dispersed systems. Definition. Classification.
Solutions
In the previous paragraph, we talked about solutions. Here we briefly recall this concept.
Solutions called homogeneous (homogeneous) systems consisting of two or more components.
homogeneous system is a homogeneous system chemical composition And physical properties which in all parts are the same or change continuously, without jumps (there are no interfaces between the parts of the system).
This definition of a solution is not quite correct. It refers more to true solutions.
At the same time, there are also colloidal solutions, which are not homogeneous, but heterogeneous, i.e. consist of different phases separated by an interface.
In order to achieve greater clarity in the definitions, another term is used - disperse systems.
Before considering disperse systems, let's talk a little about the history of their study and the emergence of such a term as colloidal solutions.
Background
Back in 1845, the chemist Francesco Selmi, while studying the properties of various solutions, noticed that biological fluids - serum and blood plasma, lymph, and others - differ sharply in their properties from ordinary true solutions, and therefore such liquids were called pseudo-solutions.
colloids and crystalloids
Further research in this direction, carried out since 1861 by the English scientist Thomas Graham, showed that some substances that quickly diffuse and pass through plant and animal membranes easily crystallize, while others have a low ability to diffuse, do not pass through membranes and do not crystallize. , and form amorphous precipitates.
The first Graham named crystalloids, and the second ones colloids(from the Greek word kolla - glue and eidos - view) or glue-like substances.
In particular, it was found that substances capable of forming amorphous precipitates, such as albumin, gelatin, gum arabic, iron and aluminum hydroxides, and some other substances, diffuse slowly in water compared to the diffusion rate of such crystalline substances like table salt, magnesium sulfate, cane sugar, etc.
The table below lists the diffusion coefficients D for some crystalloids and colloids at 18°C.
The table shows that there is an inverse relationship between the molecular weight and the diffusion coefficient.
In addition, crystalloids were found to have the ability not only to rapidly diffuse, but also be dialyzed, i.e. pass through membranes, as opposed to colloids, which have larger molecules and therefore slowly diffuse and do not penetrate membranes.
As membranes, the walls of a bovine bladder, cellophane, films of ferruginous-cyanide copper, etc. are used.
Based on the observations made, Graham found that all substances can be subdivided into crystalloids and colloids.
Russians do not agree
Against such a strict division chemical substances objected professor of Kyiv University I.G. Borschev(1869). Borshchev's opinion was later confirmed by the studies of another Russian scientist Weimarn, who proved that the same substance, depending on the conditions, can exhibit the properties of colloids or crystalloids.
So, for example, a solution of soap in water has the properties colloid, and soap dissolved in alcohol exhibits properties true solutions.
Similarly, crystalline salts, for example, table salt dissolved in water, gives true solution, and in benzene colloid solution and so on.
Hemoglobin or egg albumin, which have the properties of colloids, can be obtained in a crystalline state.
DI. Mendeleev believed that any substance, depending on the conditions and nature of the environment, can exhibit properties colloid. Currently, any substance can be obtained in a colloidal state.
Thus, there is no reason to subdivide substances into two separate classes - into crystalloids and colloids, but we can speak of a colloidal and crystalloid state of a substance.
The colloidal state of a substance means a certain degree of its fragmentation or dispersion and the presence of colloidal particles in a suspended state in a solvent.
The science that studies the physicochemical properties of heterogeneous highly dispersed and high molecular weight systems is called colloid chemistry.
Disperse systems
If one substance, which is in a crushed (dispersed) state, is evenly distributed in the mass of another substance, then such a system is called dispersed.
In such systems, the fragmented matter is called dispersed phase, and the environment in which it is distributed - dispersion medium.
So, for example, a system, which is a stirred clay in water, consists of suspended fine particles of clay - a dispersed phase and water - a dispersion medium.
dispersed(fragmented) systems are heterogeneous.
Dispersed systems, in contrast to heterogeneous ones with relatively large, continuous phases, are called microheterogeneous, and colloidal dispersed systems are called ultramicroheterogeneous.
Classification of disperse systems
The classification of dispersed systems is most often carried out on the basis of degree of dispersion or state of aggregation dispersed phase and dispersion medium.
Classification according to the degree of dispersion
All disperse systems according to the particle size of the dispersed phase can be divided into the following groups:
For reference, here are the units of sizes in the SI system:
1 m (meter) = 102 cm (centimeter) = 103 mm (millimeter) = 106 microns (micrometer) = 109 nm (nanometer).
Sometimes other units are used - mk (micron) or mmk (millimicron), and:
1 nm \u003d 10 -9 m \u003d 10 -7 cm \u003d 1 mmk;
1 µm = 10 -6 m = 10 -4 cm = 1 µm.
coarse systems.
These systems contain, as a dispersed phase, the largest particles with a diameter of 0.1 microns and above. These systems include suspensions And emulsions.
