CHAPTER 4. INITIAL INFORMATION CLASS ABOUT THE STRUCTURE OF MATTER
Solving problems on this topic should help students develop initial concepts about the molecular structure of substances.
In the tasks, it is necessary to consider, first of all, such facts, the scientific explanation of which inevitably leads to the idea that bodies consist of tiny particles - molecules.
Next, a number of problems should be solved that give an idea of the size of molecules, as well as their properties, movement and interaction. Due to the lack of mathematical preparation of students, most problems must be of high quality.
Considerable attention must also be paid to experimental problems. Students can also perform simple experimental tasks at home.
The obtained information about the molecular structure of substances is then used to explain the differences between solid, liquid and gaseous states of matter.
1. Existence of molecules. Molecular sizes
It is useful to clarify and deepen the initial concept of molecules and their sizes with the help of tasks in which photographs of molecules obtained using an electron microscope are given.
Solving problems that show the complex structure of molecules is not necessary. But in an introductory plan, especially in classes with strong academic performance, you can consider 2-3 problems showing that the molecules of complex substances consist of smaller particles - atoms.
Along with qualitative ones, you can give problems on simple calculations of the absolute and relative sizes of molecules.
43. Figure 11 shows an electron microscope photograph of a solid particle. Which
Rice. 11. (see scan)
Can a conclusion be drawn based on this photograph about the structure of a solid? Using the scale indicated in the photograph, determine the size of one particle - a molecule.
Solution. Attention is paid to the fact that all molecules are identical, located in a solid in a certain order and have such a dense packing that only small gaps remain between them.
To determine the diameter of the molecules, count their number (50) at the indicated distance of 0.00017 cm, and, by calculating, find that the diameter of the molecule is approximately 0.000003 cm.
Tell students that this is a giant molecule. A water molecule, for example, has a diameter about a hundred times smaller.
44. An optical microscope allows you to distinguish objects about 0.00003 cm in size. Is it possible to see a drop of water with a diameter of a hundred, a thousand, a million molecules in such a microscope? The diameter of a water molecule is approximately
Consequently, with an optical microscope you can only see a drop of water whose diameter is at least 1000 times larger than the diameter of a water molecule. The water molecules themselves cannot be seen with an optical microscope.
45. The number of molecules in air at normal pressure and 0°C is . Assuming that the diameter of one gas molecule is approximately 0.00000003 cm, calculate how long the “beads” would be if all these molecules could be tightly strung on an invisible thread.
Answer. 8 million km.
46(e). Place two test tubes upside down in water and place in them the bare wires attached to the poles of the battery. Observe the gas bubbles and examine their composition using a smoldering splinter. Where did the gases come from?
Solution. Based on the bright burning of the splinters in one test tube and the flash in the other, it is concluded that there was oxygen in one test tube and hydrogen in the other.
They explain that gases appeared during the decomposition of a water molecule. Consequently, the properties of a molecule are not preserved when divided into smaller parts. Students can be informed that water also decomposes into oxygen and hydrogen when water vapor is heated to a very high temperature.
Molecules come in different sizes and shapes. For clarity, we will depict the molecule in the form of a ball, imagining that it is covered by a spherical surface, inside which are the electronic shells of its atoms (Fig. 4, a). According to modern concepts, molecules do not have a geometrically defined diameter. Therefore, it was agreed to take the diameter d of the molecule as the distance between the centers of two molecules (Fig. 4, b), which are so close that the attractive forces between them are balanced by the repulsive forces.
From the chemistry course it is known that a kilogram-molecule (kilomole) of any substance, regardless of its state of aggregation, contains the same number of molecules, called Avogadro’s number, namely N A = 6.02*10 26 molecules.
Now let's estimate the diameter of a molecule, for example water. To do this, divide the volume of a kilomole of water by Avogadro's number. A kilomole of water has a mass 18 kg. Assuming that water molecules are located close to each other and its density 1000 kg/m3, we can say that 1 kmol water takes up volume V = 0.018 m3. One molecule of water accounts for the volume
Taking the molecule as a ball and using the formula for the volume of a ball, we calculate the approximate diameter, otherwise the linear size of a water molecule:
Copper molecule diameter 2.25*10 -10 m. The diameters of gas molecules are of the same order. For example, the diameter of a hydrogen molecule 2.47*10 -10 m, carbon dioxide - 3.32*10 -10 m. This means that the molecule has a diameter of the order of 10 -10 m. At length 1 cm 100 million molecules can be located nearby.
