Chemistry lesson in 8th grade. "____" _____________ 20___
Dissolution. Solubility of substances in water.
Target. To expand and deepen students' understanding of solutions and dissolution processes.
Educational tasks: to determine what a solution is, to consider the process of dissolution - as a physico-chemical process; expand the understanding of the structure of substances and chemical processes occurring in solutions; consider the main types of solutions.
Developmental tasks: To continue the development of speech skills, observation and the ability to draw conclusions based on laboratory work.
Educational tasks: to educate students' worldview through the study of solubility processes, since the solubility of substances is an important characteristic for the preparation of solutions in everyday life, medicine and other important industries and human life.
During the classes.
What is a solution? How to prepare a solution?
Experience number 1. Place a crystal of potassium permanganate in a glass of water. What are we observing? What is the process of dissolution?
Experiment No. 2. Pour 5 ml of water into a test tube. Then add 15 drops of concentrated sulfuric acid (H2SO4 conc.). What are we observing? (Answer: the test tube has warmed up, an exothermic reaction is taking place, which means that the dissolution is a chemical process).
Experience number 3. Add 5 ml of water to a test tube with sodium nitrate. What are we observing? (Answer: the test tube has become colder, an endothermic reaction is taking place, which means the dissolution is a chemical process).
The dissolution process is considered as a physicochemical process.
Page 211 complete the table.
| Signs of comparison | Physical theory | Chemical theory. |
| Proponents of the theory | Van't Hoff, Arrhenius, Ostwald | Mendeleev. |
| Definition of dissolution | The dissolution process is the result of diffusion, i.e. penetration of a solute into the spaces between water molecules | Chemical interaction of a solute with water molecules |
| Solution definition | Homogeneous mixtures consisting of two or more homogeneous parts. | A homogeneous system consisting of particles of a solute, a solvent, and products of their interaction. |
The solubility of solids in water depends on:
Task: observation of the effect of temperature on the solubility of substances.
Order of execution:
Pour water into test tubes No. 1 and No. 2 with nickel sulfate (1/3 of the volume).
Heat the test tube with No. 1, observing safety precautions.
In which of the proposed test tubes No. 1 or No. 2, the dissolution process proceeds faster?
Describe the effect of temperature on the solubility of substances.
Fig. 126 page 213
A) the solubility of potassium chloride at 30 0C is 40 g
at 65 0 With is 50 g.
B) solubility potassium sulfate at 40 0C is 10 g
at 800C is 20 y.
C) the solubility of barium chloride at 90 0C is 60 g
at 0 0 With is 30 g.
Task: observation of the influence of the nature of the solute on the dissolution process.
Order of execution:
In 3 test tubes with substances: calcium chloride, calcium hydroxide, calcium carbonate, add 5 ml of water each, close with a cork and shake well for better dissolution of the substance.
Which of the following substances dissolves well in water? Which does not dissolve?
thus, the process of dissolution depends on the nature of the solute:
Highly soluble: (three examples each)
Slightly soluble:
Practically insoluble:
3) Task: observation of the influence of the nature of the solvent on the process of dissolution of substances.
Order of execution:
Pour into 2 test tubes with copper sulfate in 5 ml of alcohol (No. 1) and 5 ml of water (No. 2),
stopper and shake well for better dissolution of the substance.
Which of the proposed solvents dissolves copper sulphate well?
Make a conclusion about the influence of the nature of the solvent on the dissolution process and
the ability of substances to dissolve in different solvents.
Solution types:
A saturated solution is a solution in which, at a given temperature, a substance no longer dissolves.
Unsaturated is a solution in which a substance can still dissolve at a given temperature.
Supersaturated is a solution in which a substance can still dissolve only when the temperature rises.
One morning I overslept.
I was going to school quickly:
Poured cold tea
Sugar poured, prevented,
But he was not sweet.
I added another spoon
He became a little sweeter.
I drank my tea to the end
And the rest was sweet
Sugar was waiting for me at the bottom!
I began to think in my mind -
Why do fate disgrace?
The culprit is solubility.
Highlight the types of solutions in the poem. What needs to be done to completely dissolve the sugar in the tea.
Physico-chemical theory of solutions.
