The solubility of liquids, its dependence on various factors. Factors affecting solubility
Some substances dissolve better in a particular solvent, others worse. It is believed that absolutely insoluble substances do not exist. Every substance is capable of solubility, even if in some cases in very small quantities (for example, mercury in water, benzene in water).
Unfortunately, to date, there is no theory by which one could predict and calculate the solubility of any substance in the corresponding solvent. This is due to the complexity and diversity of the interaction of the components of the solution with each other and the absence general theory solutions (especially concentrated ones). In this regard, the necessary data on the solubility of substances are obtained, as a rule, empirically.
Quantitatively, the ability of a substance to dissolve is most often characterized by solubility or solubility coefficient (S).
Solubility (S) shows how many grams of a substance can be maximally dissolved under given conditions (temperature, pressure) in 100 g of a solvent to form a saturated solution.
If necessary, the solubility coefficient is also determined for a different amount of solvent (for example, for 1000 g, 100 cm3, 1000 cm3, etc.).
By solubility, all substances, depending on their nature, are divided into 3 groups: 1) highly soluble; 2) slightly soluble; 3) poorly soluble or insoluble.
The solubility coefficient for substances of the first group is more than 1 g (per 100 g of solvent), for substances of the second group lies in the range of 0.01 - 1.0 g and for substances of the third group S< 0,01 г.
The solubility of substances is influenced by many factors, the main of which are the nature of the solvent and the solute, temperature, pressure, and the presence of other substances (especially electrolytes) in the solution.
Influence of the nature of substances on solubility
It has been experimentally established that in a solvent whose molecules are polar, substances formed by ionic or covalent polar bonds dissolve best. And in a solvent whose molecules are non-polar, substances formed by weakly polar or non-polar covalent bonds dissolve better. In another way, this revealed pattern can be formulated as follows: "Like dissolves in like."
The solubility of substances is largely determined by the strength and nature of their interaction with solvent molecules. The more pronounced this interaction, the greater the solubility and vice versa.
It is known that the forces acting between nonpolar and weakly polar molecules are small and nonspecific; in quantitative terms are essentially independent of the type of substance.
If similar nonpolar molecules A are introduced into a nonpolar liquid B, then the energy of interaction between particles A and B will not differ significantly from the energy of interaction between particles A and A or particles B and B. Therefore, just as any amount of the same substance is mixed , with a high probability will mix with each other indefinitely (i.e., dissolve in each other) and various non-polar liquids.
For the same reason, molecular crystals usually dissolve better in non-polar liquids.
If the interaction energy of molecules A and A or B and B is greater than A and B, then identical molecules of each component will preferentially bind to each other and their solubility in each other will decrease (Table 6).
The polarity of any solvent is often characterized by the value of its permittivity (e), which is easily determined empirically. The larger it is, the more polar the substance is.
Table 2 - Solubility of KI (wt%) in solvents of different polarity
The solubility of substances increases significantly if they are able to form hydrogen or donor-acceptor bonds with the solvent. An example of high solubility due to the formation of hydrogen bonds is a solution ethyl alcohol in H2O, and the formation of donor-acceptor bonds - a solution of NH3 in water. In this case, the solubility of alcohol is not limited, and NH3 in H2O dissolves in a volume ratio of ~ 700: 1.
Effect of pressure on the solubility of substances
The influence of pressure on the solubility of solid and liquid substances practically does not affect, because the volume of the system does not change significantly. Only at very high pressures the change in solubility becomes noticeable. For example, the solubility of NH4NO3 decreases by almost half when the pressure rises to 106 kPa (i.e., approximately 10,000 times atmospheric pressure).
Pressure has a significant effect only on the solubility of gases. Moreover, if there is no chemical interaction between the gas and the solvent, then according to Henry's law, the solubility of the gas at a constant temperature is directly proportional to its pressure over the solution
where k is a proportionality factor depending on the nature of the liquid and gas; p is the pressure of the gas above the solution.
Henry's law is valid only for dilute solutions and at low pressures.
If we are talking about the dissolution of not one gaseous substance, but a mixture consisting of several gases, then according to Dalton's law, the solubility of each component of the mixture at a constant temperature is proportional to its partial pressure over the liquid and does not depend on the total pressure of the mixture and the presence of other gases.
The partial pressure of an individual gas in a mixture (p1) is calculated by the formula:
p1 = ptot. X1
where rtosh. - total pressure of the gas mixture; X1 is the mole fraction of gas in the mixture.
If a gas mixture consists of several components, then its total pressure is determined by the sum of the partial pressures of all gases included in the mixture:
rtot. = p1 + p2 + p3 + ...
Gases that interact with a solvent (for example, NH3, SO2, HCl) when dissolved in H2O do not obey the Henry and Dalton law. Their solubility also increases with increasing pressure, but according to a more complex relationship.
A decrease in gas pressure over a solution leads to a decrease in its solubility and release from the liquid in the form of bubbles.
The change in gas solubility with a sharp decrease in pressure is due to the so-called decompression sickness, which divers working deep under water can get sick. Under these conditions, they breathe air under high pressure. At the same time, the solubility of gases in the blood increases greatly. If, after finishing work, you rise to the surface too quickly, then due to a sharp decrease in pressure, excess gases dissolved in the blood begin to be rapidly released. The resulting bubbles clog the blood vessels, which leads to circulatory disorders, numerous hemorrhages in various fabrics and organs due to rupture of capillaries.
Therefore, the ascent to the surface from great depths must be long enough and slow enough to allow excess gas to be removed from the circulatory system through the respiratory organs.
A similar picture can also arise in the case of depressurization at high altitude of aircraft cabins and cabins, as well as military pilots' spacesuits.
The effect of temperature on the solubility of substances
The solubility of most solid and liquid substances increases to some extent with increasing temperature. For some solids (especially if heat is released during their dissolution), the solubility, on the contrary, may decrease with increasing temperature.
The dependence of the solubility of substances on temperature is often clearly shown using graphs, which are called solubility curves (Fig. 5). The solubility of gases decreases with increasing temperature. Prolonged boiling can almost completely remove the dissolved gases from the liquid. Saturation of a liquid with gas, on the contrary, is more expedient to carry out at low temperatures.
The change in solubility with temperature is often used to purify substances by recrystallization. When a hot saturated solution of any salt contaminated with foreign impurities cools down, a significant part of it (salt) will be precipitated, and the contaminants will remain in the solution, since the latter will not be saturated with respect to them even in the cold.
Rice. 5
Only those solids whose solubility depends strongly on temperature can be purified in this way.
