Methods for producing colloidal solutions can also be divided into two groups: condensation and dispersion methods (a separate group is the peptization method, which will be discussed later). Another necessary condition for obtaining sols, in addition to bringing the particle sizes to colloidal ones, is the presence in the system of stabilizers - substances that prevent the process of spontaneous enlargement of colloidal particles.
Rice. Classification of production methods dispersed systems(type of systems is indicated in brackets)
Dispersion methods
Dispersion methods are based on fragmentation solids to particles of colloidal size and thus the formation of colloidal solutions. The dispersion process is carried out using various methods: mechanical grinding of the substance in the so-called. colloid mills, electric arc spraying of metals, crushing of substances using ultrasound.
Dispersion can be spontaneous or non-spontaneous. Spontaneous dispersion is characteristic of lyophilic systems and is associated with an increase in the disorder of the system (when many small particles are formed from one large piece). When dispersing at a constant temperature, the increase in entropy must exceed the change in enthalpy.
ΔH > TΔS; ΔG > 0.
The dispersion process in this case is typically non-spontaneous and is carried out due to external energy.
Dispersion is characterized by the degree of dispersion. It is determined by the ratio of the sizes of the initial product and the particles of the dispersed phase of the resulting system. The degree of dispersion can be expressed as follows:
α 1 = d n / d k; α 2 = B n / B k; α 3 = V n / V k,
where d n; d to; Bn; B to; V n; Vк - respectively diameter, surface area, volume of particles before and after dispersion.
Thus, the degree of dispersion can be expressed in terms of size (α 1), surface area (α 2) or volume (α 3) of dispersed phase particles, i.e. can be linear, superficial or volumetric.
The work W required to disperse a solid or liquid is spent on deforming the body W d and on the formation of a new phase interface W a, which is measured by the work of adhesion. Deformation is a necessary prerequisite for the destruction of a body. According to P.A. Rebinder, the work of dispersion is determined by the formula
W = W a + W d = σ*ΔB + kV,
where σ* is a value proportional to or equal to the surface tension at the interface between the dispersed phase and the dispersion medium; ΔB—increase in the phase interface as a result of dispersion; V is the volume of the original body before dispersion; k is a coefficient equivalent to the work of deformation per unit volume of a body.
Condensation methods
Condensation methods for producing dispersed systems include condensation, desublimation and crystallization. They are based on the formation of a new phase under conditions of a supersaturated state of a substance in a gas or liquid medium. In this case, the system goes from homogeneous to heterogeneous. Condensation and desublimation are characteristic of a gas medium, and crystallization is characteristic of a liquid medium.
A necessary condition for condensation and crystallization is supersaturation and uneven distribution of the substance in the dispersion medium (concentration fluctuation), as well as the formation of condensation centers or nuclei.
The degree of supersaturation β for solution and steam can be expressed as follows:
β f = s/s s , β P = r/p s ,
where p, c are the supersaturated vapor pressure and the concentration of the substance in the supersaturated solution; р s is the equilibrium pressure of saturated vapor over a flat surface; c s is the equilibrium concentration corresponding to the formation of a new phase.
To carry out crystallization, the solution or gas mixture is cooled.
The condensation methods for obtaining dispersed systems are based on the processes of crystallization, desublimation and condensation, which are caused by a decrease in the Gibbs energy (ΔG< 0) и протекают самопроизвольно.
During the nucleation and formation of particles from a supersaturated solution or gaseous medium, the chemical potential µ changes and an interface appears, which becomes the carrier of excess free surface energy.
The work spent on the formation of particles is determined by the surface tension σ and is equal to:
W 1 = 4πr 2 σ,
where 4πr 2 is the surface of spherical particles with radius r.
The chemical potential changes as follows:
Δμ = μ i // - μ i /< 0; μ i // >μ i / ,
where μ i / and μ i // are the chemical potentials of homo and heterogeneous systems (during the transition from small drops to large ones).
The change in chemical potential characterizes the transfer of a certain number of moles of a substance from one phase to another; this number n moles is equal to the volume of the particle 4πr 3 /3 divided by the molar volume Vm:
The work of formation of a new surface during the condensation process W k is equal to:
where W 1 and W 2 are, respectively, the work spent on the formation of the surface of particles, and the work on the transfer of matter from a homogeneous medium to a heterogeneous one.
The formation of dispersed systems can occur as a result of physical and chemical condensation, as well as when replacing the solvent.
Physical condensation occurs when the temperature of the gas medium containing vapor decreases various substances. When the necessary conditions are met, particles or drops of the dispersed phase are formed. A similar process takes place not only in the volume of gas, but also on a cooled solid surface, which is placed in a warmer gas environment.
Condensation is determined by the difference in chemical potentials (μ i // - μ i /)< 0, которая изменяется в результате замены растворителя. В отличие от обычной физической конденсации при solvent replacement the composition and properties of the dispersion medium do not remain constant. If alcohol or acetone solutions of sulfur, phosphorus, rosin and some others organic matter pour into water, the solution becomes supersaturated, condensation occurs and dispersed phase particles are formed. The solvent replacement method is one of the few by which sols can be obtained.
At chemical condensation the formation of a substance occurs with its simultaneous supersaturation and condensation.
9. Determine the change in the isobaric-isothermal potential of the reaction N 2 (g) + 2H 2 O (l) = NH 4 NO 2 (l) and give a conclusion about the direction of its flow under standard conditions, if for H 2 O (l) it is equal to – 237.4 kJ/mol, and for NH 4 NO 2 (l) is equal to – 115.8 kJ/mol.
The change in the isobaric-isothermal potential is less than 0, therefore, the process can proceed spontaneously towards a direct reaction.