The solute, when dissolved with water, forms hydrates.
Hydrates are fragile compounds of substances with water that exist in solution.
When dissolved, heat is absorbed or released.
As the temperature rises, the solubility of substances increases.
The composition of hydrates is not constant in solutions and is constant in crystalline hydrates.
Crystalline hydrates are salts containing water.
Copper sulfate CuSO4∙ 5H2O
Soda Na2CO3∙ 10H2O
Gypsum CaSO4∙2H2O
The solubility of potassium chloride in water at 60 0C is 50 g. Determine the mass fraction of salt in a solution saturated at a specified temperature.
Determine the solubility of potassium sulfate at 80 0C. Determine the mass fraction of salt in a solution saturated at a specified temperature.
161 g of Glauber's salt were dissolved in 180 liters of water. Determine the mass fraction of salt in the resulting solution.
Homework. Section 35
Messages.
Amazing properties of water;
Water is the most valuable compound;
Use of water in industry;
Artificial obtaining of fresh water;
The fight for clean water.
Presentation "Crystal hydrates", "Solutions - properties, application".
In an ordinary non-associated liquid, such as gasoline, for example, free molecules slide one around the other. In water, they roll rather than slide. Water molecules, as you know, are interconnected by hydrogen bonds, so before any displacement occurs, at least one of these bonds must be broken. This feature determines the viscosity of water.
The dielectric constant of water is its ability to neutralize the attraction that exists between electric charges. The dissolution of solids in water is a complex process that is determined by the interaction of solute particles and water particles.
When studying the structure of substances with the help of X-rays, it was found that most solids have a crystalline structure, that is, the particles of a substance are arranged in space in a certain order. The particles of some substances are located as if they were in the corners of a tiny cube, the particles of others - in the corners, center and middle of the sides of a tetrahedron, prism, pyramid, etc. Each of these forms is the smallest cell of larger crystals of a similar shape. Some substances have molecules at the nodes of their crystal lattice (for most organic compounds), while others (for example, inorganic salts) have ions, that is, particles consisting of one or more atoms with positive or negative charges. The forces that hold ions in a certain, spatially oriented order of the crystal lattice are the forces of electrostatic attraction of oppositely charged ions that make up the crystal lattice.
If, for example, sodium chloride is dissolved in water, then positively charged sodium ions and negatively charged chloride ions will repel each other.
This repulsion occurs because water has a high dielectric constant, i.e. higher than that of any other liquid. It reduces the force of mutual attraction between oppositely charged ions by 100 times. The reason for the strongly neutralizing effect of water must be sought in the arrangement of its molecules. The hydrogen atom in them does not share its electron equally with the oxygen atom to which it is attached. This electron is always closer to oxygen than to hydrogen. Therefore, hydrogen atoms are positively charged, while oxygen atoms are negatively charged.
When a substance, dissolving, breaks down into ions, oxygen atoms are attracted to positive ions, and hydrogen atoms to negative ones. The water molecules surrounding the positive ion send their oxygen atoms towards it, and the molecules that surround the negative ion send their hydrogen atoms towards it. Thus, water molecules form, as it were, a lattice that separates the ions from each other and neutralizes their attraction (Fig. 12). In order to separate the ions in the crystal lattice from each other and transfer them into solution, it is necessary to overcome the force of attraction of this lattice. When salts are dissolved, such a force is the attraction of lattice ions by water molecules, characterized by the so-called hydration energy. If, in this case, the energy of hydration is sufficiently large in comparison with the energy of the crystal lattice, then the ions will detach from the latter and pass into solution.
The relationship between water molecules and ions detached from the lattice in solution not only does not weaken, but becomes even closer.
As already noted, in solution, ions are surrounded and separated by water molecules, which, focusing on them with their parts opposite in charge, form the so-called hydration shell (Fig. 13). The size of this shell is different for different ions and depends on the charge of the ion, its size, and, in addition, on the concentration of ions in the solution.
For several years physical chemists studied water mainly as a solvent for electrolytes. As a result, a lot of information about electrolytes was obtained, but very little about water itself. Oddly enough, but only in recent years have there been works devoted to the study of the relationship of water to substances that are practically insoluble in it.