Effect of electrolytes on the solubility of substances
If the solvent contains impurities, then the solubility of substances in it decreases. This is especially noticeable when an electrolyte acts as such a foreign compound, and a gas is a solute. For example, about 3 cm3 of gaseous Cl2 dissolves in 1 cm3 of H2O at 20°C, and only 0.3 cm3 of chlorine dissolves in 1 cm3 of a saturated NaCl solution. Russian scientist - physiologist I.M. Sechenov established a quantitative relationship between gas solubility and electrolyte concentration in solution (Sechenov's law):
where S is the solubility of the gas in the electrolyte solution; S0 is the solubility of the gas in the solvent; C is the molar concentration of the electrolyte in the solution; k is a constant depending on the nature of the gas, electrolyte and temperature.
Sechenov's law makes it possible to investigate the solubility of gases in blood, which contains a significant number of dissolved substances, including electrolytes.
Like gases, when electrolytes are added to a solution, the solubility of some liquids and solids can also decrease.
This phenomenon is otherwise called salting out, because. salts are most often used as an electrolyte.
One of the reasons for the decrease in the solubility of substances in the presence of electrolytes may be the formation of strong hydrate (solvate) shells around the ions into which electrolytes decompose. As a result, the number of free molecules of the liquid, and hence its dissolving power, decreases.
The solubility of substances in various solvents, such as water, varies widely. If more than 10 g of a substance dissolves in 100 g of water at room temperature, then such a substance is usually called easily soluble, if less than 1 g of a substance is slightly soluble; finally, a substance is considered practically insoluble if less than 0.1 g of the substance passes into 100 g of water. Easily soluble substances include table salt (at 20 ° C, 35.8 g of NaCl dissolves in 100 g of water), copper sulfate CuSO4 5H20 (20.7 g), ammonia NH3 (67.9 g); sparingly soluble substances - gypsum CaS04 (0.195 g), slaked lime Ca(OH)2 (0.165 g); practically insoluble - barium sulfate BaSO4 (0.00023 g), silver chloride AgCl (0.00015 g), calcium carbonate CaCO3 (0.00013 g). Absolutely insoluble substances do not exist. What does the solubility of substances depend on? The solubility of substances is generally influenced by the nature of the solute and the nature of the solvent, temperature, and pressure. Influence of the nature of the solvent and the solute. A very long time ago, the rule was established empirically, according to which like dissolves into like. So, Substances with ionic (salts, alkalis) or polar (alcohols, aldehydes) type of bond are highly soluble in polar solvents, for example, in water. Conversely, the solubility of oxygen in benzene, for example, is an order of magnitude higher than in water, since the O2 and C6Hb molecules are nonpolar. The solubility of gases in liquids can vary over a very wide range. So, for example, 1.54 volumes of nitrogen, 2 volumes of hydrogen, 2.3 volumes of carbon monoxide (II), 3 volumes of oxygen, 88 volumes of carbon monoxide (IV) dissolve in 100 volumes of water at 20 ° C. Under the same conditions, more than 400 volumes of hydrogen chloride and 700 volumes of ammonia are dissolved in 1 volume of water. The high solubility of ammonia is explained by chemical interaction with water, and hydrogen chloride - by its dissociation into ions under the action of water dipoles. The influence of the nature of the solvent is illustrated by the following example: at 0 X and a pressure of 1 atm, 89.5 g of NH3 dissolves in 100 g of water, 42 g of it dissolves in methyl alcohol, and only 25 g in ethyl alcohol. The solubility of liquids in liquids depends in a very complex way on their nature. There are three classes of liquids that differ in their ability to mutually dissolve. 1. Liquids that practically do not dissolve in each other (for example, H20 - fjg, H20 - CJH *). 2. Liquids that dissolve indefinitely in each other (for example, H20 - QH5OH, H20 - CH3COOH). 3. Liquids that dissolve in each other to a limited extent (H20 - C2H5OS2H5, n20 - QHsNHz). For example, ether (QH5OQH5) dissolves in water in a small amount. Factors affecting the solubility of substances The solubility of solids in liquids is primarily determined by the nature of the chemical bonds in their crystal lattices. Molecular (or atomic) crystals, the structural units of which are atoms or molecules with a covalent non-polar type of bond, are practically insoluble in water (for example, graphite, diamond, sulfur, crystalline iodine). The influence of the nature of chemical bonds can be illustrated by a number of known substances. So, sodium salts of formic and acetic acids are very soluble in water, and soaps - salts of stearic, palmitic and oleic acids - are soluble in water to a very small extent. The solubility of phenol in water is low (QH5OH are polar molecules, but a large hydrocarbon radical). Sodium phenolate QHjONa is an ionic compound, and although the radical in the anion is the same as in phenol, the solubility of phenolate is much higher than that of phenol. The solubility of cryo is very characteristic. phenyl ammonium chloride QHjNHjCl (an organic salt with a polar bond) in polar water and non-polar benzene. In the first case, the salt is highly soluble, in benzene it is practically insoluble. Inorganic salts have different solubility in water. So, all salts of nitrogenous and nitric acid well soluble in water. The vast majority of fluorides, bromides and iodides are melting well soluble in water. Average salts of carbonic acid, with the exception of ammonium and alkali metal salts, are insoluble in water, and all bicarbonates are soluble. Of sulfates, the salts of alkaline earth metals, silver and lead are insoluble or slightly soluble. The soluble phosphates are the ammonium, sodium and potassium salts. Most ammonium and alkali metal salts are soluble. Factors affecting the solubility of substances All of the above is clearly illustrated by the table of the solubility of acids, bases and salts in water (see Appendix i) - be sure to analyze this table after reading this section.
The ability of a substance to dissolve in water or another solvent is called solubility. The quantitative characteristic of solubility is the solubility coefficient, which shows what is the maximum mass of a substance that can be dissolved in 1000 or 100 g of water at a given temperature. The solubility of a substance depends on the nature of the solvent and substance, on temperature and pressure (for gases). The solubility of solids generally increases with increasing temperature. The solubility of gases decreases with increasing temperature, but increases with increasing pressure.
According to their solubility in water, substances are divided into three groups:
- 1. Highly soluble (p.). The solubility of substances is more than 10 g in 1000 g of water. For example, 2000 g of sugar dissolves in 1000 g of water, or 1 liter of water.
- 2. Slightly soluble (m.). The solubility of substances is from 0.01 g to 10 g of a substance in 1000 g of water. For example, 2 g of gypsum (CaSO4 * 2H20) is dissolved in 1000 g of water.
- 3. Practically insoluble (n.). The solubility of substances is less than 0.01 g of a substance in 1000 g of water. For example, 1.5 * 10_3 g of AgCl dissolves in 1000 g of water.
