Solutions are homogeneous systems of variable composition, consisting of two or more substances. Gaseous, liquid and solid solutions are known. Gaseous solutions include mixtures of any gases; solid solutions include many metal alloys and glasses. Of particular importance in nature and technology are liquid solutions formed by the dissolution of gases, liquids and solids in water and other liquids. When gases and solids are dissolved in a liquid, the latter is usually called . When liquids dissolve in each other, the solvent is considered to be the one of which there is more in the solution. The amount of solute contained in a certain amount of solution or is called the concentration (see) of the solution. Solutions in which a given substance no longer dissolves and, therefore, an excess of the solute is in equilibrium with the solution are called saturated. The concentration of an unsaturated solution is less, and that of a supersaturated solution is greater than that of a saturated solution. Supersaturated solutions usually form when hot saturated solutions are slowly cooled. The ability of a substance to dissolve in a given quantity in a given solvent when forming a saturated solution is called the solubility of the substance. The solubility of gases in liquids is usually expressed by the absorption coefficient, which indicates how many volumes of gas (at t° 0° and a pressure of 1 atm) are dissolved in one volume of liquid at a given temperature and partial gas pressure equal to 1 atm. The solubility of liquids and solids in liquids is usually expressed as the number of grams of solute per 100 g of solvent or per 100 ml of saturated solution. Solubility depends on the nature of the solute and the solvent. As temperature increases, the solubility of gases decreases, while that of liquids and solids in most cases increases. The solubility of gases is directly proportional to the pressure at which the gas dissolves.
Solutions play an exceptional role in nature and technology. The waters of the World Ocean and the atmosphere are solutions. All physiological and biochemical processes are associated with solutions, since the internal environment of any organism is aqueous solutions of various kinds of substances. Many medicines are also solutions.
See also Buffer solutions, Diffusion, Isotonic solutions, Colloids, .
Solutions (true solutions) are homogeneous (homogeneous) systems of variable composition, consisting of two or more substances. Solutions differ from mechanical mixtures in their homogeneity, and from chemical compounds in their variable composition.
Solutions play an extremely important role in nature, technology and everyday life. The vast majority of known chemical reactions occur in solutions. The waters of the world's oceans and the atmosphere are solutions. Physiological fluids are also solutions. Almost all medicinal substances have their characteristic effect on the body in a dissolved state.
Depending on the state of aggregation, gaseous, liquid and solid solutions are distinguished. Gaseous substances include mixtures of any gases and vapors, including air. Hard ones include many alloys, glass, some minerals and rocks. Of particular importance for the study of life processes in health and disease are liquid solutions formed by dissolving gases, liquids or solids in liquids.
When gases or solids are dissolved in a liquid, the liquid is usually called a solvent, and gases or solids in solutions are called solutes.
In the case of dissolving one liquid in another, the solvent is considered to be the one that is present in solutions in relatively larger quantities.
A saturated solution is a solution that is in equilibrium with an excess of the solute, an unsaturated solution is a solution whose concentration is less than a saturated one, and a supersaturated solution is a solution whose concentration is greater than that of a saturated one.
Depending on the molecular weight of the dissolved substance, liquid solutions are divided into solutions of low molecular weight substances, for example, aqueous solutions of ordinary acids, alkalis and salts, and into solutions of high molecular weight compounds, which include solutions of proteins, polysaccharides, nucleic acids in water, rubber in benzene, nitrocellulose in an alcohol-ether mixture, etc. Solutions of high-molecular compounds have a number of characteristic properties inherent in typical colloidal solutions. (see colloids).
The dissolution process is accompanied by the release or absorption of heat.
The solubility of a given substance in a liquid is measured by the concentration (see) of its saturated solution in this liquid.
A number of qualitative rules for the solubility of substances in liquids have been established. Polar substances are highly soluble in polar solvents (water, alcohol, acetone, etc.) and poorly soluble in non-polar liquids (benzene, carbon tetrachloride, carbon disulfide, etc.). On the contrary, nonpolar substances are highly soluble in nonpolar solvents and poorly soluble in polar ones. The last rule forms the basis of some theories of cell permeability. This means that the membrane of many cells consists of non-polar substances - lipids.
The solubility of gases in liquids is expressed by the absorption coefficient, which indicates how many volumes of a given gas, reduced to normal conditions (t° 0° and pressure 1 atm.), dissolve in one volume of liquid at a given temperature and partial gas pressure equal to 1 atm.
The solubility of gases in liquids varies widely depending on the nature of the liquid and gas, as well as pressure and temperature. So, for example, at t° 18° the nitrogen absorption coefficient is 0.01698; oxygen - 0.03220; hydrogen chloride - 427.9; ammonia - 748.8. Oxygen is approximately twice as soluble in water as nitrogen, so the oxygen content in air dissolved in water is significantly higher than in the atmosphere (34.1% by volume at t° 18° instead of 21.2% in the atmosphere). This is of great biological importance for organisms living in water.
The dependence of gas solubility on pressure is expressed by Henry's law (see Absorption).
When a mixture of gases is dissolved, the solubility of each gas, according to Dalton's law, is proportional to its partial pressure above the solution.
As the temperature increases, the solubility of gas in liquid decreases. This property of gas is used to remove gases dissolved in them from liquids.
To do this, the solution is boiled for some time, as a result of which the gas is removed from the solution along with steam bubbles.
The indicated dependence of gas solubility on temperature is of great biological importance for organisms living in water.
As the temperature rises, the respiration of organisms and the need for oxygen increase, while its concentration in water decreases, as a result of which, when heated, the death of organisms from suffocation due to lack of oxygen can occur. When water is saturated with oxygen, organisms become less sensitive to temperature increases.
When salts and many non-electrolytes prone to hydration are dissolved in water, the solubility of gases in it, as a rule, decreases significantly in accordance with I.M. Sechenov’s law.