Many amazing things have been observed. For example, once a pipe through which natural gas flowed at t = 19 ° C turned out to be clogged with wet snow and water. It became clear that the point here is not in temperature, but in other properties of water. A number of questions arose: why did water freeze at such a high temperature, how could water combine with substances that are insoluble in it.
This mystery had not yet been solved when it was discovered that even such noble gases as argon and xenon, which do not enter into any chemical reactions, can bind with water, forming some semblance of compounds.
Rice. 13. Separation of Na + and C1 - ions by polar water molecules forming a hydration shell around them.
Interesting results on the solubility of methane in water were obtained in Illinois. Methane molecules do not form ions in water and do not accept hydrogen bonds; the attraction between them and the water molecules is very weak. However, methane is still, although poorly, soluble in water, and its dissociated molecules form compounds with it - hydrates, in which several water molecules are attached to one methane molecule. This reaction releases 10 times more heat than when methane is dissolved in hexane (methane dissolves better in hexane than in water).
The fact that methane dissolves in water is of great interest. A methane molecule is twice the size of a water molecule. In order for methane to dissolve in water, rather large “holes” must form between its molecules. This requires a significant expenditure of energy, more than for the evaporation of water (about 10,000 calories per mole). Where does so much energy come from? The forces of attraction between the molecules of methane and water are too weak, they cannot provide so much energy. Therefore, there is another possibility: the structure of the hearth changes in the presence of methane. Assume that a molecule of dissolved methane is surrounded by a shell of 10-20 water molecules. During the formation of such associations of molecules, heat is released. In the space occupied by a methane molecule, the forces of mutual attraction between water molecules disappear, and hence the internal pressure. Under such conditions, as we have seen, water freezes at temperatures above zero.
This is why molecules between methane and water can crystallize, which happened in the case described above. Frozen hydrates can be absorbed into and released from the solution. This theory is known as the iceberg theory. In practice, studies show that all non-conductive substances that have been tested form stable crystalline hydrates. At the same time, this trend is weakly expressed in electrolytes. All this leads to a completely new understanding of solubility.
It was believed that the dissolution of electrolytes occurs as a result of the action of attractive forces. Now it has been proven that the dissolution of non-electrolytes occurs not due to the forces of attraction between these substances and water, but as a result of insufficient attraction between them. Substances that do not decompose into ions combine with water, as they eliminate internal pressure and thereby contribute to the appearance of crystalline formations.
To better understand the formation of such hydrates, it is useful to consider their molecular structure.
It is proved that the resulting hydrates have a cubic structure (lattice) in contrast to the hexagonal structure of ice. Further work by researchers showed that the hydrate can have two cubic lattices: in one of them, the gaps between molecules are 12, in the other - 17 A. There are 46 water molecules in the smaller lattice, 136 in the larger one. The holes of gas molecules in the smaller lattice have 12-14 faces , and in the larger one - 12-16, moreover, they differ in size and are filled with molecules of various sizes, and not all holes can be filled. Such a model explains the actual structure of hydrates with a high degree of accuracy.
The role of such hydrates in life processes cannot be overestimated. These processes occur mainly in the spaces between water and protein molecules. In this case, water has a strong tendency to crystallize, since the protein molecule contains many non-ionic, or non-polar, groups. Any such hydrate forms at a lower density than ice, so its formation can lead to significant destructive expansion.
So, water is a peculiar and complex substance with certain and diverse chemical properties. It has a slender and at the same time changing physical structure.
The development of all living and largely inanimate nature is inextricably linked with the characteristic features of water.
Solutions play a very important role in nature, science and technology. Water, so widespread in nature, always contains dissolved substances. There are few of them in fresh water of rivers and lakes, while sea water contains about 3.5% of dissolved salts.
In the primordial ocean (at the time of the emergence of life on Earth), the mass fraction of salts was supposed to be low, about 1%.
“It was in this solution that living organisms first developed, and from this solution they received the ions and molecules necessary for their growth and life ... Over time, living organisms developed and changed, which allowed them to leave the aquatic environment and move to land and then rise into the air. They acquired this ability by preserving in their organisms an aqueous solution in the form of liquids containing the necessary supply of ions and molecules, ”this is how the famous American chemist, Nobel Prize winner Linus Pauling assesses the role of solutions in the emergence and development of life on Earth. Inside us, in each of our cells, there is a reminder of the primary ocean in which life originated, an aqueous solution that provides life itself.
