(1887) to explain the properties of aqueous solutions of electrolytes. In the future, it was developed by many scientists on the basis of the doctrine of the structure of the atom and the chemical bond. Modern content This theory can be reduced to the following three propositions:

Scheme of the dissolution of a salt crystal. Sodium and chloride ions in solution.

1. When dissolved in water, electrolytes dissociate (decompose) into ions - positively and negatively charged. (“Ion” means “wandering” in Greek. In solution, ions move randomly in different directions.)

2. Under the action of an electric current, ions acquire a directed movement: positively charged ones move towards the cathode, negatively charged ones - towards the anode. Therefore, the first are called cations, the second - anions. The directed 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 how many molecules break up into ions (dissociation), so many of them are re-formed from ions (association). Therefore, in the equations electrolytic dissociation instead of the equal sign put the sign of reversibility.

For example:

KA ↔ K + + A - ,

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

The doctrine of the chemical bond helps answer the question of why electrolytes dissociate into ions. Substances with an ionic bond dissociate most easily, since they already consist of ions (see Chemical bond). When they dissolve, the dipoles of water orient themselves around the positive and negative ions. Forces of mutual attraction arise between the ions and dipoles of water. As a result, the bond between the ions weakens, and the transition of ions from the crystal to the solution occurs. Similarly, electrolytes dissociate, the molecules of which are formed according to the type of covalent polar bond. 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, i.e., ions chemically bound to water molecules.

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

Hydrated ions have both a constant and a variable number of water molecules. A hydrate of constant composition forms hydrogen ions H + holding one water molecule - this is a hydrated proton H + (H 2 O). IN scientific literature it is customary to represent it with the formula H 3 O + (or OH 3 +) and call it a hydronium ion.

Since electrolytic dissociation is a reversible process, electrolyte solutions contain molecules along with their ions. 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 that have decayed into ions, n to total number dissolved molecules N:

The degree of dissociation of the electrolyte is determined empirically 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 decomposes into ions. Different electrolytes have different degrees of dissociation. With the dilution of the solution, it increases, and with the addition of ions of the same name (the same as 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 electrolyte concentration. More common characteristic is the dissociation constant K. It can be easily derived by applying the mass action law to the electrolyte dissociation equilibrium (1):

K = () / ,

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 larger 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 when the concentration changes.

The breakdown of electrolyte molecules into ions under the action of polar solvent molecules is called electrolytic dissociation. Substances aqueous solutions or whose melts conduct electricity are called electrolytes.

These include water, acids, bases and salts. When dissolved in water, electrolyte molecules dissociate into positive ions - cations and negative- anions. The process of electrolytic dissociation is caused by the interaction of substances with water or other solvent, which leads to the formation of hydrated ions.

So, a hydrogen ion forms a hydronium ion:

H+ + H2O «H3O+.

To simplify, the hydronium ion is written without specifying water molecules, that is, H +.

NaCl + nH2O ® Na+(H2O)x + Cl–(H2O)n-x,

or the entry is accepted: NaCl « Na+ + Cl–.

Dissociation of acids, bases, salts

acids Electrolytes are called electrolytes, during the dissociation of which only hydrogen cations are formed as cations. For example,

HNO3 « H+ + NO3–

Polybasic acids dissociate in steps. For example, hydrosulfide acid dissociates in steps:

H2S « H+ + HS– (first step)

HS– « H+ + S2– (second stage)

The dissociation of polybasic acids proceeds mainly in the first stage. This is explained by the fact that the energy that must be expended to detach an ion from a neutral molecule is minimal and becomes greater with dissociation through each next step.

grounds called electrolytes that dissociate in solution, which form only hydroxide ions as anions. For example,

NaOH ® Na+ + OH–

Polyacid bases dissociate in steps

Mg(OH)2 « MgOH+ + OH– (first step)

MgOH+ « Mg2+ + OH– (second step)

The stepwise dissociation of acids and bases explains the formation of acidic and basic salts.

There are electrolytes that dissociate simultaneously as basic and as acidic. They're called amphoteric.

