How molecules interact with each other. Interaction of molecules. Structure of solids, liquids and gases. Molecular physics. Structure and properties of solid, liquid and gaseous bodies
Since school, we know that everything around us consists of molecules, tiny particles that constantly interact with each other. Let's refresh our knowledge and remember why a stone is difficult to squeeze in your hands, and water can glue a torn leaf of a tree together.
How molecules interact with each other - mutual attraction of molecules
Everything around us: liquid and solid objects, gaseous substances consist of tiny particles - molecules that continuously and constantly move among themselves. The main reason that objects do not disintegrate into molecules is their attraction to each other. Science has proven that mutual attraction always operates. Each molecule is attracted to another and everyone else is attracted to them.
- Solids remain in their form, and liquids do not break up into droplets due to intermolecular bonding. We cannot see such attraction with our eyes, it is too small. This force operates at ultra-small distances, such as the size of the particles themselves.
- Breaking a plate and trying to put two pieces together will not restore it. Trying to zoom in on the parts of a broken plate, we zoom in only on a small part of the molecules that make it up. Most of the particles remain at a fairly large distance, insufficient for the action of molecular attraction to take effect. However, if you wet a torn leaf from a tree with water, it will stick together. We will create sufficient intermolecular attraction between water molecules and leaf molecules to glue the torn leaf together.
- In nature, the force of attraction of molecules is visible in the wetting of solids. Let's take a piece of glass and horizontally touch it to the surface of the water. When lifting up from the water, we will have to apply a little force to “tear” the glass from the surface. The lower part that came into contact with the water will be wet after lifting the glass. This means that when the glass is lifted from the surface of the water, we overcome the force of attraction between the water molecules. The break itself did not occur between glass molecules, but between water molecules. Thus, we are convinced that the attraction between molecules of different substances is not the same. Some objects have a stronger particle attraction and are more difficult to break or stretch, while others have a weaker attraction.
- It is easier to tear a sheet of paper, overcoming the attraction of molecules, than a sheet of iron. In the example above, water molecules are attracted more strongly than glass molecules. However, water is not wetted by fatty substances. For example, by dipping a piece of paraffin into water, we will pull it out dry. This will prove that the attraction of paraffin molecules is stronger than the attraction of water molecules.
How molecules interact with each other - repulsion of molecules
Molecules are attracted to each other, but do not stick together. There are gaps between the tiny particles. If the molecules are squeezed too close, they will repel each other. Intermolecular repulsion comes into force when the distance between molecules becomes less than the size of the particles themselves and tends to zero. The repulsive force is clearly demonstrated by the sponge, which, after squeezing in the hand, restores its original shape. When we compress a sponge, we forcefully compress its molecules to a very close distance, smaller than the size of the molecules, when a force of mutual repulsion of all molecules arises.
Molecules interact with each other through mutual attraction and repulsion. These processes depend on the distance at which the molecules are located from each other: if the intermolecular distance is greater than the size of the particles themselves, they attract, if less, they repel. The effect of attraction and repulsion of molecules also depends on the type of substance. Solids have a stronger attraction than liquid molecules and a weaker repulsion. The coin cannot be squeezed in your hand, and the molecules of gaseous substances repel each other more strongly, which prevents gases from forming into objects.
>> Interaction of molecules (grade 7)
- Look around and you will see many physical bodies. This is both your neighbor with whom you sit at the desk, and the desk itself. This is the chair on which you sit, and the pen with which you write, etc. All these bodies, as you already know, consist of particles separated by intervals that are constantly moving. Then why don’t the particles that make up physical bodies scatter in all directions? Moreover, bodies not only do not crumble into individual molecules - on the contrary, in order to stretch, break, tear them, you need to apply force. Let's try to figure out why this is so.
Rice. 2.19. A hanging drop of water is kept from falling by the forces of attraction between the molecules. The drop falls too heavy
1. Confirm the interaction of molecules
The reason that all the bodies around us do not break up into individual molecules is obvious: molecules attract each other. Each molecule is attracted to neighboring molecules, and they, in turn, are attracted to it. It is thanks to intermolecular attraction that solids retain their shape, liquid collects into drops (Fig. 2.19), adhesive tape sticks to paper, ink leaves a mark on the sheet, lead cylinders pressed against each other by the cuts are firmly grasped (Fig. 2.20).