Suspensions call systems in which a solid substance is in a liquid dispersion medium, for example, a suspension of starch, clay, etc. in water.
emulsions Dispersion systems of two immiscible liquids are called, where droplets of one liquid in suspension are distributed in the volume of another liquid. For example, oil, benzene, toluene in water or droplets of fat (0.1 to 22 microns in diameter) in milk, etc.
colloidal systems.
They have particle sizes of the dispersed phase from 0.1 microns to 1 microns(or from 10 -5 to 10 -7 cm). Such particles can pass through the pores of filter paper, but do not pass through the pores of animal and plant membranes.
colloidal particles if they have an electric charge and solvate-ionic shells, they remain in a suspended state and, without changing conditions, may not precipitate for a very long time.
Examples of colloidal systems are solutions of albumin, gelatin, gum arabic, colloidal solutions of gold, silver, arsenic sulfide, etc.
Molecularly dispersed systems.
Such systems have particle sizes not exceeding 1 mm. Molecular-dispersed systems include true solutions of non-electrolytes.
Ion-disperse systems.
These are solutions of various electrolytes, such as salts, bases, etc., decomposing into the corresponding ions, the dimensions of which are very small and go far beyond
10 -8 cm.
Clarification on the presentation of true solutions as dispersed systems.
From the classification given here, it can be seen that any solution (both true and colloidal) can be represented as a dispersed medium. True and colloidal solutions will differ in particle sizes of the dispersed phases. But above we wrote about the homogeneity of true solutions, and dispersion systems are heterogeneous. How to resolve this contradiction?
If speak about structure true solutions, then their homogeneity will be relative. The structural units of true solutions (molecules or ions) are much smaller than the particles of colloidal solutions. Therefore, we can say that, compared with colloidal solutions and suspensions, true solutions are homogeneous.
If we talk about properties true solutions, then they cannot be fully called dispersed systems, since the obligatory existence of dispersed systems is the mutual insolubility of the dispersed substance and the dispersion medium.
In colloidal solutions and coarse suspensions, the dispersed phase and the dispersion medium practically do not mix and do not chemically react with each other. The same cannot be said for true solutions. In them, when dissolved, substances mix and even interact with each other. For this reason, colloidal solutions differ sharply in properties from true solutions.
Sizes of some molecules, particles, cells.
As the particle size changes from the largest to the smallest and vice versa, the properties of disperse systems will change accordingly. Wherein colloidal systems occupy like intermediate position between coarse suspensions and molecular-dispersed systems.
Classification according to the state of aggregation of the dispersed phase and dispersion medium.
Foam is a dispersion of gas in a liquid, and in foams the liquid degenerates to thin films separating individual gas bubbles.
emulsions called dispersed systems in which one liquid is fragmented by another, non-dissolving liquid (for example, water in fat).
Suspensions called low-dispersion systems of solid particles in liquids.
Combinations of three types of aggregate state make it possible to distinguish nine types of dispersed systems:
Dispersed phase | Dispersion medium | Name and example |
|---|---|---|
gaseous | gaseous | The dispersion system is not formed |
gaseous | Gas emulsions and foams |
|
gaseous | Porous bodies: pumice foam rubber |
|
gaseous | Aerosols: fogs, clouds |
|
Emulsions: petroleum, cream, milk, margarine, butter |
||
Capillary systems: Liquid in porous bodies, soil, soil |
||
gaseous | Aerosols (dust, fumes), powders |
|
Suspensions: pulp, sludge, suspension, paste |
||
Solid systems: alloys, concrete |
Sols are another name for colloidal solutions.
Colloidal solutions are also known as sols(from the Latin solutus - dissolved).
Disperse systems with a gaseous dispersion medium are called aerosols. Fogs are aerosols with a liquid dispersed phase, and dust and smoke are aerosols with a solid dispersed phase. Smoke is a more highly dispersed system than dust.
Disperse systems with a liquid dispersion medium are called lysols(from the Greek "lios" - liquid).
Depending on the solvent (dispersion medium), i.e. water, benzene alcohol or ether, etc., hydrosols, alcosols, benzenes, etherosols, etc. are distinguished.
Connected disperse systems. Gels.
Disperse systems can be freely dispersed And coherently dispersed depending on the absence or presence of interaction between the particles of the dispersed phase.
TO free-dispersed systems include aerosols, lysols, dilute suspensions and emulsions. They are fluid. In these systems, particles of the dispersed phase do not have contacts, participate in random thermal motion, and move freely under the action of gravity.
The figures above show free-disperse systems:
On the drawings a B C depicted corpuscular-dispersed systems:
a,b- monodisperse systems,
V- polydisperse system,
On the image G pictured fibrous dispersed system
On the image d pictured film-dispersed system
- solid. They arise when the particles of the dispersed phase come into contact, leading to the formation of a structure in the form of a framework or network.
Such a 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 gelling).
