Topics of the Unified State Examination codifier:Electrolytic dissociation of electrolytes in introductory solutions. Strong and weak electrolytes.

- these are substances whose solutions and melts conduct electric current.

Electric current is the ordered movement of charged particles under the influence of electric field. Thus, solutions or melts of electrolytes contain charged particles. In electrolyte solutions, as a rule, electrical conductivity is due to the presence of ions.

Ions are charged particles (atoms or groups of atoms). Separate positively charged ions ( cations) and negatively charged ions ( anions).

Electrolytic dissociation - This is the process of the breakdown of an electrolyte into ions when it dissolves or melts.

Separate substances - electrolytes And non-electrolytes. TO non-electrolytes include substances with a strong covalent nonpolar bond (simple substances), all oxides (which are chemically Not interact with water), most organic substances (except for polar compounds - carboxylic acids, their salts, phenols) - aldehydes, ketones, hydrocarbons, carbohydrates.

TO electrolytes include some substances with a covalent polar bond and substances with an ionic crystal lattice.

What is the essence of the process electrolytic dissociation?

Place some sodium chloride crystals in a test tube and add water. After some time, the crystals will dissolve. What happened?
Sodium chloride is a substance with an ionic crystal lattice. NaCl crystal consists of Na+ ions and Cl - . In water, this crystal disintegrates into structural units - ions. In this case, ionic chemical bonds and some hydrogen bonds between water molecules break down. The Na + and Cl - ions that get into the water interact with water molecules. In the case of chloride ions, we can talk about the electrostatic attraction of dipole (polar) water molecules to the chlorine anion, and in the case of sodium cations, it approaches donor-acceptor in nature (when the electron pair of the oxygen atom is placed in the vacant orbitals of the sodium ion). Surrounded by water molecules, the ions become coveredhydration shell. The dissociation of sodium chloride is described by the equation: NaCl = Na + + Cl - .

When compounds with a covalent polar bond are dissolved in water, water molecules, surrounding the polar molecule, first stretch the bond in it, increasing its polarity, then break it into ions, which are hydrated and evenly distributed in the solution. For example, hydrochloric acid dissociates into ions like this: HCl = H + + Cl - .

During melting, when the crystal is heated, the ions begin to undergo intense vibrations in the nodes of the crystal lattice, as a result of which it is destroyed and a melt is formed, which consists of ions.

The process of electrolytic dissociation is characterized by the degree of dissociation of the molecules of the substance:

Degree of dissociation is the ratio of the number of dissociated (disintegrated) molecules to total number electrolyte molecules. That is, what fraction of the molecules of the original substance disintegrates into ions in a solution or melt.

α=N prodiss /N out, where:

N prodiss is the number of dissociated molecules,

N out is the initial number of molecules.

According to the degree of dissociation, electrolytes are divided into strong And weak.

Strong electrolytes (α≈1):

1. All soluble salts(including salts of organic acids - potassium acetate CH 3 COOK, sodium formate HCOONa, etc.)

2. Strong acids: HCl, HI, HBr, HNO 3, H 2 SO 4 (in the first stage), HClO 4, etc.;

3. Alkalis: NaOH, KOH, LiOH, RbOH, CsOH; Ca(OH)2, Sr(OH)2, Ba(OH)2.

Strong electrolytes disintegrate into ions almost completely in aqueous solutions, but only in. In solutions, even strong electrolytes can only partially disintegrate. Those. the degree of dissociation of strong electrolytes α is approximately equal to 1 only for unsaturated solutions of substances. In saturated or concentrated solutions, the degree of dissociation of strong electrolytes can be less than or equal to 1: α≤1.

Weak electrolytes (α<1):

1. Weak acids, incl. organic;

2. Insoluble bases and ammonium hydroxide NH 4 OH;

3. Insoluble and some slightly soluble salts (depending on solubility).

Non-electrolytes:

1. Oxides that do not interact with water (oxides that interact with water, when dissolved in water, enter into a chemical reaction to form hydroxides);

2. Simple substances;

3. Most organic substances with weakly polar or non-polar bonds (aldehydes, ketones, hydrocarbons, etc.).

How do substances dissociate? According to the degree of dissociation they distinguish strong And weak electrolytes.

