Define strengths and weaknesses. Strong and weak electrolytes

Which are in dynamic equilibrium with undissociated molecules. Weak electrolytes include most organic acids and many organic bases in aqueous and non-aqueous solutions.

Weak electrolytes are:

almost all organic acids and water;
some inorganic acids: HF, HClO, HClO 2 , HNO 2 , HCN, H 2 S, HBrO, H 3 PO 4 , H 2 CO 3 , H 2 SiO 3 , H 2 SO 3 and others;
some sparingly soluble metal hydroxides: Fe(OH) 3 , Zn(OH) 2 and others; as well as ammonium hydroxide NH 4 OH.

Literature

M. I. Ravich-Sherbo. V. V. Novikov "Physical and colloidal Chemistry" M: Higher school, 1975

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See what "Weak electrolytes" is in other dictionaries:

weak electrolytes- - electrolytes, slightly dissociating in aqueous solutions into ions. The process of dissociation of weak electrolytes is reversible and obeys the law of mass action. General chemistry: textbook / A. V. Zholnin ... Chemical terms

Substances with ionic conductivity; they are called conductors of the second kind, the passage of current through them is accompanied by the transfer of matter. Electrolytes include molten salts, oxides or hydroxides, as well as (which occurs significantly ... ... Collier Encyclopedia

In a broad sense, liquid or solid in va and systems, in which ions are present in a noticeable concentration, causing the passage of electricity through them. current (ionic conductivity); in the narrow sense into va, which decay into ions in pre. When dissolving E. ... ... Physical Encyclopedia

electrolytes- liquid or solid substances in which, as a result of electrolytic dissociation, ions are formed in any noticeable concentration, causing the passage of a direct electric current. Electrolytes in solutions ... ... Encyclopedic Dictionary of Metallurgy

In wa, in k ryh in a noticeable concentration there are ions that cause the passage of electric. current (ionic conductivity). E. also called. conductors of the second kind. In the narrow sense of the word, E. in va, molecules to ryh in p re due to electrolytic ... ... Chemical Encyclopedia

- (from Electro ... and Greek lytos decomposable, soluble) liquid or solid substances and systems in which ions are present in any noticeable concentration, causing the passage of electric current. In a narrow sense, E. ... ... Great Soviet Encyclopedia

This term has other meanings, see Dissociation. Electrolytic dissociation is the process of breaking down an electrolyte into ions when it dissolves or melts. Contents 1 Dissociation in solutions 2 ... Wikipedia

An electrolyte is a substance whose melt or solution conducts an electric current due to dissociation into ions, but the substance itself does not conduct an electric current. Examples of electrolytes are solutions of acids, salts and bases. ... ... Wikipedia

Electrolyte is a chemical term denoting a substance whose melt or solution conducts an electric current due to dissociation into ions. Examples of electrolytes are acids, salts and bases. Electrolytes are conductors of the second kind, ... ... Wikipedia

Electrolytes are classified into two groups depending on the degree of dissociation - strong and weak electrolytes. Strong electrolytes have a degree of dissociation greater than one or more than 30%, weak ones - less than one or less than 3%.

Dissociation process

Electrolytic dissociation - the process of disintegration of molecules into ions - positively charged cations and negatively charged anions. Charged particles carry electric current. Electrolytic dissociation is possible only in solutions and melts.

The driving force of dissociation is the disintegration of covalent polar bonds under the action of water molecules. Polar molecules are pulled away by water molecules. In solids, ionic bonds are broken during the heating process. High temperatures cause vibrations of ions in the nodes of the crystal lattice.

Rice. 1. The process of dissociation.

Substances that readily decompose into ions in solutions or melts and therefore conduct electricity are called electrolytes. Non-electrolytes do not conduct electricity, tk. do not decompose into cations and anions.

Depending on the degree of dissociation, strong and weak electrolytes are distinguished. Strong ones dissolve in water, i.e. completely, without the possibility of recovery, decompose into ions. Weak electrolytes decompose into cations and anions partially. The degree of their dissociation is less than that of strong electrolytes.

The degree of dissociation shows the proportion of decomposed molecules in the total concentration of substances. It is expressed by the formula α = n/N.