The solubility of liquids in liquids varies widely. There are known liquids that are unlimitedly soluble in each other, for example, alcohol and water, sulfuric acid and water, etc. There are liquids that are limitedly soluble in each other, for example, ether is soluble in water in small quantities. When large quantities are added, two layers are formed. The top layer is a saturated solution of water in ether and contains 1.2% water and 98.8% ether at t° 18°; the bottom layer, which is a saturated solution of ether in water, contains 93.5% water and 6.5% ether.
Liquids are known that are practically insoluble in each other, for example, mercury and water, benzene and water. With increasing temperature, the mutual solubility of limitedly soluble liquids in most cases increases and often when a certain temperature for each pair of liquids, called critical, is reached, the liquids completely mix with each other. For example, phenol and water at t° 68.8° (critical temperature) and above dissolve in each other in any proportions; Below the critical temperature they are only limitedly soluble in each other.
When pressure changes, the mutual solubility of liquids changes slightly.
The solubility of solids in liquids is usually expressed in grams of anhydrous solid per 100 g of solvent in a saturated solution or per 100 ml of a saturated solution. Depending on the nature of the solid and the solvent, the solubility of solids in liquids varies within very wide limits. So, for example, at 25° 257 g of AgNO 3 and only 3·10 -20 g of HgS are dissolved in 100 g of water.
The solubility of solids depends on the degree of their grinding. Small crystals, or grains, smaller than about 0.1 mm, are more soluble than large ones. Crystal hydrates of the same chemical compound that differ in their water of crystallization content have different solubility. For example, the solubility of Na 2 SO 4 ·10H 2 O in water is less than the solubility of Na 2 SO 4.
The solubility of solids in liquids is almost independent of pressure, but, as a rule, varies greatly with temperature.
Typically, the solubility of solid substances increases with increasing temperature, but substances, such as Ca(OH) 2, Ca(C 2 H 3 O 2) 2, etc., are known whose solubility decreases with increasing temperature.
See also Buffer solutions, Diffusion, Isotonic solutions, Electrolytes.
In everyday life, a person constantly encounters various solutions - drinks, medicinal mixtures, household chemicals, etc. It would seem that there is nothing simpler than preparing some kind of cocktail (a typical solution), but, meanwhile, the very nature of the process of dissolving one substance in another is quite complex.
Modern science claims that dissolution is a physico-chemical process.
One of the main physical processes occurring during dissolution is diffusion(penetration) of a solute into a solvent. Diffusion is caused by the chaotic thermal movement of molecules (atoms, ions) - first, a boundary diffusion layer is formed between two dissolved substances, within which there is intense mixing of particles of various substances with each other, caused by the difference in density of the solvent and the dissolved substance.
The dissolution process may be accompanied by the release (absorption) of heat, a change in the color of the solution and other chemical transformations, which gives reason to talk about the presence of chemical processes accompanying dissolution.
Dissolution is accompanied by the formation of chemical bonds ( solvation) between the solvent and the solute (if the solvent is water - hydration), in this case, the products of interaction of the solute with the solvent are called solvates (if water acts as a solvent - hydrates).
Solvates (hydrates) are unstable substances. Hydrates that exist in a crystalline state are called crystalline hydrates (copper sulfate, iron sulfate).
Classification of solutions:
- Liquid solutions (sweet tea).
- Solid solutions (alloys).
- Gaseous solutions (air).
Of course, the most common are liquid solutions. Usually, when talking about solutions, we mean liquid solutions.
Any liquid solution consists of solvent(liquid medium in which the dissolution process occurs) and dissolved substances, which dissolve in liquid.
Classification of liquid solutions:
- liquid + gas;
- liquid + liquid;
- liquid + solid.
What is solubility
Solubility is the ability of a substance to form solutions - some substances can dissolve in each other indefinitely; others - only in limited quantities or practically not dissolve at all.
The solubility of a particular substance depends on its nature and the nature of the solvent, as well as the conditions under which the dissolution process occurs: temperature, pressure, the presence of third substances.
The old rule says: “like dissolves in like,” i.e., non-polar substances dissolve well in non-polar solvents (benzene in hexane), and polar substances dissolve well in polar ones (ethyl alcohol in water), and vice versa - non-polar substances dissolve in polar solvents bad (benzene in water).
Saturated solution is a solution in which at a given temperature a certain substance does not dissolve (the solute is said to be in a state of equilibrium with the solution).
Accordingly, a similar solution with a smaller amount of solute than in a saturated solution is called unsaturated.
Thus, solubility can be expressed numerically as the concentration of a substance in its saturated solution.
The solubility of a particular substance is expressed through solubility coefficient(the mass of solute that saturates 100 g of solvent under certain conditions).
All substances according to their solubility in water can be divided into 3 groups:
- substances soluble in water - solubility more than 1 g per 100 ml of water;
- poorly soluble substances - 0.1..1 g per 100 ml;
- insoluble substances - less than 0.1 g per 100 ml.
The solubility of solids is highly dependent on temperature (generally, the higher the temperature, the better their solubility).
The solubility of gases in water increases with increasing pressure (anyone who has uncorked a bottle of champagne knows this well). This is also well known to divers who, after a long stay at great depths (under high pressure), are forced to gradually rise to the surface over the course of hours (or undergo adaptation in a pressure chamber), otherwise, when a diver quickly rises from great depths, decompression sickness develops, when nitrogen, dissolved in large quantities in human blood plasma, begins to be released in the form of bubbles (the blood boils), clogging small vessels and capillaries, threatening the life of the diver.
Solutions homogeneous systems containing at least two substances are called. There may be solutions of solid, liquid and gaseous substances in liquid solvents, as well as homogeneous mixtures (solutions) of solid, liquid and gaseous substances. As a rule, a substance taken in excess and in the same state of aggregation as the solution itself is considered to be a solvent, and a component taken in deficiency is considered a solvent. solute./>/>
Depending on the state of aggregation /> solvents are distinguished gaseous, liquid and solid solutions./>
Gaseous solutions /> are air and other mixtures of gases.