When substances are dissolved, saturated, unsaturated and supersaturated solutions can be formed.
A saturated solution is a solution that contains maximum amount solute under given conditions. When a substance is added to such a solution, the substance no longer dissolves.
An unsaturated solution is a solution that contains less solute than a saturated solution under given conditions. When a substance is added to such a solution, the substance still dissolves.
Sometimes it is possible to obtain a solution in which the solute contains more than in a saturated solution at a given temperature. Such a solution is called supersaturated. This solution is obtained by carefully cooling the saturated solution to room temperature. Supersaturated solutions are very unstable. Crystallization of a substance in such a solution can be caused by rubbing the walls of the vessel in which the solution is located with a glass rod. This method is used when performing some qualitative reactions.
The solubility of a substance can also be expressed by the molar concentration of its saturated solution.
The rate of the dissolution process depends on the substances being dissolved, the state of their surfaces, the temperature of the solvent, and the concentration of the final solution.
Do not confuse the concepts of "saturated" and "dilute" solution. For example, a saturated solution of silver chloride (1.5 * 10-3g / l) is yavl. very dilute, and unsaturated solution of sugar (1000 g / l) - concentrated.
Concentration of solutions and methods of its expression
According to modern ideas, the quantitative composition of the solution can be expressed both with the help of dimensionless quantities and quantities having dimensions. Dimensionless quantities are usually called fractions. 3 types of fractions are known: mass (u), volume (c), molar (h)
The mass fraction of a solute is the ratio of the mass of the solute X to the total mass of the solution:
u (X) \u003d t (X) / t
where w(X) - mass fraction solute X, expressed in fractions of a unit; m(X) -- mass of solute X, g; m is the total mass of the solution, g.
If the mass fraction of dissolved sodium chloride in the solution is 0.03, or 3%, then this means that 100 g of the solution contains 3 g of sodium chloride and 97 g of water.
Volume fraction of a substance in a solution - the ratio of the volume of a solute to the sum of the volumes of all substances involved in the formation of a solution (before mixing)
c(X)= V(X)/?V
The molar fraction of a substance in a solution is the ratio of the amount of the substance to the sum of the amounts of all the substances in the solution.
h(X)=p(X)/ ?p
Of all types of fractions in analytical chemistry, the mass fraction is most often used. The volume fraction is usually used for solutions gaseous substances and liquids (in pharmacy for ethyl alcohol solutions) The numerical value is expressed in fractions of a unit and ranges from 0 (pure solvent) to 1 (pure substance. As you know, a hundredth of a unit is called a percentage. A percentage is not a unit of measurement, but a total only a synonym for the concept of “one hundredth.” For example, if the mass fraction of NaOH in a certain solution is 0.05, then instead of five hundredths, a value of 5% can be used. Percentages cannot be mass, volume or molar, but can only be calculated from mass, volume, or quantity of a substance.
The mass fraction can also be expressed as a percentage.
For example, a 10% sodium hydroxide solution contains 10 g of NaOH and 90 g of water in 100 g of a solution.
Cmas(X) = m(X)/tcm 100%.
Volume percentage - the percentage of the volume of a substance contained in the total volume of the mixture. Indicates the number of milliliters of the substance in 100 ml of the volume of the mixture.
Sob% \u003d V / Vcm * 100
The relationship between the volume and mass of the solution (t) is expressed by the formula
where c is the density of the solution, g/ml; V is the volume of the solution, ml.
The dimensional quantities used to describe the quantitative composition of solutions include the concentration of a substance in a solution (mass, molar) and the molality of a solute. If earlier any methods of describing the quantitative composition of a solution were called concentrations of a substance, then today this concept has become narrower.
Concentration is the ratio of the mass or amount of a solute to the volume of a solution. Thus, the mass fraction, according to the modern approach, is no longer a concentration and should not be called a percentage concentration.
Mass concentration is the ratio of the mass of a solute to the volume of a solution. This type of concentration is denoted as g (X), s (X) or not to be confused with the density of the solution, s * (X)
The mass concentration unit is kg/m3 or equivalently, g/l. The mass concentration, which has the dimension g / ml, is called the titer of the solution
Molar concentration - C (X) - is the ratio of the amount of a solute (mol) to the volume of a solution (1 l) Calculated as the ratio of the amount of substance p (X) contained in a solution to the volume of this solution V:
C(X) = n(X)/ Vp= m(X)/M(X)V
where m(X) is the mass of the dissolved substance, g; M(X) is the molar mass of the solute, g/mol. The molar concentration is expressed in mol/dm3 (mol/l). The most commonly used unit is mol/l. If 1 liter of a solution contains 1 mole of a solute, then the solution is called molar (1 M). If 1 liter of solution contains 0.1 mol or 0.01 mol of a solute, then the solution is respectively called decimolar (0.1 M), centimolar (0.01 M), 0.001 mol-millimolar (0.001M)
The unit of measurement of molar concentration is mol/m3, but in practice, a multiple of the unit, mol/l, is usually used. Instead of the designation “mol / l”, you can use “M” (and the word solution is no longer necessary to write) For example, 0.1 M NaOH means the same as C (NaOH) \u003d 0.1 mol / l
A mole is a unit of chemical quantity of a substance. A mole is a portion of a substance (i.e. such an amount) that contains as many structural units as there are atoms in 0.012 kg of carbon. 0.012 kg of carbon contains 6.02*1023 carbon atoms. And this portion is 1 mol. The same number of structural units is contained in 1 mole of any substance. that is, a mole is the amount of a substance containing 6.02 * 1023 particles. This value is called the Avogadro constant.
The chemical quantity of any substances contains the same number of structural units. But for each substance, its structural unit has its own mass. Therefore, the masses of the same chemical quantities of different substances will also be different.
Molar mass is the mass of a portion of a substance with a chemical quantity of 1 mole. It is equal to the ratio of the mass m of a substance to the corresponding amount of substance n
In the International System of Units, molar mass is expressed in kg/mol, but g/mol is more commonly used in chemistry.
It should be noted. That the molar mass numerically coincides with the masses of atoms and molecules (in amu) and with the relative atomic and molecular masses.
