The mechanism of dissociation of electrolytes with ionic bonds. Chemistry lesson on the topic "electrolytic dissociation". Degree of electrolytic dissociation

Conductivity of substances electric current or lack of conductivity can be observed using a simple instrument.

It consists of carbon rods (electrodes) connected by wires to an electrical network. An electric light is included in the circuit, which indicates the presence or absence of current in the circuit. If you dip the electrodes in a sugar solution, the light bulb does not light up. But it will light up brightly if they are dipped in a sodium chloride solution.

Substances that disintegrate into ions in solutions or melts and therefore conduct electric current are called electrolytes.

Substances that, under the same conditions, do not disintegrate into ions and do not conduct electric current are called nonelectrolytes.

Electrolytes include acids, bases and almost all salts.

Non-electrolytes include most organic compounds, as well as substances whose molecules contain only covalent non-polar or low-polar bonds.

Electrolytes are conductors of the second kind. In a solution or melt, they break up into ions, which is why current flows. Obviously, the more ions in a solution, the better it conducts electric current. Pure water conducts electricity very poorly.

There are strong and weak electrolytes.

Strong electrolytes, when dissolved, completely dissociate into ions.

These include:

1) almost all salts;

2) many mineral acids, for example H 2 SO 4, HNO 3, HCl, HBr, HI, HMnO 4, HClO 3, HClO 4;

3) alkaline bases and alkaline earth metals.

Weak electrolytes When dissolved in water, they only partially dissociate into ions.

These include:

1) almost all organic acids;

2) some mineral acids, for example H 2 CO 3, H 2 S, HNO 2, HClO, H 2 SiO 3;

3) many metal bases (except alkali and alkaline earth metal bases), as well as NH 4 OH, which can be represented as ammonia hydrate NH 3 ∙H 2 O.

Water is a weak electrolyte.

Weak electrolytes cannot produce a high concentration of ions in solution.

Basic principles of the theory of electrolytic dissociation.

The breakdown of electrolytes into ions when dissolved in water is called electrolytic dissociation.

Thus, sodium chloride NaCl, when dissolved in water, completely decomposes into sodium ions Na + and chloride ions Cl -.

Water forms hydrogen ions H + and hydroxide ions OH - only in very small quantities.

To explain the characteristics of aqueous solutions of electrolytes, the Swedish scientist S. Arrhenius proposed the theory of electrolytic dissociation in 1887. Subsequently, it was developed by many scientists on the basis of the doctrine of the structure of atoms and chemical bonds.

The modern content of this theory can be reduced to the following three provisions:

1. Electrolytes, when dissolved in water, break up (dissociate) into ions - positive and negative.

Ions are in more stable electronic states than atoms. They can consist of one atom - these are simple ions (Na +, Mg 2+, Al 3+, etc.) - or of several atoms - these are complex ions (NO 3 -, SO 2- 4, PO Z- 4 etc.).

2. Under the influence of an electric current, ions acquire directional movement: positively charged ions move towards the cathode, negatively charged ions move towards the anode. Therefore, the former are called cations, the latter - anions.

The directional movement of ions occurs as a result of their attraction by oppositely charged electrodes.

3. Dissociation is a reversible process: in parallel with the disintegration of molecules into ions (dissociation), the process of combining ions (association) occurs.

Therefore, in the equations of electrolytic dissociation, instead of the equal sign, the reversibility sign is used. For example, the equation for the dissociation of the electrolyte molecule KA into the cation K + and the anion A - in general view is written like this:

KA ↔ K + + A -

The theory of electrolytic dissociation is one of the main theories in inorganic chemistry and is fully consistent with atomic-molecular science and the theory of atomic structure.

Degree of dissociation.

One of the most important concepts of Arrhenius's theory of electrolytic dissociation is the concept of the degree of dissociation.

The degree of dissociation (a) is the ratio of the number of molecules dissociated into ions (n") to the total number of dissolved molecules (n):

The degree of electrolyte dissociation is determined experimentally and is expressed in fractions of a unit or as a percentage. If α = 0, then there is no dissociation, and if α = 1 or 100%, then the electrolyte completely disintegrates into ions. If α = 20%, then this means that out of 100 molecules of a given electrolyte, 20 have broken up into ions.

Various electrolytes have varying degrees dissociation. Experience shows that it depends on the electrolyte concentration and temperature. With a decrease in electrolyte concentration, i.e. When diluted with water, the degree of dissociation always increases. As a rule, the degree of dissociation and temperature increase increase. Based on the degree of dissociation, electrolytes are divided into strong and weak.

Let us consider the shift in equilibrium established between undissociated molecules and ions during the electrolytic dissociation of a weak electrolyte - acetic acid:

CH 3 COOH ↔ CH 3 COO - + H +

When a solution of acetic acid is diluted with water, the equilibrium will shift towards the formation of ions, and the degree of dissociation of the acid increases. On the contrary, when a solution is evaporated, the equilibrium shifts towards the formation of acid molecules - the degree of dissociation decreases.