H+ + RO– « ROH « R+ + OH–

Amphotericity is explained by a small difference in the strength of the R–H and O–H bonds.

Amphoteric electrolytes include water, hydroxides of zinc, aluminum, chromium (III), tin (II, IV), lead (II, IV), etc.

dissociation amphoteric hydroxide, for example Sn(OH)2, can be expressed by the equation:

2H+ + SnO22– « Sn(OH)2 « Sn2+ + 2OH–

2H2O¯ basic properties

2H+ + 2–

acid properties

salts called electrolytes, which, upon dissociation, form metal cations, or complex cations, and anions of acid residues, or complex anions.

Medium salts, soluble in water, dissociate almost completely

Al2(SO4)3 « 2Al3+ + 2SO42–

(NH4)2CO3 « 2NH4+ + CO32–

Acid salts dissociate in steps, for example:

NaHCO3 « Na+ + HCO3– (first step)

Anions of acid salts further dissociate insignificantly:

HCO3– « H+ + CO32– (second stage)

The dissociation of the basic salt can be expressed by the equation

CuOHCl « CuOH+ + Cl– (first stage)

CuOH+ « Cu+2 + OH– (second stage)

The cations of the basic salts in the second stage dissociate to a small extent.

Double salts are electrolytes that upon dissociation form two types of metal cations. For example

KAl(SO4)2 « K+ + Al3+ + 2SO42–.

Complex salts are electrolytes, during the dissociation of which two types of ions are formed: simple and complex. For example:

Na2 « 2Na+ + 2–

The quantitative characteristic of electrolytic dissociation is degree of dissociationa, equal to the ratio of the number of molecules decomposed into ions (n) to the total number of dissolved molecules (N)

The degree of dissociation is expressed in fractions of a unit or percent.

According to the degree of dissociation, all electrolytes are divided into strong (a> 30%), weak (a<3%) и средней силы (a - 3-30%).

Strong electrolytes When dissolved in water, they completely dissociate into ions. These include:

	HCl, HBr, HJ, HNO3, H2SO4, HClO3, HClO4, HMnO4, H2SeO4
Foundations	NaOH, KOH, LiOH, RbOH, CsOH, Ba(OH)2, Ca(OH)2, Sr(OH)2
	soluble in water (appendix, table 2)

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

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

Substances that decompose into ions in solutions or melts and therefore conduct electricity are called electrolytes.

Substances that do not decompose into ions under the same conditions and do not conduct electric current are called non-electrolytes.

Electrolytes include acids, bases, and almost all salts.

Non-electrolytes include most organic compounds, as well as substances in the molecules of which there are only covalent non-polar or low-polar bonds.

Electrolytes are conductors of the second kind. In a solution or melt, they decompose into ions, due to which the current flows. Obviously, the more ions in a solution, the better it conducts electricity. Pure water conducts electricity very poorly.

Distinguish between strong and weak electrolytes.

Strong electrolytes completely dissociate into ions when dissolved.

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) bases of alkali 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 give a high concentration of ions in solution.

Basic provisions of the theory of electrolytic dissociation.

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

So, 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 features of aqueous solutions of electrolytes, the Swedish scientist S. Arrhenius in 1887 proposed the theory of electrolytic dissociation. Later it was developed by many scientists on the basis of the theory of the structure of atoms and chemical bonding.

The current content of this theory can be reduced to the following three propositions:

1. When dissolved in water, electrolytes decompose (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 from several atoms - these are complex ions (NO 3 -, SO 2- 4, PO Z- 4 etc.).

2. Under the action of an electric current, the ions acquire a directed movement: positively charged ions move towards the cathode, negatively charged ones - towards the anode. Therefore, the first are called cations, the second - anions.

The directed 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 connecting ions (association) proceeds.

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

KA ↔ K + + A -

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

Degree of dissociation.

One of the most important concepts of the Arrhenius 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 that have decayed into ions (n ​​"), to the total number of dissolved molecules (n):

The degree of dissociation of the electrolyte is determined empirically 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 decomposes into ions. If α = 20%, then this means that out of 100 molecules of this electrolyte, 20 decomposed into ions.