Unlike solids and liquids, all gaseous substances with a chemical quantity of 1 mol occupy the same volume (under the same conditions) This value is called the molar volume and is denoted
Because Since the volume of gas depends on temperature and pressure, then when performing calculations, the volumes of gases are taken under normal conditions (0? C and a pressure of 101.325 kPa). the ratio of the volume of any portion of gas to the chemical amount of gas is a constant value equal to 22.4 dm3/mol, i.e. Molar volume of any gas under normal conditions = 22.4 dm3/mol
Relationship between molar mass, molar volume and density (mass of a liter)
c= M/ Vm, g/dm3
The concept of molar concentration can refer to both the molecule or formula unit of a solute, and its equivalent. From a fundamental point of view, it doesn't matter what in question: about the concentration of sulfuric acid molecules - C (H2SO4) or "halves of sulfuric acid molecules" - C (1/2 H2SO4). The molar concentration of the equivalent of a substance used to be called the normal concentration. In addition, the molar concentration was often called molarity, although such a term is not recommended (it can be confused with molality)
The molality of a solute is the ratio of the amount of a substance in solution to the mass of the solvent. Designate molality as m(X), b(X), Cm(X):
Cm(X)= n(X)/mS
The unit of molality is mol/kg. Molality, according to modern terminology, is not a concentration. It is used in cases where the solution is under non-isothermal conditions. A change in temperature affects the volume of the solution and thereby leads to a change in concentration - while the molality remains constant.
For the quantitative characterization of standard solutions, the molar concentration (of a substance or equivalent of a substance) is usually used
Normality of solutions. Gram equivalent.
The concentration of solutions in titrimetric analysis is often expressed in terms of titer, i.e. indicate how many grams of a solute are contained in 1 ml of a solution. It is even more convenient to express it in terms of normality.
Normality is a number that indicates how many gram equivalents of a solute are contained in 1 liter of a solution.
The gram-equivalent (g-equiv) of a substance is the number of grams of it, chemically equivalent (equivalent) to one gram-atom of hydrogen in this reaction.
Cn \u003d peq / V; Cn = z n/V,
Where peq is the number of equivalents of the solute, peq = z n, V is the volume of the solution in liters, n is the number of moles of the solute, z is the effective valency of the solute
To find the gram equivalent, you need to write the reaction equation and calculate how many grams of a given substance corresponds to 1 gram of a hydrogen atom in it.
For example:
HCl + KOH KCl +H2O
One gram equivalent of an acid is equal to one gram molecule - a mole (36.46 g) of HCl, since it is this amount of acid that corresponds to one gram of hydrogen atom interacting with alkali hydroxyl ions during the reaction.
Accordingly, a gram-molecule of H2SO4 in the reactions:
H2SO4 + 2NaOH Na2SO4 + 2H2O
Corresponds to two grams of hydrogen atoms. Therefore, the gram equivalent of H2SO4 is? gram molecules (49.04 g).
Unlike a gram-molecule, a gram-atom, this number is not constant, but depends on the reaction in which the given substance is involved.
Since one gram-atom of OH- reacts with one gram-atom of H + and, therefore, is equivalent to the latter, the gram-equivalents of the bases are found similarly, but with the only difference that in this case they have to be divided by the number of gram-molecules participating in the reaction OH- ions.
Along with the gram equivalent in analytical chemistry, the concept of milligram equivalent is often used. A milligram equivalent (mg equivalent) is equal to a thousandth of a gram equivalent (E:1000) and is the equivalent weight of a substance expressed in milligrams. For example, 1 g-eq HCl is 36.46 g, and 1 meq HCl is 36.46 mg.
From the concept of an equivalent as a chemically equivalent quantity, it follows that the gram equivalents are precisely those weight quantities with which they react with each other.
It is obvious that 1 mg-eq of these substances, which is 0.001 g-eq, is in 1 ml of one-normal solutions of these substances. Therefore, the normality of a solution shows how many gram equivalents of a substance are contained in 1 liter or how many milligram equivalents of it are contained in 1 ml of a solution. The normality of solutions is denoted by the letter n. If 1 liter of solution contains 1 g-eq. substances, then such a solution is called 1 normal (1 n), 2 g-eq - two-normal (2 n), 0.5 g-eq - semi-normal, 0.1 g-eq - decinormal (0.1n), 0.01 g-eq - centinormal, 0.001 g-equiv - millinormal (0.001n). Of course, the normality of the solution, in addition, shows the number of milligram equivalents of the solute in 1 ml of the solution. For example, 1n solution contains 1 mEq, and 0.5 n - 0.5 mEq of a solute per 1 ml. The preparation of normal solutions requires the ability to calculate the gram equivalents of an acid, base or salt.
Gram-equivalent is the number of grams of a substance that is chemically equivalent (i.e. equivalent) to one gram-atom or gram-ion of hydrogen in a given reaction.
Np: HCl + NaOH= NaCl+H2O
It can be seen that one HCl gram molecule participates in the reaction with one H+ gram ion interacting with the OH- ion. Obviously, in this case, the gram equivalent of HCl is equal to its gram molecule and is 36.46 g. However, the gram equivalent of acids, bases and salts depends on the course of the reactions in which they participate. To calculate them, in each case, an equation is written and it is determined how many grams of the substance correspond to 1 gram-atom of hydrogen in this reaction. H-P, molecules of phosphoric acid H3PO4, participating in the reaction
H3PO4 + NaOH=NaH2PO4+ H2O
Gives only one H + ion and its gram equivalent is equal to a gram molecule (98.0 g). In the reaction
H3PO4 + 2NaOH = Na2HPO4+ 2H2O
each molecule corresponds to two grams of hydrogen ions. Therefore, gram-equiv. Is she equal? gram molecules, i.e. 98:2=49g
Finally, the H3PO4 molecule can also participate in the reaction with three hydrogen ions:
H3PO4 + 3NaOH=Na3PO4+ 3H2O
it is clear that in this reaction the H3PO4 gram molecule is equivalent to three H+ gram ions and the gram equivalent of the acid is 1/3 of the gram molecule, i.e. 98:3=32.67g
Gram-equiv-you bases also depend on the nature of the reaction. When calculating the gram equivalent of a base, one usually divides its gram molecule by the number of OH- ions participating in the reaction, because one OH- gram ion is equivalent to one H+ gram ion, Therefore, based on the equations
The order of conversion from one type of concentration to another. Calculations using molar concentration
In most cases, when calculating using the molar concentration, one proceeds from the proportions relating the molar concentration and the molar mass
Where C (X) is the concentration of the solution in mol / l; M is the molar mass, g / mol; m(X)/ is the mass of the solute in grams, p(X) is the amount of the solute in moles, Vp is the volume of the solution in liters. Example, calculate the molar concentration of 2 liters of 80 g of NaOH.
C(X) = m(X)/M Vp; M = 40 g/mol; C (X) \u003d 80g / 40g / mol * 2l \u003d 1 mol / l
Calculations using normality
Where Sp is the concentration of the solution in mol / l; M-molar mass, g/mol; m(X)/ is the mass of the solute in grams, p(X) is the amount of the solute in moles, Vp is the volume of the solution in liters.