Let's estimate the mass of a molecule, for example sugar (C 12 H 22 O 11). To do this you need a mass of kilomoles of sugar (μ = 342.31 kg/kmol) divided by Avogadro's number, i.e. by the number of molecules in
Kikoin A.K. A simple way to determine the size of molecules // Quantum. - 1983. - No. 9. - P.29-30.
By special agreement with the editorial board and editors of the journal "Kvant"
In molecular physics, the main “actors” are molecules, the unimaginably small particles that make up every substance in the world. It is clear that to study many phenomena it is important to know what molecules they are. In particular, what are their sizes.
When people talk about molecules, they are usually thought of as small, elastic, hard balls. Therefore, knowing the size of molecules means knowing their radius.
Despite the smallness of molecular sizes, physicists have been able to develop many ways to determine them. Physics 9 talks about two of them. One takes advantage of the property of some (very few) liquids to spread in the form of a film one molecule thick. In another, the particle size is determined using a complex device - an ion projector.
There is, however, a very simple, although not the most accurate, method of calculating the radii of molecules (or atoms). It is based on the fact that the molecules of a substance, when it is in a solid or liquid state, can be considered tightly adjacent to each other. In this case, for a rough estimate, we can assume that the volume V some mass m of a substance is simply equal to the sum of the volumes of the molecules it contains. Then we get the volume of one molecule by dividing the volume V per number of molecules N.
Number of molecules in a body weighing m equals, as is known, \(~N_a \frac(m)(M)\), where M- molar mass of the substance N A is Avogadro's number. Hence the volume V 0 of one molecule is determined from the equality
\(~V_0 = \frac(V)(N) = \frac(V M)(m N_A)\) .
This expression includes the ratio of the volume of a substance to its mass. The inverse relation \(~\frac(m)(V) = \rho\) is the density of the substance, so
\(~V_0 = \frac(M)(\rho N_A)\) .
The density of almost any substance can be found in tables accessible to everyone. Molar mass is easy to determine if the chemical formula of a substance is known.
\(~\frac(4)(3) \pi r^3 = \frac(M)(\rho N_A)\) .
from which we obtain the expression for the radius of the molecule:
\(~r = \sqrt (\frac(3M)(4 \pi \rho N_A)) = \sqrt (\frac(3)(4 \pi N_A)) \sqrt (\frac(M)(\rho) )\) .
The first of these two roots is a constant value equal to ≈ 7.4 10 -9 mol 1/3, so the formula for r pretends
\(~r \approx 7.4 \cdot 10^(-9) \sqrt (\frac(M)(\rho)) (m)\) .
For example, the radius of a water molecule calculated using this formula is equal to r B ≈ 1.9 · 10 -10 m.
The described method for determining the radii of molecules cannot be accurate simply because the balls cannot be placed so that there are no gaps between them, even if they are in contact with each other. In addition, with such a “packing” of molecules-balls, molecular movements would be impossible. Nevertheless, calculations of the sizes of molecules using the formula given above give results that almost coincide with the results of other methods, which are incomparably more accurate.
And a subsection in which modern filtration methods based on the sieve principle were reviewed in general terms. And they hinted that membrane purifiers purify water with different qualities, which depend on the size of the “cells”, called pores, in these membrane sieves. Respectively, microfiltration of water- This is the first technology of membrane water purification systems that we will consider.
Water microfiltration is water purification at the level of large molecules (macromolecules), such as asbestos particles, paint, coal dust, protozoan cysts, bacteria, rust. Whereas macrofiltration (of water) affects sand, large particles of silt, large particles of rust, etc.
We can roughly say that the particle sizes that macrofiltration screens out are particles larger than 1 micrometer (if a special one-micron cartridge is used). While the particle size that microfiltration removes is particles from 1 micron to 0.1 micron.
You may ask, "But if particles down to 0.1 microns are removed, wouldn't 100 micron particles be captured by microfiltration? Why write '1 micron to 0.1 microns' - that's a contradiction?"
In fact, there is no particular contradiction. Indeed, microfiltration of water will remove both bacteria and huge chunks of sand. But the purpose of microfiltration is not to remove large pieces of sand. The purpose of microfiltration is to “remove particles within a specified size range.” Then how would O Larger particles will simply clog the purifier and lead to additional costs.
So, let's move on to the characteristics of water microfiltration.