Rice. 2. Degree of dissociation.

Weak electrolytes

List of weak electrolytes:

dilute and weak inorganic acids - H 2 S, H 2 SO 3, H 2 CO 3, H 2 SiO 3, H 3 BO 3;
some organic acids (most organic acids are non-electrolytes) - CH 3 COOH, C 2 H 5 COOH;
insoluble bases - Al (OH) 3, Cu (OH) 2, Fe (OH) 2, Zn (OH) 2;
ammonium hydroxide - NH 4 OH.

Rice. 3. Table of solubility.

The dissociation reaction is written using the ionic equation:

HNO 2 ↔ H + + NO 2 - ;
H 2 S ↔ H + + HS -;
NH 4 OH ↔ NH 4 + + OH -.

Polybasic acids dissociate in steps:

H 2 CO 3 ↔ H + + HCO 3 -;
HCO 3 - ↔ H + + CO 3 2-.

Insoluble bases also break down in stages:

Fe(OH) 3 ↔ Fe(OH) 2 + + OH – ;
Fe(OH) 2 + ↔ FeOH 2+ + OH - ;
FeOH 2+ ↔ Fe 3+ + OH -.

Water is classified as a weak electrolyte. Water practically does not conduct electricity, because. weakly decomposes into hydrogen cations and hydroxide ion anions. The resulting ions are reassembled into water molecules:

H 2 O ↔ H + + OH -.

If water easily conducts electricity, then it contains impurities. Distilled water is non-conductive.

The dissociation of weak electrolytes is reversible. The formed ions are reassembled into molecules.

What have we learned?

Weak electrolytes include substances that partially decompose into ions - positive cations and negative anions. Therefore, such substances do not conduct electricity well. These include weak and dilute acids, insoluble bases, sparingly soluble salts. The weakest electrolyte is water. The dissociation of weak electrolytes is a reversible reaction.

Salts, their properties, hydrolysis

Pupil 8 class B school number 182

Petrova Polina

Chemistry teacher:

Kharina Ekaterina Alekseevna

MOSCOW 2009

In everyday life, we are accustomed to dealing with only one salt - table salt, i.e. sodium chloride NaCl. However, in chemistry, a whole class of compounds is called salts. Salts can be considered as products of substitution of hydrogen in an acid for a metal. Table salt, for example, can be obtained from hydrochloric acid by a substitution reaction:

2Na + 2HCl \u003d 2NaCl + H 2.

acid salt

If you take aluminum instead of sodium, another salt is formed - aluminum chloride:

2Al + 6HCl = 2AlCl 3 + 3H 2

salt- These are complex substances consisting of metal atoms and acid residues. They are products of complete or partial replacement of hydrogen in an acid with a metal or a hydroxyl group in a base with an acid residue. For example, if in sulfuric acid H 2 SO 4 we replace one hydrogen atom with potassium, we get a KHSO 4 salt, and if two - K 2 SO 4.

There are several types of salts.

Salt types	Definition	Salt examples
Medium	The product of the complete replacement of acid hydrogen by a metal. They contain neither H atoms nor OH groups.	Na 2 SO 4 sodium sulfate CuCl 2 copper (II) chloride Ca 3 (PO 4) 2 calcium phosphate Na 2 CO 3 sodium carbonate (soda ash)
Sour	The product of incomplete replacement of the hydrogen of an acid with a metal. They contain hydrogen atoms. (They are formed only by polybasic acids)	CaHPO 4 calcium hydrogen phosphate Ca (H 2 PO 4) 2 calcium dihydrogen phosphate NaHCO 3 sodium bicarbonate (baking soda)
Main	The product of incomplete replacement of the hydroxo groups of a base with an acid residue. Includes OH groups. (formed only by polyacid bases)	Cu (OH) Cl copper (II) hydroxochloride Ca 5 (PO 4) 3 (OH) calcium hydroxophosphate (CuOH) 2 CO 3 copper (II) hydroxocarbonate (malachite)
mixed	Salts of two acids	Ca(OCl)Cl - bleach
Double	Salts of two metals	K 2 NaPO 4 - sodium dipotassium orthophosphate
Crystal hydrates	Contains water of crystallization. When heated, they dehydrate - they lose water, turning into anhydrous salt.	CuSO4. 5H 2 O - copper (II) sulfate pentahydrate (copper sulfate) Na 2 CO 3. 10H 2 O - decahydrate sodium carbonate (soda)

Methods for obtaining salts.