14. Determine molecularity and order chemical reaction on specific examples.
The molecularity of a reaction is determined by the minimum number of molecules simultaneously participating in the elementary act of a given reaction. The molecularity and order of the reaction are numerically the same only for the simplest reactions. For complex processes, these reaction characteristics will be different (the order of the reaction is less than its molecularity). Consequently, the formal concept of the order of a reaction in most cases does not reflect its complex mechanism, i.e. the presence of several intermediate elementary reactions (stages). However, knowledge of the experimental order of the reaction makes it possible to judge its proposed mechanism by comparing the calculated and experimentally observed values of n. When the order of a reaction found experimentally does not correspond to the number of moles of reagents participating in the reaction, this indicates that the reaction is not an elementary process, but proceeds through a complex mechanism. For a complex mechanism, the rate of the overall reaction is determined by the rate of the slowest stage of the multi-stage process. Thus, if a reaction proceeds in one stage, then its order is equal to molecularity; if a reaction proceeds in several stages, then the order of each stage of the reaction is equal to the molecularity of only this stage. Therefore, experimental determination of the order of a reaction can serve as a method for studying its mechanism.
If only one particle (molecule) is needed to carry out an elementary act, then such a reaction is called monomolecular.
For an elementary process with the simultaneous participation of two particles, the reaction will be called bimolecular, etc.
For example:
The reaction is monomolecular, the reaction order is 1/3.
C (t) + H 2 O (g) CO (g) + H 2 (g)
The reaction is bimolecular, the order of the reaction is 2/2 = 1.
The reaction is trimolecular, the order of the reaction is 2/3 (from three molecules of reacting substances two molecules of the reaction product are obtained).
29. The change in free energy that accompanies a chemical reaction, its connection with the equilibrium constant. Calculation of the thermal effect of the reaction.
The change in the Gibbs free energy, or the change in the isobaric-isothermal potential, is the maximum part of the energy of the system that, under given conditions, can be converted into useful work. When the reaction occurs spontaneously.
In accordance with the law of mass action for an arbitrary reaction
a A + b B = c C + d D (1)
the rate equation for the forward reaction can be written:
, (2)
and for the rate of reverse reaction
. (3)
As reaction (1.33) proceeds from left to right, the concentrations of substances A and B will decrease and the rate of the forward reaction will decrease. On the other hand, as reaction products C and D accumulate, the rate of the reaction from right to left will increase. There comes a moment when the speeds υ 1 and υ 2 become the same, the concentrations of all substances remain unchanged, therefore,
Where does K c = k 1 / k 2 = 
.
The constant value Kc, equal to the ratio of the rate constants of the forward and reverse reactions, quantitatively describes the state of equilibrium through the equilibrium concentrations of the starting substances and the products of their interaction (to the extent of their stoichiometric coefficients) and is called the equilibrium constant. The equilibrium constant is constant only for a given temperature, i.e. K c = f (T). The equilibrium constant of a chemical reaction is usually expressed as a ratio, the numerator of which is the product of the equilibrium molar concentrations of the reaction products, and the denominator is the product of the concentrations of the starting substances.
If the reaction components are a mixture of ideal gases, then the equilibrium constant (K p) is expressed in terms of the partial pressures of the components:
K p = 
. (5)
From equation (6) it follows that K p = K c provided that the reaction proceeds without changing the number of moles in the gas phase, i.e. when (c + d) = (a + b).
If reaction (1) occurs spontaneously at constant P and T or V and T, then the values of G and this reaction can be obtained from the equation:
where Р А, Р В, Р С, Р D are the partial pressures of the starting substances and reaction products.
Equation (7) is called the Van't Hoff chemical reaction isotherm equations. This relationship makes it possible to calculate the values of G and F of the reaction and determine its direction at different concentrations of the starting substances.
It should be noted that for both gas systems and solutions, when solids participate in the reaction (i.e., for heterogeneous systems), the concentration of the solid phase is not included in the expression for the equilibrium constant, since this concentration is practically constant. Yes, for reaction
2 CO (g) = CO 2 (g) + C (t)
the equilibrium constant is written as
The dependence of the equilibrium constant on temperature (for temperature T 2 relative to temperature T 1) is expressed by the following van't Hoff equation:
, (8)
whereН 0 – thermal effect of the reaction.
34. Osmosis, osmotic pressure. Van't Hoff equation and osmotic coefficient.
Osmosis is the spontaneous movement of solvent molecules through a semi-permeable membrane that separates solutions of different concentrations, from a solution of lower concentration to a solution of higher concentration, which leads to the dilution of the latter. A cellophane film is often used as a semi-permeable membrane, through the small holes of which only small-volume solvent molecules can selectively pass through and large or solvated molecules or ions are retained - for high-molecular substances, and a copper ferrocyanide film for low-molecular substances. The process of solvent transfer (osmosis) can be prevented if external hydrostatic pressure is applied to a solution with a higher concentration (under equilibrium conditions this will be the so-called osmotic pressure, denoted by the letter ). To calculate the value of in solutions of non-electrolytes, the empirical Van't Hoff equation is used:
= C R T,
where C is the molal concentration of the substance, mol/kg;
R – universal gas constant, J/mol K.
The magnitude of osmotic pressure is proportional to the number of molecules (in general, the number of particles) of one or more substances dissolved in a given volume of solution, and does not depend on their nature and the nature of the solvent. In solutions of strong or weak electrolytes, the total number of individual particles increases due to the dissociation of molecules, therefore, an appropriate proportionality coefficient, called the isotonic coefficient, must be introduced into the equation for calculating osmotic pressure.
i C R T,
where i is the isotonic coefficient, calculated as the ratio of the sum of the numbers of ions and undissociated electrolyte molecules to the initial number of molecules of this substance.