From this expression it is obvious that α can vary from 0 (no dissociation) to 1 (complete dissociation). The degree of dissociation is often expressed as a percentage. The degree of electrolyte dissociation can only be determined experimentally, for example, by measuring the freezing point of the solution, by the electrical conductivity of the solution, etc.

Dissociation mechanism

Substances with ionic bonds dissociate most easily. As you know, these substances consist of ions. When they dissolve, the water dipoles are oriented around the positive and negative ions. Forces arise between ions and water dipoles mutual attraction. As a result, the bond between the ions weakens, and the ions move from the crystal to the solution. In this case, hydrated ions are formed, i.e. ions chemically bonded to water molecules.

Electrolytes, whose molecules are formed according to the type of polar covalent bond (polar molecules), dissociate similarly. Around each polar molecule of a substance, water dipoles are also oriented, which are attracted by their negative poles to the positive pole of the molecule, and by their positive poles - to the negative pole. As a result of this interaction, the connecting electron cloud (electron pair) is completely shifted towards the atom with higher electronegativity, the polar molecule turns into an ionic one and then hydrated ions are easily formed:

Dissociation of polar molecules can be complete or partial.

Thus, electrolytes are compounds with ionic or polar bonds - salts, acids and bases. And they can dissociate into ions in polar solvents.

Dissociation constant.

Dissociation constant. A more accurate characteristic of electrolyte dissociation is the dissociation constant, which does not depend on the concentration of the solution.

The expression for the dissociation constant can be obtained by writing the equation for the dissociation reaction of the AA electrolyte in general form:

A K → A - + K + .

Since dissociation is a reversible equilibrium process, the law of mass action is applied to this reaction, and the equilibrium constant can be defined as:

where K is the dissociation constant, which depends on the temperature and nature of the electrolyte and solvent, but does not depend on the concentration of the electrolyte.

The range of equilibrium constants for different reactions is very large - from 10 -16 to 10 15. For example, high value TO for reaction

means that if metallic copper is added to a solution containing silver ions Ag +, then at the moment equilibrium is reached, the concentration of copper ions is much greater than the square of the concentration of silver ions 2. Against, low value TO in reaction

indicates that by the time equilibrium was reached, a negligible amount of silver iodide AgI had dissolved.

Please pay Special attention on the form of writing expressions for the equilibrium constant. If the concentrations of some reactants do not change significantly during the reaction, then they are not written into the expression for the equilibrium constant (such constants are denoted K 1).

So, for the reaction of copper with silver the expression will be incorrect:

The correct form would be:

This is explained by the fact that the concentrations of metallic copper and silver are introduced into the equilibrium constant. Copper and silver concentrations are determined by their densities and cannot be changed. Therefore, there is no point in taking these concentrations into account when calculating the equilibrium constant.

The expressions for the equilibrium constants when dissolving AgCl and AgI are explained in a similar way

Product of solubility. The dissociation constants of poorly soluble metal salts and hydroxides are called the product of solubility of the corresponding substances (denoted PR).

For the water dissociation reaction

the constant expression will be:

This is explained by the fact that the concentration of water during reactions in aqueous solutions changes very little. Therefore, it is assumed that the concentration of [H 2 O] remains constant and is introduced into the equilibrium constant.

Acids, bases and salts from the standpoint of electrolytic dissociation.

Using the theory of electrolytic dissociation, they define and describe the properties of acids, bases and salts.

Acids are electrolytes whose dissociation produces only hydrogen cations as cations.

For example:

НCl ↔ Н + + С l - ;

CH 3 COOH ↔ H + + CH 3 COO -

The dissociation of a polybasic acid occurs mainly through the first step, to a lesser extent through the second, and only to a small extent through the third. Therefore, in an aqueous solution of, for example, phosphoric acid, along with H 3 PO 4 molecules, there are ions (in successively decreasing quantities) H 2 PO 2-4, HPO 2-4 and PO 3-4

N 3 PO 4 ↔ N + + N 2 PO - 4 (first stage)

N 2 PO - 4 ↔ N + + NPO 2- 4 (second stage)

NRO 2- 4 ↔ N+ PO Z- 4 (third stage)

The basicity of an acid is determined by the number of hydrogen cations that are formed during dissociation.

So, HCl, HNO 3 - monobasic acids - one hydrogen cation is formed;

H 2 S, H 2 CO 3, H 2 SO 4 - dibasic,

H 3 PO 4, H 3 AsO 4 are tribasic, since two and three hydrogen cations are formed, respectively.

Of the four hydrogen atoms contained in the acetic acid molecule CH 3 COOH, only one, included in the carboxyl group - COOH, is capable of being cleaved off in the form of the H + , - cation acetic acid monobasic.

Dibasic and polybasic acids dissociate stepwise (gradually).