1. Salts can be obtained by acting with acids on metals, basic oxides and bases:

Zn + 2HCl ZnCl 2 + H 2

zinc chloride

3H 2 SO 4 + Fe 2 O 3 Fe 2 (SO 4) 3 + 3H 2 O

iron(III) sulfate

3HNO 3 + Cr(OH) 3 Cr(NO 3) 3 + 3H 2 O

chromium(III) nitrate

2. Salts are formed by the reaction of acid oxides with alkalis, as well as acid oxides with basic oxides:

N 2 O 5 + Ca (OH) 2 Ca (NO 3) 2 + H 2 O

calcium nitrate

SiO 2 + CaO CaSiO 3

calcium silicate

3. Salts can be obtained by reacting salts with acids, alkalis, metals, non-volatile acid oxides and other salts. Such reactions proceed under the condition of evolution of gas, precipitation, evolution of an oxide of a weaker acid, or evolution of a volatile oxide.

Ca 3 (PO4) 2 + 3H 2 SO 4 3CaSO 4 + 2H 3 PO 4

calcium orthophosphate calcium sulfate

Fe 2 (SO 4) 3 + 6NaOH 2Fe (OH) 3 + 3Na 2 SO 4

iron(III) sulfate sodium sulfate

CuSO 4 + Fe FeSO 4 + Cu

copper(II) sulfate iron(II) sulfate

CaCO 3 + SiO 2 CaSiO 3 + CO 2

calcium carbonate calcium silicate

Al 2 (SO 4) 3 + 3BaCl 2 3BaSO 4 + 2AlCl 3

sulfate chloride sulfate chloride

aluminum barium barium aluminum

4. Salts of oxygen-free acids are formed by the interaction of metals with non-metals:

2Fe + 3Cl 2 2FeCl 3

iron(III) chloride

physical properties.

Salts are solids of various colors. Their solubility in water is different. All salts of nitric and acetic acids, as well as sodium and potassium salts, are soluble. The water solubility of other salts can be found in the solubility table.

Chemical properties.

1) Salts react with metals.

Since these reactions proceed in aqueous solutions, Li, Na, K, Ca, Ba and other active metals, which react with water under normal conditions, cannot be used for experiments, or reactions can be carried out in a melt.

CuSO 4 + Zn ZnSO 4 + Cu

Pb(NO 3) 2 + Zn Zn(NO 3) 2 + Pb

2) Salts react with acids. These reactions take place when a stronger acid displaces a weaker acid, releasing gas or precipitates.

When carrying out these reactions, they usually take a dry salt and act with concentrated acid.

BaCl 2 + H 2 SO 4 BaSO 4 + 2HCl

Na 2 SiO 3 + 2HCl 2NaCl + H 2 SiO 3

3) Salts react with alkalis in aqueous solutions.

This is a method for obtaining insoluble bases and alkalis.

FeCl 3 (p-p) + 3NaOH(p-p) Fe(OH) 3 + 3NaCl

CuSO 4 (p-p) + 2NaOH (p-p) Na 2 SO 4 + Cu(OH) 2

Na 2 SO 4 + Ba(OH) 2 BaSO 4 + 2NaOH

4) Salts react with salts.

The reactions proceed in solutions and are used to obtain practically insoluble salts.

AgNO 3 + KBr AgBr + KNO 3

CaCl 2 + Na 2 CO 3 CaCO 3 + 2NaCl

5) Some salts decompose when heated.