Strong electrolytes dissociate completely (in saturated solutions), in one step, all molecules disintegrate into ions, almost irreversibly. Please note that during dissociation in solution, only stable ions are formed. The most common ions can be found in the solubility table - your official cheat sheet for any exam. The degree of dissociation of strong electrolytes is approximately equal to 1. For example, during the dissociation of sodium phosphate, Na + and PO 4 3– ions are formed:

Na 3 PO 4 → 3Na + +PO 4 3-

NH 4 Cr(SO 4) 2 → NH 4 + + Cr 3+ + 2SO 4 2–

Dissociation weak electrolytes : polyacid acids and polyacid bases occurs stepwise and reversibly. Those. During the dissociation of weak electrolytes, only a very small part of the original particles disintegrates into ions. For example, carbonic acid:

H 2 CO 3 ↔ H + + HCO 3 –

HCO 3 – ↔ H + + CO 3 2–

Magnesium hydroxide also dissociates in 2 steps:

Mg(OH) 2 ⇄ Mg(OH) + OH –

Mg(OH) + ⇄ Mg 2+ + OH –

Acid salts also dissociate stepwise, ionic bonds are broken first, then polar covalent bonds. For example, potassium hydrogen carbonate and magnesium hydroxychloride:

KHCO 3 ⇄ K + + HCO 3 – (α=1)

HCO 3 – ⇄ H + + CO 3 2– (α< 1)

Mg(OH)Cl ⇄ MgOH + + Cl – (α=1)

MgOH + ⇄ Mg 2+ + OH – (α<< 1)

The degree of dissociation of weak electrolytes is much less than 1: α<<1.

The main provisions of the theory of electrolytic dissociation are thus:

1. When dissolved in water, electrolytes dissociate (break up) into ions.

2. The reason for the dissociation of electrolytes in water is its hydration, i.e. interaction with water molecules and breaking of chemical bonds in it.

3. Under the influence of an external electric field, positively charged ions move towards a positively charged electrode - the cathode; they are called cations. Negatively charged electrons move towards the negative electrode - the anode. They are called anions.

4. Electrolytic dissociation occurs reversibly for weak electrolytes, and practically irreversibly for strong electrolytes.

5. Electrolytes can dissociate into ions to varying degrees, depending on external conditions, concentration and nature of the electrolyte.

6. The chemical properties of ions differ from the properties of simple substances. The chemical properties of electrolyte solutions are determined by the properties of the ions that are formed from it during dissociation.

Examples.

1. With incomplete dissociation of 1 mol of salt, the total number of positive and negative ions in the solution was 3.4 mol. Salt formula – a) K 2 S b) Ba(ClO 3) 2 c) NH 4 NO 3 d) Fe(NO 3) 3

Solution: First, let's determine the strength of electrolytes. This can be easily done using the solubility table. All salts given in the answers are soluble, i.e. strong electrolytes. Next, we write down the equations of electrolytic dissociation and use the equation to determine the maximum number of ions in each solution:

A) K 2 S ⇄ 2K + + S 2– , with the complete decomposition of 1 mole of salt, 3 moles of ions are formed; more than 3 moles of ions cannot be obtained;

b) Ba(ClO 3) 2 ⇄ Ba 2+ + 2ClO 3 –, again, during the decomposition of 1 mole of salt, 3 moles of ions are formed, more than 3 moles of ions are not formed;

V) NH 4 NO 3 ⇄ NH 4 + + NO 3 –, during the decomposition of 1 mole of ammonium nitrate, a maximum of 2 moles of ions are formed; no more than 2 moles of ions are formed;

G) Fe(NO 3) 3 ⇄ Fe 3+ + 3NO 3 –, with the complete decomposition of 1 mole of iron (III) nitrate, 4 moles of ions are formed. Consequently, with incomplete decomposition of 1 mole of iron nitrate, the formation of a smaller number of ions is possible (incomplete decomposition is possible in a saturated salt solution). Therefore, option 4 suits us.

There are close to 1 such electrolytes.

Strong electrolytes include many inorganic salts, some inorganic acids and bases in aqueous solutions, as well as in solvents with high dissociating ability (alcohols, amides, etc.).

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SOLUTIONS
THEORY OF ELECTROLYTIC DISSOCIATION

ELECTROLYTIC DISSOCIATION
ELECTROLYTES AND NON-ELECTROLYTES

Electrolytic dissociation theory

(S. Arrhenius, 1887)

1. When dissolved in water (or melted), electrolytes break down into positively and negatively charged ions (subject to electrolytic dissociation).

2. Under the influence of electric current, cations (+) move towards the cathode (-), and anions (-) move towards the anode (+).