Bases are electrolytes whose dissociation produces only hydroxide ions as anions.

For example:

KOH ↔ K + + OH - ;

NH 4 OH ↔ NH + 4 + OH -

Bases that dissolve in water are called alkalis. There are not many of them. These are the bases of alkali and alkaline earth metals: LiOH, NaOH, KOH, RbOH, CsOH, FrOH and Ca(OH) 2, Sr(OH) 2, Ba(OH) 2, Ra(OH) 2, as well as NH 4 OH. Most bases are slightly soluble in water.

The acidity of a base is determined by the number of its hydroxyl groups (hydroxy groups). For example, NH 4 OH is a one-acid base, Ca(OH) 2 is a two-acid base, Fe(OH) 3 is a three-acid base, etc. Two- and polyacid bases dissociate stepwise

Ca(OH) 2 ↔ Ca(OH) + + OH - (first stage)

Ca(OH) + ↔ Ca 2+ + OH - (second stage)

However, there are electrolytes that, upon dissociation, simultaneously form hydrogen cations and hydroxide ions. These electrolytes are called amphoteric or ampholytes. These include water, zinc, aluminum, chromium hydroxides and a number of other substances. Water, for example, dissociates into H + and OH - ions (in small quantities):

H 2 O ↔ H + + OH -

Consequently, she has equally both acidic properties due to the presence of hydrogen cations H + and alkaline properties due to the presence of OH - ions are expressed.

The dissociation of amphoteric zinc hydroxide Zn(OH) 2 can be expressed by the equation

2OH - + Zn 2+ + 2H 2 O ↔ Zn(OH) 2 + 2H 2 O ↔ 2- + 2H +

Salts are electrolytes, upon dissociation of which metal cations are formed, as well as ammonium cation (NH 4) and anions of acid residues

For example:

(NH 4) 2 SO 4 ↔ 2NH + 4 + SO 2- 4;

Na 3 PO 4 ↔ 3Na + + PO 3- 4

This is how medium salts dissociate. Acidic and basic salts dissociate stepwise. In acidic salts, metal ions are first eliminated, and then hydrogen cations. For example:

KHSO 4 ↔ K + + HSO - 4

HSO - 4 ↔ H + + SO 2- 4

In basic salts, acid residues are eliminated first, and then hydroxide ions.

Mg(OH)Cl ↔ Mg(OH) + + Cl -

Spontaneous partial or complete disintegration of dissolved electrolytes (see) into ions is called electrolytic dissociation. The term “ions” was introduced by the English physicist M. Faraday (1833). The theory of electrolytic dissociation was formulated by the Swedish scientist S. Arrhenius (1887) to explain the properties of aqueous solutions of electrolytes. Subsequently, it was developed by many scientists on the basis of the doctrine of the structure of the atom and chemical bonds. The modern content of this theory can be reduced to the following three provisions:

1. Electrolytes, when dissolved in water, dissociate (break up) into ions - positively and negatively charged. (“Ion” is Greek for “wandering.” In a solution, ions move randomly in different directions.)

2. Under the influence of electric current, ions acquire directional movement: positively charged ones move towards the cathode, negatively charged ones move towards the anode. Therefore, the former are called cations, the latter - anions. The directional movement of ions occurs as a result of the attraction of their oppositely charged electrodes.

3. Dissociation is a reversible process. This means that a state of equilibrium occurs in which as many molecules break up into ions (dissociation), so many of them are formed again from ions (association).

Therefore, in the equations of electrolytic dissociation, instead of the equal sign, the reversibility sign is used.

For example:

where KA is an electrolyte molecule, is a cation, A is an anion.

The doctrine of chemical bonding helps answer the question of why electrolytes dissociate into ions. Substances with ionic bonds dissociate most easily, since they already consist of ions (see Chemical bonding). When they dissolve, the water dipoles are oriented around the positive and negative ions. Mutual attractive forces arise between the ions and dipoles of water. As a result, the bond between the ions weakens, and the ions move from the crystal to the solution. Electrolytes, whose molecules are formed according to the type of covalent polar bond, dissociate similarly. The dissociation of polar molecules can be complete or partial - it all depends on the degree of polarity of the bonds. In both cases (during the dissociation of compounds with ionic and polar bonds), hydrated ions are formed, that is, ions chemically bonded to water molecules (see figure on p. 295).

The founder of this view of electrolytic dissociation was honorary academician I. A. Kablukov. In contrast to the Arrhenius theory, which did not take into account the interaction of the solute with the solvent, I. A. Kablukov applied the chemical theory of solutions of D. I. Mendeleev to explain electrolytic dissociation. He showed that upon dissolution occurs chemical reaction dissolved substance with water, which leads to the formation of hydrates, and then they dissociate into ions. I. A. Kablukov believed that an aqueous solution contains only hydrated ions. Currently, this idea is generally accepted. So, ion hydration is the main cause of dissociation. In other, non-aqueous electrolyte solutions, the chemical bond between the particles (molecules, ions) of the solute and the solvent particles is called solvation.