On the drawings a B C depicted bonded disperse systems:
A- gel,
b- coagulate with a dense structure,
V- coagulate with a loose - "arched" structure
On the drawings d, d depicted capillary-dispersed systems
Powders (pastes), foams are examples of connected dispersed systems.
The soil, formed as a result of contact and compaction of dispersed particles of soil minerals and humic (organic) substances, is also a coherently dispersed system.
A continuous mass of a substance can be pierced by pores and capillaries, forming capillary-dispersed systems. These include, for example, wood, leather, paper, cardboard, fabrics.
Lyophilicity and lyophobicity
A common characteristic of colloidal solutions is the property of their dispersed phase to interact with the dispersion medium. In this regard, two types of sols are distinguished:
1. Lyophobic(from Greek phobia - hatred) And
2.Lyophilic(from Greek philia - love).
At lyophobic Sol particles have no affinity for the solvent, weakly interact with it, and form around themselves a thin shell of solvent molecules.
In particular, if the dispersion medium is water, then such systems are called hydrophobic, for example, metal sols of iron, gold, arsenic sulfide, silver chloride, etc.
IN lyophilic systems between the dispersed substance and the solvent there is an affinity. The particles of the dispersed phase, in this case, acquire a more voluminous shell of solvent molecules.
In the case of an aqueous dispersion medium, such systems are called hydrophilic such as solutions of protein, starch, agar-agar, gum arabic, etc.
coagulation of colloids. Stabilizers.
Substance at the interface.
All liquids and solids are limited by an outer surface where they come into contact with phases of a different composition and structure, such as vapor, another liquid, or a solid.
The properties of matter in this interface, several diameters of atoms or molecules thick, differ from the properties inside the volume of the phase.
Inside the volume of a pure substance in a solid, liquid or gaseous state, any molecule is surrounded by similar molecules.
In the boundary layer, the molecules are in interaction either with a different number of molecules (different in comparison with the interaction inside the bulk of the substance).
This happens, for example, at the boundary of a liquid or solid body with their ferry. Or, in the boundary layer, molecules of a substance interact with molecules of a different chemical nature, for example, at the boundary of two mutually slightly soluble liquids.
As a result, differences in the nature of the interaction inside the phase volume and at the phase boundary arise force fields associated with this inconsistency. (More on this in the section Surface Tension of a Liquid.)
The greater the difference in the intensity of the intermolecular forces acting in each of the phases, the greater the potential energy of the interfacial surface, briefly called surface energy.
Surface tension
To estimate the surface energy, a quantity such as the specific free surface energy is used. It is equal to the work expended on the formation of a unit area of a new phase interface (assuming a constant temperature).
In the case of a boundary between two condensed phases, this quantity is called boundary tension.
When talking about the boundary of a liquid with its vapors, this value is called surface tension.
Coagulation of colloids
All spontaneous processes occur in the direction of decreasing the energy of the system (isobaric potential).
Similarly, processes spontaneously occur at the phase boundary in the direction of a decrease in the free surface energy.
The free energy is the smaller, the smaller the phase separation surface.
And the interface, in turn, is related to the degree of dispersion of the dissolved substance. The higher the dispersity (the finer the particles of the dispersed phase), the larger the interface between the phases.
Thus, in dispersed systems, there are always forces that lead to a decrease in the total interface, i.e. to particle enlargement. Therefore, small droplets merge in fogs, rain clouds and emulsions - aggregation of fine particles into larger formations.
All this leads to the destruction of dispersed systems: fogs and rain clouds rain, emulsions separate, colloidal solutions coagulate, i.e. are separated into a precipitate of the dispersed phase (coagulate) and a dispersion medium or, in the case of elongated particles of the dispersed phase, turn into a gel.
The ability of fragmented systems to maintain their inherent degree of dispersion is called aggregative stability.
Disperse system stabilizers
As stated earlier, dispersed systems are fundamentally thermodynamically unstable. The higher the dispersion, the greater the free surface energy, the greater the tendency to a spontaneous decrease in dispersion.
Therefore, to obtain stable, i.e. long-lasting suspensions, emulsions, colloidal solutions, it is necessary not only to achieve a given dispersion, but also to create conditions for its stabilization.
In view of this, stable disperse systems consist of at least three components: a dispersed phase, a dispersion medium, and a third component - disperse system stabilizer.
The stabilizer can be both ionic and molecular, often high molecular, in nature.
The ionic stabilization of the sols of lyophobic colloids is associated with the presence of low concentrations of electrolytes, which form ionic boundary layers between the dispersed phase and the dispersion medium.
High-molecular compounds (proteins, polypeptides, polyvinyl alcohol, and others) added to stabilize disperse systems are called protective colloids.
Being adsorbed at the phase boundary, they form network and gel-like structures in the surface layer, creating a structural-mechanical barrier that prevents the particles of the dispersed phase from combining.