Concentration of solutions and methods of its expression (Chemical analysis in thermal power engineering, Moscow. MPEI Publishing House, 2008)
The quantitative ratios between the masses of reacting substances are expressed by the law of equivalents. Chemical elements and their compounds enter into chemical reactions with each other in strictly defined mass quantities corresponding to their chemical equivalents.
Let the following reaction take place in the system:
aX+ b Y > Reaction products.
The reaction equation can also be written as
X + b/a Y > Reaction products,
which means that one particle of substance X is equivalent to b/a particles of substance Y.
Attitude
Equivalence factor, a dimensionless value not exceeding 1. Its use as a fractional value is not very convenient. More often, the reciprocal of the equivalence factor is used - the equivalence number (or equivalent number) z;
The value of z is determined by the chemical reaction in which a given substance participates.
There are two definitions of the equivalent:
- 1. An equivalent is a certain real or conditional particle that can attach, release, or in some other way be equivalent to one hydrogen ion in acid-base reactions or one electron in redox reactions.
- 2. Equivalent - a conditional particle of a substance, z times smaller than its corresponding formula unit. Formula units in chemistry are actually existing particles, such as atoms, molecules, ions, radicals, conditional molecules of crystalline substances and polymers.
The unit of the amount of substance equivalents is mole or mmol (previously g-eq or mg-eq). The value required for calculations is the molar mass of the equivalent of the substance Meq (Y), g / mol, equal to the ratio of the mass of the substance mY to the amount of substance equivalents neq (Y):
Meq(Y) = mY / neq(Y)
since neq
Consequently
Meq(Y) =MY / zY
where MY is the molar mass of substance Y, g/mol; nY is the amount of substance Y, mol; zY is the equivalence number.
The concentration of a substance is a physical quantity (dimensional or dimensionless) that determines the quantitative composition of a solution, mixture or melt. Various methods are used to express the concentration of a solution.
Molar concentration of substance B or concentration of the amount of substance - the ratio of the amount of dissolved substance B to the volume of the solution, mol / dm3,
St = nv / Vp = mv / Mv Vp
where nv is the amount of substance, mol; Vp is the volume of the solution, dm3; MB -- molar mass of the substance, g/mol; mB is the mass of the solute, g.
The abbreviated form of the molar concentration unit M = mol/dm3 is convenient to use.
Molar concentration of equivalents of substance B - the ratio of the number of equivalents of substance B to the volume of the solution, mol / dm3? n:
Seq (V) \u003d n equiv (V) / Vp \u003d mv / Mv Vp \u003d mv zv / Mv Vp
where neq is the amount of substance equivalents, mole; Meq -- molar mass of substance equivalents, g/mol; zB is an equivalence number.
The use of the terms "normality" and "normal concentration" and units of measurement g-eq/dm3, mg-eq/dm3 is not recommended, as well as the symbol N, for the abbreviated designation of the molar concentration of substance equivalents.
Mass concentration of substance B - the ratio of the mass of the dissolved substance B to the volume of the solution, g / dm3,
Mass fraction of solute B is the ratio of the mass of solute B to the mass of the solution:
Sv = mv / mr = mv / s Vp
where mr is the mass of the solution, g; c is the density of the solution, g/cm3.
The use of the term "percent concentration" is not recommended.
The molar fraction of a solute B is the ratio of the amount of this substance to the total amount of all substances that make up the solution, including the solvent,
XV= nV / ? ni, ? ni = nВ + n1 + n2 +.....+ ni
The molality of substance B in solution is the amount of solute B contained in 1 kg of solvent, mol / kg,
Cm \u003d nv / ms \u003d mv / Mv ms
where ms is the mass of the solvent, kg.
Titer - The titer of a solution of substance B is the concentration of a standard solution, equal to the mass substance B contained in 1 cm3 of solution, g/cm3,
At present, the use of many terms is not recommended, but in the practice of water treatment and in production, specialists use these terms and units of measurement, therefore, in order to eliminate discrepancies, the usual terms and units of measurement will be used in the future, and new terminology will be indicated in brackets.
According to the law of equivalents, substances react in equivalent quantities:
neq (X) = neq (Y), and neq (X) = Seq (X) Vx and neq (Y) = Seq (Y) Vy
therefore, one can write
Seq (X) Vx = Seq (Y) Vy
where neqv(X) and neqv(Y) -- the amount of substance equivalents, mol; Seq (X) and Seq (Y) - normal concentrations, g-eq / dm3 (molar concentrations of substance equivalents, mol / dm3); VX and VY are volumes of reacting solutions, dm3.
Let us assume that it is necessary to determine the concentration of a solution of a titrated substance X-- Ceq(X). To do this, accurately measure an aliquot of this VX solution. Then, a titration reaction is carried out with a solution of substance Y with a concentration of Ceq (Y) and note how much solution is used for titration of VY - titrant. Further, according to the law of equivalents, we can calculate the unknown concentration of a solution of substance X:
Equilibrium in solutions. True solutions and suspensions. Equilibrium in the "precipitate - saturated solution" system. Chemical equilibrium
Chemical reactions can proceed in such a way that the substances taken are completely converted into reaction products - as they say, the reaction goes to the end. Such reactions are called irreversible. An example of an irreversible reaction is the decomposition of hydrogen peroxide:
2H2O2 = 2H2O + O2 ^
Reversible reactions proceed simultaneously in 2 opposite directions. because the products obtained as a result of the reaction interact with each other to form the starting substances. For example: when iodine vapor interacts with hydrogen at 300 ° C, hydrogen iodide is formed:
However, at 300?C, hydrogen iodide decomposes:
Both reactions can be expressed in one general equation, replacing the equal sign with the reversibility sign:
The reaction between the starting substances is called a direct reaction, and its rate depends on the concentration of the starting substances. A chemical reaction between products is called a reverse reaction, and its rate depends on the concentration of the starting substances. A chemical reaction between products is called a reverse reaction, and its rate depends on the concentration of the substances obtained. At the beginning of a reversible process, the rate of the forward reaction is maximum, and the rate of the reverse is zero. As the process proceeds, the rate of the direct reaction decreases, because the concentration of the taken substances decreases, and the rate of the reverse reaction increases, as the concentration of the obtained substances increases. When the rates of both reactions become equal, a state called chemical equilibrium sets in. In chemical equilibrium, neither the forward nor the reverse reactions stop; they both move at the same speed. Therefore, the chemical equilibrium is a mobile, dynamic equilibrium. The state of chemical equilibrium is influenced by the concentration of reacting substances, temperature, and for gaseous substances - pressure in the system.