Liquid solutions include homogeneous mixtures of gases, liquids bones and solids with liquids./>
Solid solutions /> There are many alloys, for example, metals with each other, glass. The most important are liquid mixtures in which the solvent is liquid. The most common inorganic solvent is, of course, water. Among organic substances, methanol, ethanol, diethyl alcohol are used as solvents. ether, acetone, benzene, carbon tetrachloride, etc.
During the dissolution process, particles (ions or molecules) of the solute, under the influence of chaotically moving solvent particles, pass into the solution, forming, as a result of the random movement of the particles, a qualitatively new homogeneous system. The ability to form solutions is expressed indifferent substances to varying degrees.Some substances are capablemix with each other in any quantities (water and alcohol),others - in limited quantities (sodium chloride and water)./>
The essence of the solution formation process can be shown in /> example of dissolving a solid in a liquid. From a point of viewAccording to the molecular kinetic theory, dissolution proceeds as follows: when a solid substance, for example, table salt, is added to the solvent, particles of ions Na+ and Cl — , located on the surface, as a result of the oscillatory motion, which increases upon collision with solvent particles, can break off and pass into the solvent. This process extends to subsequent layers of particles, which are exposed in the crystal after the surface layer is removed. So gradually the particles that form the crystal (ions or molecules) pass into solution. A visual diagram of the destruction of the ionic crystal lattice is given. Na WITH l when dissolved in water consisting of polar molecules.
Particles that have passed into the solution are distributed throughout the entire volume of the solvent due to diffusion. On the other hand, as the concentration increases, particles (ions, molecules) that are in continuous motion, when colliding with a solid surface of a substance that has not yet dissolved, can linger on it, i.e. dissolution is always accompanied by the opposite phenomenon - crystallization. A moment may come when the same number of particles (ions, molecules) are released from the solution at the same time as they enter the solution—equilibrium occurs. />
Based on the ratio of the predominance of the number of particles passing into the solution or removing from the solution, solutions are distinguished between saturated, unsaturated and supersaturated. Based on the relative amounts of solute and solvent, solutions are divided into dilute and concentrated./>/>
A solution in which a given substance no longer dissolves at a given temperature, i.e. a solution in equilibrium with the solute is called saturated, and a solution in which an additional amount of this substance can still be dissolved is called unsaturated.
A saturated solution contains the maximum possible (for given conditions) amount of solute. Therefore, a saturated solution is one that is in equilibrium with an excess of solute. The concentration of a saturated solution (solubility) for a given substance under strictly defined conditions (temperature, solvent) is a constant value./>
A solution containing more solute than it should be under given conditions in a saturated solution is called supersaturated. Supersaturated solutions are unstable, nonequilibrium systems in which a spontaneous transition to an equilibrium state is observed. This releases excess solute and the solution becomes saturated.
Saturated and unsaturated solutions should not be confused with dilute and concentrated solutions. Dilute solutions- solutions with a small content of dissolved substance; concentrated solutions- solutions with a high content of dissolved substance. It must be emphasized that the concepts of dilute and concentrated solutions are relative, expressing only the ratio of the amounts of solute and solvent in the solution. />
Comparing the solubility of various substances, we see that saturated solutions of poorly soluble substances are dilute, and highly soluble substances, although unsaturated, are quite concentrated./>
Depending on electrically neutral or charged particles are the components of the solution; they are divided into molecular (non-electrolyte solutions) and ionic (electrolyte solutions). One of the characteristic features of electrolyte solutions is that they conduct electricity.
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Plan
Introduction
1. Dispersed systems. True solutions
2. Solubility of solids and liquids in liquids. Influence of the nature of substances and temperature on solubility
3. Methods of expressing the concentration of solutions: mass - C%, molar - C m and normal (equivalent) - C n
4. Electrolyte solutions. Electrolytic dissociation of acids, bases, salts. Step dissociation
5. Classification of electrolytes. Degree of dissociation. Strong and weak electrolytes
6. Exchange reactions in electrolyte solutions and conditions for their occurrence. Ionic equations
7. Ionic product of water. Hydrogen indicator pH of solutions. Indicators
Conclusion
List of sources used
Introduction
Solutions are homogeneous systems containing at least two substances. There may be solutions of solid, liquid and gaseous substances in liquid solvents, as well as homogeneous mixtures (solutions) of solid, liquid and gaseous substances. As a rule, a substance taken in excess and in the same state of aggregation as the solution itself is considered to be a solvent, and a component taken in deficiency is considered a dissolved substance. Depending on the state of aggregation of the solvent, gaseous, liquid and solid solutions are distinguished. Gaseous solutions are air and other mixtures of gases. Liquid solutions include homogeneous mixtures of gases, liquids and solids with liquids. Solid solutions are many alloys, for example, metals with each other, glass. The most important are liquid mixtures in which the solvent is liquid. The most common inorganic solvent is, of course, water. Among organic substances, methanol, ethanol, diethyl ether, acetone, benzene, carbon tetrachloride, etc. are used as solvents. During the dissolution process, particles (ions or molecules) of the solute under the influence of chaotically moving solvent particles pass into solution, forming as a result of random movement of particles a qualitatively new homogeneous system. The ability to form solutions is expressed in different substances to varying degrees. Some substances can be mixed with each other in any quantities (water and alcohol), others - in limited quantities (sodium chloride and water). The essence of the process of solution formation can be illustrated by the example of the dissolution of a solid in a liquid. From the point of view of molecular kinetic theory, dissolution proceeds as follows: when any solid substance, for example, table salt, is added to the solvent, particles of Na+ and Cl- ions located on the surface, as a result of oscillatory motion, which increases upon collision with solvent particles, can break off and go into the solvent. This process extends to subsequent layers of particles, which are exposed in the crystal