A typical example of such a reaction is the burning of limestone, the main component of which is calcium carbonate:

CaCO 3 CaO + CO2 calcium carbonate

1. Some salts are able to crystallize with the formation of crystalline hydrates.

Copper (II) sulfate CuSO 4 is a white crystalline substance. When it dissolves in water, it heats up and forms a blue solution. The release of heat and color change are signs of a chemical reaction. When the solution is evaporated, the CuSO 4 crystalline hydrate is released. 5H 2 O (copper sulfate). The formation of this substance indicates that copper (II) sulfate reacts with water:

CuSO 4 + 5H 2 O CuSO 4 . 5H2O+Q

white blue blue

The use of salts.

Most salts are widely used in industry and in everyday life. For example, sodium chloride NaCl, or table salt, is indispensable in cooking. In industry, sodium chloride is used to produce sodium hydroxide, NaHCO 3 soda, chlorine, and sodium. Salts of nitric and orthophosphoric acids are mainly mineral fertilizers. For example, potassium nitrate KNO 3 is potassium nitrate. It is also found in gunpowder and other pyrotechnic mixtures. Salts are used to obtain metals, acids, in the production of glass. Many plant protection products against diseases, pests, and some medicinal substances also belong to the class of salts. Potassium permanganate KMnO 4 is often called potassium permanganate. Limestone and gypsum - CaSO 4 - are used as building materials. 2H 2 O, which is also used in medicine.

Solutions and solubility.

As previously stated, solubility is an important property of salts. Solubility - the ability of a substance to form with another substance a homogeneous, stable system of variable composition, consisting of two or more components.

Solutions are homogeneous systems consisting of solvent molecules and solute particles.

So, for example, a solution of table salt consists of a solvent - water, a solute - ions Na +, Cl -.

ions(from the Greek ión - going), electrically charged particles formed when electrons (or other charged particles) are lost or gained by atoms or groups of atoms. The concept and term "ion" was introduced in 1834 by M. Faraday, who, studying the effect of electric current on aqueous solutions of acids, alkalis, and salts, suggested that the electrical conductivity of such solutions is due to the movement of ions. Positively charged ions moving in solution towards the negative pole (cathode) Faraday called cations, and negatively charged ions moving towards the positive pole (anode) - anions.

According to the degree of solubility in water, substances are divided into three groups:

1) Highly soluble;

2) Slightly soluble;

3) Practically insoluble.

Many salts are highly soluble in water. When deciding on the solubility of other salts in water, you will have to use the solubility table.

It is well known that some substances in dissolved or molten form conduct electric current, while others do not conduct current under the same conditions.

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. Electrolytes themselves do not conduct electricity. In solutions and melts, they decompose into ions, due to which the current flows.

The breakdown of electrolytes into ions when they are dissolved in water is called electrolytic dissociation. Its content boils down to the following three provisions:

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

2) Under the action of an electric current, ions acquire a directed movement: positively charged ions move towards the cathode and are called cations, and negatively charged ions move towards the anode and are called anions.

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

reversibility

Strong and weak electrolytes.

To quantitatively characterize the ability of an electrolyte to decompose into ions, the concept of the degree of dissociation (α), t . E. The ratio of the number of molecules decomposed into ions to the total number of molecules. For example, α = 1 indicates that the electrolyte has completely decomposed into ions, and α = 0.2 means that only every fifth of its molecules has dissociated. When a concentrated solution is diluted, as well as when heated, its electrical conductivity increases, since the degree of dissociation increases.

Depending on the value of α, electrolytes are conditionally divided into strong ones (they dissociate almost completely, (α 0.95) of medium strength (0.95

Strong electrolytes are many mineral acids (HCl, HBr, HI, H 2 SO 4 , HNO 3, etc.), alkalis (NaOH, KOH, Ca(OH) 2, etc.), almost all salts. Solutions of some mineral acids (H 2 S, H 2 SO 3, H 2 CO 3 , HCN, HClO), many organic acids (for example, acetic CH 3 COOH), an aqueous solution of ammonia (NH 3 . 2 O), belong to the weak ones, water, some mercury salts (HgCl 2). Medium-strength electrolytes often include hydrofluoric HF, orthophosphoric H 3 PO 4 and nitrous HNO 2 acids.

Salt hydrolysis.

The term "hydrolysis" comes from the Greek words hidor (water) and lysis (decomposition). Hydrolysis is usually understood as an exchange reaction between a substance and water. Hydrolytic processes are extremely common in the nature around us (both animate and inanimate), and are also widely used by humans in modern production and household technologies.

