solids dissolved in water. A manual in chemistry for applicants to higher educational institutions
There are several interpretations of the term solubility.
Solubility is the ability of a substance to dissolve in water or another solvent.
Solubility is the ability of substances to dissolve in each other, quantitatively characterized by the solubility coefficient (k or p) - this is the mass of a solute per 100 or 1000 g of solvent, in a saturated solution - at a certain temperature.
The solubility of a substance depends on various factors: the nature of the substance and solvent, the state of aggregation, temperature and pressure (for gases).
There is a statement "Like dissolves into like.”This means that molecular and ionic compounds with a polar bond dissolve well in polar solvents, while substances with a nonpolar bond dissolve in nonpolar ones.
chief solvent is water. But not all substances, especially organic ones, dissolve in water. Various solvents are used for dissolution, such as acetone, alcohol, benzene, ether, chloroform, methanol, etc. Mixtures of solvents are also used, for example mixtures of alcohol with water.
In order to dissolve a solid, it must be crushed very finely (grind with a fork or grind in a mill). This is done in order to increase the contact surface of the solute and solvent. When stirring or shaking, the process of obtaining a solution is accelerated. Often, a reflux condenser is put on the container in which the solution is prepared. It is used mainly for the preparation of solutions by boiling. This reduces the loss of solvent. The mixture vapors formed during heating are deposited in the refrigerator and flow back. This is especially important for flammable solvents, whose vapors from an open vessel could catch fire from contact with the heating element.
Solubility substances happens :
- unlimited
(Examples: water and alcohol; potassium chloride and potassium bromide; potassium and rubidium) - these substances are mixed in any ratio.
- limited (Example: water and table salt) - a certain amount of a solute
According to the degree of solubility, all substances are divided into:
- Highly soluble (solubility at 20 0 С more than 1 g)
- Slightly soluble (solubility at 20 0 С from 0.01 to 1.0 g)
- Insoluble (solubility at 20 0 С no more than 0.01 g)
A substance is said to be highly soluble if more than 10 g of it dissolves well in 100 g of water.
A substance is said to be insoluble if less than 1 g dissolves in 100 g of water.
And insoluble - these are substances, less than 0.01 g of which goes into solution.
There are no completely insoluble substances. Even when water is poured into a glass vessel, an insignificant part of the glass molecules goes into solution.
What gives us knowledge about the solubility of substances in the production of cosmetics? There are many options for the composition of cosmetic products. To prevent potential incompatibility of the components in them, knowledge of the solubility of substances is necessary for this. Knowing how and in what substances dissolve, they select the correct, sequential introduction into the reactor of all the necessary components in the manufacture of cosmetics. concept "solubility" widely used in pharmacology. By definition of solubility, the purity of the substance and excipients is judged.
In the manufacture of medicines, biologically active additives (BAA), knowing about solubility, special technological methods are used:
- Change the sequence of dissolution (mixing) of the ingredients.
- Use the methods of separate dissolution of the components.
- Mix parts medicinal substances, various bases and then combine these parts into a single whole
Knowing the solubility of substances, various co-solvents, solubilizers and stabilizers are selected to create durable dosage forms.
solubilities of substances in different solvents are usually given in private articles on substances or excipients.
The solubility of substances in the pharmacopoeia means conditional terms, which are given in Table No. 1 (1):
Table #1:
Knowledge of the solubility of the drug is very important for taking medicines and dietary supplements. The drug penetrates more easily in dissolved form into the gastrointestinal tract, thus bringing a faster relief effect to the patient, in contrast to slightly soluble or hardly soluble dosage forms.
How is the solubility of substances determined?
A sample of the test substance is taken, placed in a measured amount of solvent, the solution is shaken for 10 minutes.
All determinations are carried out at a temperature of (18-22) 0 C.
For slowly soluble substances (the dissolution time of which is more than 20 minutes), heating in a water bath up to 30 0 C is possible.
After vigorous shaking for two minutes and cooling the solution to (18-22) 0 C, the result is visually recorded.
For slowly soluble substances, the solubility conditions are specified in private articles.
A substance is considered dissolved if no particles are found in the solution when viewed in transmitted light.
If the solubility of a substance is not known, then the test procedure is as follows:
Take 1 g of the substance, add 1 ml of solvent and carry out the test as described above. If the substance is completely dissolved, it is considered soluble very easily.
If the dissolution is not complete, then take 100 mg of the powdered substance, add 1 ml of the solvent and dissolve again. The sample dissolved completely - they conclude that the substance easily soluble.
If the dissolution is not complete, add 2 ml of solvent to this solution and continue the test. The sample dissolved - it is believed that the substance soluble.
If the dissolution is not complete, then another 7 ml of solvent is added to the solution and the dissolution is carried out again, as described above. If, when observed in transmitted light, particles are not visually observed, then the dissolution has passed. Such a substance is considered moderately soluble.