In every living organism, endlessly flows through the vessels - arteries, veins and capillaries - a magical solution that forms the basis of blood, the mass fraction of salts in it is the same as in the primary ocean - 0.9%. Complex physicochemical processes occurring in human and animal organisms also occur in solutions. Assimilation of food is associated with the transfer of nutrients into solution. Natural aqueous solutions are involved in the processes of soil formation and supply plants with nutrients. Many technological processes in the chemical and other industries, such as the production of soda, fertilizers, acids, metals, and paper, proceed in solutions. The study of the properties of solutions occupies a very important place in modern science. So what is a solution?
The difference between the solution and other mixtures is that the particles of the constituent parts are distributed evenly in it, and the composition is the same in any microvolume of such a mixture.
Therefore, solutions were understood as homogeneous mixtures consisting of two or more homogeneous parts. This idea was based on the physical theory of solutions.
Supporters of the physical theory of solutions, which was developed by van't Hoff, Arrhenius and Ostwald, believed that the dissolution process is the result of diffusion, i.e., the penetration of a solute into the gaps between water molecules.
In contrast to the ideas of the physical theory of solutions, D. I. Mendeleev and supporters of the chemical theory of solutions argued that dissolution is the result of the chemical interaction of a solute with water molecules. Therefore, it is more correct (more accurate) to define a solution as a homogeneous system consisting of particles of a dissolved substance, a solvent, and the products of their interaction.
As a result of the chemical interaction of a dissolved substance with water, compounds are formed - hydrates. Chemical interaction is indicated by such signs of chemical reactions as thermal phenomena during dissolution. For example, remember that the dissolution of sulfuric acid in water proceeds with the release of such a large amount of heat that the solution can boil, and therefore acid is poured into water (and not vice versa).
The dissolution of other substances, such as sodium chloride, ammonium nitrate, is accompanied by the absorption of heat.
M. V. Lomonosov found that solutions freeze at a lower temperature than the solvent. In 1764, he wrote: “The frosts of salty brine cannot conveniently turn into ice, as they overcome fresh.”
Hydrates are fragile compounds of substances with water that exist in solution. Indirect evidence of hydration is the existence of solid crystalline hydrates - salts, which include water. In this case, it is called crystallization. For example, the well-known blue salt, copper sulfate CuSO 4 5H 2 O, belongs to crystalline hydrates. Anhydrous copper (II) sulfate is white crystals. The change in the color of copper (II) sulfate to blue when it is dissolved in water and the existence of blue crystals of copper sulphate is another proof of the hydrate theory of D. I. Mendeleev.
At present, a theory has been adopted that combines both points of view - the physicochemical theory of solutions. It was predicted back in 1906 by D. I. Mendeleev in his wonderful textbook "Fundamentals of Chemistry": will most likely lead to a general theory of solutions, because the same general laws govern both physical and chemical phenomena.
The solubility of substances in water depends on temperature. As a rule, the solubility of solids in water increases with increasing temperature (Fig. 126), and the solubility of gases decreases, so water can be almost completely freed from the gases dissolved in it by boiling.
Rice. 126.
Solubility of substances depending on temperature
If potassium chloride KCl, which is used as a fertilizer, is dissolved in water, then at room temperature (20 ° C) only 34.4 g of salt can be dissolved in 100 g of water; no matter how much the solution is mixed with the rest of the undissolved salt, no more salt will dissolve - the solution will be saturated with this salt at a given temperature.
If at this temperature less than 34.4 g of potassium chloride is dissolved in 100 g of water, then the solution will be unsaturated.
It is comparatively easy to obtain supersaturated solutions from some substances. These include, for example, crystalline hydrates - Glauber's salt (Na 2 SO 4 10H 2 O) and copper sulfate (CuSO 4 5H 2 O).