after the surface layer is removed. So gradually the particles that form the crystal (ions or molecules) pass into solution. Figure shows a visual diagram of the destruction of the NaCl ionic crystal lattice when dissolved in water, consisting of polar molecules. Particles that have passed into the solution are distributed throughout the entire volume of the solvent due to diffusion. On the other hand, as the concentration increases, particles (ions, molecules) that are in continuous motion, when colliding with a solid surface of a substance that has not yet dissolved, can linger on it, i.e. dissolution is always accompanied by the opposite phenomenon - crystallization. A moment may come when the same number of particles (ions, molecules) are released from the solution at the same time as they enter the solution - equilibrium occurs. Based on the ratio of the predominance of the number of particles passing into the solution or being removed from the solution, solutions are distinguished between saturated, unsaturated and supersaturated. Based on the relative amounts of solute and solvent, solutions are divided into dilute and concentrated. A solution in which a given substance no longer dissolves at a given temperature, i.e. a solution in equilibrium with the solute is called saturated, and a solution in which an additional amount of this substance can still be dissolved is called unsaturated. A saturated solution contains the maximum possible (for given conditions) amount of solute. Therefore, a saturated solution is one that is in equilibrium with an excess of solute. The concentration of a saturated solution (solubility) for a given substance under strictly defined conditions (temperature, solvent) is a constant value. A solution containing more solute than it should be under given conditions in a saturated solution is called supersaturated. Supersaturated solutions are unstable, nonequilibrium systems in which a spontaneous transition to an equilibrium state is observed. This releases excess solute and the solution becomes saturated. Saturated and unsaturated solutions should not be confused with dilute and concentrated solutions. Dilute solutions - solutions with a small content of dissolved substance; concentrated solutions - solutions with a high content of dissolved substance. It must be emphasized that the concepts of dilute and concentrated solutions are relative, expressing only the ratio of the amounts of solute and solvent in the solution. Comparing the solubility of various substances, we see that saturated solutions of poorly soluble substances are dilute, and highly soluble substances, although unsaturated, are quite concentrated. Depending on whether the components of the solution are electrically neutral or charged particles, they are divided into molecular (non-electrolyte solutions) and ionic (electrolyte solutions). One of the characteristic features of electrolyte solutions is that they conduct electricity.
1. Dispersed systems. True solutions
Crystals of any substance, such as sugar or sodium chloride, can be obtained in different sizes - large and small. Whatever the size of the crystals, they all have the same internal structure for a given substance - a molecular or ionic crystal lattice.
When sugar and sodium chloride crystals are dissolved in water, molecular and ionic solutions are formed, respectively. Thus, the same substance can be in different degrees of fragmentation: macroscopically visible particles (>0.2 mm, eye resolution), microscopically visible particles (from 0.2-0.1 mm to 400-300 nm, microscope resolution when illuminated with white light) and in the molecular (or ionic) state.
If the thickness of films, the diameter of fibers or particles (corpuscles) is less than the resolution of an optical microscope, then they cannot be detected with it. Such particles, invisible under an optical microscope, are called colloidal, and the crushed (dispersed) state of substances with particle sizes from 400-300 nm to 1 nm is called the colloidal state of the substance.
Dispersed (fragmented) systems are heterogeneous. They consist of a continuous continuous phase - a dispersion medium and crushed particles of one size or another shape - the dispersed phase - located in this medium.
Since the dispersed (discontinuous) phase is in the form of separate small particles, dispersed systems, in contrast to heterogeneous ones with continuous phases, are called microheterogeneous, and colloidal disperse systems are also called ultra micro heterogeneous to emphasize that in these systems the phase boundary is not can be detected in a light microscope.
When a substance is in the environment in the form of molecules or ions, then such solutions are called true, i.e. homogeneous single-phase solutions.
A prerequisite for obtaining dispersed systems is the mutual insolubility of the dispersible substance and the dispersion medium. For example, colloidal solutions of sugar or sodium chloride cannot be obtained in water, but they can be obtained in kerosene or benzene, in which these substances are practically insoluble.
Dispersed systems are classified according to their dispersity, the state of aggregation of the dispersed phase and the dispersion medium, the intensity of interaction between them, the absence or formation of structures in dispersed systems.
A quantitative characteristic of the dispersion of a substance is the degree of dispersity (D) - a value reciprocal to the size (a) of dispersed particles.
Here a is equal to either the diameter of spherical or fibrous particles, or the length of the edge of cubic particles, or the thickness of the films.
The degree of dispersion is numerically equal to the number of particles that can be tightly packed in a row (or stack of films) over one centimeter.
If all particles of the dispersed phase have the same size, then such systems are called monodisperse. Particles of the dispersed phase of unequal size form polydisperse systems.
With increasing dispersity, an increasing number of atoms of a substance are located in the surface layer, at the phase boundary, compared to their number inside the volume of particles of the dispersed phase. The relationship between surface and volume is characterized by specific surface area:
which for spherical particles is equal to and for cubic particles where r is the radius of the ball; d is its diameter; l is the length of the cube edge.
Consequently, with increasing dispersion of a substance, its properties determined by surface phenomena become increasingly important, i.e. a set of processes occurring in the interphase surface. Thus, the uniqueness of dispersed systems is determined by the large specific surface area of the dispersed phase and the physicochemical effect of the dispersed medium at the phase interface.
The diversity of disperse systems is due to the fact that the phases that form them can be in any of three states of aggregation (L, G, T).
Dispersed systems with a gaseous dispersed medium are called aerosols. Fogs are aerosols with a liquid dispersed phase, and dust and smoke are aerosols with a solid dispersed phase; Dust is formed by the dispersion of substances, and smoke is formed by the condensation of volatile substances.