Hydrated ions have both a constant and variable number of water molecules. A hydrate of constant composition forms hydrogen ions that hold one molecule, this is a hydrated proton. In scientific literature, it is usually represented by a formula and called hydronium ion.

Since electrolytic dissociation is a reversible process, in solutions of electrolytes, along with their ions, there are also molecules. Therefore, electrolyte solutions are characterized by the degree of dissociation (denoted by the Greek letter a). The degree of dissociation is the ratio of the number of molecules dissociated into ions n to the total number of dissolved molecules:

The degree of electrolyte dissociation is determined experimentally and is expressed in fractions of a unit or as a percentage. If there is no dissociation, and if or 100%, then the electrolyte completely disintegrates into ions. Different electrolytes have different degrees of dissociation. With dilution of the solution it increases, and with the addition of ions of the same name (the same as the electrolyte ions) it decreases.

However, to characterize the ability of an electrolyte to dissociate into ions, the degree of dissociation is not a very convenient value, since it depends on the concentration of the electrolyte. More general characteristic is the dissociation constant K. It can be easily derived by applying the law of mass action to the electrolyte dissociation equilibrium:

where KA is the equilibrium concentration of the electrolyte, and are the equilibrium concentrations of its ions (see Chemical equilibrium). K does not depend on concentration. It depends on the nature of the electrolyte, solvent and temperature.

For weak electrolytes, the greater the K (dissociation constant), the stronger the electrolyte, the more ions in the solution.

Strong electrolytes do not have dissociation constants. Formally, they can be calculated, but they will not be constant as the concentration changes.

Polybasic acids dissociate in stages, which means that such acids will have several dissociation constants - one for each stage. For example:

First stage:

Second stage:

Third stage:

Always, i.e., a polybasic acid, when dissociated in the first step, behaves as a stronger acid than in the second or third.

Polyacid bases also undergo stepwise dissociation. For example:

Acidic and basic salts also dissociate stepwise. For example:

In this case, in the first step, the salt completely disintegrates into ions, which is due to the ionic nature of the bond between and; and dissociation in the second stage is insignificant, since charged particles (ions) undergo further dissociation as very weak electrolytes.

From the point of view of the theory of electrolytic dissociation, definitions are given and the properties of such classes are described. chemical compounds as acids, bases, salts.

Acids are electrolytes whose dissociation produces only hydrogen ions as cations. For example:

All common characteristic properties of acids - sour taste, change in color of indicators, interaction with bases, basic oxides, salts - are due to the presence of hydrogen ions, more precisely.

Bases are electrolytes whose dissociation produces only hydroxide ions as anions:

According to the theory of electrolytic dissociation, all general alkaline properties of solutions - soapiness to the touch, change in color of indicators, interaction with acids, acid anhydrides, salts - are due to the presence of hydroxide ions.

True, there are electrolytes, during the dissociation of which both hydrogen ions and hydroxide ions are simultaneously formed. These electrolytes are called amphoteric or ampholytes. These include water, zinc, aluminum, chromium hydroxides and a number of other substances. Water, for example, in small quantities dissociates into ions and:

Since all reactions in aqueous solutions of electrolytes represent the interaction of ions, the equations for these reactions can be written in ionic form.

The significance of the theory of electrolytic dissociation is that it explained numerous phenomena and processes occurring in aqueous solutions of electrolytes. However, it does not explain the processes occurring in non-aqueous solutions. So, if ammonium chloride in an aqueous solution behaves like a salt (dissociates into ions and ), then in liquid ammonia it exhibits the properties of an acid - it dissolves metals with the release of hydrogen. How does the base behave? Nitric acid, dissolved in liquid hydrogen fluoride or anhydrous sulfuric acid.

All these factors contradict the theory of electrolytic dissociation. They are explained by the protolytic theory of acids and bases.

The term “dissociation” itself means the breakdown of molecules into several simpler particles. In chemistry, in addition to electrolytic dissociation, thermal dissociation is distinguished. This is a reversible reaction that occurs when the temperature increases. For example, thermal dissociation of water vapor:

calcium carbonate:

iodine molecules:

The equilibrium of thermal dissociation obeys the law of mass action.

The main reason for dissociation is the polarization interaction of polar solvent molecules with solute molecules. For example, a water molecule is polar, its dipole moment is μ = 1.84 D, i.e. it has a strong polarizing effect. Depending on the structure of the soluble substance in the anhydrous state, its dissociation proceeds differently. The two most typical cases are:

Rice. 4.8 Dissolution of a substance with an ionic crystal lattice

1. Solute with an ionic bond (NaCl, KCl, etc.). Crystals of such substances already consist of ions. When they dissolve, polar water molecules (dipoles) will be oriented towards the ions with their opposite ends. Mutual attractive forces arise between the ions and dipoles of water (ion-dipole interaction), as a result the bond between the ions weakens, and they pass into solution in a hydrated form (Fig. 4.8). In the case under consideration, dissociation of molecules occurs simultaneously with dissolution. Substances with ionic bonds dissociate most easily.