So, if the degree of dissociation of the electrolyte, i.e. the ratio of the number of molecules disintegrated into ions to the total number of molecules of the dissolved substance is equal to and the electrolyte molecule disintegrates into n ions, then the isotonic coefficient is calculated as follows:
i = 1 + (n – 1) · ,(i > 1).
For strong electrolytes, we can take = 1, then i = n, and the coefficient i (also greater than 1) is called the osmotic coefficient.
The phenomenon of osmosis is of great importance for plant and animal organisms, since the membranes of their cells in relation to solutions of many substances have the properties of a semi-permeable membrane. In pure water, the cell swells greatly, in some cases to the point of rupture of the membrane, and in solutions with a high concentration of salts, on the contrary, it decreases in size and wrinkles due to large loss of water. Therefore, when preserving food products, it is added a large number of salt or sugar. Microbial cells under such conditions lose a significant amount of water and die.
Osmotic pressure ensures the movement of water in plants due to the difference in osmotic pressure between the cell sap of plant roots (5-20 bar) and the soil solution, which is further diluted during irrigation. Osmotic pressure causes water to rise in the plant from the roots to the top. Thus, leaf cells, losing water, osmotically absorb it from stem cells, and the latter take it from root cells.
49. Calculate the emf of a copper-zinc galvanic cell in which the concentration of C ionsu 2 + is equal to 0.001 mol/l, and ionsZn 2+ 0.1 mol/l. When making calculations, take into account the standard EMF values:
ε o (Zn 2+ /Zn 0) = – 0.74 V and ε o (Cu 2 + /Cu 0) = + 0.34 V.
To calculate the EMF value, the Nernst equation is used 
54. Methods for obtaining dispersed systems, their classification and a brief description of. Which method of obtaining dispersed systems is most beneficial from a thermodynamic point of view?
Dispersion method. It consists of mechanical crushing of solids to a given dispersion; dispersion by ultrasonic vibrations; electrical dispersion under the influence of alternating and direct current. To obtain dispersed systems by the dispersion method, mechanical devices are widely used: crushers, mills, mortars, rollers, paint grinders, shakers. Liquids are atomized and sprayed using nozzles, grinders, rotating disks, and centrifuges. Dispersion of gases is carried out mainly by bubbling them through a liquid. In foam polymers, foam concrete, and foam gypsum, gases are produced using substances that release gas at elevated temperatures or in chemical reactions.
Despite the widespread use of dispersion methods, they cannot be used to obtain disperse systems with a particle size of -100 nm. Such systems are obtained by condensation methods.
Condensation methods are based on the process of formation of a dispersed phase from substances in a molecular or ionic state. A necessary requirement for this method is the creation of a supersaturated solution from which a colloidal system should be obtained. This can be achieved under certain physical or chemical conditions.
Physical methods of condensation:
1) cooling of vapors of liquids or solids during adiabatic expansion or mixing them with a large volume of air;
2) gradual removal (evaporation) of the solvent from the solution or replacing it with another solvent in which the dispersed substance is less soluble.
Thus, physical condensation refers to the condensation of water vapor on the surface of airborne solid or liquid particles, ions or charged molecules (fog, smog).
Solvent replacement results in the formation of a sol when another liquid is added to the original solution, which mixes well with the original solvent but is a poor solvent for the solute.
Chemical condensation methods are based on performing various reactions, as a result of which an undissolved substance is precipitated from a supersaturated solution.
Chemical condensation can be based not only on exchange reactions, but also on redox reactions, hydrolysis, etc.
Dispersed systems can also be obtained by peptization, which consists of converting sediments, the particles of which already have colloidal sizes, into a colloidal “solution”. The following types of peptization are distinguished: peptization by washing the sediment; superficial peptization – active substances; chemical peptization.
For example, a freshly prepared and quickly washed precipitate of iron hydroxide turns into a red-brown colloidal solution by adding a small amount of FeCl 3 solution (adsorption peptization) or HCl (dissolution).
The mechanism of formation of colloidal particles using the peptization method has been studied quite fully: chemical interaction of particles on the surface occurs according to the following scheme:
Next unit 
adsorbs Fe +3 or FeO + ions, the subsequent ones are formed as a result of the hydrolysis of FeCl 3 and the micelle core receives a positive charge. The micelle formula can be written as:
From the point of view of thermodynamics, the dispersion method is the most advantageous.
1) The diffusion coefficient for a spherical particle is calculated using the Einstein equation:
,
where N А is Avogadro’s number, 6 10 23 molecules/mol;
h – viscosity of the dispersion medium, N s/m 2 (Pa s);
r – particle radius, m;
R – universal gas constant, 8.314 J/mol K;
T – absolute temperature, TO;
number 3.14.
2) Root mean square displacement:
·D·
where mean square displacement (averaged shift value) of a disperse particle, m 2 ;
time during which the particle is displaced (diffusion duration), s;
D diffusion coefficient, m 2. s -1 .
·D·=2*12.24*10 -10 *5=12.24*10 -9 m 2
Answer: 12.24*10 -9 m 2 .
74. Surfactants. Describe the causes and mechanism of manifestation of their surface activity.
At low concentrations, surfactants form true solutions, i.e. particles are dispersed and they are reduced to individual molecules (or ions). As the concentration increases, micelles appear. in aqueous solutions, the organic parts of the molecules in micelles are combined into a liquid hydrocarbon core, and the polar hydrated groups are in water, while the total area of contact of the hydrophobic parts of the molecules with water is sharply reduced. Due to the hydrophilicity of the polar groups surrounding the micelle, the surface (interfacial) tension at the core-water interface is reduced to values that ensure the thermodynamic stability of such aggregates compared to a molecular solution and the surfactant macrophase.