Foams are a dispersion of gas in a liquid, and in foams the liquid degenerates into thin films that separate and separate individual gas bubbles. Emulsions are dispersed systems in which one liquid is crushed into another liquid that does not dissolve it. Low-disperse systems of solid particles in liquids are called suspensions, or suspensions, and extremely highly dispersed systems are called colloidal solutions, or sols, often lysols, to emphasize that the dispersed medium is a liquid. If the dispersed medium is water, then such sols are called hydrosols, and if an organic liquid is called organosols.
Dispersed systems can be freely dispersed and coherently dispersed, depending on the absence or presence of interaction between particles of the dispersed phase. Freely dispersed systems include aerosols, lyosols, diluted suspensions and emulsions. They are fluid. In these systems, particles of the dispersed phase have no contacts, participate in random thermal motion, and move freely under the influence of gravity. Cohesive-disperse systems are solid; they arise when particles of the dispersed phase come into contact, leading to the formation of a structure in the form of a frame or network. This structure limits the fluidity of the dispersed system and gives it the ability to retain its shape. Such structured colloidal systems are called gels. The transition of a sol to a gel, which occurs as a result of a decrease in the stability of the sol, is called gelation (or gelation). The highly elongated and film-leaf shape of dispersed particles increases the likelihood of contact between them and favors the formation of gels at low concentrations of the dispersed phase. Powders, concentrated emulsions and suspensions (pastes), foams are examples of coherently dispersed systems. The soil formed as a result of contact and compaction of dispersed particles of soil minerals and humus (organic) substances is also a cohesive disperse system.
A continuous mass of substance can be penetrated by pores and capillaries, forming capillary-dispersed systems. These include, for example, wood, various membranes and diaphragms, leather, paper, cardboard, and fabrics.
2. Solubility of solids and liquids in liquids. Influence of the nature of substances and temperature on solubility
A solution is a solid or liquid homogeneous system consisting of two or more components, the relative quantities of which can vary within wide limits.
Every solution consists of dissolved substances and a solvent, i.e. an environment in which these substances are evenly distributed in the form of molecules or ions. Typically, a solvent is considered to be a component that, in its pure form, exists in the same state of aggregation as the resulting solution. If both components were in the same state of aggregation before dissolution, then the component present in a larger quantity is considered the solvent.
The homogeneity of solutions makes them very similar to chemical compounds. The release of heat during the dissolution of some substances also indicates a chemical interaction between the solvent and the solute. The difference between solutions and chemical compounds is that the composition of the solution can vary within wide limits. In addition, in the properties of a solution one can detect many properties of its individual components, which are not observed in the case of a chemical compound. The variability of the composition of solutions brings them closer to mechanical mixtures, but they differ sharply from the latter in their homogeneity. Thus, solutions occupy an intermediate position between mechanical mixtures and chemical compounds.
Solubility is the ability of a substance to dissolve in a particular solvent. A measure of the solubility of a substance under given conditions is its content in a saturated solution. Therefore, solubility can be expressed numerically in the same ways as composition, for example, by the percentage of the mass of the solute to the mass of a saturated solution or the amount of solute contained in 1 liter of a saturated solution. Solubility is often also expressed as the number of units of mass of an anhydrous substance that saturates 100 units of mass of solvent under given conditions; Sometimes the solubility expressed in this way is called the solubility coefficient.
The solubility of various substances in water varies widely. If more than 10 g of a substance dissolves in 100 g of water, then such a substance is usually called highly soluble; if less than 1 g of a substance dissolves, it is slightly soluble and, finally, practically insoluble, if less than 0.01 g of a substance goes into solution.
The principles for predicting the solubility of a substance are not yet known. However, usually substances consisting of polar molecules and substances with an ionic type of bond are better dissolved in polar solvents (water, alcohols, liquid ammonia), and non-polar substances are better dissolved in non-polar solvents (benzene, carbon disulfide).
The dissolution of most solids is accompanied by the absorption of heat. This is explained by the expenditure of a significant amount of energy on the destruction of the crystal lattice of a solid, which is usually not fully compensated by the energy released during the formation of hydrates (solvates). Applying Le Chatelier's principle to the equilibrium between a substance in a crystalline state and its saturated solution
Crystal + Solvent Saturated solution ± Q
We come to the conclusion that in cases where a substance dissolves with the absorption of energy, an increase in temperature should lead to an increase in its solubility. If, however, the hydration energy is high enough that the formation of a solution is accompanied by the release of energy, solubility decreases with increasing temperature. This occurs, for example, when many salts of lithium, magnesium, and aluminum are dissolved in water.
When solids are dissolved in water, the volume of the system usually changes only slightly. Therefore, the solubility of substances in the solid state is practically independent of pressure.
Liquids can also dissolve in liquids. Some of them are unlimitedly soluble in one another, i.e. mix with each other in any proportions, such as alcohol and water, others dissolve mutually only to a certain limit. So, if you shake diethyl ether with water, two layers are formed: the upper one is a saturated solution of water in ether, and the lower one is a saturated solution of ether in water. In most such cases, with increasing temperature, the mutual solubility of liquids increases until a temperature is reached at which both liquids mix in any proportions.
The temperature at which the limited mutual solubility of liquids becomes unlimited is called the critical solution temperature. aggregate molecular heterogeneous
As in the case of the dissolution of solids, the mutual dissolution of liquids is usually not accompanied by a significant change in volume. Therefore, the mutual solubility of liquids depends little on pressure and increases noticeably only at very high pressures (on the order of thousands of atmospheres).
If a third substance is introduced into a system consisting of two immiscible liquids, capable of dissolving in each of these liquids, then the solute will be distributed between both liquids in proportion to its solubility in each of them. This implies the law of distribution, according to which a substance capable of dissolving 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 dissolved substance:
Here C 1 and C 2 are the concentrations of the dissolved substance in the first and second solvents; K is the so-called distribution coefficient.