Since microfiltration removes particles measuring 0.1-1 microns, we can say that microfiltration is a membrane technology for water purification, which occurs on membrane sieves with a pore cell diameter of 0.1-1 microns. That is, on such membranes all substances larger than 0.5-1 microns are removed:
How completely they are removed depends on the diameter of the pores and the actual size of, say, bacteria. So, if the bacterium is long but thin, then it will easily fit through the pores of the microfiltration membrane. And the thicker spherical bacterium will remain on the surface of the “sieve”.
The most common use of microfiltration is in the food industry(for skimming milk, concentrating juices) and in medicine(for the primary preparation of medicinal raw materials). Microfiltration is also used in industrial drinking water treatment- mainly in Western countries (for example, in Paris). Although there are rumors that one of the water treatment plants in Moscow also uses microfiltration technology. Perhaps this is true :)
But there are also household filters based on microfiltration.
The most common example is track microfiltration membranes. Track from the word "track", that is, a trace, and this name is associated with how membranes of this type are made. The procedure is very simple:
- The polymer film is bombarded by particles, which, due to their own high energy, burn traces in the film - depressions of approximately the same size, since the particles with which the surface is bombarded are of the same size.
- Then this polymer film is etched in a solution, for example, acid, so that the traces of particle impacts become through.
- Well, then a simple procedure of drying and fixing the polymer film on the substrate - and that’s it, the track microfiltration membrane is ready!
As a result, these membranes have a fixed pore diameter and low porosity compared to other membrane water treatment systems. And the conclusion: these membranes will remove particles only of a certain size.
There is also a more sophisticated version of microfiltration household membranes - microfiltration membranes coated with activated carbon. That is, the steps listed above include one more step - applying a thin layer of. These membranes remove not only bacteria and mechanical impurities, but also
- smell,
- organic matter,
- etc.
It should be taken into account that for microfiltration membranes there is danger. Thus, bacteria that did not pass through the membrane begin to live on this membrane and issue products of your life into purified water. That is, it arises secondary water poisoning. To avoid this, it is necessary to follow the manufacturer's instructions for regular disinfection of membranes.
The second danger is that bacteria will begin to eat these membranes on their own. And they will make huge holes in them, which will allow the substances that the membrane should retain to pass through. To prevent this from happening, you should purchase filters based on bacteria-resistant substances (for example, ceramic microfiltration membranes) or be prepared to frequently replace microfiltration membranes.
Frequent replacement of microfiltration membranes is also encouraged by the fact that they not equipped with a flushing mechanism. And the pores of the membrane are simply clogged with dirt. Membranes fail.
In principle, everything is about microfiltration. Microfiltration is a fairly high-quality method of water purification. However,
The actual purpose of microfiltration is not to prepare water for drinking (due to the danger of bacterial contamination), but to pre-treat water before the next stages.
The microfiltration stage removes most of the burden from subsequent water treatment stages.
Based on materials How to choose a water filter: http://voda.blox.ua/2008/07/Kak-vybrat-filtr-dlya-vody-22.html
When two or more atoms chemically bond with each other, molecules are formed. It does not matter whether these atoms are the same or whether they are completely different from each other, both in shape and in size. We will figure out what the size of molecules is and what it depends on.
What are molecules?
For thousands of years, scientists have pondered the mystery of life, what exactly happens when it begins. According to the most ancient cultures, life and everything in this world consists of the basic elements of nature - earth, air, wind, water and fire. However, over time, many philosophers began to put forward the idea that all things are made up of tiny, indivisible things that cannot be created or destroyed.
However, it was only after the advent of atomic theory and modern chemistry that scientists began to postulate that particles, taken together, gave rise to the basic building blocks of all things. This is how the term appeared, which in the context of modern particle theory refers to the smallest units of mass.
By its classical definition, a molecule is the smallest particle of a substance that helps maintain its chemical and physical properties. It consists of two or more atoms, or groups of identical or different atoms, held together by chemical forces.
What is the size of the molecules? In the 5th grade, natural history (a school subject) gives only a general idea of sizes and shapes; this issue is studied in more detail in high school in chemistry lessons.
Examples of molecules
Molecules can be simple or complex. Here are some examples:
- H 2 O (water);
- N 2 (nitrogen);
- O 3 (ozone);
- CaO (calcium oxide);
- C 6 H 12 O 6 (glucose).