At low micellar concentrations, spherical micelles (Hartley micelles) with a liquid apolar core are formed.
Surface activity is related to the chemical composition of the substance. As a rule, it increases with decreasing polarity of the surfactant (for aqueous solutions).
According to Langmuir, during adsorption, the polar group, which has a high affinity for the polar phase, is drawn into the water, and the hydrocarbon non-polar radical is pushed out. the resulting decrease in Gibbs energy limits the size of the surface layer to one molecule thick. in this case, a so-called monomolecular layer is formed.
Depending on the structure, surfactant molecules are divided into nonionic, built on the basis of esters, including ethoxy groups, and ionic, based on organic acids and bases.
Ionic surfactants dissociate in solution to form surface-active ions, for example:
If surface active anions are formed during dissociation, surfactants are called anionic (salts of fatty acids, soaps). If surface-active cations are formed during dissociation, surfactants are called cationic (salts of primary, secondary and tertiary amines).
There are surfactants that, depending on the pH of the solution, can be either cationic or aninoactive (proteins, amino acids).
The peculiarity of surfactant molecules is that they have high surface activity towards water, which reflects the strong dependence of the surface tension of an aqueous surfactant solution on its concentration.
At low surfactant concentrations, adsorption is proportional to concentration.
Surface activity is related to the chemical composition of the substance. As a rule, it increases with decreasing polarity of the surfactant (for aqueous solutions). For example, for carboxylic acids the activity value is higher than for their salts.
When studying homologous series, a clear dependence of activity on the length of the hydrocarbon radical was discovered.
Based on a large amount of experimental material at the end of the 19th century, Duclos and Traube formulated a rule: surface activity in a series of homologs increases 3-3.5 times with an increase in the hydrocarbon chain by one CH 2 group.
As the concentration increases, adsorption on the surface of the liquid first increases sharply and then approaches a certain limit, called the limiting adsorption.
Based on this fact and a large number of studies, Langmuir put forward the idea of the orientation of molecules in surface layer. According to Langmuir, during adsorption, a polar group, which has a high affinity for the polar phase - water, is drawn into the water, and the hydrocarbon non-polar radical is pushed out. The resulting decrease in Gibbs energy limits the size of the surface layer to one molecule thick. In this case, a so-called monomolecular layer is formed.
Monomolecular films on the surface of water can exist in three states: gaseous, liquid and solid. Liquid and solid surface films are also called condensed films.
If the forces acting between the molecules in the film are relatively small, then the surfactant molecules are freely distributed over the surface of the water, moving away from each other as much as possible, which determines the surface pressure acting in the direction opposite to surface tension, such a film can be considered a two-dimensional gas, since the molecules This gas cannot break away from the surface of the water and can only move in two dimensions. Substances that form two-dimensional gaseous films on water include, for example, fatty acids with the number of hydrocarbon atoms from 12 to 20-22, aliphatic alcohols and amines with a not very high molecular weight.
If the tangential forces between the hydrocarbon radicals of the surfactant molecules in the surface film are large, then the molecules stick together, forming large condensed “islands” in which the thermal movement of the molecules is hindered. In such "islands" the molecules are usually oriented parallel to each other and perpendicular to the surface of the water. It should be noted, however, that, for example, with increasing temperature, condensed films can turn into gaseous ones.
Condensed films are usually liquid, and the molecules in them move quite freely. if the interaction forces between radicals are so strong that the molecules cannot move, then the condensed films can be considered solid. Such films form carboxylic acids with the number of carbon atoms more than 20-24.
The presence of solid-state properties in surface films can be verified by spraying powder onto the surface. If the film is solid, then when carefully blown off the powder remains motionless; if the film is liquid, the powder moves over the surface.
It should be noted that in addition to gaseous and condensed films, there are also so-called stretched films that occupy an intermediate position.
Such films can form from condensed ones with increasing temperature. It is believed that in stretched films, the hydrocarbon radicals of surfactant molecules are not oriented in parallel, but are intertwined with each other, lying “flat” on the water, which prevents unlimited spreading of the film, while polar groups move relatively freely in the surface layer.
The ability of substances to form certain films for ionic surfactants depends on the pH of the solution. Higher fatty acids in acidic and neutral solutions(i.e., with practically undissociated groups) at a certain temperature give stretched films at the interface with air. At the same temperature in an alkaline environment, gaseous films are formed on the surface of the solution, which is due to the repulsion of like charges neighboring groups, resulting from their dissociation.
89. Write the formula for the structure of a sol micelle formed as a result of the interaction of the indicated substances (an excess of one, then another substance): CdCl 2 + Na 2 S; FeCl 3 + NaOH. Name the constituent components of a micelle.
1) CdCl 2 + Na 2 S
Excess CdCl 2 gives a micelle:
[ (CdCl 2) Cd 2+ Cl – ] + x Cl –
germ: (CdCl 2)
core: [ (CdCl 2) Cd 2+
granule: [ (CdCl 2) Cd 2+ Cl – ] +
Excess Na 2 S gives a micelle:
–xNa+
germ: (NaCl)
core: (NaCl)2Cl -
granule: [ (CdCl 2) Cd 2+ Cl – ] +
2) FeCl 3 + NaOH
Excess FeCl 3 gives a micelle:
[ (FeCl 3) Fe 3+ 2Cl – ] + x Cl –
germ: (FeCl 3)
core: (FeCl 3) Fe 3+
granule: [ (FeCl 3) Fe 3+ 2Cl – ] +
Excess NaOH gives a micelle:
–xNa+
germ: (NaCl)
core: 3 (NaCl) 3 Cl –
granule: –
94. Protection of colloidal particles using an IUD. Mechanism of protective action. Proteins, carbohydrates, pectins as colloidal protection.