If undissolved sample particles are found, tests are carried out with 10 mg of the ground substance, adding 10 ml of solvent to it. In the event that it has completely dissolved, the substance is considered slightly soluble.
If the dissolution is not complete, take 10 mg of the powdered substance, add 100 ml of solvent to it and carry out the test again, as described in the procedure. The substance is completely dissolved very slightly soluble.
If not dissolved - it is considered that the substance practically insoluble in this solvent.
For substances with known solubility, test according to the above procedure, but only for the extreme values of the specified solubility term. For example, if the substance soluble then 100 mg of it should not dissolve in 1 ml, but completely dissolve in 3 ml of solvent. Literature.
State Pharmacopoeia Russian Federation. X II edition. Part 1, Moscow, 2007, pp. 92-93.
Section 5. SOLUTIONS. THEORY OF ELECTROLYTIC DISSOCIATION
§ 5.2. Solubility of substances in water
Solubility is the property of a substance to dissolve in water or another solvent. Solid, liquid and gaseous substances can dissolve in water.
For solubility in water, all substances are divided into three groups: 1) highly soluble; 2) slightly soluble; and 3) practically insoluble. The latter are also called insoluble substances. However, it should be noted that there are no absolutely insoluble substances. If you immerse a glass rod or a piece of gold or silver in water, they still dissolve in water in negligible amounts. As you know, solutions of argentum or aurum in water kill microbes. Glass, silver, gold are examples of substances that are practically insoluble in water (solids). They also include kerosene, vegetable oil (liquid substances), noble gases (gas substances). Many substances dissolve quite well in water. Examples of such substances are sugar, copper sulphate, sodium hydroxide (solid substances), alcohol, acetone (liquid substances), chlorine water, ammonia (gas substances).
From the above examples it follows that solubility primarily depends on the nature of the substances, in addition, it depends on temperature and pressure. The dissolution process itself is due to the interaction of the particles of the solute and the solvent; it is a spontaneous process.
The process of dissolution of solids in liquids can be represented as follows: under the influence of a solvent, individual ions or molecules gradually detach from the surface of a solid and are evenly distributed throughout the entire volume of the solvent. If the solvent is in contact with a large amount of a substance, then after a while the solution becomes saturated.
A saturated solution is one that is in dynamic equilibrium with an excess of solute.
To prepare a saturated solution, you need to add a substance to water at a given temperature with stirring until a precipitate forms, that is, an excess of the substance remains insoluble. In this case, a dynamic equilibrium will be established between the solution and the excess of the substance, it dissolves: how many particles of the substance will pass into the solution, the same number will be released (crystallized) from the solution. A saturated solution at a given temperature contains the maximum amount of solute possible.
An unsaturated solution contains fewer substances, and a saturated solution contains more than a saturated solution. Supersaturated solutions are rather unstable. Gently shaking the vessel or adding a salt crystal to the solution causes the excess of the solute to precipitate. Saturated solutions form sucrose, Na 2 SO 4 ∙ 10H 2 O, Na 2 S 2 O 3 ∙ 5H 2 O, CH 3 COOHa, Na 2 B 4 O 7 ∙10H 2 O, etc.
Often poorly soluble and practically insoluble substances are combined by one name - slightly soluble. In this case, only soluble and poorly soluble substances are spoken of. Quantitatively, solubility is expressed by the concentration of a saturated solution. Most often, it is expressed as the maximum number of grams of a substance that can be dissolved in 100 g of a solvent at a given temperature. This amount of a substance is sometimes called the solubility coefficient or simply the solubility of the substance. For example, at 18 ° C, 51.7 g of lead(II) nitrate salt G will dissolve in 100 g of water. b (NO 3) 2 , that is, the solubility of this salt at 18°C is 51.7. If, at the same temperature, in excess of this amount, more lead (II) nitrate salts are added, then it will not dissolve, but will precipitate in the form of a precipitate.
When talking about the solubility of a substance, the temperature of dissolution should be indicated. Most often, the solubility of solids with increasing temperature p amu p and c p remains. This is clearly illustrated by the solubility curves (Fig. 5.2). The temperature is plotted on the abscissa axis, and the solubility coefficient is plotted on the ordinate axis. However, the solubility of some substances increases slightly with increasing temperature (for example, NaCl, A l C l 3 ) or even decreases [for example, Ca( O H) 2, Li 2 SO 4 , Ca(CH 3 COO) 2]. On the solubility factor solid body in water, the pressure has little effect, since there is no noticeable change in the volume of the system during dissolution. With the help of solubility curves, it is easy to calculate how much salt will fall out of the solution when it is cooled. For example, if you take 100 g of water and prepare a saturated solution of potassium nitrate at 45 ° C, and then cool it to 0 ° C, then, as follows from the solubility curve (see Fig. 5.2), 60 g of salt crystals should fall out. Solubility curves are used to easily determine the solubility coefficient of substances at different temperatures.