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

Let us consider the shift of equilibrium established between non-dissociated 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 - the degree of dissociation of the acid increases. On the contrary, when the solution is evaporated, the equilibrium shifts towards the formation of acid molecules - the degree of dissociation decreases.

It is obvious from this expression 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 an ionic bond dissociate most easily. As you know, these substances are composed of ions. When they dissolve, the dipoles of water orient themselves around the positive and negative ions. Forces of mutual attraction arise between the ions and dipoles of water. As a result, the bond between the ions weakens, and the transition of ions from the crystal to the solution occurs. In this case, hydrated ions are formed, i.e. ions chemically bound to water molecules.

Similarly, electrolytes, whose molecules are formed according to the type of polar covalent bond (polar molecules), also dissociate. Water dipoles are also oriented around each polar molecule of the substance, 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 binding electron cloud (electron pair) is completely shifted to an atom with a higher electronegativity, the polar molecule turns into an ionic one, and then hydrated ions are easily formed:

The dissociation of polar molecules can be complete or partial.

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

dissociation constant.

dissociation constant. A more accurate characteristic of the 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 reaction equation for the dissociation of the AK electrolyte in a general form:

A K → A - + K + .

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

where K is the dissociation constant, which depends on the temperature and the 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, a high value TO for reaction

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

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

Pay special attention to the form of writing expressions for the equilibrium constant. If the concentrations of some reagents do not change significantly during the reaction, then they are not written in the expression for the equilibrium constant (such constants are denoted by 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. The concentrations of copper and silver are determined by their density and cannot be changed. Therefore, it makes no sense to take these concentrations into account when calculating the equilibrium constant.

The expressions for the equilibrium constants in the dissolution of AgCl and AgI are explained similarly

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

For the water dissociation reaction

the constant expression would be:

This is explained by the fact that the concentration of water during reactions in aqueous solutions changes very slightly. 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, definitions are given and the properties of acids, bases and salts are described.

Electrolytes are called acids, during the dissociation of which only hydrogen cations are formed as cations.

For example:

HCl ↔ H + + C l - ;

CH 3 COOH ↔ H + + CH 3 COO -

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

H 3 RO 4 ↔ N + + H 2 RO - 4 (first stage)

H 2 RO - 4 ↔ H + + HPO 2- 4 (second stage)

NRO 2- 4 ↔ H + 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 molecule of acetic acid CH 3 COOH, only one, which is part of the carboxyl group - COOH, can be split off in the form of an H + cation, - monobasic acetic acid.

Two - and polybasic acids dissociate stepwise (gradually).

Bases are called electrolytes, during the dissociation of which only hydroxide ions are formed as anions.

For example:

KOH ↔ K + + OH - ;

NH 4 OH ↔ NH + 4 + OH -

Bases that are soluble in water are called alkalis. There are few 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, and also 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 two-acid, Fe (OH) 3 is three-acid, etc. Two- and polyacid bases dissociate in steps

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

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

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

H 2 O ↔ H + + OH -

Consequently, it has equally pronounced acidic properties, due to the presence of hydrogen cations H +, and alkaline properties, due to the presence of OH - ions.

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 called electrolytes, during the dissociation of which metal cations are formed, as well as the 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 middle salts dissociate. Acid and basic salts dissociate in steps. In acid salts, metal ions are first split off, and then hydrogen cations. For example:

KHSO 4 ↔ K + + HSO - 4

HSO - 4 ↔ H + + SO 2- 4

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

Mg(OH)Cl ↔ Mg(OH) + + Cl -

Substances whose solutions (or melts) conduct electricity are called e le c t r o l i t a m i Often, the solutions of these substances themselves are also called electrolytes. These solutions (melts) of electrolytes are conductors of the second kind, since the transmission of electricity is carried out in them by movement i o n o v - charged particles. A particle that is positively charged is called cation (Ca +2), a particle carrying a negative charge - anion (IS HE -). Ions can be simple (Ca +2, H +) and complex (RO 4 ־ 3, HCO 3 ־2).