2. A solute with a polar covalent bond (for example, HCl, H 2 SO 4, H 2 S, etc.). Here, too, around each polar molecule of the substance, water dipoles are oriented accordingly to form hydrates. As a result of such a dipole-dipole interaction, the connecting electron cloud (electron pair) will almost completely shift to an atom with higher electronegativity, while the polar molecule turns into an ionic one (the stage of ionization of the molecule) and then breaks up into ions, which pass into the solution in a hydrated form (Fig. .4.9). Dissociation can be complete or partial - it all depends on the degree of polarity of the bonds in the molecule.

ionization dissociation

Rice. 4.9 Dissolution of a substance with a polar covalent bond

The difference between the considered cases is that in the case of an ionic bond, the ions existed in the crystal, and in the case of a polar bond, they were formed during the dissolution process. Compounds containing both ionic and polar bonds are first dissociated along ionic and then through covalent polar bonds. For example, sodium hydrogen sulfate NaHSO 4 dissociates completely at the Na-O bond, partially at H-O bonds and practically does not dissociate through low-polar bonds of sulfur with oxygen.

Thus , upon dissolution, only compounds with ionic and covalent polar bonds dissociate and only in polar solvents.

Degree of dissociation. Strong and weak electrolytes

A quantitative characteristic of electrolytic dissociation is the degree of dissociation of the electrolyte in solution. This characteristic was introduced by Arrhenius. Degree of dissociation – α - this is the ratio of the number of molecules N that have broken up into ions to the total number of molecules of the dissolved electrolyte N 0:

α is expressed in fractions of a unit or in %.

Based on the degree of dissociation, electrolytes are divided into strong or weak.

When dissolved in water strong electrolytes dissociate almost completely, the dissociation process in them is irreversible. For strong electrolytes, the degree of dissociation in solutions is equal to unity (α = 1) and almost does not depend on the concentration of the solution. In the dissociation equations of strong electrolytes, the sign “=” or “ ” is used. For example, the dissociation equation for the strong electrolyte sodium sulfate has the form

Na 2 SO 4 = 2Na + + SO 4 2 - .

Strong electrolytes in aqueous solutions include almost all salts, bases of alkali and alkaline earth metals, acids: H 2 SO 4, HNO 3, HCl, HBr, HI, HСlO 4, HClO 3, HBrO 4, HBrO 3, HIO 3, H 2 SeO 4, HMnO 4, H 2 MnO 4, etc.

To the weak electrolytes include electrolytes, the degree of dissociation of which in solutions is less than unity (α<1) и она уменьшается с ростом концентрации.

The process of dissociation of weak electrolytes proceeds reversibly until equilibrium is established in the system between the undissociated molecules of the dissolved substance and its ions. In the dissociation equations of weak electrolytes, the sign of “reversibility” is indicated. For example, the dissociation equation for the weak electrolyte ammonium hydroxide has the form

NH 4 + OH NH 4 + + OH -

Weak electrolytes include water, almost all organic acids (formic, acetic, benzoic, etc.), a number of inorganic acids (H 2 SO 3, HNO 2, H 2 CO 3, H 3 AsO 4, H 3 AsO 3, H 3 BO 3, H 3 PO 4, H 2 SiO 3, H 2 S, H 2 Se, H 2 Te, HF, HCN, HCNS), bases of p-, d-, f- elements (Al(OH) 3 , Cu(OH)2, Fe(OH)2, etc.), ammonium hydroxide, magnesium and beryllium hydroxides, some salts (CdI2, CdCl2, HgCl2, Hg(CN)2, Fe(CNS)3 etc.).

Depending on the degree of dissociation, electrolytes are distinguished between strong and weak. Electrolytes with a degree of dissociation greater than 30% are usually called strong, with a degree of dissociation from 3 to 30% - medium, less than 3% - weak electrolytes.

Numerical the value of the degree of electrolytic dissociation depends on various factors:

1 . Nature of the solvent.

This is due to the dielectric constant of the solvent ε. As follows from Coulomb's law, the force of electrostatic attraction between two oppositely charged particles depends not only on the magnitude of their charges and the distance between them, but also on the nature of the medium in which the charged particles interact, i.e. from ε:

For example, at 298 K ε(H 2 O) = 78.25, and ε(C 6 H 6) = 2.27. Salts such as KCl, LiCl, NaCl, etc., are completely dissociated into ions in water, i.e. behave as strong electrolytes; in benzene these salts dissociate only partially, i.e. are weak electrolytes. Thus, the same substances may exhibit different dissociation abilities depending on the nature of the solvent.

2 . Temperature.