The release of a substance from a solution with a decrease in temperature is called crystallization. If the solution contained impurities, then, due to crystallization, the substance is always obtained pure, since the solution remains unsaturated with respect to impurities even when the temperature is lowered, and impurities do not precipitate. This is the basis of the method of purification of substances, called recrystallization.
During the dissolution of gases in water, heat is released. Therefore, according to Le Chatelier's principle, with increasing
Rice. 5.2. Solubility curves for solids
temperature, the solubility of gases decreases, and as it decreases, it increases (Fig. 5.3). The solubility of gases increases with increasing pressure. Since the volume of gas that dissolves in a given volume of water does not depend on pressure, the solubility of a gas is usually expressed as the number of milliliters that dissolves in 100 g of solvent (see Fig. 5.3).
Rice. 5.3. Gas solubility curves
V Everyday life people rarely encounter pure substances. Most objects are mixtures of substances.
A solution is a homogeneous mixture in which the components are uniformly mixed. There are several types according to particle size: coarse systems, molecular solutions and colloidal systems, which are often called sols. In this article we are talking about molecular (or true) solutions. The solubility of substances in water is one of the main conditions affecting the formation of compounds.
Solubility of substances: what is it and why is it needed
To understand this topic, you need to know what solutions and solubility of substances are. in plain language, is the ability of a substance to combine with another and form a homogeneous mixture.
From a scientific point of view, a more complex definition can be considered.
The solubility of substances is their ability to form homogeneous (or heterogeneous) compositions with one or more substances with a dispersed distribution of components. There are several classes of substances and compounds:
- soluble;
- sparingly soluble;
- insoluble.
What is the measure of the solubility of a substance
a substance in a saturated mixture is a measure of its solubility. As mentioned above, for all substances it is different. Soluble are those that can dissolve more than 10g of themselves in 100g of water. The second category is less than 1 g under the same conditions. Practically insoluble are those in the mixture of which less than 0.01 g of the component passes. In this case, the substance cannot transfer its molecules to water.
What is the solubility coefficient
The solubility coefficient (k) is an indicator of the maximum mass of a substance (g) that can be dissolved in 100 g of water or another substance.
Solvents
This process involves a solvent and a solute. The first differs in that initially it is in the same state of aggregation, which is the final mixture. As a rule, it is taken in larger quantities.
However, many people know that water occupies a special place in chemistry. There are separate rules for it. A solution in which H2O is present is called an aqueous solution.
When talking about them, the liquid is an extractant even when it is in a smaller amount. An example is an 80% solution of nitric acid in water.
The proportions here are not equal. Although the proportion of water is less than that of acids, it is incorrect to call the substance a 20% solution of water in nitric acid.There are mixtures that do not contain H2O. They will bear the name seine. Such electrolyte solutions are ionic conductors. They contain single or mixtures of extractants. They are composed of ions and molecules. They are used in industries such as medicine, the production of household chemicals, cosmetics and other areas.
They can combine several desired substances with different solubility. The components of many products that are applied externally are hydrophobic. In other words, they do not interact well with water. In such mixtures, the solvents may be volatile, non-volatile, or combined.
Organic substances in the first case dissolve fats well. The volatiles include alcohols, hydrocarbons, aldehydes, and others. They are often included in household chemicals. Non-volatile are most often used for the manufacture of ointments. These are fatty oils, liquid paraffin, glycerin and others.
Combined is a mixture of volatile and non-volatile, for example, ethanol with glycerin, glycerin with dimexide. They may also contain water.
A saturated solution is a mixture chemical substances, containing the maximum concentration of one substance in the solvent at a certain temperature. It will not breed further.
In the preparation of a solid substance, precipitation is noticeable, which is in dynamic equilibrium with it.
This concept means a state that persists in time due to its flow simultaneously in two opposite directions (forward and reverse reactions) at the same speed.
If a substance can still decompose at a constant temperature, then this solution is unsaturated. They are stable. But if you continue to add a substance to them, then it will be diluted in water (or other liquid) until it reaches its maximum concentration.
Another type is oversaturated. It contains more solute than can be at a constant temperature. Due to the fact that they are in an unstable equilibrium, crystallization occurs when they are physically affected.
How can you tell a saturated solution from an unsaturated one?
This is easy enough to do. If the substance is a solid, then a precipitate can be seen in a saturated solution.
In this case, the extractant can thicken, as, for example, in a saturated composition, water to which sugar has been added.
But if you change the conditions, increase the temperature, then it will no longer be considered saturated, since at more high temperature the maximum concentration of this substance will be different.
Theories of interaction of components of solutions
There are three theories regarding the interaction of elements in a mixture: physical, chemical and modern. The authors of the first one are Svante August Arrhenius and Wilhelm Friedrich Ostwald.