By changing these conditions, it is possible to shift the equilibrium to the right (in this case, the product yield will increase) or to the left. Offset chem. equilibrium obeys the Le Chatelier principle:
Under steady state equilibrium, the product of the concentrations of the reaction products divided by the product of the concentrations of the starting materials (for a given reaction, T=const) is a constant value called the equilibrium constant.
When external conditions change, the chemical equilibrium shifts in the direction of the reaction that weakens this external influence. So, with an increase in the concentration of reacting substances, the equilibrium shifts towards the formation of reaction products. The introduction of additional amounts of any of the reactants into the equilibrium system accelerates the reaction in which it is consumed. An increase in the concentration of the starting substances shifts the equilibrium towards the formation of reaction products. An increase in the concentration of reaction products shifts the equilibrium towards the formation of starting materials.
Reactions occurring in the process of chemical analysis. Types of reactions. Characteristic. Types chemical reactions
Chemical reactions can be classified into four main types:
decomposition |
connections |
substitution |
|
Decomposition reaction- is called such a chem. reaction, in a cat. from one complex thing-va it turns out two or more. simple or complex substances: 2H2O > 2H2^ +O2^3 |
A compound reaction is such a reaction, in the result of which one more complex substance is formed from two or more simple or complex substances: |
A substitution reaction is a reaction that occurs between simple and complex substances, with a cat. atoms is simple. things replace the atoms of one of the elements in a complex substance: Fe+CuCl2> Cu+FeCl2 Zn+CuCl2>ZnCl2+Cu |
An exchange reaction is a reaction in which two complex substances exchanges its constituent parts, forming two new substances: NaCl+AgNO3=AgCl+NaNO3 |
According to the release and absorption of energy, chemical reactions are divided into exothermic, going with the release of heat in environment and endothermic, going with the absorption of heat from the environment
The science of methods for analyzing the composition of an analyte, (in a broad sense) and methods for a comprehensive chemical study of substances that surround us on Earth is called analytical chemistry. The subject of analytical chemistry is the theory and practice of various methods of analysis. The analysis of a substance is carried out in order to establish its qualitative or quantitative chemical composition.
The task of qualitative analysis is the discovery of elements, sometimes compounds that make up the substance under study. Quantitative analysis makes it possible to determine the quantitative ratio of these components.
In a qualitative analysis, to establish the composition of the analyte, other substances are added to it, causing such chemical transformations, which are accompanied by the formation of new compounds with specific properties:
- - certain physical condition(precipitate, liquid, gas)
- - known solubility in water, acids, alkalis and other solvents
- - characteristic color
- - crystalline or amorphous structure
- - smell
Qualitative analysis in the study of the composition of an unknown substance always precedes quantitative, because. choice of quantitation method constituent parts of the analyte depends on the data obtained by qualitative analysis. The results of a qualitative analysis do not make it possible to judge the properties of the materials under study, since the properties are determined not only by what parts the object under study consists of, but also by their quantitative ratio. When starting a quantitative analysis, it is necessary to know exactly the qualitative composition of the substance under study; knowing the qualitative composition of the substance and the approximate content of the components, it is possible to choose the right method for the quantitative determination of the element of interest to us.
In practice, the task facing the analyst is usually greatly simplified due to the fact that the qualitative composition of most of the studied materials is well known.
Methods of quantitative analysis
Methods of quantitative analysis, depending on the nature of the experimental technique used for the final determination of the constituent parts of the analyte, are divided into 3 groups:
- - chemical
- - physical
- - physico-chemical (instrumental)
Physical methods - methods of analysis with which you can determine the composition of the substance under study, without resorting to the use of chemical reactions. Physical methods include:
- - spectral analysis - based on studies of emission spectra (or emission and absorption of the substances under study)
- - luminescent (fluorescent) - analysis based on the observation of luminescence (glow) of the analyzed substances, caused by the action of ultraviolet rays
- - X-ray diffraction-based on the use x-rays to study the structure of matter
- - mass spectrometric analysis
- - methods based on measuring the density of the studied compounds
Physico-chemical methods are based on the study physical phenomena that occur during chemical reactions, accompanied by a change in the color of the solution, color intensity (colorimetry), electrical conductivity (conductometry)
Chemical methods are based on the use chemical properties elements or ions.
Chemical |
Physico-chemical |
||
Gravimetric |
Titrimetric |
colorimetric |
Electrochemical |
The method of quantitative analysis consists in the exact measurement of the mass of the analyzed component of the sample, isolated in the form of a compound of known composition or in the form of an element. The classical name of the weight method |
The method of quantitative analysis is based on measuring the volume (or mass) of a solution of a reagent of known concentration, consumed for the reaction with the analyte. They are divided according to the type of reactions into 4 methods:
|
Method of quantitative analysis based on the assessment of the color intensity of the solution (visually or with the help of appropriate instruments). Photometric determination is possible only if the color of the solutions is not too intense, therefore, highly diluted solutions are used for such measurements. In practice, photometric determinations are especially often used when the content of the corresponding element in the object under study is low and when the methods of gravimetric and titrimetric analysis are unsuitable. The rapidity of the determination contributes to the widespread use of the photometric method. |
The method of quantitative analysis, it retains the usual principle of titrimetric determinations, but the moment of completion of the corresponding reaction is set by measuring the electrical conductivity of the solution (conductometric method), or by measuring the potential of one or another electrode immersed in the test solution (potentiometric method) |
In quantitative analysis, macro-, micro- and semi-micro methods are distinguished.
In macroanalysis, relatively large (about 0.1 g or more) samples of the investigated solid or large volumes of solutions (several tens of milliliters or more) are taken. The main working tool in this method is an analytical balance, which allows weighing with an accuracy of 0.0001-0.0002 g, depending on the design of the balance (i.e., 0.1-0.2 mg).
In micro- and semi-micro methods of quantitative analysis, weighings from 1 to 50 mg and solution volumes from tenths of a milliliter to several milliliters are used. for these methods, more sensitive balances are used, such as microbalances (weighing accuracy up to 0.001 mg), as well as more accurate equipment for measuring the volumes of solutions.
Volumetric analysis, essence and characteristics of the method. The concept of titration, titre. General titration techniques, titer setting methods
Titrimetric (volumetric) analysis Essence of analysis.
Titrimetric analysis offers a huge advantage over gravimetric analysis in terms of speed. In titrimetric analysis, the volume of a reagent solution consumed for the reaction is measured, the concentration (or titer) of which is always exactly known. A titer is usually understood as the number of grams or milligrams of a solute contained in 1 ml of a solution. Thus, in titrimetric analysis, quantification chemical substances It is most often carried out by accurately measuring the volumes of solutions of two substances that react with each other.