Structural-mechanical stabilization is of decisive importance for the stabilization of suspensions, pastes, foams, concentrated emulsions.
7.1 Basic concepts and definitions. Topic structure 3
7.1.1 Classification of solutions 3
7.1.2 Structure of topic 4
7.2. Disperse systems (mixtures) their types 5
7.2.1 Coarse systems 6
7.2.2. Finely dispersed systems (colloidal solutions) 6
7.2.3. Highly dispersed systems (true solutions) 9
7.3. Concentration, ways of expressing it 10
7.3.1 Solubility of substances. 10
7.3.2. Methods for expressing the concentration of solutions. eleven
7.3.2.1 Interest 12
7.3.2.2 Molar 12
7.3.2.3 Normal 12
7.3.2.4 Molar 12
7.3.2.5 Mole fraction 12
7.4. Physical laws of solutions 13
7.4.1 Raoult's law 13
7.4.1.1 Changing freezing temperatures 14
7.4.1.2 Changing boiling points 15
7.4.2 Henry's Law 15
7.4.3 Van't Hoff's law. Osmotic pressure 15
7.4.4 Ideal and real solutions. 16
7.4.4.1. Activity - concentration for real systems 17
7.5.Theory of solutions 17
7.5.1 Physical theory 18
7.5.2 Chemical theory 18
7.6 Theory of electrolytic dissociation 19
7.6.1 Electrolyte solutions 20
7.6.1.1 Dissociation constant 20
7.6.1.2 Degree of dissociation. Strong and weak electrolytes 24
7.6.1.3 Ostwald's Dilution Law 27
7.6.2 Electrolytic dissociation of water 27
7.6.2.1 Ionic product of water 28
7.6.2.2. Hydrogen index. Acidity and basicity of solutions 29
7.6.2.3 Acid-base indicators 29
7.7. Ion exchange reactions. 31
7.7.1 Formation of a weak electrolyte 32
7.7.2 Gas evolution 34
7.7.3 Precipitation formation 34
7.7.3.1 Precipitation condition. Solubility product 34
7.7.4. Hydrolysis of salts 36
7.7.4.1. Equilibrium shift during hydrolysis 38
Basic concepts and definitions. Theme Structure
Dispersed systems or mixtures are multicomponent systems in which one or more substances are uniformly distributed in the form of particles in the medium of another substance.
In dispersed systems, a dispersed phase is distinguished - a finely divided substance and a dispersion medium - a homogeneous substance in which the dispersed phase is distributed. For example, in muddy water containing clay, the dispersed phase is solid particles of clay, and the dispersion medium is water; in fog, the dispersed phase is liquid particles, the dispersion medium is air; in smoke, the dispersed phase is solid particles of coal, the dispersion medium is air; in milk - dispersed phase - fat particles, dispersion medium - liquid, etc. Disperse systems can be both homogeneous and heterogeneous.
A homogeneous dispersed system is a solution.
Classification of solutions
According to the size of the dissolved substances, all multicomponent solutions are divided into:
coarse systems (mixtures);
finely dispersed systems (colloidal solutions);
highly dispersed systems (true solutions).
According to the phase state, solutions are:
According to the composition of dissolved substances, liquid solutions are considered as:
electrolytes;
non-electrolytes.
Theme Structure
Dispersed systems (mixtures) their types
Dispersion system - a mixture of two or more substances that do not mix at all or practically and do not chemically react with each other. The first of the substances dispersed phase) is finely distributed in the second ( dispersion medium). The phases are separated by an interface and can be physically separated from each other (centrifuged, separated, etc.).
The main types of disperse systems: aerosols, suspensions, emulsions, sols, gels, powders, fibrous materials such as felt, foams, latexes, composites, microporous materials; in nature - rocks, soils, precipitation.
By kinetic properties dispersed phase, dispersed systems can be divided into two classes:
Freely dispersed systems in which the dispersed phase is mobile;
Cohesive-dispersed systems, the dispersion medium of which is solid, and the particles of their dispersed phase are interconnected and cannot move freely.
By particle size dispersed phase are distinguished coarse systems(suspensions) with a particle size of more than 500 nm and finely dispersed(colloidal solutions or colloids) with particle sizes from 1 to 500 nm.
Table 7.1. Variety of dispersed systems.
|
Dispersion medium |
Dispersed phase |
Name of the disperse system |
Examples of disperse systems |
|
Liquid |
Aerosol |
Fog, clouds, carburetor mixture of gasoline and air in a car engine. |
|
|
Solid |
Aerosol |
Smoke, smog, dust in the air |
|
|
Liquid |
Carbonated drinks, whipped cream |
||
|
Liquid |
emulsions |
Milk, mayonnaise, body fluids (blood plasma, lymph), liquid contents of cells (cytoplasm, karyoplasm) |
|
|
Solid |
Sol, suspension |
River and sea silt, mortars, pastes. |
|
|
Solid |
hard foam |
Ceramics, foam plastics, polyurethane, foam rubber, aerated chocolate. |
|
|
Liquid |
Jelly, gelatin, cosmetic and medical products (ointments, mascara, lipstick) |
||
|
Solid |
solid sol |
Rocks, colored glass, some alloys. |
Pure substances are very rare in nature. Mixtures of different substances in different states of aggregation can form heterogeneous and homogeneous systems - dispersed systems and solutions.