They assumed that, due to diffusion, the particles of the solvent and the solute were evenly distributed throughout the volume of the mixture, but there was no interaction between them. The chemical theory put forward by Dmitri Ivanovich Mendeleev is the opposite of it.
According to it, as a result of chemical interaction between them, unstable compounds of constant or variable composition are formed, which are called solvates.
At present, the unified theory of Vladimir Aleksandrovich Kistyakovsky and Ivan Alekseevich Kablukov is used. It combines physical and chemical. The modern theory says that in a solution there are both non-interacting particles of substances and the products of their interaction - solvates, the existence of which Mendeleev proved.When the extractant is water, they are called hydrates. The phenomenon in which solvates (hydrates) are formed is called solvation (hydration). It affects all physical and chemical processes and changes the properties of the molecules in the mixture.
Solvation occurs due to the fact that the solvation shell, consisting of molecules of the extractant closely associated with it, surrounds the solute molecule.
Factors affecting the solubility of substances
Chemical composition of substances. The rule “like attracts like” applies to reagents as well. similar in physical and chemical properties substances can dissolve mutually faster. For example, non-polar compounds interact well with non-polar ones.
Substances with polar molecules or an ionic structure are diluted in polar ones, for example, in water. Salts, alkalis and other components decompose in it, while non-polar ones do the opposite. A simple example can be given. To prepare a saturated solution of sugar in water, you need large quantity substances than in the case of salt.
What does it mean? Simply put, you can dilute much more sugar in water than salt.
Temperature. To increase the solubility of solids in liquids, you need to increase the temperature of the extractant (works in most cases). An example can be shown. If you put a pinch of sodium chloride (salt) in cold water, this process will take a long time.
If you do the same with a hot medium, then the dissolution will be much faster. This is because as the temperature rises, the kinetic energy, a significant amount of which is often spent on the destruction of bonds between molecules and ions of a solid.
However, when the temperature rises in the case of lithium, magnesium, aluminum and alkali salts, their solubility decreases.
Pressure. This factor only affects gases. Their solubility increases with increasing pressure. After all, the volume of gases is reduced.
Changing the dissolution rate
Do not confuse this indicator with solubility. After all, different factors influence the change in these two indicators.
The degree of fragmentation of the solute.
This factor affects the solubility of solids in liquids. In the whole (lumpy) state, the composition is diluted longer than the one that is broken into small pieces. Let's take an example.
A solid block of salt will take much longer to dissolve in water than salt in the form of sand.
Stirring speed. As is known, this process can be catalyzed by stirring. Its speed is also important, because the faster it is, the faster the substance will dissolve in the liquid.
Why is it important to know the solubility of solids in water?
First of all, such schemes are needed to correctly solve chemical equations. In the solubility table there are charges of all substances. They need to be known in order to correctly record the reagents and draw up the equation of a chemical reaction. Solubility in water indicates whether the salt or base can dissociate.
Aqueous compounds that conduct current have strong electrolytes in their composition. There is another type. Those that conduct current poorly are considered weak electrolytes. In the first case, the components are substances that are completely ionized in water.
Whereas weak electrolytes show this indicator only to a small extent.
Chemical reaction equations
There are several types of equations: molecular, complete ionic and short ionic. In fact, the last option is a shortened form of molecular. This is the final answer. The complete equation contains the reactants and products of the reaction. Now comes the turn of the solubility table of substances.
First you need to check whether the reaction is feasible, that is, whether one of the conditions for the reaction is met. There are only 3 of them: the formation of water, the release of gas, precipitation. If the first two conditions are not met, you need to check the last one.
To do this, you need to look at the solubility table and find out if there is an insoluble salt or base in the reaction products. If it is, then this will be the sediment. Further, the table will be required to write the ionic equation.
Since all soluble salts and bases are strong electrolytes, they will decompose into cations and anions. Further, unbound ions are reduced, and the equation is written in a short form. Example:- K2SO4+BaCl2=BaSO4↓+2HCl,
- 2K+2SO4+Ba+2Cl=BaSO4↓+2K+2Cl,
- Ba+SO4=BaSO4↓.
Thus, the table of solubility of substances is one of the key conditions for solving ionic equations.
A detailed table helps you find out how much component you need to take to prepare a rich mixture.
Solubility table
This is what the usual incomplete table looks like. It is important that the temperature of the water is indicated here, as it is one of the factors that we have already mentioned above.
How to use the table of solubility of substances?
The table of solubility of substances in water is one of the main assistants of a chemist. It shows how various substances and compounds interact with water. The solubility of solids in a liquid is an indicator without which many chemical manipulations are impossible.
The table is very easy to use. Cations (positively charged particles) are written on the first line, anions (negatively charged particles) are written on the second line. Most of the table is occupied by a grid with certain symbols in each cell.
These are the letters "P", "M", "H" and the signs "-" and "?".