Supersaturated solutions are prepared as follows. Prepare a saturated salt solution at a high temperature, for example at the boiling point. The excess salt is filtered off, the flask with the hot filtrate is covered with cotton wool and carefully, avoiding shaking, slowly cooled to room temperature. The solution prepared in this way, protected from shocks and dust, can be stored for quite a long time. But as soon as a glass rod is introduced into such a supersaturated solution, on the tip of which there are several grains of this salt, its crystallization from the solution immediately begins (Fig. 127).
Rice. 127.
Instantaneous crystallization of a substance from a supersaturated solution
Glauber's salt is widely used as a raw material in chemical plants. It is mined in winter in the Kara-Bogaz-Gol Bay, which is relatively isolated from the Caspian Sea. In summer, due to the high rate of water evaporation, the bay is filled with a highly concentrated salt solution. In winter, due to a decrease in temperature, its solubility decreases and the salt crystallizes, which underlies its extraction. In summer, the salt crystals dissolve, and its extraction stops.
In the most salty of the seas of the world - the Dead Sea - the concentration of salts is so great that bizarre crystals grow on any object placed in the water of this sea (Fig. 128).
Rice. 128.
In the water of the Dead Sea, beautiful bizarre crystals grow from the salts dissolved in it.
When working with substances, it is important to know their solubility in water. A substance is considered highly soluble if more than 1 g of this substance dissolves in 100 g of water at room temperature. If under such conditions less than 1 g of a substance dissolves in 100 g of water, then such a substance is considered poorly soluble. Practically insoluble substances include those whose solubility is less than 0.01 g in 100 g of water (Table 9).
Table 9
Solubility of some salts in water at 20 °C
Completely insoluble substances do not exist in nature. For example, even silver atoms slightly go into solution from products placed in water. As you know, a solution of silver in water kills microbes.
Keywords and phrases
- Solutions.
- Physical and chemical theory of solutions.
- Thermal phenomena during dissolution.
- Hydrates and crystalline hydrates; water of crystallization.
- Saturated, unsaturated and supersaturated solutions.
- Highly soluble, slightly soluble and practically insoluble substances.
Work with computer
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Questions and tasks
- Why does a piece of sugar dissolve faster in hot tea than in cold tea?
- Give examples of highly soluble, slightly soluble and practically insoluble substances in water of various classes, using the solubility table.
- Why can't aquariums be filled with quickly cooled boiled water (it must stand for several days)?
- Why do wounds washed with water in which silver items were placed heal faster?
- Using figure 126, determine the mass fraction of potassium chloride contained in the saturated solution at 20 °C.
- Can a dilute solution be saturated at the same time?
- To 500 g of a solution of magnesium sulfate saturated at 20 ° C (see Fig. 126), a volume of a solution of barium chloride sufficient for the reaction was added. Find the mass of the precipitate.
Solubility is the property of a substance to form homogeneous mixtures with various solvents. As we already mentioned, the amount of solute required to obtain a saturated solution determines this substance. In this regard, solubility has the same measure as composition, for example, the mass fraction of a solute in its saturated solution or the amount of a solute in its saturated solution.
All substances in terms of their solubility can be classified into:
- Highly soluble - more than 10 g of the substance can dissolve in 100 g of water.
- Slightly soluble - less than 1 g of the substance can dissolve in 100 g of water.
- Insoluble - less than 0.01 g of the substance can dissolve in 100 g of water.
It is known that if polarity solute is similar to the polarity of the solvent, it is more likely to dissolve. If the polarities are different, then with a high degree of probability the solution will not work. Why is this happening?
polar solvent is a polar solute.
Let's take a solution of common salt in water as an example. As we already know, water molecules are polar in nature with a partial positive charge on each hydrogen atom and a partial negative charge on the oxygen atom. And ionic solids, like sodium chloride, contain cations and anions. So when table salt is placed in water, the partial positive charge on the hydrogen atoms of the water molecules is attracted to the negatively charged chloride ion in NaCl. Similarly, the partial negative charge on the oxygen atoms of water molecules is attracted by the positively charged sodium ion in NaCl. And, since the attraction of water molecules for sodium and chlorine ions is stronger than the interaction that holds them together, the salt dissolves.
Non-polar solvent is a non-polar solute.