3. Electrolytic dissociation is a reversible process (the reverse reaction is called molarization).

4. Degree of electrolytic dissociation ( a ) depends on the nature of the electrolyte and solvent, temperature and concentration. It shows the ratio of the number of molecules broken up into ions ( n ) to the total number of molecules introduced into the solution ( N).

a = n / N 0< a <1

Mechanism of electrolytic dissociation of ionic substances

When dissolving compounds with ionic bonds ( for example NaCl ) the hydration process begins with the orientation of water dipoles around all the protrusions and faces of the salt crystals.

Orienting around the ions of the crystal lattice, water molecules form either hydrogen or donor-acceptor bonds with them. This process releases a large amount of energy, which is called hydration energy.

The energy of hydration, the magnitude of which is comparable to the energy of the crystal lattice, is used to destroy the crystal lattice. In this case, the hydrated ions pass layer by layer into the solvent and, mixing with its molecules, form a solution.

Mechanism of electrolytic dissociation of polar substances

Substances whose molecules are formed according to the type of polar covalent bond (polar molecules) dissociate similarly. Around each polar molecule of matter ( for example HCl ), water dipoles are oriented in a certain way. As a result of interaction with water dipoles, the polar molecule becomes even more polarized and turns into an ionic molecule, then free hydrated ions are easily formed.

Electrolytes and non-electrolytes

The electrolytic dissociation of substances, which occurs with the formation of free ions, explains the electrical conductivity of solutions.

The process of electrolytic dissociation is usually written down in the form of a diagram, without revealing its mechanism and omitting the solvent ( H2O ), although he is the main participant.

CaCl 2 « Ca 2+ + 2Cl -

KAl(SO 4) 2 « K + + Al 3+ + 2SO 4 2-

HNO 3 « H + + NO 3 -

Ba(OH) 2 « Ba 2+ + 2OH -

From the electrical neutrality of molecules it follows that the total charge of cations and anions should be equal to zero.

For example, for

Al 2 (SO 4) 3 ––2 (+3) + 3 (-2) = +6 - 6 = 0

KCr(SO 4) 2 ––1 (+1) + 3 (+3) + 2 (-2) = +1 + 3 - 4 = 0

Strong electrolytes

These are substances that, when dissolved in water, almost completely disintegrate into ions. As a rule, strong electrolytes include substances with ionic or highly polar bonds: all highly soluble salts, strong acids ( HCl, HBr, HI, HClO4, H2SO4, HNO3 ) and strong bases ( LiOH, NaOH, KOH, RbOH, CsOH, Ba (OH) 2, Sr (OH) 2, Ca (OH) 2).

In a strong electrolyte solution, the solute is mainly in the form of ions (cations and anions); undissociated molecules are practically absent.

Weak electrolytes

Substances that partially dissociate into ions. Solutions of weak electrolytes contain undissociated molecules along with ions. Weak electrolytes cannot produce a high concentration of ions in solution.

Weak electrolytes include:

1) almost all organic acids ( CH 3 COOH, C 2 H 5 COOH, etc.);

2) some inorganic acids ( H 2 CO 3, H 2 S, etc.);

3) almost all salts, bases and ammonium hydroxide that are slightly soluble in water(Ca 3 (PO 4) 2; Cu (OH) 2; Al (OH) 3; NH 4 OH);

4) water.

They conduct electricity poorly (or almost not at all).

СH 3 COOH « CH 3 COO - + H +

Cu(OH) 2 «[CuOH] + + OH - (first stage)

[CuOH] + « Cu 2+ + OH - (second stage)

H 2 CO 3 « H + + HCO - (first stage)

HCO 3 - « H + + CO 3 2- (second stage)

Non-electrolytes

Substances whose aqueous solutions and melts do not conduct electric current. They contain covalent non-polar or low-polar bonds that do not break down into ions.

Gases, solids (non-metals), and organic compounds (sucrose, gasoline, alcohol) do not conduct electric current.

Degree of dissociation. Dissociation constant

The concentration of ions in solutions depends on how completely a given electrolyte dissociates into ions. In solutions of strong electrolytes, the dissociation of which can be considered complete, the concentration of ions can be easily determined from the concentration (c) and the composition of the electrolyte molecule (stoichiometric indices), For example :

The concentrations of ions in solutions of weak electrolytes are qualitatively characterized by the degree and dissociation constant.