- "P" - the compound is dissolved;
- "M" - dissolves a little;
- "H" - does not dissolve;
- "-" - connection does not exist;
- "?" - no information about the existence of the connection.
There is one empty cell in this table - this is water.
Simple example
Now about how to work with such material. Suppose you need to find out if a salt is soluble in water - MgSo4 (magnesium sulfate). To do this, you need to find the Mg2+ column and go down it to the SO42- line. At their intersection is the letter P, which means the compound is soluble.
Conclusion
So, we have studied the issue of the solubility of substances in water and not only. Without a doubt, this knowledge will be useful in the further study of chemistry. After all, the solubility of substances plays an important role there. It will be useful in deciding chemical equations, and various tasks.
Solubility of various substances in water
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 simple 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 dissolving gases, heat is released, therefore, with increasing temperature, their solubility decreases (for example, the solubility of NH3 at 0 ° C is 1100 dm3 / 1 dm3 of water, and at 25 ° C - 700 dm3 / 1 dm3 of water).
The dependence of gas solubility on pressure obeys Henry's law: The mass of dissolved gas at constant temperature is directly proportional to pressure.
Expression of the quantitative composition of solutions
Along with temperature and pressure, the main parameter of the state of a 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 a 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 dissolved substance contained in 1 dm3 of the solution (s or M, mol / dm3)
v) equivalent concentration, or normality , shows the number of equivalents of a solute contained in 1 dm3 of a solution (ce or n, mol / dm3)
G) molar mass concentration, or molality , shows the number of moles of a solute contained in 1000 g of solvent (cm, mol / 1000 g)
e) titer solution is the number of grams of solute in 1 cm3 of solution (T, g / cm3)
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 a dissolved substance (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 dissolved substance (ν1) to the total amount of all components of the solution, that is, ν1 + ν2 (where ν2 is the amount of solvent)
Nr.v.= ν1/(ν1+ ν2)= mr.v./Mr.v./(mr.v./Mr.v+mr-l./Mr-l).
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 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 semipermeable partition (dotted line in the figure) into two parts filled to the same O-O level. Solvent is placed on the left side, solution is placed on the right side.
solvent solution
The concept 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 semi-permeable partition into the solution, diluting it.
The driving force for the predominant 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.
One-way 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 the solvent to pass through the semi-permeable partition into the given solution.
It is equal to the additional pressure that must be applied to the solution so that osmosis stops (the action 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 the 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
Rosm= withRT,
where Rosm is the osmotic pressure, kPa; c is the molar concentration, mol/dm3; R is the gas constant equal to 8.314 J/mol∙K; T is temperature, K.
This expression is similar in form to the Mendeleev-Clapeyron equation for ideal gases, but these equations describe different processes. Osmotic pressure occurs 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 legal cosmic pressure The 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 as a solution 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 saturation 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.
Raul showed that the pressure of a saturated vapor of a solvent over a solution P is less than over a pure solvent P °. The difference P ° - P \u003d P 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 saturated vapor pressure of the solvent over the solution is equal to the mole fraction of the dissolved non-volatile substance
(Р°-Р)/Р°= N= ν1/(ν1+ ν2)= mr.v./Mr.v./(mr.v./Mr.v+mr-la./Mr-la)= XA
where XA is the mole fraction of the solute. And since ν1 \u003d mr.v. / Mr.v, using this law, you can determine the molar mass of the 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 principle of Le Chatelier's equilibrium shift.
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 a part of the vapor (the reaction of the system to a decrease in the water concentration when the substance is dissolved), which causes a decrease in the 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 \u200b\u200bare proportional to the molar concentration of the solute - non-electrolyte, that is:
t= K∙sT= K∙t∙1000/M∙a,
where cm is the molar concentration of the solution; a is the mass of the solvent. Proportionality factor TO , when the boiling point rises, it is called ebullioscopic constant for a given solvent (E ), and to lower the freezing temperature - cryoscopic constant(TO ).
These constants, numerically different for the same solvent, characterize an increase in the boiling point and a decrease in the freezing point of a one molar solution, i.e. by 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 dissolved substance, but depend only on the nature of the solvent and are characterized by the dimension kg∙deg/mol.
The concept of solutions. Solubility of substances
Solutions- homogeneous (homogeneous) systems of variable composition, which contain two or more components.
Liquid solutions are the most common. They consist of a solvent (liquid) and solutes (gaseous, liquid, solid):
Liquid solutions may be aqueous or non-aqueous. Aqueous solutions are solutions in which the solvent is water. Non-aqueous solutions- these are solutions in which other liquids are solvents (benzene, alcohol, ether, etc.). In practice, aqueous solutions are most often used.
Dissolution of substances
Dissolution- complex physical chemical process. The destruction of the structure of the dissolved substance and the distribution of its particles between solvent molecules is a physical process. At the same time, the solvent molecules interact with the particles of the dissolved substance, i.e. chemical process. As a result of this interaction, solvates are formed.
solvates- products of variable composition, which are formed during the chemical interaction of particles of a solute with solvent molecules.