Molecules consisting of two or more elements are called compounds. Thus, water, calcium oxide and glucose are compounds. Not all compounds are molecules, but all molecules are compounds. How big can they be? What is the size of the molecule? It is a known fact that almost everything around us consists of atoms (except light and sound). Their total weight will be the mass of the molecule.
Molecular mass
When talking about the size of molecules, most scientists start from molecular weight. This is the total weight of all atoms included in it:
- Water, consisting of two hydrogen atoms (having one atomic mass unit each) and one oxygen atom (16 atomic mass units), has a molecular weight of 18 (more precisely, 18.01528).
- Glucose has a molecular weight of 180.
- DNA, which is very long, can have a molecular weight that is about 1010 (the approximate weight of one human chromosome).
Measurement in nanometers
In addition to mass, we can also measure how big molecules are in nanometers. A unit of water is about 0.27 Nm across. DNA reaches 2 nm in diameter and can stretch up to several meters in length. It is difficult to imagine how such dimensions can fit into one cell. The length-to-thickness ratio of DNA is amazing. It is 1/100,000,000, which is like a human hair the length of a football field.
Shapes and sizes
What is the size of the molecules? They come in different shapes and sizes. Water and carbon dioxide are among the smallest, proteins are among the largest. Molecules are elements made up of atoms that are bonded to each other. Understanding the appearance of molecules has traditionally been a part of chemistry. Besides their incomprehensibly strange chemical behavior, one of the important characteristics of molecules is their size.
Where might knowing how big molecules are be especially useful? The answer to this and many other questions helps in the field of nanotechnology, since the concept of nanorobots and smart materials necessarily deals with the effects of molecular sizes and shapes.
What is the size of the molecules?
In the 5th grade, natural history on this topic provides only general information that all molecules consist of atoms that are in constant random motion. In high school, you can already see structural formulas in chemistry textbooks that resemble the actual shape of molecules. However, it is impossible to measure their length using a regular ruler, and to do this, you need to know that molecules are three-dimensional objects. Their image on paper is a projection onto a two-dimensional plane. The length of a molecule is changed by the relationships between the lengths of its angles. There are three main ones:
- The angle of a tetrahedron is 109° when all bonds of that atom to all other atoms are single (only one dash).
- The angle of a hexagon is 120° when one atom has one double bond with another atom.
- The line angle is 180° when an atom has either two double bonds or one triple bond with another atom.
Actual angles often differ from these angles because a number of different effects must be taken into account, including electrostatic interactions.
How to imagine the size of molecules: examples
What is the size of the molecules? In grade 5, the answers to this question, as we have already said, are general. Students know that the size of these compounds is very small. For example, if you turn a molecule of sand in one single grain of sand into a whole grain of sand, then under the resulting mass you could hide a house of five floors. What is the size of the molecules? The short answer, which is also more scientific, is as follows.
Molecular mass is equated to the ratio of the mass of the entire substance to the number of molecules in the substance or the ratio of the molar mass to Avogadro's constant. The unit of measurement is kilogram. On average, the molecular weight is 10 -23 -10 -26 kg. Let's take water for example. Its molecular weight will be 3 x 10 -26 kg.
How does molecular size affect attractive forces?
Responsible for the attraction between molecules is the electromagnetic force, which manifests itself through the attraction of opposite charges and the repulsion of similar charges. The electrostatic force that exists between opposite charges dominates interactions between atoms and between molecules. The gravitational force is so small in this case that it can be neglected.
In this case, the size of the molecule affects the force of attraction through the electron cloud of random distortions that arise during the distribution of the electrons of the molecule. In the case of non-polar particles, which exhibit only weak van der Waals interactions or dispersion forces, the size of the molecules has a direct effect on the size of the electron cloud surrounding said molecule. The larger it is, the larger the charged field that surrounds it.
A larger electron cloud means that more electronic interactions can occur between neighboring molecules. As a result, one part of the molecule develops a temporary positive partial charge, while the other develops a negative partial charge. When this happens, a molecule can polarize the electron cloud of its neighbor. Attraction occurs because the partial positive side of one molecule is attracted to the partial negative side of another.
Conclusion
So how big are the molecules? In natural history, as we have found out, one can only find a figurative idea of the mass and size of these smallest particles. But we know that there are simple and complex compounds. And the second category includes such a concept as a macromolecule. It is a very large unit, such as a protein, that is usually created by polymerizing smaller subunits (monomers). They are usually made up of thousands of atoms or more.