Salt hydrolysis is the reaction of the interaction of ions that make up the salt with water, which leads to the formation of a weak electrolyte and is accompanied by a change in the solution medium.

Three types of salts undergo hydrolysis:

a) salts formed by a weak base and a strong acid (CuCl 2, NH 4 Cl, Fe 2 (SO 4) 3 - cation hydrolysis proceeds)

NH 4 + + H 2 O NH 3 + H 3 O +

NH 4 Cl + H 2 O NH 3. H2O + HCl

The reaction of the medium is acidic.

b) salts formed by a strong base and a weak acid (K 2 CO 3, Na 2 S - anion hydrolysis occurs)

SiO 3 2- + 2H 2 O H 2 SiO 3 + 2OH -

K 2 SiO 3 + 2H 2 O H 2 SiO 3 + 2KOH

The reaction of the medium is alkaline.

c) salts formed by a weak base and a weak acid (NH 4) 2 CO 3, Fe 2 (CO 3) 3 - hydrolysis proceeds along the cation and anion.

2NH 4 + + CO 3 2- + 2H 2 O 2NH 3. H 2 O + H 2 CO 3

(NH 4) 2 CO 3 + H 2 O 2NH 3. H 2 O + H 2 CO 3

Often the reaction of the environment is neutral.

d) salts formed by a strong base and a strong acid (NaCl, Ba (NO 3) 2) are not subject to hydrolysis.

In some cases, hydrolysis proceeds irreversibly (as they say, goes to the end). So, when solutions of sodium carbonate and copper sulfate are mixed, a blue precipitate of a hydrated basic salt precipitates, which, when heated, loses part of the water of crystallization and becomes green - turns into anhydrous basic copper carbonate - malachite:

2CuSO 4 + 2Na 2 CO 3 + H 2 O (CuOH) 2 CO 3 + 2Na 2 SO 4 + CO 2

When mixing solutions of sodium sulfide and aluminum chloride, hydrolysis also goes to the end:

2AlCl 3 + 3Na 2 S + 6H 2 O 2Al(OH) 3 + 3H 2 S + 6NaCl

Therefore, Al 2 S 3 cannot be isolated from an aqueous solution. This salt is obtained from simple substances.

Distinguish between strong and weak electrolytes. Strong electrolytes in solutions are almost completely dissociated. This group of electrolytes includes most salts, alkalis and strong acids. Weak electrolytes include weak acids and weak bases and some salts: mercury (II) chloride, mercury (II) cyanide, iron (III) thiocyanate, and cadmium iodide. Solutions of strong electrolytes at high concentrations have a significant electrical conductivity, and it increases slightly with dilution of the solutions.

Solutions of weak electrolytes at high concentrations are characterized by insignificant electrical conductivity, which increases greatly with dilution of the solutions.

When a substance is dissolved in any solvent, simple (non-solvated) ions are formed, neutral molecules of the solute, solvated (hydrated in aqueous solutions) ions (for example, etc.), ion pairs (or ion twins), which are electrostatically associated groups of oppositely charged ions (for example,), the formation of which is observed in the vast majority of non-aqueous electrolyte solutions, complex ions (for example,), solvated molecules, etc.

In aqueous solutions of strong electrolytes, only simple or solvated cations and anions exist. There are no solute molecules in their solutions. Therefore, it is incorrect to assume the presence of molecules or the presence of long-term bonds between or and in an aqueous solution of sodium chloride.

In aqueous solutions of weak electrolytes, the solute can exist in the form of simple and solvated (-hydrated) ions and undissociated molecules.

In non-aqueous solutions, some strong electrolytes (for example, ) are not completely dissociated even at moderately high concentrations. In most organic solvents, the formation of ion pairs of oppositely charged ions is observed (for more details, see Book 2).

In some cases, it is impossible to draw a sharp line between strong and weak electrolytes.

Interionic forces. Under the action of interionic forces around each freely moving ion, other ions are grouped symmetrically, charged with the opposite sign, forming the so-called ionic atmosphere, or ionic cloud, which slows down the movement of the ion in solution.