If the solvent is water, then the resulting solvates are called hydrates. The process of formation of solvates is called solvation. The process of hydrate formation is called hydration. Hydrates of some substances can be isolated in crystalline form by evaporating solutions. For instance:
What is a crystalline substance and how is it formed of blue color? When copper (II) sulfate is dissolved in water, it dissociates into ions:
The resulting ions interact with water molecules:
When the solution is evaporated, copper sulfate (II) crystalline hydrate - CuSO4 5H2O is formed.
Crystalline substances containing water molecules are called crystalline hydrates. The water included in their composition is called water of crystallization. Examples of crystalline hydrates:
For the first time, the idea of the chemical nature of the dissolution process was expressed by D. I. Mendeleev in his chemical (hydrate) theory of solutions(1887). The proof of the physicochemical nature of the dissolution process is the thermal effects during dissolution, i.e., the release or absorption of heat.
Thermal effect of dissolution is equal to the sum thermal effects of physical and chemical processes. The physical process proceeds with the absorption of heat, the chemical - with the release.
If as a result of hydration (solvation) more heat is released than it is absorbed during the destruction of the structure of the substance, then dissolution is an exothermic process. The release of heat is observed, for example, when such substances as NaOH, AgNO3, H2SO4, ZnSO4, etc. are dissolved in water.
If more heat is needed to destroy the structure of a substance than it is generated during hydration, then dissolution is an endothermic process. This happens, for example, when NaNO3, KCl, K2SO4, KNO2, NH4Cl, etc. are dissolved in water.
Solubility of substances
We know that some substances dissolve well, others poorly. When substances are dissolved, saturated and unsaturated solutions are formed.
saturated solution is a solution that contains maximum amount solute at a given temperature.
unsaturated solution is a solution that contains less solute than saturated at a given temperature.
The quantitative characteristic of solubility is solubility factor. The solubility coefficient shows what is the maximum mass of a substance that can be dissolved in 1000 ml of solvent at a given temperature.
Solubility is expressed in grams per liter (g/l).
By solubility in water, substances are divided into 3 groups:
Table of solubility of salts, acids and bases in water:
The solubility of substances depends on the nature of the solvent, on the nature of the solute, temperature, pressure (for gases). The solubility of gases decreases with increasing temperature, and increases with increasing pressure.
The dependence of the solubility of solids on temperature is shown by solubility curves. The solubility of many solids increases with increasing temperature.Solubility curves can be used to determine: 1) the coefficient of solubility of substances at different temperatures; 2) the mass of the solute that precipitates when the solution is cooled from t1oC to t2oC.
The process of isolating a substance by evaporating or cooling its saturated solution is called recrystallization. Recrystallization is used to purify substances.
Solutions play a key role in nature, science and technology. Water is the basis of life, always contains dissolved substances. Fresh water rivers and lakes contain few dissolved substances, while sea water contains about 3.5% dissolved salts.
The primordial ocean (during the birth of life on Earth) is thought to have contained only 1% dissolved salts.
“It was in this environment that living organisms first developed, from this solution they scooped up the ions and molecules that are necessary for their further growth and development ... Over time, living organisms developed and transformed, so they were able to leave the aquatic environment and move to land and then rise to air. They obtained these abilities by preserving in their organisms an aqueous solution in the form of liquids that contain a vital supply of ions and molecules” – these are the words that describe the role of solutions in nature by the famous American chemist, laureate Nobel Prize Linus Pauling. Inside each of us, in every cell of our body, there are memories of the primordial ocean, the place where life originated, an aqueous solution that provides life itself.
In any living organism, an unusual solution constantly flows through the vessels - arteries, veins and capillaries, which 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 the human and animal body also interact in solutions. The process of assimilation of food is associated with the transfer of highly nutritious substances into solution. Natural aqueous solutions are directly related to the processes of soil formation, plant supply nutrients. Such technological processes in the chemical and many other industries, such as the production of fertilizers, metals, acids, paper, occur in solutions. modern science studies the properties of solutions. Let's find out what is a solution?
Solutions differ from other mixtures in that the particles constituent parts are located in them evenly, and in any microvolume of such a mixture, the composition will be the same.
That is why solutions were understood as homogeneous mixtures, which consist of two or more homogeneous parts. This idea was based on the physical theory of solutions.
Adherents of the physical theory of solutions, which van't Hoff, Arrhenius and Ostwald were engaged in, believed that the dissolution process is the result of diffusion.
D. I. Mendeleev and supporters of the chemical theory believed that dissolution is the result of the chemical interaction of a solute with water molecules. Thus, it will be more accurate to define a solution as a homogeneous system that consists of particles of a solute, a solvent, and also the products of their interaction.