3. Methods of expressing the concentration of a solutionov: mass - C%, molar - Cm andnormal (equivalent) - Cn
Mass fraction- the ratio (usually percentage) of the mass of the dissolved substance to the mass of the solution. For example, a 15% (wt.) aqueous solution of sodium chloride is a solution in 100 mass units of which contains 15 mass units of NaCl and 85 mass units of water.
Molar concentration,ormolarity- the ratio of the amount of dissolved substance to the volume of solution. Typically, molarity is denoted CM or (after the numerical value of molarity) M. Thus, 2MH 2 SO 4 means a solution, each liter of which contains 2 moles of sulfuric acid, i.e. C M = 2 mol/l.
Equivalentornormal concentration- the ratio of the number of equivalents of a dissolved substance to the volume of the solution. The concentration expressed in this way is denoted by CH or (after the numerical value of normality) by the letter n. So 2 n H 2 SO 4 means a solution, each liter of which contains 2 equivalents of sulfuric acid, i.e. CH (1/2H 2 SO 4) = 2 mol/l.
4. Electrolyte solutions. Electrolytic dissociation of acids, bases, salts. Step dissociation
Aqueous solutions of salts, acids and bases have a special feature - they conduct electric current. At the same time, most solid salts and bases in an anhydrous state, as well as anhydrous acids, have very weak electrical conductivity: water also conducts electricity poorly. It is obvious that when solutions are formed, such substances undergo some changes that cause the appearance of high electrical conductivity. These changes consist in the dissociation of the corresponding substances into ions, which serve as carriers of electric current.
Substances that conduct electric current with their ions are called electrolytes. When dissolved in water and a number of non-aqueous solvents, salts, acids and bases exhibit the properties of electrolytes. Electrolytes are also many molten salts, oxides and hydroxides, some salts and oxides in the solid state.
Acids
When any acid dissociates, hydrogen ions are formed. Therefore, all properties that are common to aqueous solutions of acids are explained by the presence of hydrated hydrogen ions. They cause litmus to turn red, give acids a sour taste, etc. With the elimination of hydrogen ions, for example during neutralization, the acidic properties also disappear. Therefore, the theory of electrolytic dissociation defines acids as electrolytes that dissociate in solutions to form hydrogen ions. In strong acids, which completely dissociate, the properties of acids are manifested to a greater extent, in weak ones, to a lesser extent. The better the acid dissociates, the stronger it is.
Grounds
Since all solutions of bases have in common the presence of hydroxide ions in them, it is clear that the carrier of the basic properties is the hydroxide ion. Therefore, from the point of view of the theory of electrolytic dissociation, bases are electrolytes that dissociate in solutions with the elimination of hydroxide ions.
The strength of bases, like the strength of acids, depends on the value of the dissociation constant. The greater the dissociation constant of a given base, the stronger it is.
Salts
Salts can be defined as electrolytes that, when dissolved in water, dissociate, releasing positive ions other than hydrogen ions and negative ions other than hydroxide ions. There are no ions that are common to aqueous solutions of all salts; Therefore, salts do not have general properties. As a rule, salts dissociate well, and the lower the charges of the ions forming the salt, the better.
When acid salts are dissolved in a solution, metal cations, complex anions of the acidic residue, as well as ions that are products of the dissociation of this complex acidic residue, including H + ions, are formed.
When basic salts dissociate, acid anions and complex cations consisting of metal and hydroxyl groups are formed. These complex cations are also capable of dissociation. Therefore, OH - ions are present in the basic salt solution.
The laws of chemical equilibrium can be applied to the equilibrium that is established between molecules and ions in a weak electrolyte solution. The equilibrium constant corresponding to the dissociation of a weak electrolyte is called the dissociation constant. The value of K depends on the nature of the electrolyte and solvent, as well as on temperature, but does not depend on the C of the solution. It characterizes the ability of a given acid or base to dissociate into ions: the higher K, the easier the electrolyte dissociates.
Polybasic acids, as well as bases of two or more valent metals, dissociate stepwise. In solutions of these substances, complex equilibria are established in which ions of different charges participate.
The first equilibrium - dissociation in the first step - is characterized by a dissociation constant, denoted K 1, and the second - dissociation in the second step - by the dissociation constant K 2. The quantities K, K 1 and K 2 are related to each other by the relation
During the stepwise dissociation of substances, the decomposition in the subsequent step always occurs to a lesser extent than in the previous one. The following inequality holds:
K 1 > K 2 > K 3 ...
This is explained by the fact that the energy that must be expended to remove an ion is minimal when it is separated from a neutral molecule and becomes greater during dissociation at each subsequent step.
5. Classification of electrolytes. Degree of dissociation. Strong and weak electrolytes
If electrolytes were completely dissociated into ions, then the osmotic pressure (and other quantities proportional to it) would always be an integer number of times greater than the values observed in solutions of non-electrolytes. But Van't Hoff also established that the coefficient i is expressed in fractional numbers, which increase with dilution of the solution, approaching whole numbers.
Arrhenius explained this fact by the fact that only part of the electrolyte dissociates into ions in solution, and introduced the concept of the degree of dissociation. The degree of dissociation of an electrolyte is the ratio of the number of its molecules disintegrated into ions in a given solution to the total number of its molecules in the solution.
It was later discovered that electrolytes can be divided into two groups: strong and weak electrolytes. Strong electrolytes in aqueous solutions are almost completely dissociated. The concept of the degree of dissociation is essentially inapplicable to them, and the deviation of the isotonic coefficient i from integer values is explained by other reasons. Weak electrolytes dissociate only partially in aqueous solutions. Therefore, the number of ions in solutions of strong electrolytes is greater than in solutions of weak electrolytes of the same concentration. And if in solutions of weak electrolytes the C of ions is small, the distance between them is large and the interaction of ions with each other is insignificant, then in not very dilute solutions of strong electrolytes the average distance between the ions due to the significant concentration is relatively small. In such solutions, the ions are not completely free; their movement is constrained by mutual attraction to each other. Thanks to this attraction, each ion is surrounded by a spherical swarm of oppositely charged ions, called the “ionic atmosphere.”