For example, in a solution, chloride ions cluster around moving potassium ions, and an atmosphere of potassium ions is created near moving chloride ions.

Ions, the mobility of which is weakened by the forces of interionic extension, exhibit a reduced chemical activity in solutions. This causes deviations in the behavior of strong electrolytes from the classical form of the law of mass action.

Foreign ions present in a solution of a given electrolyte also have a strong influence on the mobility of its ions. The higher the concentration, the more significant the interionic interaction and the stronger the foreign ions affect the ion mobility.

Weak acids and bases have a hydrogen or hydroxyl bond in their molecules that is largely covalent rather than ionic; therefore, when weak electrolytes are dissolved in solvents that differ by a very high dielectric constant, most of their molecules do not decompose into ions.

Solutions of strong electrolytes differ from solutions of weak electrolytes in that they do not contain undissociated molecules. This is confirmed by modern physical and physico-chemical studies. For example, the study of crystals of strong electrolytes of the type by X-ray diffraction confirms the fact that the crystal lattices of salts are built from ions.

When dissolved in a solvent with a high dielectric constant, solvate (hydrated in water) shells are formed around the ions, preventing their combination into molecules. Thus, since strong electrolytes, even in the crystalline state, do not contain molecules, they do not contain molecules in solution even more so.

However, it has been experimentally found that the electrical conductivity of aqueous solutions of strong electrolytes is not equivalent to the electrical conductivity that could be expected during the dissociation of molecules of dissolved electrolytes into ions.

Using the theory of electrolytic dissociation proposed by Arrhenius, it turned out to be impossible to explain this and a number of other facts. To explain them, new scientific provisions were put forward.

At present, the discrepancy between the properties of strong electrolytes and the classical form of the law of mass action can be explained using the theory of strong electrolytes proposed by Debye and Hückel. The main idea of this theory is that forces of mutual attraction arise between ions of strong electrolytes in solutions. These interionic forces cause the behavior of strong electrolytes to deviate from the laws of ideal solutions. The presence of these interactions causes mutual deceleration of cations and anions.

Influence of dilution on interionic attraction. Interionic attraction causes deviations in the behavior of real solutions in the same way as intermolecular attraction in real gases entails deviations in their behavior from the laws of ideal gases. The greater the concentration of the solution, the denser the ionic atmosphere and the lower the mobility of the ions, and hence the electrical conductivity of the electrolytes.

Just as the properties of a real gas at low pressures approach those of an ideal gas, so the properties of solutions of strong electrolytes approach those of ideal solutions at high dilution.

In other words, in dilute solutions, the distances between the ions are so large that the mutual attraction or repulsion experienced by the ions is extremely small and practically reduces to zero.

Thus, the observed increase in the electrical conductivity of strong electrolytes upon dilution of their solutions is explained by the weakening of the interionic forces of attraction and repulsion, which causes an increase in the speed of ion movement.

The less dissociated the electrolyte and the more dilute the solution, the less the interionic electric influence and the less deviations from the law of mass action are observed, and, conversely, the greater the concentration of the solution, the greater the interionic electric influence and the more deviations from the law of mass action are observed.

For the above reasons, the law of mass action in its classical form cannot be applied to aqueous solutions of strong electrolytes, as well as to concentrated aqueous solutions of weak electrolytes.

All substances can be divided into electrolytes and non-electrolytes. Electrolytes include substances whose solutions or melts conduct electric current (for example, aqueous solutions or melts of KCl, H 3 PO 4 , Na 2 CO 3). Non-electrolyte substances do not conduct electric current when melted or dissolved (sugar, alcohol, acetone, etc.).

Electrolytes are divided into strong and weak. Strong electrolytes in solutions or melts completely dissociate into ions. When writing the equations of chemical reactions, this is emphasized by an arrow in one direction, for example:

HCl → H + + Cl -

Ca (OH) 2 → Ca 2+ + 2OH -

Strong electrolytes include substances with a heteropolar or ionic crystal structure (table 1.1).