The substance that is present in a smaller amount and distributed in the volume of another is called the dispersed phase. It may consist of several substances.
The substance present in more, in the volume of which the dispersed phase is distributed, is called the dispersion medium. There is an interface between it and the particles of the dispersed phase; therefore, disperse systems are called heterogeneous (non-uniform).
Both the dispersion medium and the dispersed phase can be represented by substances in various states of aggregation - solid, liquid and gaseous.
Depending on the combination of the state of aggregation of the dispersion medium and the dispersed phase, 8 types of such systems can be distinguished (Table 11).
Table 11
Examples of disperse systems
According to the particle size of the substances that make up the dispersed phase, dispersed systems are divided into coarse (suspensions) with particle sizes of more than 100 nm and finely dispersed (colloidal solutions or colloidal systems) with particle sizes from 100 to 1 nm. If the substance is fragmented to molecules or ions smaller than 1 nm in size, a homogeneous system is formed - a solution. It is homogeneous (homogeneous), there is no interface between the particles of the dispersed phase and the medium.
Already a cursory acquaintance with dispersed systems and solutions shows how important they are in Everyday life and in nature (see Table 11).
Judge for yourself: without the Nile silt would not have taken place great civilization ancient egypt; without water, air, rocks and minerals, a living planet would not exist at all - our common Home- Earth; without cells there would be no living organisms, etc.
The classification of disperse systems and solutions is shown in Scheme 2.
Scheme 2
Classification of disperse systems and solutions
suspension
Suspensions are dispersed systems in which the particle size of the phase is more than 100 nm. These are opaque systems, individual particles of which can be seen with the naked eye. The dispersed phase and the dispersion medium are easily separated by settling. Such systems are divided into three groups:
- emulsions (both the medium and the phase are liquids insoluble in each other). These are milk, lymph, water-based paints, etc., well known to you;
- suspensions (the medium is a liquid, and the phase is a solid insoluble in it). These are building solutions (for example, “milk of lime” for whitewashing), river and sea silt suspended in water, a living suspension of microscopic living organisms in sea water - plankton, which giant whales feed on, etc .;
- aerosols - suspensions in a gas (for example, in air) of small particles of liquids or solids. Distinguish between dust, smoke, fog. The first two types of aerosols are suspensions of solid particles in a gas (larger particles in dusts), the last one is a suspension of small liquid droplets in a gas. For example, natural aerosols: fog, thunderclouds - a suspension of water droplets in the air, smoke - small solid particles. And the smog hanging over major cities the world, also an aerosol with a solid and liquid dispersed phase. Inhabitants settlements near cement plants, they suffer from the finest cement dust that is always hanging in the air, which is formed during the grinding of cement raw materials and the product of its firing - clinker. Similar harmful aerosols - dust - are also found in cities with metallurgical industries. Smoke from factory chimneys, smog tiny droplets saliva flying out of the mouth of a patient with the flu is also harmful aerosols.
Aerosols play an important role in nature, everyday life and human production activities. Cloud accumulations, chemical treatment of fields, spray painting, fuel spraying, dry dairy products, treatment respiratory tract(inhalation) - examples of those phenomena and processes where aerosols are beneficial.
Aerosols - fogs over the sea surf, near waterfalls and fountains, the rainbow that arises in them gives a person joy, aesthetic pleasure.
For chemistry highest value have dispersed systems in which the medium is water.
colloid systems
Colloidal systems are such dispersed systems in which the particle size of the phase is from 100 to 1 nm. These particles are not visible to the naked eye, and the dispersed phase and the dispersion medium in such systems are separated by settling with difficulty.
They are divided into sols (colloidal solutions) and gels (jelly).
1. Colloidal solutions, or sols. This is the majority of fluids of a living cell (cytoplasm, nuclear juice - karyoplasm, the contents of organelles and vacuoles) and a living organism as a whole (blood, lymph, tissue fluid, digestive juices, humoral fluids, etc.). Such systems form adhesives, starch, proteins, and some polymers.