All salts are strong electrolytes; The most important acids and bases include HNO 3, H 2 SO 4, HClO 4, HCl, HBr, HI, KOH, NaOH, Ba(OH) 2, and Ca(OH) 2.
Weak electrolytes include most organic acids, and the most important inorganic compounds include H 2 CO 3, H 2 S, HCN, H 2 SiO 3 and NH 4 OH.
The degree of dissociation is usually denoted by the Greek letter a and expressed either as a fraction of a unit or as a percentage.
6. Exchange reactions in electrolyte solutions and conditions for their occurrence. Ionic equations
When any strong acid is neutralized by any strong base, about 57.6 kJ of heat is released for each mole of water formed. This suggests that such reactions are reduced to one process. If we rewrite the equation, writing strong electrolytes in ionic form, since they exist in solution in the form of ions, and weak electrolytes in molecular form, since they exist in solution primarily in the form of molecules.
Considering the resulting equation, we see that during the reaction the Na + and Cl - ions did not undergo changes. Therefore, we will rewrite the equation again, eliminating these ions from both sides of the equation. We get:
Thus, the reaction of neutralization of any strong acid with any strong base comes down to the same process - the formation of water molecules from hydrogen ions and hydroxide ions. It is clear that the thermal effects of these reactions must also be the same.
Strictly speaking, the reaction of the formation of water from ions is reversible, which can be expressed by the equation:
Water is a very weak electrolyte and dissociates only to a negligible extent. The equilibrium between water molecules and ions is strongly shifted towards the formation of molecules. Therefore, practically the reaction of neutralization of a strong acid with a strong base proceeds to completion
When mixing a solution of any silver salt with hydrochloric acid or with a solution of any of its salts, a characteristic white cheesy precipitate of silver chloride is always formed:
Such reactions also come down to one process. In order to obtain its ionic-molecular equation, we rewrite the equation of the first reaction, writing strong electrolytes in ionic form and the substance in the sediment in molecular form:
As can be seen, the H + and NO 3 - ions do not undergo changes during the reaction. Therefore, we exclude them and rewrite the equation again:
This is the ion-molecular equation of the process under consideration.
Here we must also keep in mind that the silver chloride precipitate is in equilibrium with the Ag + and Cl - ions in solution, so the process expressed by the last equation is reversible:
However, due to the low solubility of silver chloride, this equilibrium is very strongly shifted to the right. Therefore, we can assume that the reaction of the formation of AgCl from ions is almost complete.
To draw up ion-molecular equations, you need to know which salts are soluble in water and which are practically insoluble.
Ionic-molecular equations help to understand the characteristics of reactions between electrolytes.
7. Ionic product of water. Hydrogen indicator pH of solutions. Indicators
Pure water conducts electricity very poorly, but still has measurable electrical conductivity, which is explained by the slight dissociation of water into hydrogen ions and hydroxide ions. For water and dilute aqueous solutions at a constant temperature, the product of the concentrations of hydrogen ions and hydroxide ions is a constant value. This constant is called the ionic product of water. Solutions in which the concentrations of hydrogen ions and hydroxide ions are the same are called neutral solutions.
If the concentration of hydrogen ions in an aqueous solution is known, then the concentration of hydroxide ions is also determined. Therefore, both the degree of acidity and the degree of alkalinity of a solution can be quantitatively characterized by the concentration of hydrogen ions. The acidity and alkalinity of a solution can be expressed in another, more convenient way: instead of the concentration of hydrogen ions, indicate its decimal logarithm, taken with the opposite sign. This value is called the hydrogen index and is denoted by pH:
There are various methods for measuring pH. The approximate reaction of a solution can be determined using special reagents called indicators, the color of which changes depending on the concentration of hydrogen ions. The most common indicators are methyl orange, methyl red, and phenolphtholein.
Conclusion
Solutions are homogeneous systems of variable composition in which the solute is in the form of atoms, ions or molecules uniformly surrounded by atoms, ions or molecules of the solvent. Any solution consists of at least two substances, one of which is considered a solvent and the other a solute. A solvent is a component whose state of aggregation is the same as the state of aggregation of the solution. This division is quite arbitrary, and for substances that are mixed in any ratio (water and acetone, gold and silver), it makes no sense. In this case, the solvent is the component present in the solution in larger quantities.
The composition of solutions can vary over a fairly wide range; in this respect, solutions are similar to mechanical mixtures. Based on other characteristics, such as homogeneity, the presence of a thermal effect and color, solutions are similar to chemical compounds. Solutions can exist in a gaseous, liquid or solid state of aggregation. Air, for example, can be considered a solution of oxygen and other gases in nitrogen; sea water is an aqueous solution of various salts in water. Metal alloys refer to solid solutions of some metals in others. The dissolution of substances is a consequence of the interaction of particles of the solute and the solvent. At the initial moment of time, dissolution occurs at a high speed, but as the amount of dissolved substance increases, the rate of the reverse process - crystallization - increases. Crystallization is the separation of a substance from solution and its precipitation. At some point, the rates of dissolution and precipitation will become equal, and a state of dynamic equilibrium will occur. A solution in which a substance no longer dissolves at a given temperature, or otherwise, a solution that is in equilibrium with the solute, is called saturated. For most solids, solubility in water increases with increasing temperature. If a solution that is saturated when heated is carefully cooled so that crystals do not precipitate, a supersaturated solution is formed. A solution is called supersaturated if it contains more solute at a given temperature than a saturated solution. A supersaturated solution is extremely unstable and when conditions change (vigorous shaking or the introduction of active crystallization centers - salt crystals, dust particles) a saturated solution and salt crystals are formed. A solution containing less solute than a saturated solution is called an unsaturated solution.