Table 1.1 Strong electrolytes

Weak electrolytes decompose into ions only partially. Along with ions, in melts or solutions of these substances, the vast majority of non-dissociated molecules are present. In solutions of weak electrolytes, in parallel with dissociation, the reverse process proceeds - association, that is, the combination of ions into molecules. When writing the reaction equation, this is emphasized by two oppositely directed arrows.

CH 3 COOH D CH 3 COO - + H +

Weak electrolytes include substances with a homeopolar type of crystal lattice (table 1.2).

Table 1.2 Weak electrolytes

The equilibrium state of a weak electrolyte in an aqueous solution is quantitatively characterized by the degree of electrolytic dissociation and the electrolytic dissociation constant.

The degree of electrolytic dissociation α is the ratio of the number of molecules decomposed into ions to the total number of dissolved electrolyte molecules:

The degree of dissociation shows what part of the total amount of the dissolved electrolyte decomposes into ions and depends on the nature of the electrolyte and solvent, as well as on the concentration of the substance in the solution, has a dimensionless value, although it is usually expressed as a percentage. With infinite dilution of the electrolyte solution, the degree of dissociation approaches unity, which corresponds to the complete, 100%, dissociation of the solute molecules into ions. For solutions of weak electrolytes α<<1. Сильные электролиты в растворах диссоциируют полностью (α =1). Если известно, что в 0,1 М растворе уксусной кислоты степень электрической диссоциации α =0,0132, это означает, что 0,0132 (или 1,32%) общего количества растворённой уксусной кислоты продиссоциировало на ионы, а 0,9868 (или 98,68%) находится в виде недиссоциированных молекул. Диссоциация слабых электролитов в растворе подчиняется закону действия масс.

In general, a reversible chemical reaction can be represented as:

a A+ b B D d D+ e E

The reaction rate is directly proportional to the product of the concentration of reacting particles in powers of their stoichiometric coefficients. Then for the direct reaction

V 1 = k 1[A] a[B] b,

and the rate of the reverse reaction

V 2 = k 2[D] d[E] e.

At some point in time, the rates of the forward and reverse reactions will equalize, i.e.

This state is called chemical equilibrium. From here

k 1[A] a[B] b=k 2[D] d[E] e

Grouping the constants on one side and the variables on the other side, we get:

Thus, for a reversible chemical reaction in a state of equilibrium, the product of the equilibrium concentrations of the reaction products in powers of their stoichiometric coefficients, related to the same product for the starting substances, is a constant value at a given temperature and pressure. Numerical value of the chemical equilibrium constant To does not depend on the concentration of reactants. For example, the equilibrium constant for the dissociation of nitrous acid, in accordance with the law of mass action, can be written as:

HNO 2 + H 2 OD H 3 O + + NO 2 -

the value K a called the dissociation constant of the acid, in this case nitrous.

The dissociation constant of a weak base is expressed similarly. For example, for the ammonia dissociation reaction:

NH 3 + H 2 O DNH 4 + + OH -

the value K b called the dissociation constant of the base, in this case ammonia. The higher the dissociation constant of the electrolyte, the more the electrolyte dissociates and the higher the concentration of its ions in solution at equilibrium. There is a relationship between the degree of dissociation and the dissociation constant of a weak electrolyte:

This is a mathematical expression of the Ostwald dilution law: when a weak electrolyte is diluted, the degree of its dissociation increases. For weak electrolytes at To≤1∙10 -4 and With≥0.1 mol/l use the simplified expression:

To= α 2 ∙With or α

Example1. Calculate the degree of dissociation and concentration of ions and [ NH 4 + ] in 0.1 M ammonium hydroxide solution if To NH 4 OH \u003d 1.76 ∙ 10 -5

Given: NH 4 OH

To NH 4 OH \u003d 1.76 ∙ 10 -5

Decision:

Since the electrolyte is rather weak ( To NH 4 OH =1,76∙10 –5 <1∙ 10 - 4) и раствор его не слишком разбавлен, можно принять, что:

or 1.33%

The concentration of ions in a binary electrolyte solution is equal to C∙α, since the binary electrolyte ionizes with the formation of one cation and one anion, then \u003d [ NH 4 + ] \u003d 0.1 1.33 10 -2 \u003d 1.33 10 -3 (mol / l).