Let's try to dissolve a piece of carbon tetrabromide in carbon tetrachloride. In the solid state, carbon tetrabromide molecules are held together by a very weak dispersion interaction. When placed in carbon tetrachloride, its molecules will be arranged more randomly, i.e. the entropy of the system increases and the compound dissolves.
Equilibria in dissolution
Consider a solution of a poorly soluble compound. In order for equilibrium to be established between a solid and its solution, the solution must be saturated and in contact with the undissolved portion of the solid.
For example, let the equilibrium be established in a saturated solution of silver chloride:
AgCl (tv) \u003d Ag + (aq.) + Cl - (aq.)
The compound in question is ionic and is present in dissolved form as ions. We already know that in heterogeneous reactions the concentration of a solid remains constant, which allows us to include it in the equilibrium constant. So the expression for will look like this:
K = [ Cl - ]
Such a constant is called solubility product PR, provided that the concentrations are expressed in mol/L.
PR \u003d [ Cl - ]
Solubility product is equal to the product of the molar concentrations of the ions participating in the equilibrium, in powers equal to the corresponding stoichiometric coefficients in the equilibrium equation.
It is necessary to distinguish between the concept of solubility and the product of solubility. The solubility of a substance can change when another substance is added to the solution, and the solubility product does not depend on the presence of additional substances in the solution. Although these two values are interconnected, which allows knowing one value to calculate the other.
Solubility as a function of temperature and pressure
Water plays an important role in our life, it is able to dissolve a large number of substances, which is of great importance to us. Therefore, we will focus on aqueous solutions.
Solubility gases increases with rising pressure gas above the solvent, and the solubility of solid and liquid substances depends on pressure insignificantly.
William Henry first came to the conclusion that the amount of gas that dissolves at a constant temperature in a given volume of liquid is directly proportional to its pressure. This statement is known as Henry's law and is expressed as follows:
C \u003d k P,
where C is the solubility of the gas in the liquid phase
P - gas pressure over the solution
k is Henry's constant
The following figure shows the solubility curves of some gases in water temperature at constant gas pressure over the solution (1 atm)
As can be seen, the solubility of gases decreases with increasing temperature, in contrast to most ionic compounds, the solubility of which increases with increasing temperature.
Effect of temperature on solubility depends on the enthalpy change that occurs during the dissolution process. During the course of the endothermic process, the solubility increases with increasing temperature. This follows from what we already know : if you change one of the conditions under which the system is in equilibrium - concentration, pressure or temperature - then the equilibrium will shift in the direction of the reaction that counteracts this change.
Imagine that we are dealing with a solution in equilibrium with a partially dissolved substance. And this process is endothermic, i.e. goes with the absorption of heat from outside, then:
Substance + solvent + heat = solution
According to principle of Le Chatelier, at endothermic process, the equilibrium shifts in the direction that reduces the heat input, i.e. to the right. Thus, the solubility increases. If the process exothermic, then an increase in temperature leads to a decrease in solubility.
dependence of the solubility of ionic compounds on temperature It is known that there are solutions of liquids in liquids. Some of them can dissolve in each other in unlimited quantities, like water and ethyl alcohol, while others can only partially dissolve. So, if you try to dissolve carbon tetrachloride in water, then two layers are formed: the upper one is a saturated solution of water in carbon tetrachloride and the lower one is a saturated solution of carbon tetrachloride in water. As the temperature rises, in general, the mutual solubility of such liquids increases. This happens until the critical temperature is reached, at which both liquids are mixed in any proportions. The solubility of liquids is practically independent of pressure.
When a substance that can be dissolved in either of these two liquids is introduced into a mixture consisting of two immiscible liquids, its distribution between these liquids will be proportional to the solubility in each of them. Those. according to distribution law a substance that can dissolve in two immiscible solvents is distributed between them so that the ratio of its concentrations in these solvents at a constant temperature remains constant, regardless of the total amount of the solute:
C 1 / C 2 \u003d K,
where C 1 and C 2 are the concentrations of a substance in two liquids
K is the distribution coefficient.
Categories ,The ability of a given substance to dissolve in a given solvent is called solubility.
On the quantitative side, the solubility of a solid characterizes the solubility coefficient or simply solubility - this is the maximum amount of a substance that can dissolve in 100 g or 1000 g of water under given conditions to form a saturated solution.