In the analysis, a titrated reagent solution is placed in a measuring vessel called a burette, and it is gradually poured into the test solution until it is established in one way or another that the spent amount of the reagent is equivalent to the amount of the analyte. This operation is called titration.
A titratable substance is a substance whose solution concentration is to be determined. In this case, the volume of the solution of the titratable substance must be known.
A titrant is a solution of a reagent used for titration, the concentration of which is known with high accuracy. It is often referred to as a standard (working) or titrated solution.
The solution can be prepared in several ways:
- - according to the exact weight of the starting substance (only chemically pure stable compounds, the composition of which strictly corresponds to the chemical formula, as well as easily cleaned substances, can be used as starting substances);
- - according to fixanal (according to a strictly defined amount of a substance, usually 0.1 mol or its fraction, placed in a glass ampoule);
- - by an approximate sample with subsequent determination of the concentration according to the primary standard (it is necessary to have a primary standard - a chemically pure substance of exactly known composition that meets the relevant requirements);
- - by diluting a previously prepared solution with a known concentration.
Titration is the main method of titrimetric analysis, which consists in the gradual addition of a reagent solution of known concentration from a burette (titrant) to the analyzed solution until the equivalence point is reached. Often fixing the equivalence point. It is possible due to the fact that the colored reagent changes its color during the reaction (during oxidizability titration). Or substances are added to the test solution that undergo any change during titration and thereby allow fixing the equivalence point, these substances are called indicators. The main characteristic of indicators is considered to be not the value of the end point of the titration, but the interval of the indicator color transition. The color change of the indicator becomes noticeable to the human eye not at a specific pT value,
Transition interval of acid-base indicators
Indicator |
transition, pH |
acid form |
Main form |
||
Alizarin yellow |
purple |
||||
thymolphthalein |
Colorless |
||||
Phenolphthalein |
Colorless |
||||
Cresol purple |
Purple |
||||
Phenol red |
|||||
Bromothymol blue |
|||||
methyl red |
|||||
methyl orange |
|||||
Bromophenol blue |
However, even if indicators are available, their use is not always possible. In general, strongly colored or cloudy solutions should not be titrated with indicators, as the color change of the indicator becomes difficult to distinguish.
In such cases, the equivalence point is sometimes fixed by changing some physical properties solution during titration. Electrotitrimetric methods of analysis are based on this principle. For example, the conductometric method, in which the equivalence point is found by measuring the electrical conductivity of the solution; potentiometric method based on measuring the redox potential of a solution (potentiometric titration method).
In addition, it is necessary that the added titrated reagent solution be used exclusively for the reaction with the analyte, i.e. during the titration, no side reactions should occur that make an accurate calculation of the results of the analysis impossible. In the same way, the absence of substances in the solution that interfere with the course of the reaction or prevent the fixation of the equivalence point is necessary.
As a reaction, you can use only those chemical interactions between a titrated substance and a titrant that meet the following requirements:
- 1) the reaction must be strictly stoichiometric, i.e. chemical composition titratable substance, titrant and reaction products must be strictly defined and unchanged;
- 2) the reaction must proceed quickly, since changes can occur in the solution for a long time (due to competing reactions), the nature and influence of which on the main titration reaction is quite difficult to predict and take into account;
- 3) the reaction must proceed quantitatively (if possible completely), i.e. the equilibrium constant of the titration reaction should be as high as possible;
- 4) there must be a way to determine the end of the reaction. .
In titrimetry, the following titration options are distinguished:
- - direct titration method. The titrant is directly added to the substance to be titrated. This method is used if all the requirements for the titration reaction are met;
- - back titration method. A known excess of titrant is added to the substance to be titrated, the reaction is brought to completion, and then the excess of unreacted titrant is titrated with another titrant, i.e. the titrant used in the first part of the experiment is itself converted into the titratable substance in the second part of the experiment. This method is used if the reaction rate is low, it is not possible to select an indicator, side effects are observed (for example, losses of the analyte due to its volatility), or the reaction is not stoichiometric; - method of indirect titration by substituent. A stoichiometric reaction of the titratable compound with another reagent is carried out, and the new compound resulting from this reaction is titrated with a suitable titrant. The method is used if the reaction is non-stoichiometric or occurs slowly.
Solubility depends on the nature of the solute and solvent, temperature and pressure.
1)The nature of the solute.
Fig.8. The nature of the solute.
Crystalline substances are divided into:
P - highly soluble (more than 1.0 g per 100 g of water);
M - slightly soluble (0.1 g - 1.0 g per 100 g of water);
H - insoluble (less than 0.1 g per 100 g of water).
Table 1.
Table of solubility of acids, bases and salts in water
(in grams per 100g of water at 20C)
H+ | NH4+ | K+ | Na+ | Ag+ | Ba 2+ | Ca2+ | Sr2+ | Mg2+ | Zn2+ | Cu2+ | Hg2+ | Pb 2+ | Fe2+ | sn 2+ | Mn2+ | Bi 3+ | Fe3+ | Al 3+ | |
oh- | R | R | R | - | P | M | M | H | H | H | - | H | H | H | H | H | H | H | |
Cl- | R | R | R | R | H | R | R | R | R | R | R | R | H | R | R | R | - | R | R |
br- | R | R | R | R | H | R | R | R | R | R | R | R | H | R | R | R | - | R | R |
I- | R | R | R | R | H | R | R | R | R | R | R | R | H | R | R | R | - | R | R |
NO 3 - | R | R | R | R | R | R | R | R | R | R | R | R | R | R | - | R | R | R | R |
S2- | R | R | R | R | H | R | M | R | R | H | H | H | H | H | H | H | H | - | - |
SO 3 2- | R | R | R | R | M | H | H | H | H | H | H | H | H | H | - | H | H | - | - |
SO 4 2- | R | R | R | R | M | H | M | H | R | R | R | R | H | R | R | R | - | R | R |
CO 3 2- | R | R | R | R | H | H | H | H | H | H | H | H | H | H | - | H | H | - | - |
PO 4 3- | R | R | R | R | H | H | H | H | M | H | H | H | H | H | H | H | H | M | H |
SiO 3 2- | H | - | R | R | - | H | H | H | H | H | H | - | H | H | - | H | - | H | H |
CH 3 COO - | R | R | R | R | R | R | R | R | R | R | R | R | R | R | - | R | - | R | M |
2) The nature of the solvent. When a solution is formed, the bonds between the particles of each of the components are replaced by bonds between the particles of different components. In order for new bonds to form, the components of the solution must have bonds of the same type, i.e. be of the same nature. Therefore, ionic substances dissolve in polar solvents and poorly in non-polar, and
molecular substances are the opposite.
