. a small “hard” lithium ion holds water molecules more strongly than a large and “diffuse” cesium ion having the same charge. The thermodynamic stability of the resulting solvates is determined by the change in the Gibbs energy during solvation (solvG) = (solvH) - T(solvS). The more negative this value is, the more stable the solvate. This is mainly determined by negative values ​​of the enthalpy of solvation.
The concept of solutions and theories of solutions. True solutions are obtained spontaneously when two or more substances come into contact, due to the destruction of bonds between particles of one type and the formation of bonds of another type, and the distribution of the substance throughout the volume due to diffusion. Solutions according to their properties are divided into ideal and real, solutions of electrolytes and non-electrolytes, diluted and concentrated, unsaturated, saturated and supersaturated. The properties of rasters depend on the nature and magnitude of the IMF. These interactions can be of a physical nature (van der Waals forces) and a complex physicochemical nature (hydrogen bond, ion-molecular, charge transfer complexes, etc.). The process of solution formation is characterized by the simultaneous manifestation of attractive and repulsive forces between interacting particles. In the absence of repulsive forces, particles would merge (stick together) and liquids could be compressed indefinitely; in the absence of attractive forces, liquids or solids could not be obtained. In the previous lecture we looked at the physical and chemical theories of solutions.
However, the creation of a unified theory of solutions encounters significant difficulties and at present it has not yet been created, although research is being carried out using the most modern methods of quantum mechanics, statistical thermodynamics and physics, crystal chemistry, X-ray diffraction analysis, optical methods, and NMR methods. Reactive field. Continuing our consideration of the forces of intermolecular interaction, let us consider the concept of a “reactive field,” which is important for understanding the structure and structure of condensed matter and real gases, in particular the liquid state, and therefore the entire physical chemistry of liquid solutions.
The reactive field occurs in mixtures of polar and nonpolar molecules, for example, for mixtures of hydrocarbons and naphthenic acids. Polar molecules influence a field of a certain symmetry (the symmetry of the field is determined by the symmetry of vacant molecular orbitals) and intensity H on non-polar molecules. The latter are polarized due to charge separation, which leads to the appearance (induction) of a dipole. A molecule with an induced dipole, in turn, affects a polar molecule, changing its electromagnetic field, i.e. excites a reactive (response) field. The emergence of a reactive field leads to an increase in the interaction energy of particles, which is expressed in the creation of strong solvation shells of polar molecules in a mixture of polar and non-polar molecules.
The reactive field energy is calculated using the following formula: where:
sign “-” - determines the attraction of molecules
S – static electrical permittivity
infinite – dielectric constant due to the electronic and atomic polarizability of molecules
NA - Avogadro's number
VM – volume occupied by 1 mole of a polar substance in an isotropic liquid v = dipole moment
ER - energy of 1 mole of polar substance in solution
The "reactive field" concept will allow us to better understand the structure of pure liquids and solutions. The quantum chemical approach to the study of the reactive field was developed in the works of M. V. Bazilevsky and his colleagues at the Scientific Research Institute of Physics and Chemistry named after. L. Ya. Karpova Thus, the problem of the liquid state awaits its young researchers. The cards are in your hands.