Degree of dissociation (a) - the ratio of the number of molecules disintegrated into ions ( n ) to the total number of dissolved molecules ( N):

a=n/N

and is expressed in fractions of a unit or in % ( a = 0.3 – conventional limit of division into strong and weak electrolytes).

Example

Determine the molar concentration of cations and anions in 0.01 M solutions KBr, NH 4 OH, Ba (OH) 2, H 2 SO 4 and CH 3 COOH.

Degree of dissociation of weak electrolytes a = 0.3.

Solution

KBr, Ba(OH)2 and H2SO4 - strong electrolytes that dissociate completely(a = 1).

KBr « K + + Br -

0.01 M

Ba(OH) 2 « Ba 2+ + 2OH -

0.01 M

0.02 M

H 2 SO 4 « 2H + + SO 4

0.02 M

[ SO 4 2- ] = 0.01 M

NH 4 OH and CH 3 COOH – weak electrolytes(a = 0.3)

NH 4 OH + 4 + OH -

0.3 0.01 = 0.003 M

CH 3 COOH « CH 3 COO - + H +

[H + ] = [ CH 3 COO - ] = 0.3 0.01 = 0.003 M

The degree of dissociation depends on the concentration of the weak electrolyte solution. When diluted with water, the degree of dissociation always increases, because the number of solvent molecules increases ( H2O ) per molecule of solute. According to Le Chatelier’s principle, the equilibrium of electrolytic dissociation in this case should shift in the direction of the formation of products, i.e. hydrated ions.

The degree of electrolytic dissociation depends on the temperature of the solution. Typically, as the temperature increases, the degree of dissociation increases, because bonds in molecules are activated, they become more mobile and are easier to ionize. The concentration of ions in a weak electrolyte solution can be calculated by knowing the degree of dissociationaand initial concentration of the substancec in solution.

Example

Determine the concentration of undissociated molecules and ions in a 0.1 M solution NH4OH , if the degree of dissociation is 0.01.

Solution

Molecular concentrations NH4OH , which at the moment of equilibrium will disintegrate into ions, will be equal toac. Ion concentration NH 4 - and OH - - will be equal to the concentration of dissociated molecules and equalac(according to the electrolytic dissociation equation)

NH4OH		NH4+		OH-
c - a c

A c = 0.01 0.1 = 0.001 mol/l

[NH 4 OH] = c - a c = 0.1 – 0.001 = 0.099 mol/l

Dissociation constant ( K D ) is the ratio of the product of equilibrium ion concentrations to the power of the corresponding stoichiometric coefficients to the concentration of undissociated molecules.

It is the equilibrium constant of the electrolytic dissociation process; characterizes the ability of a substance to disintegrate into ions: the higher K D , the greater the concentration of ions in the solution.

Dissociations of weak polybasic acids or polyacid bases occur in steps; accordingly, each step has its own dissociation constant:

First stage:

H 3 PO 4 « H + + H 2 PO 4 -

K D 1 = () / = 7.1 10 -3

Second stage:

H 2 PO 4 - « H + + HPO 4 2-

K D 2 = () / = 6.2 10 -8

Third stage:

HPO 4 2- « H + + PO 4 3-

K D 3 = () / = 5.0 10 -13

K D 1 > K D 2 > K D 3

Example

Derive an equation relating the degree of electrolytic dissociation of a weak electrolyte ( a ) with dissociation constant (Ostwald dilution law) for a weak monoprotic acid ON .

HA « H + + A +

K D = () /

If the total concentration of a weak electrolyte is denotedc, then the equilibrium concentrations H + and A - are equal ac, and the concentration of undissociated molecules ON - (c - a c) = c (1 - a)

K D = (a c a c) / c(1 - a ) = a 2 c / (1 - a )

In the case of very weak electrolytes ( a £ 0.01)

K D = c a 2 or a = \ é (K D / c )

Example

Calculate the degree of dissociation of acetic acid and the ion concentration H + in 0.1 M solution, if K D (CH 3 COOH) = 1.85 10 -5