Since most solids absorb energy when dissolved in water, according to Le Chatelier's principle, the solubility of many solids increases with increasing temperature.
The solubility of gases in a liquid characterizes absorption coefficient- the maximum volume of gas that can dissolve at n.o. in one volume of solvent. When gases are dissolved, heat is released, therefore, with increasing temperature, their solubility decreases (for example, the solubility of NH 3 at 0 ° C is 1100 dm 3 / 1 dm 3 of water, and at 25 ° C - 700 dm 3 / 1 dm 3 of water). The dependence of the solubility of gases on pressure obeys Henry's law: the mass of a dissolved gas at a constant temperature is directly proportional to the pressure.
Expression of the quantitative composition of solutions
Along with temperature and pressure, the main parameter of the state of the solution is the concentration of the dissolved substance in it.
solution concentration called the content of a solute in a certain mass or in a certain volume of a solution or solvent. The concentration of a solution can be expressed in different ways. In chemical practice, the following methods of expressing concentrations are most commonly used:
a) mass fraction of solute shows the number of grams (mass units) of a solute contained in 100 g (mass units) of a solution (ω, %)
b) molar volume concentration, or molarity , shows the number of moles (amount) of the solute contained in 1 dm 3 solution (s or M, mol / dm 3)
in) equivalent concentration, or normality , shows the number of equivalents of a solute contained in 1 dm 3 of a solution (s e or n, mol / dm 3)
G) molar mass concentration, or molality , shows the number of moles of a solute contained in 1000 g of solvent (s m , mol / 1000 g)
e) titer a solution is the number of grams of a solute in 1 cm 3 of a solution (T, g / cm 3)
T = m r.v. /V.
In addition, the composition of the solution is expressed in terms of dimensionless relative values - fractions. Volume fraction - the ratio of the volume of the solute to the volume of the solution; mass fraction - the ratio of the mass of the solute to the volume of the solution; mole fraction is the ratio of the amount of solute (number of moles) to the total amount of all components of the solution. The most commonly used value is the mole fraction (N) - the ratio of the amount of solute (ν 1) to the total amount of all components of the solution, that is, ν 1 + ν 2 (where ν 2 is the amount of solvent)
N r.v. \u003d ν 1 / (ν 1 + ν 2) \u003d m r.v. /M r.v. / (m r.v. / M r.v + m r-la. / M r-la).
Dilute solutions of non-electrolytes and their properties
In the formation of solutions, the nature of the interaction of the components is determined by their chemical nature, which makes it difficult to identify general patterns. Therefore, it is convenient to resort to some idealized solution model, the so-called ideal solution. A solution whose formation is not associated with a change in volume and thermal effect is called ideal solution. However, most solutions do not fully possess the properties of ideality and general patterns can be described using the examples of so-called dilute solutions, that is, solutions in which the content of the solute is very small compared to the content of the solvent and the interaction of molecules of the solute with the solvent can be neglected. Solutions have olligative properties are the properties of solutions that depend on the number of particles of the solute. The colligative properties of solutions include:
osmotic pressure;
saturated steam pressure. Raoult's law;
increase in boiling point;
freezing temperature drop.
Osmosis. osmotic pressure.
Let there be a vessel divided by a semi-permeable partition (dotted line in the figure) into two parts filled to the same level O-O. The solvent is placed on the left side, the solution is placed on the right side.
solvent solution
To the concept of the phenomenon of osmosis
Due to the difference in solvent concentrations on both sides of the partition, the solvent spontaneously (in accordance with the Le Chatelier principle) penetrates through the semipermeable partition into the solution, diluting it. The driving force of preferential diffusion of the solvent into the solution is the difference between the free energies of the pure solvent and the solvent in the solution. When the solution is diluted due to spontaneous diffusion of the solvent, the volume of the solution increases and the level moves from position O to position II. Unilateral diffusion of a certain kind of particles in solution through a semi-permeable partition is called osmosis.