The founder of the theory of electrolytic dissociation is the Swedish scientist S. Arrhenius. According to the theory electrolytic dissociation called the disintegration of molecules into ions when they are dissolved in water, and this occurs without the influence of an electric current. However, this theory did not answer the questions: what causes the appearance of ions in solutions and why positive ions, colliding with negative ones, do not form neutral particles.

Russian scientists made their contribution to the development of this theory: D.I. Mendeleev, I. A. Kablukov - supporters of the chemical theory of solutions, who paid attention to the effect of the solvent in the dissociation process. Kablukov argued that a solute interacts with a solvent ( solvation process ) forming products of variable composition ( s o l v a t y ).

The solvate is an ion surrounded by solvent molecules (solvate shell), which can be of different amounts (it is due to this that a variable composition is achieved). If the solvent is water, then the process of interaction of the molecules of the solute and the solvent is called g i d r a t a c i e y, and the interaction product is g i d r a t o m.

Thus, the cause of electrolytic dissociation is solvation (hydration). And it is the solvation (hydration) of ions that prevents the reverse connection into neutral molecules.

Quantitatively, the dissociation process is characterized by the value degrees of electrolytic dissociation (α), which is the ratio of the amount of matter decomposed into ions to the total amount of dissolved matter. It follows that for strong electrolytes α = 1 or 100% (solute ions are present in the solution), for weak electrolytes 0< α < 1 (в растворе присутствуют наряду с ионами растворенного вещества и его недиссоциированные молекулы), для неэлектролитов α = 0 (there are no ions in the solution). In addition to the nature of the solute and solvent, the quantity α depends on the solution concentration and temperature.

If the solvent is water, strong electrolytes include:

1) all salts;

2) the following acids: HCl, HBr, HI, H 2 SO 4 , HNO 3 , HClO 4 ;

3) the following bases: LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH) 2 , Sr(OH) 2 , Ba(OH) 2 .

The process of electrolytic dissociation is reversible, therefore, it can be characterized by the value of the equilibrium constant, which, in the case of a weak electrolyte, is called dissociation constant (K D) .

The larger this value, the easier the electrolyte decomposes into ions, the more its ions are in solution. For example: HF ═ H + + F־

This value is constant at a given temperature and depends on the nature of the electrolyte, solvent.

Polybasic acids and polyacid bases dissociate in steps. For example, sulfuric acid molecules first remove one hydrogen cation:

H 2 SO 4 ═ H + + HSO 4 ־.

Elimination of the second ion according to the equation

HSO 4 ־ ═ H + + SO 4 ־2

goes much more difficult, since it has to overcome the attraction from the doubly charged SO 4 ־2 ion, which, of course, attracts the hydrogen ion to itself more strongly than the singly charged HSO 4 ־ ion. Therefore, the second stage of dissociation occurs to a much lesser extent than the first.

Bases containing more than one hydroxyl group in the molecule also dissociate in steps. For example:

Ba(OH) 2 ═ BaOH + + OH - ;

BaOH + \u003d Ba 2+ + OH -.

Medium (normal) salts always dissociate into metal ions and acid residues:

CaCl 2 \u003d Ca 2+ + 2Cl -;

Na 2 SO 4 \u003d 2Na + + SO 4 2-.

Acid salts, like polybasic acids, dissociate in steps. For example:

NaHCO 3 \u003d Na + + HCO 3 -;

HCO 3 - \u003d H + + CO 3 2-.

However, the degree of dissociation in the second stage is very small, so that the acid salt solution contains only a small number of hydrogen ions.

Basic salts dissociate into ions of basic and acid residues. For example:

Fe(OH)Cl 2 = FeOH 2+ + 2Cl -.

The secondary dissociation of ions of the main residues into metal and hydroxyl ions almost does not occur.