Due to the chemical interaction of a solute with water, compounds are formed - hydrates. Chemical interaction is usually accompanied by thermal phenomena. For example, the dissolution of sulfuric acid in water takes place with the release of such an enormous amount of heat that the solution can boil, which is why acid is poured into water, and not vice versa. The dissolution of substances such as sodium chloride, ammonium nitrate, accompanied by the absorption of heat.
M. V. Lomonosov proved that solutions turn into ice at a lower temperature than the solvent.
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DISSOLUTION.
SOLUBILITY OF SUBSTANCES IN WATER.
I DISSOLUTION AND SOLUTIONS.
DISSOLUTION. SOLUTIONS.
Physical theory (van't Hoff,
Ostwald, Arrhenius).
Dissolution is a diffusion process
a solutions are homogeneous mixtures.
chemical theory (Mendeleev,
Kablukov, Kistyakovsky).
Dissolution is a chemical process
solute interactions
with water, - the process of hydration,
a solutions These compounds are hydrates.
Modern theory.
Dissolution- This is a physico-chemical process that occurs between the solvent and the particles of the solute and is accompanied by the process of diffusion.
Solutions- these are homogeneous (homogeneous) systems consisting of particles of a solute, a solvent and the products of their interaction - hydrates.
II SIGNS OF CHEMICAL INTERACTION DURING DISSOLUTION.
1. Thermal phenomena.
ü Exothermic - these are phenomena accompanied by the release of heat /dissolution of concentrated sulfuric acid H2SO4 in water/.
ü Endothermic- these are phenomena accompanied by the absorption of heat /dissolution of crystals of ammonium nitrate NH4NO3 in water/.
2. Color change.
CuSO4 + 5H2O → CuSO4∙ 5H2O
white blue crystals
crystals
3. Volume change.
III DEPENDENCE OF SOLID SUBSTANCES ON DISSOLUTION.
1. From the nature of substances:
ü highly soluble in water / more than 10 g of substance per 100 g of water /;
ü slightly soluble in water /less than 1g/;
ü practically insoluble in water /less than 0.01g/.
2. From temperature.
IV TYPES OF SOLUTIONS BY SOLUBILITY.
Ø According to the degree of solubility:
ü unsaturated solution - a solution in which, at a given temperature and pressure, further dissolution of the substance already contained in it is possible.
ü saturated solution - a solution that is in phase equilibrium with the solute.
ü Supersaturated solution - an unstable solution in which the content of a solute is greater than in a saturated solution of the same substance at those values of temperature and pressure.
Ø According to the ratio of the solute to the solvent:
ü concentrated;
ü diluted.
THEORY OF ELECTROLYTIC DISSOCIATION (TED).
I. Theory electrolytic dissociation(TED) was proposed by the Swedish scientist Svante Arrhenius in 1887
Later, TED developed and improved. The modern theory of aqueous solutions of electrolytes, in addition to the theory of electrolytic dissociation by S. Arrhenius, includes ideas about the hydration of ions (,), the theory of strong electrolytes (, 1923).
II. SUBSTANCES
electrolytes - substances, solutions
or whose melts conduct
electricity.
/acids, salts, bases/
Non-electrolytes Substances whose solutions or melts do not conduct electricity.
/simple substances/
IONS are charged particles.
ü cations /kat+/ are positively charged particles.
ü anions /an-/– negatively charged particles
III. MAIN PROVISIONS OF TED:
ü The spontaneous process of decomposition of an electrolyte into ions in a solution or in a melt is called electrolytic dissociation .
ü In aqueous solutions, ions are not in free, but in hydrated state, i.e., surrounded by dipoles of water and chemically associated with them. Ions in the hydrated state differ in properties from ions in the gaseous state of matter.
ü For the same solute, the degree of dissociation increases as the solution is diluted.
ü In solutions or melts of electrolytes, ions move randomly, but when an electric current is passed through a solution or melt of an electrolyte, the ions move in a direction: cations - to the cathode, anions - to the anode.
MECHANISM OF ELECTROLYTIC DISSOCIATION
1. ED of ionic substances:
ü Orientation of water dipoles relative to crystal ions.
ü The disintegration of the crystal into ions (proper dissociation).
ü Hydration of ions.
2. ED of substances with a covalent polar type of chemical bond.
ü Destruction hydrogen bonds between water molecules, the formation of water dipoles.
ü Orientation of water dipoles relative to the dipoles of a polar molecule.
ü Strong bond polarization, as a result of which the common electron pair is completely shifted to the atomic particle of a more electronegative element.
ü The disintegration of matter into ions (proper dissociation).
ü Hydration of ions.
DEGREE OF ELECTROLYTIC DISSOCIATION /α/
1. Degree of ED is the ratio of the number of decayed molecules to total number particles in solution.
α = ─ ∙ 100%
Ntotal
2. According to the magnitude of the degree of ED, substances are divided:
ü strong electrolytes /HCl; H2SO4; NaOH; Na2CO3/
ü medium strength electrolytes /H3PO4/
ü weak electrolytes /H2CO3; H2SO3/.