Science has established that attraction between molecules always operates. Why then does a broken cup not become whole after its fragments are pressed together? No matter how hard we press the parts of a broken pencil against each other, they will also not unite into a whole pencil.
The fact is that the attraction between molecules becomes noticeable only at very small distances (such that can be compared with the size of the particles themselves). By pressing the fragments of a cup or parts of a broken pencil, we bring only a very small number of molecules closer to such distances. The distance between most of them remains such that the molecules practically do not interact. Now it becomes clear why, in order for lead cylinders to stick together, it is necessary to first polish the sections, and pieces of soft wax or plasticine will easily stick together without any grinding.
Rice. 2.20. Lead bars pressed together with fresh cuts stick together so tightly that they can withstand the weight of a large weight.
Rice. 2.21 Experience in determining the conditions of intermolecular attraction
It is impossible to bring two dry sheets close enough to join. However, if you wet the sheets with water, they will stick together, since the water molecules will approach the paper molecules so much that intermolecular attraction will already hold the sheets near each other (Fig. 2.21).
Intermolecular attraction is also the reason why the body is wetted or not wetted by certain liquids (Fig. 2.22).
2. Confirm intermolecular repulsion
Above we proved that there is attraction between molecules. Given this, a number of questions arise. Why do gas molecules, moving in disorder and constantly colliding with each other, not stick together into one big lump? Why, if you squeeze, for example, a sponge, will it restore its shape after a while?
Rice. 2.22. A droplet of water spreads over the surface of clean glass (wet it) because the attraction between the molecules of the liquid is greater than between the molecules of the liquid and the glass (o). The attraction between water molecules is greater than between the molecules of water and fat that cover the feathers of waterfowl, so water does not wet them (remember the expression “water off a duck’s back”) (b)
The fact is that molecules not only attract each other, but also repel. If the distance between them becomes very small (slightly smaller than the size of the molecule), then intermolecular repulsion becomes stronger than attraction. Try squeezing a coin, for example. You will not be able to significantly reduce its size, since the molecules of the coin will repel each other. Also, you will not be able to significantly reduce the volume of liquid even with the help of a powerful press.
It is intermolecular attraction and repulsion that holds the molecules of liquids and solids at more or less certain distances, which are approximately equal to the size of the molecules themselves. If the distance decreases, the molecules begin to repel each other, and if the distance increases, they begin to attract each other, therefore, both to bring molecules closer together and to move them apart, it is necessary to apply force.
- Let's sum it up
Molecules interact with each other: they attract and repel at the same time. Intermolecular interaction manifests itself at distances that can be compared with the dimensions of the molecules themselves.
- Control questions
1. Why do solids and liquids not break down into individual molecules?
2. Under what conditions does the attraction between molecules become noticeable?
3. Under what condition is the repulsion of molecules observed?
4. Why is it impossible to connect two fragments of a cup, even pressing them strongly against each other, but two pieces of plasticine easily stick together?
5. It is known that there is attraction between molecules. Why then do the molecules, for example, of air not gather in one place?
- Exercises
1. No matter how carefully you connect two pieces of a ruler, they will not connect. Why is the attraction of molecules not affected in this case?
2. Why does it take force to break the cord?
3. For what purpose is it laid with paper strips when storing sheet glass?
4. Liquid glue ensures a strong connection between the two bodies. Explain why this happens.
5. What are the similarities and differences between the processes of welding and soldering metals?
6. The feathers of waterfowl are covered with a thin layer of fat. How does this benefit the birds?
- Experimental tasks
1. Using a soft spring (or thin rubber band), a clean metal (or glass) plate, and a saucer of water, demonstrate that attractive forces exist between water molecules and metal (glass) molecules.
2. Using sheets of paper, vessels with vegetable oil and water, get answers to such questions. Will two sheets stick together if they are wetted with water? oil? What if one is moistened with water and the other with oil? Justify the results of the experiment.