Colloidal solutions can be obtained as a result chemical reactions; for example, when solutions of potassium or sodium silicates (“soluble glass”) interact with acid solutions, a colloidal solution of silicic acid is formed. The sol is also formed during the hydrolysis of iron (III) chloride in hot water. Colloidal solutions are outwardly similar to true solutions. They are distinguished from the latter by the resulting "luminous path" - a cone when a beam of light passes through them. This phenomenon is called the Tyndall effect. Larger than in a true solution, the particles of the dispersed phase of the sol reflect light from their surface, and the observer sees a luminous cone in a vessel with a colloidal solution. It does not form in true solution. A similar effect, but only for an aerosol rather than a liquid colloid, can be observed in cinemas when a beam of light from a movie camera passes through the air of the cinema hall.
Particles of the dispersed phase of colloidal solutions often do not settle even during long-term storage due to continuous collisions with solvent molecules due to thermal motion. They do not stick together even when approaching each other due to the presence of the same name on their surface. electric charges. But under certain conditions, the process of coagulation can occur.
Coagulation- the phenomenon of adhesion of colloidal particles and their precipitation - is observed when the charges of these particles are neutralized, when an electrolyte is added to the colloidal solution. In this case, the solution turns into a suspension or gel. Some organic colloids coagulate when heated (glue, egg white) or when the acid-base environment of the solution changes.
2. The second subgroup of colloidal systems is gels, or jellies y representing gelatinous sediments formed during the coagulation of sols. They include a large number of polymer gels, confectionery, cosmetic and medical gels so well known to you (gelatin, aspic, jelly, marmalade, bird's milk soufflé cake) and, of course, an infinite number of natural gels: minerals (opal), bodies of jellyfish, cartilage, tendons, hair, muscle and nervous tissue, etc. The history of the development of life on Earth can be simultaneously considered the history of the evolution of the colloidal state of matter. Over time, the structure of the gels is broken - water is released from them. This phenomenon is called syneresis.
DEFINITION
Disperse systems- formations consisting of two or more phases that practically do not mix and do not react with each other. A substance that is finely distributed in another substance (dispersion medium) is called dispersed phase.
There is a classification of dispersed systems according to the particle size of the dispersed phase. Isolate, molecular-ionic (< 1 нм) – глюкоза, сахароза, коллоидные (1-100 нм) – эмульсии (масло) и суспензии (раствор глины) и грубодисперсные (>100 nm) systems.
There are homogeneous and heterogeneous dispersed systems. Homogeneous systems are otherwise called true solutions.
Solutions
DEFINITION
Solution- a homogeneous system consisting of two or more components.
According to the state of aggregation, solutions are divided into gaseous (air), liquid, solid (alloys). In liquid solutions, there is the concept of a solvent and a solute. In most cases, the solvent is water, but it can also be non-aqueous solvents (ethanol, hexane, chloroform).
Methods for expressing the concentration of solutions
To express the concentration of solutions, use: mass fraction of the dissolved substance (, %), which shows how many grams of a solute are contained in 100 g of a solution.
Molar concentration (C M, mol/l) shows how many moles of a solute are contained in one liter of solution. Solutions with a concentration of 0.1 mol / l are called decimolar, 0.01 mol / l - centimolar, and with a concentration of 0.001 mol / l - millimolar.
Normal concentration (C H, mol-eq / l) shows the number of equivalents of a solute in one liter of solution.
Molar concentration (С m, mol / 1 kg H 2 O) is the number of moles of solute per 1 kg of solvent, i.e. per 1000 g of water.
Mole fraction of solute (N) is the ratio of the number of moles of a solute to the number of moles of a solution. For gas solutions, the mole fraction of a substance coincides with the volume fraction ( φ ).
Solubility
DEFINITION
Solubility(s, g / 100 g H 2 O) - the property of a substance to dissolve in water or another solvent.
By solubility, solutions and substances are divided into 3 groups: highly soluble (sugar), slightly soluble (benzene, gypsum) and practically insoluble (glass, gold, silver). There are no absolutely insoluble substances in water, there are no instruments with which it is possible to calculate the amount of a substance that has dissolved. Solubility depends on temperature (Fig. 1), the nature of the substance and pressure (for gases). As the temperature rises, the solubility of the substance increases.
Rice. 1. An example of the dependence of some salts in water on temperature
The concept of a saturated solution is closely related to the concept of solubility, since solubility characterizes the mass of a solute in a saturated solution. While the substance is able to dissolve, the solution is called unsaturated, if the substance ceases to dissolve, it is called saturated; for a while, you can create a supersaturated solution.
Vapor pressure of solutions
A vapor that is in equilibrium with a liquid is said to be saturated. At a given temperature, the saturation vapor pressure over each liquid is a constant value. Therefore, each liquid has a saturation vapor pressure. Consider this phenomenon using the following example: a solution of a non-electrolyte (sucrose) in water - sucrose molecules are much larger than water molecules. Saturated vapor pressure in a solution creates a solvent. If we compare the pressure of the solvent and the pressure of the solvent over the solution at the same temperature, then in the solution the number of molecules that have passed into vapor above the solution is less than in the solution itself. It follows that the saturated vapor pressure of a solvent over a solution is always lower than over a pure solvent at the same temperature.