For strong electrolytes, the degree of dissociation decreases with increasing temperature; for weak electrolytes, when the temperature rises to 60°C, α increases and then begins to decrease.

3 . Solution concentration.

If we consider dissociation as an equilibrium chemical process, then, in accordance with Le Chatelier's principle, the addition of a solvent (dilution with water), as a rule, increases the number of dissociated molecules, which leads to an increase in α. The process of forming molecules from ions as a result of dilution becomes more difficult: for the formation of a molecule, a collision of ions must occur, the probability of which decreases with dilution.

4 . Presence of ions of the same name.

The addition of like ions reduces the degree of dissociation, which is also consistent with Le Chatelier's principle. For example, in a solution of weak nitrous acid, during electrolytic dissociation, an equilibrium is established between undissociated molecules and ions:

НNO 2 Н + + NO 2 - .

When nitrite ions NO 2 ˉ are introduced into a solution of nitrous acid (by adding a solution of potassium nitrite KNO 2), the equilibrium will shift to the left, therefore, the degree of dissociation α will decrease. A similar effect will be achieved by introducing H + ions into the solution.

It should be noted that the concepts of “strong electrolyte” and “good solubility” should not be confused. For example, the solubility of CH 3 COOH in H 2 O is unlimited, but acetic acid is a weak electrolyte (α = 0.014 in a 0.1 M solution). On the other hand, BaSO 4 is a sparingly soluble salt (at 20°C solubility is less than 1 mg in 100 g of H 2 O), but it belongs to strong electrolytes, since all molecules that go into solution disintegrate into Ba 2+ and SO 4 ions 2 - .

Dissociation constant

A more accurate characteristic of electrolyte dissociation is dissociation constant, which does not depend on the concentration of the solution.

The expression for the dissociation constant can be obtained by writing the equation for the dissociation reaction of the AA electrolyte in general form:

AK A – + K + .

Since dissociation is a reversible equilibrium process, we apply the law of mass action to this reaction, and the equilibrium constant can be determined as:

where K is the dissociation constant, which depends on the temperature and nature of the electrolyte and solvent, but does not depend on the concentration of the electrolyte.

The range of equilibrium constants for different reactions is very large - from 10 –16 to 10 15.

Dissociation of substances consisting of more than two ions occurs in steps. For a reaction of the form

A n K m nA – m + mK + n

the dissociation constant has the form

For example, sulfurous acid dissociates in steps:

H 2 SO 3 H + + HSO 3 –

HSO 3 – H + + SO 3 2–

Each stage of dissociation is described by its own constant:

At the same time, it is clear that

During the stepwise dissociation of substances, the decomposition in the subsequent step always occurs to a lesser extent than in the previous one. In other words:

K d1 > K d2 >…

If the concentration of the electrolyte breaking up into two ions is equal to C in, and the degree of its dissociation is α, then the concentration of the resulting ions will be C to α, and the concentration of undissociated molecules is C in (1–α). The expression for the constant takes next view:

This equation expresses Ostwald's dilution law . It allows one to calculate the degree of dissociation at various electrolyte concentrations if its dissociation constant is known. For weak electrolytes α<<1, тогда (1–α) → 1. Уравнение в этом случае принимает вид:

This equation clearly shows that the degree of dissociation increases as the solution is diluted.

In aqueous solutions, strong electrolytes are usually completely dissociated, so the number of ions in them is greater than in solutions of weak electrolytes of the same concentration. In this case, the forces of interionic attraction and repulsion are quite large. In such solutions, the ions are not completely free; their movement is constrained by mutual attraction to each other. Thanks to this attraction, each ion is surrounded by a spherical swarm of oppositely charged ions, called the “ionic atmosphere.”

THEORY OF ELECTROLYTIC DISSOCIATION

Solutions of all substances can be divided into two groups: they conduct electric current or are not conductors.

You can get acquainted with the characteristics of the dissolution of substances experimentally by studying the electrical conductivity of solutions of these substances using the device shown in the figure.

Observe the following experiment " Study of electrical conductivity of substances."

To explain the characteristics of aqueous solutions of electrolytes to a Swedish scientist S. Arrhenius in 1887 it was proposed electrolytic dissociation theory . Subsequently, it was developed by many scientists on the basis of the doctrine of the structure of atoms and chemical bonds. The modern content of this theory can be reduced to the following three provisions:

1. Electrolytes when dissolved in water or melted break apart (dissociate) to ions – positive (cations) and negative (anions) charged particles.

Ions are in more stable electronic states than atoms. They can consist of one atom - this is simple ions ( Na + , Mg 2+ , Al 3+ etc.) - or from several atoms - this is complex ions ( NO 3 - ,SO 2- 4 , RO Z-4, etc.).

2. In solutions and melts electrolytes conduct electricity .

Under the influence of an electric current, ions acquire directional movement: positively charged ions move towards the cathode, negatively charged ions move towards the anode. Therefore, the former are called cations, the latter - anions. The directional movement of ions occurs as a result of their attraction by oppositely charged electrodes.