Water is a unique, most common and available solvent.
Rice. 9. Dependence of solubility
substances from the nature of solubility.
Aqueous solutions of inorganic (salts, acids, bases) and organic (amino acids, nitrogenous bases, nucleic acids) substances, low- and high-molecular compounds, electrolytes and non-electrolytes, form the basis of the most important biological fluids in which all physico-chemical processes occur that ensure the vital activity of the body. For example, in medicine, artificial analogues of biological fluids are used - colloidal and saline blood substitute solutions.
Solutions of substances with a molar mass of less than 5000 g/mol are called solutions of low molecular weight compounds (NMS), and solutions of substances with a molar mass of more than 5000 g/mol are called solutions of high molecular weight compounds (HMC).
Solutions of low-molecular compounds (electrolytes and non-electrolytes) are called true, in contrast to colloidal solutions. True solutions are characterized by a homogeneous composition and the absence of an interface between the solute and the solvent. The size of dissolved particles (ions and molecules) is less than 10 -9 m.
Most IUDs are polymers whose molecules (macromolecules) are composed of a large number repeating groupings or monomeric units interconnected by chemical bonds. IUD solutions are called polyelectrolyte solutions. Polyelectrolytes include polyacids (heparin, polyadenylic acid, polyaspartic acid, etc.), polybases (polylysine), polyampholytes (proteins, nucleic acids).
The properties of HMS solutions differ significantly from those of NMS solutions.
3)The effect of temperature. If the dissolution of a substance is an exothermic process, then its solubility decreases with increasing temperature (for example, Ca (OH) 2 in water) and vice versa. Most salts are characterized by an increase in solubility when heated. Almost all gases dissolve with the release of heat. The solubility of gases in liquids decreases with increasing temperature and increases with decreasing temperature.
Rice. 10. Dependency Graph
solubility of gases on temperature.
4) Pressure influence. With increasing pressure, the solubility of gases in liquids increases, and with decreasing pressure, it decreases.
Fig.11. dependency graph
gas solubility on pressure
1) NATURE OF MIXED SUBSTANCES. We have already seen that in substances with polar molecules (especially those with hydrogen bonds) and in ionic substances there is a strong mutual attraction of particles. Therefore, such substances will not be easily crushed (mixed with others) if there is no strong attraction between particles of different substances in the solution, i.e. a large value of ΔH 1 must be completely or almost completely compensated by a negative value of ΔH 2 . Hence it follows that substances with ionic bonds or with polar molecules should dissolve much better in polar or ionic solvents than in solvents with non-polar molecules. Accordingly, substances with non-polar molecules dissolve better in non-polar solvents and worse - in polar ones, and metals - in metals. This rule was formulated by the alchemists: like dissolves into like. Do not confuse polar bonds with polar molecules. The C-Cl bond is polar, but in the CCl 4 molecule these bonds are arranged symmetrically and their dipole moments add up to zero, so the molecule as a whole is non-polar. The water molecule is polar only because it is angular. If it were linear, like CO 2 , it would be non-polar, Tbp. would be much lower.
Thus, if there is a greasy stain on the clothes, it is better to wash it off not with water, but with gasoline, CCl 4 or another non-polar solvent, and if the stain is from salt or sugar, then it is better to wash it off with water, not gasoline. Similarly, in metallurgy: metals in a liquid state usually dissolve each other well and poorly dissolve substances with an ionic bond (own oxides, phosphates, silicates, fluorides), which form a separate liquid phase - slag.
2) TEMPERATURE. Here, as in any other equilibrium, Le Chatelier's principle applies. When heated, the solubility increases if ΔHsol > 0 (and the stronger, the larger ΔH), and decreases if ΔHsol< 0. Для твердых веществ более характерно первое, а для газов - второе, хотя бывает и наоборот. Это особенно наглядно в случае солей, образующих кристаллогидраты. При растворении кристаллогидрата в воде не может быть сильной гидратации, поскольку вещество уже гидратировано. Поэтому преобладает первое слагаемое, и ΔHраств >0. If we take the same salt in an anhydrous form, but we know that it is capable of producing a crystalline hydrate, then we can expect that the second term prevails in it, and ΔHsol< 0. Поэтому графики зависимости растворимости от температуры у кристаллогидрата и безводной соли часто имеют противоположный наклон.
Thus, more often when dissolving solid or liquid substances in liquids, the solubility increases with increasing temperature, and for gases it decreases.
3) PRESSURE. As already discussed, pressure mainly affects processes involving gases.
The mass of a gas that dissolves at a constant temperature in a given volume of liquid is directly proportional to the partial pressure of the gas.
it Henry's law. It can be expressed by the equation:
ω B = k B p B
where ω В is the mass fraction of gas in a saturated solution, р В is the partial pressure of the gas above the solution, k В is the proportionality coefficient, called Henry's constant characterizing the solubility of a given gas in a given solvent.
It is valid only for dilute solutions, at not very high pressures, and provided that there is neither dissociation nor association during dissolution (otherwise the reaction equation will change). For example, for HCl in water it is not applicable, but for O 2 , N 2 , NO it is applicable. According to the equation of state of a gas, its volume is inversely proportional to its pressure. Therefore, the volume of gas that can be dissolved in a given amount of solvent, according to Henry's law, does not depend on pressure. We can say: 31 ml of oxygen dissolves in 1 liter of water at 20 ° C, without indicating the pressure. If you increase the pressure, then the number of oxygen molecules in the solution will increase, but the volume of dissolved gas will be the same.
Everyone who opened a bottle of lemonade, beer or champagne saw the dependence of the solubility of gases on pressure. There is increased pressure inside the bottle, and carbon dioxide is in solution. When opening, the pressure drops, the gas mixes with air, and the partial pressure of CO 2 drops even more. The solution becomes supersaturated and gas bubbles are released from it.
4) THE PRESENCE OF A THIRD SUBSTANCE. Its influence can be varied. The most important cases:
a) this substance is highly solvated, binds many solvent molecules and thereby reduces solubility; example: alcohol in relation to salt solutions;
b) this substance binds molecules or ions of the dissolved substance and thereby increases solubility; example: ammonia, which binds copper ions and increases the solubility of Cu(OH) 2 ;
c) this substance gives ions of the same name with the ions of the dissolved substance, and thereby shifts the equilibrium of dissolution to the left; example: in a saturated solution of CaSO 4 there is an equilibrium CaSO 4 (tv) \u003d Ca 2+ (solution) + SO 4 2- (solution). By adding a strong solution of calcium chloride, we increase the concentration of calcium ions, and part of the sulfate falls out.
When HCl(g) is added to a saturated NaCl solution, causes (a) and (c) act.