If we denote the pressure of the saturated vapor of the solvent over the pure solvent p 0, and over the solution - p, then the relative decrease in vapor pressure over the solution will be (p 0 -p) / p 0.
Based on this, F.M. Raul deduced the law: the relative decrease in the saturated vapor of the solvent over the solution is equal to the mole fraction of the solute: (p 0 -p) / p 0 = N (molar fraction of the solute).
Cryoscopy. Ebullioscopy. Raoult's second law
The concepts of cryoscopy and ebullioscopy are closely related to the freezing and boiling points of solutions, respectively. Thus, the boiling point and crystallization of solutions depend on the vapor pressure over the solution. Any liquid boils at the temperature at which its saturated vapor pressure reaches the external (atmospheric) pressure.
Upon freezing, crystallization begins at the temperature at which the saturation vapor pressure over the liquid phase is equal to the saturation vapor pressure over the solid phase. Hence - the second Raoult's law: a decrease in the crystallization temperature and an increase in the boiling point of a solution are proportional to the concentrations of the solute. The mathematical expression of this law is:
Δ T crist \u003d K × C m,
Δ T bale \u003d E × C m,
where K and E are cryoscopic and ebullioscopic constants, depending on the nature of the solvent.
Examples of problem solving
EXAMPLE 1
| Exercise | What amount of water and 80% acetic acid solution should be taken to obtain 200 g of an 8% solution? |
| Solution |
Let the mass of an 80% solution of acetic acid be x g. Find the mass of the substance dissolved in it: m r.v-va (CH 3 COOH) \u003d m p-ra × / 100% m r.v-va (CH 3 COOH) 1 \u003d x × 0.8 (g) Find the mass of the solute in a solution of 8% acetic acid: m r.v-va (CH 3 COOH) 2 \u003d 200 (g) × 0.08 \u003d 16 (g) m r.v-va (CH 3 COOH) 2 \u003d x × 0.8 (g) \u003d 16 (g) Let's find x: x \u003d 16 / 0.8 \u003d 20 The mass of an 80% solution of acetic acid is 20 (g). Find the required amount of water: m (H 2 O) \u003d m r-ra2 - m r-ra1 m (H 2 O) \u003d 200 (g) - 20 (g) \u003d 180 (g) |
| Answer | m solution (CH 3 COOH) 80% = 20 (g), m (H 2 O) = 180 (g) |
EXAMPLE 2
| Exercise | 200 g of water and 50 g of sodium hydroxide were mixed. Determine the mass fraction of sodium hydroxide in the solution. | |||||||||||||||||||||||||
| Solution | We write down the formula for finding the mass fraction:
Find the mass of sodium hydroxide solution: m solution (NaOH) \u003d m (H 2 O) + m (NaOH) m solution (NaOH) = 200 +50 = 250 (g) Find the mass fraction of sodium hydroxide. Sections: Chemistry Class: 11 After studying the topic of the lesson, you will learn:
Pure substances are very rare in nature. Crystals of pure substances - sugar or table salt, for example, can be obtained in different sizes - large and small. Whatever the size of the crystals, they all have the same given substance internal structure - a molecular or ionic crystal lattice. In nature, mixtures of various substances are most often found. Mixtures of different substances in different states of aggregation can form heterogeneous and homogeneous systems. We will call such systems dispersed. A dispersed system is a system consisting of two or more substances, one of which, in the form of very small particles, is evenly distributed in the volume of the other. The substance breaks up into ions, molecules, atoms, which means it “splits up” into the smallest particles. “Crushing” > dispersion, i.e. substances are dispersed to different particle sizes, visible and invisible. A substance that is present in a smaller amount, disperses and is distributed in the volume of another, is called dispersed phase. It may consist of several substances. A substance that is present in a larger amount, in the volume of which the dispersed phase is distributed, is called dispersed medium. Between it and the particles of the dispersed phase there is an interface, therefore, disperse systems are called heterogeneous (non-uniform). Both the dispersed medium and the dispersed phase can represent substances that are in various states of aggregation - solid, liquid and gaseous. Depending on the combination of the state of aggregation of the dispersed medium and the dispersed phase, 9 types of such systems can be distinguished. Table
According to the particle size of the substances that make up the dispersed phase, dispersed systems are divided into coarse (suspensions) with particle sizes over 100 nm and finely dispersed (colloidal solutions or colloidal systems) with particle sizes from 100 to 1 nm. If the substance is fragmented to molecules or ions smaller than 1 nm in size, a homogeneous system is formed - solution. It is homogeneous, there is no interface between the particles and the medium. Dispersed systems and solutions are very important in everyday life and in nature. Judge for yourself: without the Nile silt, the great civilization of Ancient Egypt would not have taken place; without water, air, rocks and minerals, there would be no living planet at all - our common home - the Earth; without cells, there would be no living organisms, and so on. SUSPENSIONSSuspensions are dispersed systems in which the particle size of the phase is more than 100 nm. These are opaque systems, individual particles of which can be seen with the naked eye. The dispersed phase and the dispersed medium are easily separated by settling, filtering. Such systems are divided into:
For chemistry, the most important are dispersed systems in which the medium is water and liquid solutions. Natural water always contains dissolved substances. Natural aqueous solutions participate in the processes of soil formation and supply plants with nutrients. The complex life processes that occur in human and animal organisms also occur in solutions. Many technological processes in the chemical and other industries, such as the production of acids, metals, paper, soda, fertilizers, proceed in solutions. COLLOID SYSTEMSColloid systems – these are dispersed systems in which the particle size of the phase is from 100 to 1 nm. These particles are not visible to the naked eye, and the dispersed phase and the dispersed medium in such systems are separated by settling with difficulty. From the course general biology you know that particles of this size can be detected using an ultramicroscope, which uses the principle of light scattering. Due to this, the colloidal particle in it appears as a bright dot on a dark background. They are divided into sols (colloidal solutions) and gels (jelly). 1. Colloidal solutions, or sols. This is the majority of fluids of a living cell (cytoplasm, nuclear juice - karyoplasm, the contents of organelles and vacuoles). And the living organism as a whole (blood, lymph, tissue fluid, digestive juices, etc.) Such systems form adhesives, starch, proteins, and some polymers. Colloidal solutions can be obtained as a result of chemical reactions; for example, when solutions of potassium or sodium silicates (“soluble glass”) interact with acid solutions, a colloidal solution of silicic acid is formed. The sol is also formed during the hydrolysis of iron (III) chloride in hot water. A characteristic property of colloidal solutions is their transparency. Colloidal solutions are outwardly similar to true solutions. They are distinguished from the latter by the resulting “luminous path” - a cone when a beam of light passes through them. This phenomenon is called the Tyndall effect. Larger than in a true solution, the particles of the dispersed phase of the sol reflect light from their surface, and the observer sees a luminous cone in a vessel with a colloidal solution. It does not form in true solution. A similar effect, but only for an aerosol rather than a liquid colloid, can be observed in the forest and in cinemas when a beam of light from a movie camera passes through the air of the cinema hall.
Passing a beam of light through solutions; a - a true solution of sodium chloride; Particles of the dispersed phase of colloidal solutions often do not settle even during long-term storage due to continuous collisions with solvent molecules due to thermal motion. They do not stick together when approaching each other due to the presence of similar electric charges on their surface. This is explained by the fact that substances in a colloidal, i.e., in a finely divided state, have a large surface. Either positively or negatively charged ions are adsorbed on this surface. For example, silicic acid adsorbs negative ions SiO 3 2-, which are abundant in solution due to the dissociation of sodium silicate:
Particles with like charges repel each other and therefore do not stick together. But under certain conditions, the process of coagulation can occur. When boiling some colloidal solutions, desorption of charged ions occurs, i.e. colloidal particles lose their charge. They start to thicken and settle down. The same is observed when adding any electrolyte. In this case, the colloidal particle attracts an oppositely charged ion and its charge is neutralized. Coagulation - the phenomenon of sticking together of colloidal particles and their precipitation - is observed when the charges of these particles are neutralized, when an electrolyte is added to the colloidal solution. In this case, the solution turns into a suspension or gel. Some organic colloids coagulate when heated (glue, egg white) or when the acid-base environment of the solution changes. 2. Gels or jellies are gelatinous precipitates formed during the coagulation of sols. These include a large number of polymer gels, confectionery, cosmetic and medical gels so well known to you (gelatin, jelly, marmalade, Bird's Milk cake) and, of course, an infinite number of natural gels: minerals (opal), jellyfish bodies, cartilage, tendons , hair, muscle and nerve tissue, etc. The history of development on Earth can be simultaneously considered the history of the evolution of the colloidal state of matter. Over time, the structure of the gels is broken (peeled off) - water is released from them. This phenomenon is called syneresis.
Complete laboratory experiments on the topic (group work, in a group of 4 people). You have been given a sample of the disperse system. Your task is to determine which disperse system you have been given. Issued to students: sugar solution, iron (III) chloride solution, a mixture of water and river sand, gelatin, aluminum chloride solution, common salt solution, a mixture of water and vegetable oil. Instructions for performing a laboratory experiment
Sediments or exfoliates within a few minutes, or with difficulty over a long period of time, or does not settle. Fill in column No. 2 of the table. If you do not observe particle settling, examine it for coagulation. Pour a little solution into two test tubes and add 2-3 drops of yellow blood salt to one and 3-5 drops of alkali to the other, what do you observe?
Writing rules cinquain:
Summer 2008 Vienna. Schönbrunn.
Summer 2008 Nizhny Novgorod region.
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