Colloidal protection – stabilization of a dispersed system by forming an adsorption protective shell around particles of the dispersed phase. Proteins, pectins and carbohydrates act as stabilizers of dispersed systems, protecting systems from further coagulation or sedimentation.
110. Foams, conditions of their formation and properties. The role of foaming for food and examples of the use of foams.
Foams are highly concentrated disperse systems (volume fraction of gas more than 60-80%), in which the dispersed phase is gas, and the dispersion medium is liquid or solid (foam concrete, foam gypsum, foam polymers, etc.). Foams are coarse systems, the size of the bubbles in which is from 0.01 cm to 0.1 cm or more. Most often, foams with a liquid dispersion medium are obtained by dispersing gas in a liquid in the presence of a stabilizer, which in this case is called a foaming agent.
Food products that are foams include such foams as whipped cream in balloons; milkshakes are also prepared by whipping and initially its components form foam. With the help of foaming in the food industry, they extract valuable impurities from solutions, which is especially effective in dry foams. But in the production of sugar, foam interferes with the normal course of processes and in this case defoaming is carried out.
LITERATURE
Akhmetov B.V. Problems and exercises in physical and colloidal chemistry. – L.: Chemistry, 1989.
Gameeva O. S. Physical and colloidal chemistry. – M.: graduate School, 1983.
Evstratova K. I., Kupina N. A., Malakhova E. M. Physical and colloidal chemistry. – M.: Higher School, 1990.
Zimon A. D., Leshchenko N. F. Colloid chemistry. – M.: Chemistry, 2001.
Zimon A. D., Leshchenko N. F. Physical chemistry. – M.: Chemistry, 2000.
Kiselev E.V. Collection of examples and problems in physical chemistry. – M.: Higher School, 1983.
Knorre D. G. Physical chemistry. – M.: Higher School, 1990.
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Khmelnitsky R. A. Physical and colloidal chemistry. – M.: Higher School, 1988.
Condensation methods, compared to dispersion methods, make it possible to obtain colloidal systems of higher dispersion. In addition, they usually do not involve the use of special machines.
Condensation methods for producing dispersed systems are based on creating conditions under which the future dispersion medium is supersaturated with the substance of the future dispersed phase. Depending on the methods for creating these conditions, the condensation method is divided into physical And chemical.
Physical condensation includes:
A) Vapor condensation by passing them through a cold liquid, resulting in the formation of lyosols. So, when passing vapors of boiling mercury, sulfur, selenium into cold water their colloidal solutions are formed.
b) Solvent replacement. The method is based on the fact that the substance from which the sol is to be obtained is dissolved in a suitable solvent, then a second liquid is added, which is a poor solvent for the substance, but mixes well with the original solvent. The initially dissolved substance is released from the solution in a highly dispersed state. For example, in this way it is possible to obtain hydrosols of sulfur, phosphorus, rosin, paraffin and many other organic substances by pouring their alcohol solution into water.
Chemical condensation differs from all the methods discussed above in that the dispersible substance is not taken in finished form, but is obtained directly in solution by a chemical reaction, as a result of which the desired compound, insoluble in the given medium, is formed. The task boils down to obtaining the precipitate that falls out in a finely dispersed state. When pouring solutions, it is necessary to achieve such conditions that many crystallization centers arise, then the resulting crystals will be very small in size. Optimal conditions for obtaining sols (concentration of solutions, draining order, draining speed, component ratio, temperature) are usually found experimentally.
Chemical condensation methods use any reaction leading to the formation of a new phase: double exchange reaction, decomposition, oxidation-reduction, etc. Electrochemical reactions can be used, for example, the reduction of metals by electrolysis.
Below are some examples of the synthesis of colloidal systems using various reactions. The stabilizer of a colloidal solution is usually one of the reaction participants or a by-product, from which adsorption layers of the ionic or molecular type are formed at the particle-medium interface, preventing the particles from sticking together and precipitating.
When gaseous NH 3 and HCl interact, an aerosol (smoke) of solid ammonium chloride is formed (compound reaction):
NH 3 + HCl = NH 4 Cl
By reacting sodium thiosulfate with sulfuric acid, a sulfur hydrosol can be obtained (oxidation-reduction reaction):
Na 2 S 2 O 3 + H 2 SO 4 = S¯ + Na 2 SO 4 + SO 2 + H 2 O
Many sols can be synthesized using exchange reactions:
Na 2 SiO 3 + 2HCl = H 2 SiO 3 ¯ + 2NaCl
KJ + AgNO 3 = AgJ¯ + KNO 3.
The resulting sols are contaminated with impurities of low molecular weight substances.
Cleaning dispersed systems
To purify dispersed systems from dissolved low molecular weight substances, Graham proposed using the ability of finely porous films (membranes) to retain particles of the dispersed phase and freely allow ions and molecules to pass through. This method is called dialysis.
The dispersed system to be cleaned is placed in a vessel made of fine-porous material or having a fine-porous bottom (Fig. 9.33 a). The vessel is washed with running water (distilled). According to the laws of diffusion, ions and molecules of the dissolved substance contained in the dispersed system in the form of impurities penetrate through the pores of the membrane into distilled water, and particles of the dispersed phase are retained and remain in the dispersed system.