Physics. 7th grade: Textbook / F. Ya. Bozhinova, N. M. Kiryukhin, E. A. Kiryukhina. - X.: Publishing house "Ranok", 2007. - 192 p.: ill.
The fact that molecules interact with each other follows at least from the fact that liquids and solids exist: otherwise they would break up into separate molecules, turning into gases!
How do molecules interact? The answer to this question can be obtained by studying the properties of solids in the following simple experiments.
Try to squeeze the stone - it is unlikely that you will succeed. The fact is that in solids the molecules are located close to each other and therefore, when compressed, the molecules seem to “rest” against one another. In other words, when molecules are very close together, they repel each other.
Thanks to this repulsion, you do not fall through the floor: the molecules that make up the material of the soles “rest” against the molecules that make up the floor. These repulsive forces between molecules are shown schematically in Fig. 6.3, a.
However, solids resist not only compression, but also tension. This means that as the distance increases, repulsion between molecules is replaced by attraction.
Rice. 6.3. We do not fall through the floor due to the repulsion of molecules from each other (a); trying to break the thread, you feel the forces of attraction between the molecules in a small section of the thread (b)
Let's put experience
To feel how strong the attractive forces between molecules are, try tearing a nylon thread with a cross-section of 1 mm 2 with your hands. Difficult? But the efforts of your body are opposed by the attractive forces of tiny molecules in the small cross-section of the thread. These forces are shown schematically in Fig. 6.3, b.
Observations and experiments show that not only molecules of the same substance are attracted to each other, but also molecules of different substances.
Why does wet hair stick together?
Molecular physics made easy!
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.
With the chaotic movement of molecules, numerous collisions of gas molecules occur with each other.
The distance that a molecule travels between two successive collisions is called the mean free path and is denoted λ The mean free paths between individual collisions of molecules can differ significantly from each other. Therefore, we use the mean free path λ 1:
λ = (λ 1 + λ 2 +…+ λz) / z.
If z denotes the average number of collisions of a molecule in 1 second, then
λ = v/z.
Brownian motion- the movement of small particles suspended in a liquid or gas under the influence of uncompensated impacts of the molecules of the substance.
Diffusion- the process of equalizing concentrations caused by the transfer of a substance through molecular movement.
Mass and size of molecules.
Molecules are extremely small in size. Simple monatomic molecules have a size of the order of 10–10 m. Complex polyatomic molecules can have sizes hundreds and thousands of times larger. (1 nm = 10 -9 m). For example: the diameter of a water molecule (H 2 O) is 0.26 nm.
In molecular kinetic theory, the amount of matter is considered to be proportional to the number of particles. The unit of quantity of a substance is called a mole (mole).
A mole is an amount of substance containing as many particles (molecules) as there are atoms in 0.012 kg of carbon 12C. A carbon molecule consists of one atom.
Thus, one mole of any substance contains the same number of particles (molecules). This number is called Avogadro's constant N A:
Avogadro's constant is one of the most important constants in molecular kinetic theory.
The amount of substance ν is defined as the ratio of the number N of particles (molecules) of the substance to Avogadro’s constant N A:
Molar mass is expressed in kilograms per mole (kg/mol). For substances whose molecules consist of a single atom, the term atomic mass is often used.
The unit of mass of atoms and molecules is taken to be 1/12 of the mass of an atom of the carbon isotope 12 C (with mass number 12). It is called the atomic mass unit (a.m.u.):
This value almost coincides with the mass of a proton or neutron. The ratio of the mass of an atom or molecule of a given substance to 1/12 of the mass of a carbon atom 12 C is called relative mass.
Avogadro's law: Equal volumes of different gases at the same pressure and temperature contain the same number of molecules.
Ideal gas.
An ideal gas is one that satisfies the following conditions:
· the volume of all gas molecules can be neglected compared to the volume of the vessel in which this gas is located;
· the time of collision of molecules with each other is negligible compared to the time between two collisions;
· molecules interact with each other only in direct collision;
· the forces of attraction between the molecules of an ideal gas are negligible and can be neglected;
· the movement of molecules obeys Newton's law.
An ideal gas exerts pressure on the walls of the container due to the elastic impacts of its molecules on the walls.