Solution

Let's use Ostwald's dilution law

\é (K D / c ) = \é((1.85 10 -5) / 0.1 )) = 0.0136 or a = 1.36%

[H+] = a c = 0.0136 0.1 mol/l

Solubility product

Definition

Place some sparingly soluble salt in a beaker, for example AgCl and add distilled water to the sediment. In this case, the ions Ag+ and Cl- , experiencing attraction from the surrounding water dipoles, gradually break away from the crystals and go into solution. Colliding in solution, ions Ag+ and Cl- form molecules AgCl and deposited on the surface of the crystals. Thus, two mutually opposite processes occur in the system, which leads to dynamic equilibrium, when the same number of ions pass into the solution per unit time Ag+ and Cl- , how many of them are deposited. Ion accumulation Ag+ and Cl- stops in solution, it turns out saturated solution. Consequently, we will consider a system in which there is a precipitate of a sparingly soluble salt in contact with a saturated solution of this salt. In this case, two mutually opposite processes occur:

1) Transition of ions from precipitate to solution. The rate of this process can be considered constant at a constant temperature: V 1 = K 1 ;

2) Precipitation of ions from solution. The speed of this process V 2 depends on ion concentration Ag + and Cl - . According to the law of mass action:

V 2 = k 2

Since this system is in a state of equilibrium, then

V 1 = V 2

k 2 = k 1

K 2 / k 1 = const (at T = const)

Thus, the product of ion concentrations in a saturated solution of a sparingly soluble electrolyte at a constant temperature is constant size. This quantity is calledsolubility product(ETC ).

In the given example ETC AgCl = [Ag + ] [Cl - ] . In cases where the electrolyte contains two or more identical ions, the concentration of these ions must be raised to the appropriate power when calculating the solubility product.

For example, PR Ag 2 S = 2; PR PbI 2 = 2

In general, the expression for the product of solubility for an electrolyte is A m B n

PR A m B n = [A] m [B] n .

The values ​​of the solubility product are different for different substances.

For example, PR CaCO 3 = 4.8 10 -9; PR AgCl = 1.56 10 -10.

ETC easy to calculate, knowing ra c solubility of a compound at a given t°.

Example 1

The solubility of CaCO 3 is 0.0069 or 6.9 10 -3 g/l. Find the PR of CaCO 3.

Solution

Let's express the solubility in moles:

S CaCO 3 = ( 6,9 10 -3 ) / 100,09 = 6.9 10 -5 mol/l

MCaCO3

Since every molecule CaCO3 gives one ion when dissolved Ca 2+ and CO 3 2-, then
[Ca 2+ ] = [ CO 3 2- ] = 6.9 10 -5 mol/l ,
hence, PR CaCO 3 = [Ca 2+ ] [CO 3 2- ] = 6.9 10 –5 6.9 10 -5 = 4.8 10 -9

Knowing the PR value , you can, in turn, calculate the solubility of a substance in mol/l or g/l.

Example 2

Solubility product PR PbSO 4 = 2.2 10 -8 g/l.

What is solubility? PbSO 4 ?

Solution

Let's denote solubility PbSO 4 via X mol/l. Having gone into solution, X moles of PbSO 4 will give X Pb 2+ and X ions ionsSO 4 2- , i.e.:

= = X

ETCPbSO 4 = = = X X = X 2

X =\ é(ETCPbSO 4 ) = \ é(2,2 10 -8 ) = 1,5 10 -4 mol/l.

To go to the solubility expressed in g/l, we multiply the found value by the molecular weight, after which we get:

1,5 10 -4 303,2 = 4,5 10 -2 g/l.

Precipitation formation

If

[ Ag + ] [ Cl - ] < ПР AgCl- unsaturated solution

[ Ag + ] [ Cl - ] = PRAgCl- saturated solution

[ Ag + ] [ Cl - ] > PRAgCl- supersaturated solution

A precipitate is formed when the product of concentrations of ions of a poorly soluble electrolyte exceeds the value of its solubility product at a given temperature. When the ionic product becomes equal to the valueETC, precipitation stops. Knowing the volume and concentration of the mixed solutions, it is possible to calculate whether a precipitate of the resulting salt will precipitate.

Example 3

Does a precipitate form when mixing equal volumes 0.2MsolutionsPb(NO 3 ) 2 AndNaCl.
ETCPbCl 2 = 2,4 10 -4 .

Solution

When mixed, the volume of the solution doubles and the concentration of each substance decreases by half, i.e. will become 0.1 M or 1.0 10 -1 mol/l. These are there will be concentrationsPb 2+ AndCl - . Hence,[ Pb 2+ ] [ Cl - ] 2 = 1 10 -1 (1 10 -1 ) 2 = 1 10 -3 . The resulting value exceedsETCPbCl 2 (2,4 10 -4 ) . Therefore part of the saltPbCl 2 precipitates. From all of the above, we can conclude about the influence of various factors on the formation of precipitation.