TESTING SUBSTANCES FOR ELECTRICAL CONDUCTIVITY

REMINDER

ELECTROLYTES AND NON-ELECTROLYTES

THERMAL EFFECTS WHEN DISSOLVING SUBSTANCES IN WATER

3. Dissociation - reversible process: in parallel with the disintegration of molecules into ions (dissociation), the process of combining ions (association) occurs.

Therefore, in the equations of electrolytic dissociation, instead of the equal sign, the reversibility sign is put. For example, the dissociation equation of the electrolyte molecule K Ainto the K + cation and the A - anion, in general it is written as follows:

KA ↔K + + A -

Consider the process of dissolving electrolytes in water

In general, a water molecule is not charged. But inside the moleculeH 2 O The hydrogen and oxygen atoms are arranged so that positive and negative charges are at opposite ends of the molecule (Fig. 1). Therefore, a water molecule is a dipole.

Dissolution of substances with ionic chemical bonds in water

(using the example of sodium chloride - table salt)

Mechanism of electrolytic dissociationNaCl when table salt is dissolved in water (Fig. 2), it consists of the sequential elimination of sodium and chlorine ions by polar water molecules. Following the transition of ions Na + and Сl – From the crystal to the solution, hydrates of these ions are formed.

Dissolution of substances with covalent highly polar chemical bonds in water

(using the example of hydrochloric acid)

When hydrochloric acid is dissolved in water (in moleculesHCl the bond between atoms is covalent, highly polar), the nature of the chemical bond changes. Under the influence of polar water molecules, a covalent polar bond turns into an ionic one. The resulting ions remain bound to water molecules - hydrated. If the solvent is non-aqueous, then the ions are called solvated (Fig. 3).

Key points:

Electrolytic dissociation - This is the process of decomposition of an electrolyte into ions when it is dissolved in water or melted.

Electrolytes– these are substances that, when dissolved in water or in a molten state, disintegrate into ions.

Ions are atoms or groups of atoms that have a positive ( cations) or negative ( anions) charge.

Ions differ from atoms both in structure and properties

Example 1. Let's compare the properties of molecular hydrogen (consists of two neutral hydrogen atoms) with the properties of the ion.

Example 2. Let's compare the properties of atomic and molecular chlorine with the properties of the ion.

Remember!

1. Ions differ from atoms and molecules in structure and properties;

2. A common and characteristic feature of ions is the presence of electrical charges;

3. Solutions and melts of electrolytes conduct electric current due to the presence of ions in them.

All substances, according to their ability to conduct electric current in solution or in a molten state, can be divided into two groups: electrolytes and non-electrolytes.

Electrolytes are substances whose solutions or melts conduct electric current. Electrolytes include acids, bases and salts.

Non-electrolytes are substances whose solutions or melts do not conduct electric current. For example, many organic substances.

The ability of electrolytes (type II conductors) to conduct electric current is fundamentally different from the electrical conductivity of metals (type I conductors): the electrical conductivity of metals is due to the movement of electrons, and the electrical conductivity of electrolytes is associated with the movement of ions.

It was discovered that in solutions of acids, bases and salts, the experimentally found values ​​of p, tcrystal, tboil, pcm, are greater than those theoretically calculated for the same solution based on its molar concentration in i once ( i- isotonic coefficient). Moreover, the number of particles in the NaCl solution increased almost 2 times, and in the CaCl2 solution - 3 times.

To explain the peculiarities of the behavior of electrolytes, the Swedish scientist S. Arrhenius in 1887 proposed a theory called electrolytic dissociation theories. The essence of the theory is as follows:

• 1. Electrolytes, when dissolved in water, disintegrate (dissociate) into charged particles (ions) - positively charged cations (Na+, K+, Ca2+, H+) and negatively charged anions (Cl-, SO42-, CO32-, OH-). The properties of ions are completely different from those of the atoms that formed them. The decomposition of a neutral substance into ions as a result of chemical interaction with a solvent is called electrolytic dissociation.
• 2. Under the influence of an electric current, ions acquire directional movement: cations move towards a negatively charged electrode (cathode), anions move towards a positively charged electrode (anode).
• 3. Dissociation is a reversible and equilibrium process. This means that in parallel with the disintegration of molecules into ions (dissociation), there is a process of combining ions into molecules (association): KA K+ + A-.
• 4. In solution, ions are in a hydrated state.

To quantify electrolytic dissociation, the concept is used degree of electrolytic dissociation() - the ratio of the number of molecules disintegrated into ions to the total number of dissolved molecules. The degree of dissociation is determined empirically and is expressed in fractions or percentages. The degree of electrolytic dissociation depends on the nature of the solvent and solute, the temperature and concentration of the solution:

• 1. The more polar the solvent, the higher the degree of dissociation of the electrolyte in it.
• 2. Substances with ionic and covalent polar bonds undergo dissociation.
• 3. An increase in temperature increases the dissociation of weak electrolytes.
• 4. As the electrolyte concentration decreases (dilute), the degree of dissociation increases.