Rice. 9.33. Schemes of the dialyzer (a) and electrodialyzer (b)
The dialysis rate is very low, but it can be significantly increased (10-20 times) by taking advantage of the action electric field to ions of dissolved impurities. This method of purifying dispersed systems from electrolyte impurities is called electrodialysis.
An electrodialyzer (Fig. 9.33. b) is a vessel divided by membranes into three compartments, of which the middle one is filled with a dispersed system to be purified, and electrodes are placed in the outermost ones; A liquid homogeneous with the substance of the dispersion medium of the system being cleaned circulates through these same compartments. When a sufficient potential difference (several hundred volts) is applied to the electrodes, the dispersed system is relatively quickly cleared of electrolyte.
Currently, dialysis is used in many industries. It is especially effective in medicine. For example, the operation of the “artificial kidney” device is based on the principle of electrolysis, which allows the patient’s blood to be purified from harmful waste products of the body.
Ultrafiltration – a method of purifying sols by forcing a dispersion medium with low molecular weight impurities through ultrafilters. Ultrafilters - these are membranes with a pore size through which impurities and solvent pass, but particles of sol (or high-molecular compounds) do not pass through.
The sol to be purified is poured into a bag made from an ultrafilter and pressed through the membrane under pressure. The dispersion medium is renewed by adding a pure solvent to the sol. Pure sol remains in the bag.
Thus, to obtain dispersed systems, they use both methods of grinding large particles ( dispersion), and methods based on combining molecular particles to colloidal sizes ( condensation). Dispersion methods make it possible to obtain coarsely dispersed systems with large sizes particles. Condensation methods make it possible to obtain highly dispersed sols. Dispersed systems are purified from low-molecular impurities using fine-porous filters - membranes.
In terms of degree of dispersion, colloidal systems occupy an intermediate position between true solutions (molecular or ion-dispersed systems) and coarsely dispersed systems. Therefore, there are two groups of methods for obtaining dispersed systems: Group 1 – dispersion, i.e. crushing of particles of the dispersed phase of coarsely dispersed systems, group 2 is based on aggregation (condensation) processes in which molecules under the influence of adhesion forces unite and first give rise to the germ of a new phase, and then - real particles of the new phase
One more a necessary condition obtaining sols, in addition to bringing the particle sizes to colloidal ones, is the presence in the system of stabilizers - substances that prevent the process of spontaneous enlargement of colloidal particles.
Dispersion methods
Dispersion methods are based on the crushing of solids into particles of colloidal size and thus the formation of colloidal solutions. The dispersion process is carried out by various methods: mechanical grinding of the substance in colloid mills, electric arc spraying of metals, crushing of the substance using ultrasound.
Condensation methods
A substance in a molecularly dispersed state can be converted into a colloidal state by replacing one solvent with another - those. solvent replacement method. An example is the production of rosin sol, which is insoluble in water, but highly soluble in ethanol. When an alcoholic solution of rosin is gradually added to water, the solubility of rosin sharply decreases, resulting in the formation of a colloidal solution of rosin in water. Sulfur hydrosol can be prepared in a similar manner.
Colloidal solutions can also be obtained using the method chemical condensation, based on chemical reactions accompanied by the formation of insoluble or slightly soluble substances. For this purpose, various types of reactions are used - decomposition, hydrolysis, redox, etc. Thus, red gold sol is obtained by reducing the sodium salt of gold acid with formaldehyde:
NaAuO 2 + HCOH + Na 2 CO 3 ––> Au + HCOONa + H 2 O
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Physical chemistry
Tutorial Krasnoyarsk 2007 UDC 541.128: BBK 35.514 I 73
And development
All known chemical reactions, regardless of the nature of the reactants, are accompanied by various physical phenomena - the release or absorption of heat, light, changes in
Ideal gases. Equations of state of gases
The equation of state for an ideal gas is the Clapeyron-Mendeleev equation; The simplest equation of state of a real gas is the van der Waals equation.
Here follows
Internal energy, heat, work Internal energy U characterizes the total energy reserve of motion and interaction of all particles that make up the system. It includes the energy of progressive and rotational movement
molecules, ene
First law of thermodynamics
The first law of thermodynamics is a postulate. This means that this law cannot be proven logically, but follows from the sum of human experience. Just
The first law of thermodynamics under isobaric, isochoric, isothermal and adiabatic conditions for ideal gas systems
Hess's law. Corollaries from Hess's law
Thermochemistry is a branch of physical chemistry that studies the thermal effects of chemical reactions. The thermal effect of a chemical reaction is the heat that
Standard Thermal Effects
For the convenience of comparing thermal effects, as well as other thermodynamic functions, the idea of the standard state of matter is introduced. For solids and liquids as standard with
First corollary of Hess's law
This consequence is related to the heats of formation of compounds. The heat (enthalpy) of formation of a compound is the amount of heat released or absorbed during the formation of 1 mol
Second corollary of Hess's law
In some cases, it is more convenient to calculate the thermal effect of a reaction from the heats (enthalpies) of combustion of the substances participating in the reaction. The heat (enthalpy) of combustion of a compound is called
Kirchhoff's equation. Dependence of the thermal effect of the reaction on temperature
Differentiating with respect to temperature (at constant pressure) the equality DН = Н2 − Н1 we obtain ¶(
The concept of entropy. Statistical thermodynamics and the physical meaning of entropy
All processes occurring in nature can be divided into spontaneous and non-spontaneous. Spontaneous processes occur without external energy expenditure; for pro
Change in entropy as a criterion for the spontaneous occurrence of a process in an isolated system
Spontaneous processes occur without the expenditure of energy from the outside. The spontaneous course of the process is associated with irreversibility. Irreversible in thermodynamics