Literature
1. Glinka N.L. General chemistry: - L.: Chemistry 1985.-704p. Ed. V.A. Rabinovich.
2. Frolov V.V. Chemistry: - M.: Higher. School, 1986.- 543 p.
3. Kireev V.A. "Course of physical chemistry", M. 1975
4. Glinka. N.L. "General Chemistry", M. 2000
5. Dey M.K., D. Selbin “Theoretical inorganic chemistry”, M. 1971
6. Nikolaev L.A. "General and inorganic chemistry" M. 1974
7. Krasnov K.S. "Physical Chemistry" M. 2001
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Consisting of particles of a dissolved substance, solvent and products of their interaction. “Homogeneous” means that each of the components is distributed in the mass of the other in the form of its own particles, that is, atoms, molecules or ions. .
Solution- a single-phase system of variable, or heterogeneous, composition, consisting of two or more components.
The formation of one or another type of solution is determined by the intensity of intermolecular, interatomic, interionic or other type of interaction, that is, the same forces that determine the occurrence of one or another state of aggregation. Differences: the formation of a solution depends on the nature and intensity of the interaction of particles different substances
Compared to individual substances, solutions are more complex in structure.
Solutions are gaseous, liquid and solid.
Solid, liquid, gaseous solutions
More often, a solution means a liquid substance, for example a solution of salt or alcohol in water (or even a solution of gold in amalgam).
Dissolution
Dissolution is the transition of molecules of a substance from one phase to another ( solution, dissolved state). Occurs as a result of the interaction of atoms (molecules) of the solvent and the dissolved substance and is accompanied by an increase in entropy during the dissolution of solids and its decrease during the dissolution of gases. Upon dissolution, the interphase boundary disappears, and many physical properties of the solution (for example, density, viscosity, sometimes color, and others) change.
In the case of chemical interaction between a solvent and a solute, the chemical properties also change greatly - for example, when hydrogen chloride gas is dissolved in water, liquid hydrochloric acid is formed.
Solutions of electrolytes and non-electrolytes
Electrolytes are substances that conduct electric current in melts or aqueous solutions. In melts or aqueous solutions they dissociate into ions. Nonelectrolytes are substances whose aqueous solutions and melts do not conduct electric current, since their molecules do not dissociate into ions. Electrolytes, when dissolved in suitable solvents (water, other polar solvents), dissociate into ions. Strong physicochemical interaction during dissolution leads to a strong change in the properties of the solution (chemical theory of solutions).
Substances that, under the same conditions, do not disintegrate into ions and do not conduct electric current are called nonelectrolytes.
Electrolytes include acids, bases and almost all salts; non-electrolytes include most organic compounds, as well as substances whose molecules contain only covalent non-polar or low-polar bonds.
Polymer solutions
Solutions of high-molecular substances of the Navy - proteins, carbohydrates, etc. simultaneously possess many of the properties of true and colloidal solutions. Average molecular weight of dissolved...
Concentration of solutions
Depending on the purpose, different physical quantities are used to describe the concentration of solutions.
Mnemonic rules
In cases of preparing solutions of strong acids, according to safety regulations, the acid must be added to water, but in no case vice versa. There are several mnemonic rules for remembering this laboratory technique:
“aged cognac” (acid in water)
see also
Notes
Literature
- Streitwieser Andrew Introduction to Organic Chemistry. - 4th ed.. - Macmillan Publishing Company, New York, 1992. - ISBN ISBN 0-02-418170-6
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Synonyms:See what “Solution” is in other dictionaries:
Solution- Single-phase system consisting of a solute, solvent and products of their interaction Source ... Dictionary-reference book of terms of normative and technical documentation
Solution- - a homogeneous mixture of two or more components, uniformly distributed in the form of atoms, ions or molecules in a liquid or solid. [Tarasov V.V. Materials science. Technology of structural materials: a textbook for... ... Encyclopedia of terms, definitions and explanations of building materials
Ushakov's Explanatory Dictionary
1. SOLUTION1, solution, male. 1. The angle formed by the spread apart blades of scissors, legs of compasses, etc. (colloquial). Compass solution. Narrow solution. 2. A hole created when a double-hung window, gate, door, etc. is opened. 3. Small shopping... ... Ushakov's Explanatory Dictionary
Composition, mixture; ash pan, gravy, sol, essence, collodion, liquid, syrup, emulsoid; hole, corner Dictionary of Russian synonyms. solution see composition 1 Dictionary of synonyms of the Russian language. Practical guide. M.: Russian language. Z. E. Alexandrov ... Synonym dictionary
SOLUTION, in chemistry, a liquid (SOLVENT) containing another substance (SOLVED). Unlike mixtures, two or more individual chemical compounds in a solution cannot be separated by filtration. The amount of substance... ... Scientific and technical encyclopedic dictionary
1. SOLUTION, a; m. 1. The angle formed by the spread legs of a compass, blades of scissors, etc. R. compass. Wide river 2. A hole created when a double-hung window, door, gate, etc. is opened. Wide river window. Stand in the doorway. 3. One... ... encyclopedic Dictionary
Construction, a mixture of binder, sand and water, acquiring a stone-like state over time. There are solutions: cement, lime, gypsum, mixed; for stone (mainly brick) masonry, finishing (including ... Modern encyclopedia
In medicine, a liquid dosage form is a homogeneous transparent mixture of a drug (solid or liquid) and some liquid (solvent) ...
A building mixture of sand, binder and water, which acquires a stone-like state over time. The main types of mortars are cement, lime, gypsum, mixed. There are mortars for stone (mainly brick) masonry,... ... Big Encyclopedic Dictionary
SOLUTION 1, a, m. Ozhegov's Explanatory Dictionary. S.I. Ozhegov, N.Yu. Shvedova. 1949 1992 … Ozhegov's Explanatory Dictionary
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