Depending on the degree of dissociation, conditionally electrolytes (at a concentration of their solutions of 0.1 M) are divided into:

Based on the type of ions formed during dissociation, all electrolytes can be divided into acids, bases, and salts.

Acids- electrolytes that dissociate with the formation of only H+ cations and an acidic residue (Cl- - chloride, NO3- - nitrate, SO42- - sulfate, HCO3 bicarbonate, CO32 carbonate). For example: HCl H++Cl-, H2SO4 2H++SO42-.

The presence of a hydrogen ion, or more precisely, a hydrated H3O+ ion, in acid solutions determines the general properties of acids (sour taste, effect on indicators, interaction with alkalis, interaction with metals with the release of hydrogen, etc.).

In polybasic acids, dissociation occurs in steps, with each step characterized by its own degree of dissociation. Thus, orthophosphoric acid dissociates in three steps:

And 3<2<1, т.е. распад электролита на ионы протекает, в основном, по первой ступени и в растворе ортофосфорной кислоты будут находиться преимущественно ионы Н+ и H2РO4-. Причины этого в том, что ионы водорода значительно сильнее притягиваются к трехзарядному иону РO43- и двухзарядному иону HРO42-, чем к однозарядному H2РO4-. Кроме того, на 2-ой и 3-ей ступенях имеет место смещение равновесия в сторону исходной формы по принципу Ле-Шателье за счет накапливающихся ионов водорода.

Grounds- electrolytes that dissociate to form only hydroxide ions (OH-) as anions. After the separation of OH-, the following cations remain: Na+, Ca2+, NH4+. For example: NaOH Na+ + OH-, Ca(OH)2 Ca2+ + 2 OH-.

The general properties of bases (soapiness to the touch, effect on an indicator, interaction with acids, etc.) are determined by the presence of the hydroxo group OH- in solutions of bases.

Polyacid bases are characterized by stepwise dissociation:

Dissociation of amphoteric hydroxides occurs both as a base and as an acid. Thus, the dissociation of zinc hydroxide can proceed in the following directions (in this case, the equilibrium shifts depending on the environment according to Le Chatelier’s principle):

Salts- these are electrolytes that dissociate into metal cations (or groups replacing it) and anions of the acid residue.

Medium salts dissociate completely: CuSO4 Cu2+ + SO42-. Unlike average salts, acidic and basic salts dissociate stepwise:

Moreover, the degree of dissociation of salts in the second step is very small.

Exchange reactions in electrolyte solutions- These are reactions between ions. A necessary condition for the occurrence of exchange reactions in electrolyte solutions is the formation of weakly dissociating compounds or compounds released from the solution in the form of a precipitate or gas.

When writing reaction equations in ionic-molecular form, weakly dissociating, gaseous and sparingly soluble compounds are written in the form molecules, and soluble strong electrolytes - in the form ions. When writing ionic equations, you should be sure to follow the table of solubility of acids, bases and salts in water (Appendix A).

Let's look at the technique of writing ionic equations using examples.

Example 1. Write the reaction equation in ionic-molecular form:

BaCl2 + K2SO4 = BaSO4 + 2KCl

Solution: Salts are strong electrolytes and almost completely dissociate into ions. Since BaSO4 is a practically insoluble compound (see table in Appendix A), the main part of barium sulfate will be in undissociated form, so we will write this substance in the form of molecules, and the remaining salts, which are soluble, in the form of ions:

Ba2+ + 2Cl- + 2K+ + SO42- = BaSO4 + 2K+ + 2Cl-

As can be seen from the resulting complete ion-molecular equation, the K+ and Cl- ions do not interact, therefore, excluding them, we obtain a short ion-molecular equation:

Ba2+ + SO42- = BaSO4,

The arrow indicates that the resulting substance precipitates.

Ionic equations can be used to represent any reactions occurring in solutions between electrolytes. Moreover, the essence of any chemical reaction is reflected in a short ion-molecular equation. Based on the ion-molecular equation, you can easily write the molecular equation.

Example 2. Match the molecular equation to the following ionic-molecular equation: 2H+ + S2- = H2S.

Solution: Hydrogen ions are formed during the dissociation of any strong acid, such as HCl. To the hydrogen ions in the short ionic equation you need to add two chlorine ions. Cations (for example, 2K+) should be added to sulfide ions to form a soluble, well-dissociating electrolyte. Then the same ions need to be written on the right side. Then the complete ion-molecular and molecular equations will look like:

• 2H+ + 2Cl- + 2K+ + S2- = H2S + 2K+ + 2Cl-
• 2 HCl + K2S = H2S + 2 KCl-