Planck's postulate. (Third Law of Thermodynamics)
Unlike internal energy and enthalpy, entropy can be defined in absolute terms. This possibility appears when using Planck's postulate, which
Thermodynamic potentials
The mathematical apparatus of thermodynamics is based on the combined equation of the first and second laws of thermodynamics for reversible processes: dU = T d
Changes in Gibbs energy in chemical reactions
Calculation of DG for chemical processes can be done in two ways. The first method uses relation (27): DG = D
Chemical Potential
Let us consider systems in which the amounts of substances change. These changes can occur as a result of chemical reactions or phase transitions. At the same time they change
Gibbs phase rule
Component - a chemically homogeneous substance contained in the system that can be isolated from the system and can exist in isolated form for a long time
Single component systems
When kn = 1, the phase rule equation takes the form: C = 3 - F. If there is 1 phase in equilibrium, then C = 2, then
Phase diagram of water
The phase diagram of water in p - T coordinates is presented in Fig. 8. It is composed of 3 phase fields - areas of different (p, T)-values, at which
Sulfur phase diagram
Crystalline sulfur exists in the form of two modifications - orthorhombic (Sр) and monoclinic (Sm). Therefore it is possible that
Clausius–Clapeyron equation
Movement along the lines of two-phase equilibrium on the phase diagram (C=1) means a consistent change in pressure and temperature, i.e. p = f(T). General form such a function for single-component
Entropy of evaporation
The molar entropy of evaporation DSsp = DHsp/Tboil is equal to the difference Spara - Sliquid. Since Sp
Chemical equilibrium
Thermodynamic equilibrium is a state of a system whose characteristics (temperature, pressure, volume, concentration) do not change over time at constant
Law of mass action. Equilibrium constants
Quantitative characteristics chemical equilibrium is the equilibrium constant, which can be expressed in terms of the equilibrium concentrations of Ci,
Isobar and isochore of a chemical reaction
To obtain the dependence of the equilibrium constant Kp on temperature, we use the Gibbs-Helmholtz equation:
Thermodynamics of solutions
The existence of absolutely pure substances is impossible - every substance necessarily contains impurities, or, in other words, every homogeneous system is multicomponent.
The solution is a homogeneous system
Formation of solutions. Solubility
The concentration of a component in a solution can vary from zero to a certain maximum value, called the solubility of the component. Solubility is the concentration of a component in a saturated
Solubility of gases in liquids
The solubility of gases in liquids depends on a number of factors: the nature of the gas and liquid, pressure, temperature, concentration of substances dissolved in the liquid (especially strong
Mutual solubility of liquids
Depending on their nature, liquids can be mixed in any ratio (in this case they speak of unlimited mutual solubility), they can be practically
Solubility of solids in liquids
The solubility of solids in liquids is determined by the nature of the substances and, as a rule, depends significantly on temperature; information about the solubility of target solids
The relative content of components in steam, as a rule, differs from their content in solution - steam is relatively richer in the component whose boiling point is lower. This fact
Saturated vapor pressure of dilute solutions. Raoult's law
Let's imagine that some substance B is introduced into the equilibrium system liquid A - vapor A. When a solution is formed, the mole fraction of the solvent XA becomes
Deviations from Raoult's law
If both components of a binary (consisting of two components) solution are volatile, then the vapor above the solution will contain both components. Consider a binary solution, soc
Crystallization temperature of dilute solutions
A solution, unlike a pure liquid, does not solidify entirely at a constant temperature. At a certain temperature, called the crystallization onset temperature
Boiling point of dilute solutions
The boiling point of solutions of a non-volatile substance is always higher than the boiling point of a pure solvent at the same pressure. Consider the p – T diagram with
Solute Activity Concept
If the solute concentration does not exceed 0.1 mol/L, then the nonelectrolyte solution is usually considered dilute. In such solutions, the interaction between molecules
Colligative properties of solutions
Some properties of solutions depend only on the concentration of dissolved particles and do not depend on their nature. Such properties of a solution are called colligative. At the same time, sales
Theory of electrolytic dissociation. Degree of dissociation
Electrolytes are substances whose melts or solutions conduct electric current due to dissociation into ions.
To explain the peculiarities of the properties of electrolyte solutions, S. Arrhenius proposed
Weak electrolytes. Dissociation constant
The process of dissociation of weak electrolytes is reversible. A dynamic equilibrium is established in the system, which can be quantified by the constant pa
Strong electrolytes
Strong electrolytes in solutions of any concentration completely dissociate into ions and, therefore, the laws obtained for weak electrolytes cannot be applied to strong electrolytes b
Electrical conductivity of electrolyte solutions Electricity
is the ordered movement of charged particles. Electrolyte solutions have ionic conductivity due to the movement of ions in the electric
When a metal electrode (conductor with electronic conductivity) comes into contact with a polar solvent (water) or an electrolyte solution, two
Galvanic cell. EMF of a galvanic cell
Let's consider the simplest Daniel-Jacobi galvanic cell, consisting of two half-cells - zinc and copper plates, placed in solutions of zinc and copper sulfates, respectively, which are connected
Electrode potential. Nernst equation
It is convenient to represent the EMF of a galvanic cell E as the difference in some quantities characterizing each of the electrodes - electrode potentials; O
Reference electrodes
To determine the potential of the electrode, it is necessary to measure the EMF of a galvanic cell composed of the electrode under test and an electrode with an accurately known potential
Indicator electrodes
Hydrogen ion reversible electrodes are used in practice to determine the activity of these ions in a solution (and hence the pH of the solution) by potentiome
Redox electrodes
In contrast to the described electrode processes, in the case of redox electrodes, the processes of receiving and donating electrons by atoms or ions occur
Chemical reaction rate
Basic concept chemical kinetics– rate of chemical reaction.