Answer:α=1.33%; \u003d [ NH 4 + ] \u003d 1.33 ∙ 10 -3 mol / l.

Theory of strong electrolytes

Strong electrolytes in solutions and melts completely dissociate into ions. However, experimental studies of the electrical conductivity of solutions of strong electrolytes show that its value is somewhat underestimated compared to the electrical conductivity that should be at 100% dissociation. This discrepancy is explained by the theory of strong electrolytes proposed by Debye and Hueckel. According to this theory, in solutions of strong electrolytes, there is an electrostatic interaction between ions. Around each ion, an “ionic atmosphere” is formed from ions of opposite charge, which slows down the movement of ions in solution when a direct electric current is passed. In addition to the electrostatic interaction of ions, in concentrated solutions it is necessary to take into account the association of ions. The influence of interionic forces creates the effect of incomplete dissociation of molecules, i.e. apparent degree of dissociation. The value of α determined experimentally is always somewhat lower than the true α. For example, in a 0.1 M Na 2 SO 4 solution, the experimental value α = 45%. To take into account electrostatic factors in solutions of strong electrolytes, the concept of activity is used (a). The activity of an ion is called the effective or apparent concentration, according to which the ion acts in solution. Activity and true concentration are related by the expression:

where f- activity coefficient, which characterizes the degree of deviation of the system from the ideal due to electrostatic interactions of ions.

The activity coefficients of ions depend on the value of µ, called the ionic strength of the solution. The ionic strength of a solution is a measure of the electrostatic interaction of all ions present in a solution and is equal to half the sum of the products of the concentrations (with) of each of the ions present in the solution per square of its charge number (z):

In dilute solutions (µ<0,1М) коэффициенты активности меньше единицы и уменьшаются с ростом ионной силы. Растворы с очень низкой ионной силой (µ < 1∙10 -4 М) можно считать идеальными. В бесконечно разбавленных растворах электролитов активность можно заменить истинной концентрацией. В идеальной системе a = c and the activity factor is 1. This means that there are practically no electrostatic interactions. In very concentrated solutions (µ>1M), the activity coefficients of ions can be greater than unity. The relationship of the activity coefficient with the ionic strength of the solution is expressed by the formulas:

at µ <10 -2

at 10 -2 ≤ µ ≤ 10 -1

+ 0,1z2µ at 0.1<µ <1

The equilibrium constant expressed in terms of activities is called thermodynamic. For example, for the reaction

a A+ b B d D+ e E

the thermodynamic constant has the form:

It depends on temperature, pressure and the nature of the solvent.

Since the activity of the particle , then

where To C is the concentration equilibrium constant.

Meaning To C depends not only on temperature, the nature of the solvent and pressure, but also on the ionic strength m. Since thermodynamic constants depend on the smallest number of factors, they are, therefore, the most fundamental characteristics of equilibrium. Therefore, in reference books, it is thermodynamic constants that are given. The values of thermodynamic constants of some weak electrolytes are given in the appendix of this manual. \u003d 0.024 mol / l.

With an increase in the charge of the ion, the activity coefficient and the activity of the ion decrease.

Questions for self-control:

What is an ideal system? Name the main reasons for the deviation of a real system from an ideal one.
What is the degree of dissociation of electrolytes?
Give examples of strong and weak electrolytes.
What is the relationship between the dissociation constant and the degree of dissociation of a weak electrolyte? Express it mathematically.
What is activity? How are the activity of an ion and its true concentration related?
What is an Activity Factor?
How does the charge of an ion affect the value of the activity coefficient?
What is the ionic strength of a solution, its mathematical expression?
Write down the formulas for calculating the activity coefficients of individual ions depending on the ionic strength of the solution.
Formulate the law of mass action and express it mathematically.
What is the thermodynamic equilibrium constant? What factors influence its value?
What is the concentration equilibrium constant? What factors influence its value?
How are thermodynamic and concentration equilibrium constants related?
To what extent can the value of the activity coefficient change?
What are the main provisions of the theory of strong electrolytes?