It is possible to quantitatively characterize the osmotic properties of a solution (with respect to a pure solvent) by introducing the concept of osmotic pressure. The latter is a measure of the tendency of a solvent to pass through a semi-permeable partition into a given solution. It is equal to the additional pressure that must be applied to the solution so that osmosis stops (the effect of pressure is reduced to an increase in the release of solvent molecules from the solution).
Solutions with the same osmotic pressure are called isotonic. In biology, solutions with an osmotic pressure greater than that of the intracellular contents are called hypertensive, with less hypotonic. The same solution is hypertonic for one cell type, isotonic for another, and hypotonic for a third.
Most of the tissues of organisms have the properties of semi-permeability. Therefore, osmotic phenomena are of great importance for the vital activity of animal and plant organisms. The processes of digestion, metabolism, etc. are closely related to the different permeability of tissues for water and certain solutes. The phenomena of osmosis explain some of the issues related to the relationship of the organism to the environment. For example, they are due to the fact that freshwater fish cannot live in sea water, and marine fish in river water.
Van't Hoff showed that the osmotic pressure in a non-electrolyte solution is proportional to the molar concentration of the solute
R osm = withRT,
where R osm - osmotic pressure, kPa; c - molar concentration, mol / dm 3; R is the gas constant, equal to 8.314 J/mol∙K; T - temperature, K.
This expression is similar in form to the Mendeleev-Clapeyron equation for ideal gases, but these equations describe different processes. Osmotic pressure arises in a solution when an additional amount of solvent penetrates into it through a semi-permeable partition. This pressure is the force that prevents further equalization of concentrations.
van't Hoff formulated law of osmotic pressure: osmotic pressure is equal to the pressure that a solute would produce if it, in the form of an ideal gas, occupied the same volume that the solution occupies at the same temperature.
Saturated steam pressure. Raul's law.
Consider a dilute solution of a non-volatile (solid) substance A in a volatile liquid solvent B. In this case, the total saturated vapor pressure over the solution is determined by the partial vapor pressure of the solvent, since the vapor pressure of the solute can be neglected.
Raoult showed that the saturated vapor pressure of a solvent over a solution P is less than that over a pure solvent P°. The difference Р° - Р = Р is called the absolute decrease in vapor pressure over the solution. This value, referred to the vapor pressure of a pure solvent, that is, (P ° -P) / P ° \u003d P / P °, is called the relative decrease in vapor pressure.
According to Raoult's law, the relative decrease in the pressure of the saturated vapor of the solvent over the solution is equal to the mole fraction of the dissolved non-volatile substance
(P ° -P) / P ° \u003d N \u003d ν 1 / (ν 1 + ν 2) \u003d m r.v. /M r.v. / (m r.v. / M r.v + m r-la. / M r-la) \u003d X A
where X A is the mole fraction of the solute. And since ν 1 = m r.v. /M r.v, then using this law, you can determine the molar mass of a solute.
Consequence of Raoult's law. The decrease in vapor pressure over a solution of a non-volatile substance, for example, in water, can be explained using the Le Chatelier equilibrium shift principle. Indeed, with an increase in the concentration of a non-volatile component in a solution, the equilibrium in the water-saturated steam system shifts towards the condensation of part of the vapor (the reaction of the system to a decrease in the concentration of water when the substance is dissolved), which causes a decrease in vapor pressure.
A decrease in vapor pressure over a solution compared to a pure solvent causes an increase in the boiling point and a decrease in the freezing point of solutions compared to a pure solvent (t). These values are proportional to the molar concentration of the solute - non-electrolyte, that is:
t= K∙s t = K∙t∙1000/M∙a,
where c m is the molar concentration of the solution; a is the mass of the solvent. Proportionality factor To , in the case of an increase in the boiling point, is called ebullioscopic constant for a given solvent (E ), and to lower the freezing point - cryoscopic constant(To ). These constants, which are numerically different for the same solvent, characterize an increase in the boiling point and a decrease in the freezing point of a 1-molar solution, i.e. when dissolving 1 mol of non-volatile non-electrolyte in 1000 g of solvent. Therefore, they are often referred to as the molar increase in the boiling point and the molar decrease in the freezing point of the solution.
The criscopic and ebullioscopic constants do not depend on the concentration and nature of the solute, but depend only on the nature of the solvent and are characterized by the dimension kg∙deg/mol.