The rate of a chemical reaction is the change in the concentration of reactants per unit time.
Matematich
Basic postulate of chemical kinetics
(law of mass action in chemical kinetics) Chemical kinetics is based on the basic postulate of chemical kinetics: The rate of a chemical reaction is directly proportional
Zero order reactions
Let us substitute expression (71) into equation (74), taking into account the fact that the calculation is carried out using the starting substance A (which determines the choice of the “minus” sign):
First order reactions
Let us substitute expression (71) into equation (75): Integration
Second order reactions
Let us consider the simplest case, when the kinetic equation has the form (76). In this case, taking into account (71), we can write: CH3COOC2H5 + H2O ––> CH3COOH + C2H5OH If this reaction is carried out at close concentrations of ethyl acetate and water, then
general order
reaction is equal to two and the kinetic equation has the following form:
Methods for determining reaction order
To determine particular reaction orders, the method of excess concentrations is used. It lies in the fact that the reaction is carried out under conditions where the concentration of one of the reagents is high various reaction products can simultaneously form, for example, two or more isomers:
Chain reactions
These reactions consist of a series of interconnected steps, with the particles resulting from each step generating subsequent steps. Usually, chain reactions proceed with the participation of free
Van't Hoff and Arrhenius equations
The reaction rate constant k in equation (72) is a function of temperature; An increase in temperature generally increases the rate constant. The first attempt to take into account the influence of temperature was made
Photochemical reactions
Overcoming the activation barrier during the interaction of molecules can be accomplished by supplying energy to the system in the form of light quanta. Reactions in which particle activation
Catalysis
The rate of a chemical reaction at a given temperature is determined by the rate of formation of the activated complex, which, in turn, depends on the amount of energy
Michaelis equation
Enzymatic catalysis - catalytic reactions occurring with the participation of enzymes - biological catalysts of protein nature. Enzyme catalysis has two characteristic features:
Molecular kinetic properties of disperse systems
It is typical for crushed particles Brownian motion. The smaller the particle diameter and the lower the viscosity of the medium, the more intense it is. With a particle diameter of 3-4 microns, Brownian motion is
Optical properties of colloidal systems
Colloidal systems are characterized by a matte (usually bluish) glow, which can be observed against a dark background when a beam of light is passed through them. This is the glow on
Adsorption. Gibbs equation
Adsorption is the phenomenon of spontaneous thickening in the surface layer of a mass of substance, which by its presence reduces surface tension. Adsorption value (G, mol/m
Adsorption at the solid-gas interface
In the adsorption of gases on solids, the description of the interaction between the molecules of the adsorbate (the substance that is adsorbed) and the adsorbent (the substance that adsorbs) is very complex
Adsorption from solutions
Surfactants (surfactants) Surfactants (surfactants) reduce surface tension. Surfactant molecules adsorbed at the water interface
Micelle formation
Like adsorption, the phenomenon of micellization is associated with the molecular interactions of its polar molecules (parts of molecules) and the hydrophobic bonds of the hydrocarbon chain.
Higher
When considering the structure of the micelle, it was shown that an electric double layer (EDL) is formed on the surface of colloidal particles. The first theory of the structure of DES was developed by Helmholtz and Perret
Sol is a dispersed system with a solid-particle dispersed phase. Aerosol corresponds to gaseous dispersed environment, and lyosol (hydrosol) - a liquid dispersed medium.
Dispersion of liquids is usually called atomization when it occurs in the gas phase, and emulsification when it is carried out in another liquid that is immiscible with the first.
Dispersion- fine grinding of solids or liquids, resulting in powders, suspensions, emulsions ( emulsification, or emulsification). When solids are dispersed, their mechanical destruction occurs.Dispersion methods
– mechanical dispersion– carried out under the influence of external mechanical work. Methods: abrasion, crushing, splitting, spraying, bubbling (passing a stream of air through a liquid), shaking, explosion, the action of sound and ultrasonic waves. This method is used to produce flour, powdered sugar, cocoa powder, spices, ground coffee and others. Size of particles obtained by this method, c.p. quite large, at least 100 nm. Equipment: mortars, mills, crushers various types, millstone.
To increase efficiency, mechanical dispersion is carried out in a liquid medium. Liquids (surfactant solutions, electrolytes) that wet a solid body are adsorbed on it and reduce the strength during machining. This is called adsorption reduction in the strength of solids or Rebinder effect(founded in 1982 by P.A. Rebinder).
– electrical dispersion– is based on the formation of a voltaic arc between electrodes made of sprayed metal placed in a cooled DS. Metals evaporate at the temperature of a voltaic arc and then condense in a cold DS. This method mainly produces metal hydrosols (the dispersion medium is water), such as silver, gold and platinum.
– ultrasonic dispersion– based on the influence of ultrasonic vibrations with a frequency above 20 thousand per 1 s, which are not detected by the human ear, effective only for substances with low strength. These include sulfur, graphite, starch, rubber, gelatin, etc.
To physico-chemicaldispersion applies method peptization. It consists of converting freshly prepared loose sediments into a colloidal solution under the influence of special stabilizing additives (peptizers - electrolytes, surfactant solutions). The action of a peptizer is that the sediment particles are separated from each other and become suspended, forming a sol. This method can be used to obtain, for example, iron (III) hydroxide hydrosol). The peptization method can only be used for freshly prepared sediments, since during storage processes of recrystallization and aging occur, leading to the merging of particles with each other. The particle sizes obtained by this method are about 1 nm.