In the dissociation of acids, the role of cations is played by hydrogen ions(H +), no other cations are formed during the dissociation of acids:

HF ↔ H + + F - HNO 3 ↔ H + + NO 3 -

It is hydrogen ions that give acids their characteristic properties: sour taste, red coloring of the indicator, and so on.

Negative ions (anions) split off from an acid molecule are acid residue.

One of the characteristics of the dissociation of acids is their basicity - the number of hydrogen ions contained in an acid molecule that can be formed during dissociation:

• monobasic acids: HCl, HF, HNO 3 ;
• dibasic acids: H 2 SO 4, H 2 CO 3;
• tribasic acids: H 3 PO 4 .

The process of splitting off hydrogen cations in polybasic acids occurs in steps: first one hydrogen ion is split off, then another (third).

Stepwise dissociation of dibasic acid:

H 2 SO 4 ↔ H + + HSO 4 - HSO 4 - ↔ H + + HSO 4 2-

Stepwise dissociation of a tribasic acid:

H 3 PO 4 ↔ H + + H 2 PO 4 - H 2 PO 4 - ↔ H + + HPO 4 2- HPO 4 2- ↔ H + + PO 4 3-

In the dissociation of polybasic acids, the highest degree of dissociation falls on the first stage. For example, when dissociating phosphoric acid, the degree of dissociation of the first stage is 27%; the second - 0.15%; third - 0.005%.

Base dissociation

In the dissociation of bases, the role of anions is played by hydroxide ions(OH -), no other anions are formed during the dissociation of bases:

NaOH ↔ Na + + OH -

The acidity of the base is determined by the number of hydroxide ions formed during the dissociation of one base molecule:

• single acid bases - KOH, NaOH;
• diacid bases - Ca (OH) 2;
• triacid bases - Al (OH) 3.

Polyacid bases dissociate, by analogy with acids, also in steps - at each stage, one hydroxide ion is split off:

Some substances, depending on the conditions, can act both as acids (dissociate with the elimination of hydrogen cations) and as bases (dissociate with the elimination of hydroxide ions). Such substances are called amphoteric(see Acid-base reactions).

Dissociation of Zn(OH) 2 as a base:

Zn(OH) 2 ↔ ZnOH + + OH - ZnOH + ↔ Zn 2+ + OH -

Dissociation of Zn(OH) 2 as acids:

Zn(OH) 2 + 2H 2 O ↔ 2H + + 2-

Salt dissociation

Salts dissociate in water into anions of acid residues and cations of metals (or other compounds).

Salt dissociation classification:

• Normal (medium) salts obtained by the complete simultaneous replacement of all hydrogen atoms in the acid with metal atoms - these are strong electrolytes, completely dissociate in water with the formation of metal catoins and a single acid residue: NaNO 3, Fe 2 (SO 4) 3, K 3 PO 4.
• Acid salts contain in their composition, in addition to metal atoms and an acid residue, one more (several) hydrogen atoms - they dissociate stepwise with the formation of metal cations, anions of an acid residue and a hydrogen cation: NaHCO 3 , KH 2 PO 4 , NaH 2 PO 4 .
• Basic salts contain in their composition, in addition to metal atoms and an acid residue, one more (several) hydroxyl groups - they dissociate with the formation of metal cations, anions of an acid residue and a hydroxide ion: (CuOH) 2 CO 3, Mg (OH) Cl.
• double salts are obtained by the simultaneous replacement of hydrogen atoms in the acid with atoms of various metals: KAl(SO 4) 2.
• mixed salts dissociate into metal cations and anions of several acid residues: CaClBr.

Normal salt dissociation: K 3 PO 4 ↔ 3K + + PO 4 3- Acid salt dissociation: NaHCO 3 ↔ Na + + HCO 3 - HCO 3 - ↔ H+ + CO 3 2- Base salt dissociation: Mg(OH)Cl ↔ Mg (OH) + + Cl - Mg(OH) + ↔ Mg 2+ + OH - Double salt dissociation: KAl(SO 4) 2 ↔ K + + Al 3+ + 2SO 4 2- Mixed salt dissociation: CaClBr ↔ Ca 2+ + Cl - + Br -