CHEMICAL DICTATION
ON THE TOPIC: "ELETROLYTIC DISSOCIATION"
1. All water-soluble bases are strong electrolytes.
2. Only water-soluble salts undergo hydrolysis.
3. Dissociation is a reversible process.
4. The essence of the neutralization reaction, CH3COOH + KOH → CH3COOH + H2O, reflected in the form of a short ionic equation of a chemical reaction is: H++ OH- → H2O.
5. BaSO4 ; AgCl are water-insoluble salts, so they do not dissociate into ions.
6. Is the dissociation equation for the following salts correct:
ü Na2SO4 → 2Na+ + SO42-
ü KCl → K+ + Cl-
7. Dissociation equation sulfurous acid has the following form: H2 SO3 → 2 H+ + SO3 2- .
8. The true degree of dissociation of a strong electrolyte is less than 100%.
9. As a result of the neutralization reaction, salt and water are always formed.
10. Only water-soluble bases - alkalis, are electrolytes.
11. Equations below chemical reactions are ion exchange reactions:
ü 2KOH + SiO2 → K2SiO3 + H2O
ü Al2O3 + 2NaOH → 2NaAlO2 + H2O
ü CuO + 2HCl → CuCl2 + H2O
12. Sulfurous acid is a weak acid, so it decomposes into water (H2O) and sulfur dioxide (SO2).
H2SO3 → H2O + SO2.
THE CODE
1. No /excluding NH3∙H2O/
2. No: Al2S3 + 2H2O → 2AlOHS + H2S
3. No. /The dissociation of only weak electrolytes is reversible process, strong electrolytes dissociate irreversibly/.
4. No: CH3COOH + OH - → CH3COO= + H2O.
5. No. /These salts are insoluble in relation to water, but they are able to dissociate/.
6. No. /These salts are strong electrolytes, so they dissociate irreversibly/.
7. No. /Polybasic acids dissociate stepwise/.
8. No. /The true degree of dissociation is equal to 100%/.
9. No: NH3(g.) + HCl(g.) → NH4Cl, water formation remains questionable.
10. No. /All bases are electrolytes/.
11. No. /These are exchange reactions, but ionic/.
12. No. /Decomposition of sulphurous acid occurs because it is a fragile acid/.
REGULATIONS
COMPILATION OF IONIC EQUATIONS OF CHEMICAL REACTIONS.
1. Simple substances, oxides, as well as insoluble acids, salts and bases do not decompose into ions.
2. Solutions are used for the ion exchange reaction, so even poorly soluble substances are in solutions in the form of ions. /If a poorly soluble substance is the original compound, then it is decomposed into ions when compiling ionic equations of chemical reactions/.
3. If the poorly soluble is formed as a result of the reaction, then when writing the ionic equation it is considered insoluble.
4. Amount electric charges on the left side of the equation must be equal to the sum of the electric charges on the right side.
CONDITIONS
ION EXCHANGE REACTIONS
1. The formation of a low-dissociating substance of water - H2O:
ü HCl + NaOH → NaCl + H2O
H+ + Cl - + Na+ + OH- → Na+ + Cl - + H2O
H+ + OH - → H2O
ü Cu(OH)2 + H2SO4 → CuSO4 + 2H2O
Cu(OH)2 + 2H+ + SO42- → Cu2+ + SO42- + 2H2O
Cu(OH)2 + 2H+ → Cu2+ + 2H2O
2. Precipitation:
ü FeCl3 + 3NaOH → Fe(OH)3↓ + 3NaCl
Fe3++ 3Cl - + 3Na+ + 3OH- → Fe(OH)3↓ + 3Na++ 3Cl-
Fe3++ 3OH - → Fe(OH)3↓
ü BaCl2 + H2SO4 → BaSO4↓ + 2HCl
Ba2++ 2Cl - + 2H++ SO42- → BaSO4↓ + 2H++ 2Cl-
Ba2++ SO42- → BaSO4↓
ü AgNO3 + KBr → AgBr↓ + KNO3
Ag+ + NO3- + K++ Br - → AgBr↓ + K++ NO3-
Ag+ + Br - → AgBr↓
3. Gas release:
ü Na2CO3 + 2HCl → 2NaCl + H2O + CO2
2Na++ CO32-+ 2H++ 2Cl- → 2Na++ 2Cl - + H2O + CO2
CO32-+ 2H+ → H2O + CO2
ü FeS + H2SO4 → FeSO4 + H2S
FeS + 2H++ SO42-→ Fe2++ SO42-+ H2S
FeS + 2H+ → Fe2++ H2S
ü K2SO3 + 2HNO3 → 2KNO3 + H2O + SO2
2K++ SO32-+ 2H++ 2NO3- → 2K++ 2NO3- + H2O + SO2