Effect of solution concentration

A sparingly soluble electrolyte with a sufficiently large valueETCcannot be precipitated from dilute solutions.For example, sedimentPbCl 2 will not fall out when mixing equal volumes 0.1MsolutionsPb(NO 3 ) 2 AndNaCl. When mixing equal volumes, the concentrations of each substance will become0,1 / 2 = 0,05 Mor 5 10 -2 mol/l. Ionic product[ Pb 2+ ] [ Cl 1- ] 2 = 5 10 -2 (5 10 -2 ) 2 = 12,5 10 -5 .The resulting value is lessETCPbCl 2 , therefore, precipitation will not occur.

Influence of the amount of precipitant

For the most complete precipitation possible, an excess of precipitant is used.

For example, precipitate saltBaCO 3 : BaCl 2 + Na 2 CO 3 ® BaCO 3 ¯ + 2 NaCl. After adding an equivalent amountNa 2 CO 3 ions remain in solutionBa 2+ , the concentration of which is determined by the valueETC.

Increasing ion concentrationCO 3 2- caused by the addition of excess precipitant(Na 2 CO 3 ) , will entail a corresponding decrease in the concentration of ionsBa 2+ in solution, i.e. will increase the completeness of precipitation of this ion.

Influence of the same ion

The solubility of sparingly soluble electrolytes decreases in the presence of other strong electrolytes that have ions of the same name. If to an unsaturated solutionBaSO 4 add solution little by littleNa 2 SO 4 , then the ionic product, which was initially smaller ETCBaSO 4 (1,1 10 -10 ) , will gradually reachETCand will exceed it. Precipitation will begin to form.

Effect of temperature

ETCis a constant value at constant temperature. With increasing temperature ETC increases, so precipitation is best carried out from cooled solutions.

Dissolution of sediments

The solubility product rule is important for converting poorly soluble precipitates into solution. Suppose we need to dissolve the precipitateBaWITHO 3 . The solution in contact with this precipitate is relatively saturatedBaWITHO 3 .
It means that[ Ba 2+ ] [ CO 3 2- ] = PRBaCO 3 .

If you add an acid to a solution, the ionsH + will bind the ions present in the solutionCO 3 2- into molecules of fragile carbonic acid:

2H + + CO 3 2- ® H 2 CO 3 ® H 2 O+CO 2 ­

As a result, the ion concentration will sharply decreaseCO 3 2- , the ionic product will become less thanETCBaCO 3 . The solution will be unsaturated relativelyBaWITHO 3 and part of the sedimentBaWITHO 3 will go into solution. By adding enough acid, the entire precipitate can be brought into solution. Consequently, the dissolution of the precipitate begins when, for some reason, the ionic product of the poorly soluble electrolyte becomes less thanETC. In order to dissolve the precipitate, an electrolyte is introduced into the solution, the ions of which can form a slightly dissociated compound with one of the ions of the sparingly soluble electrolyte. This explains the dissolution of sparingly soluble hydroxides in acids

Fe(OH) 3 + 3HCl® FeCl 3 + 3H 2 O

IonsOH - bind into slightly dissociated moleculesH 2 O.

Table.Solubility product (SP) and solubility at 25AgCl

1,25 10 -5

1,56 10 -10

AgI

1,23 10 -8

1,5 10 -16

Ag 2 CrO4

1,0 10 -4

4,05 10 -12

BaSO4

7,94 10 -7

6,3 10 -13

CaCO3

6,9 10 -5

4,8 10 -9

PbCl 2

1,02 10 -2

1,7 10 -5

PbSO 4

1,5 10 -4

2,2 10 -8

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

Process of dissociation

Electrolytic dissociation is the process of breakdown 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 for dissociation is the disintegration of polar covalent bonds under the action of water molecules. Polar molecules are attracted by water molecules. In solids, ionic bonds are broken during heating. High temperatures cause vibrations of ions at the nodes of the crystal lattice.

Rice. 1. The process of dissociation.

Substances that easily disintegrate into ions in solutions or melts and, therefore, conduct electric current are called electrolytes. Non-electrolytes do not conduct electricity because do not break down 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, disintegrate into ions. Weak electrolytes partially break down into cations and anions. The degree of their dissociation is less than that of strong electrolytes.

The degree of dissociation shows the proportion of disintegrated 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. Solubility table.

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 stepwise:

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

Insoluble bases also decompose 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 electric current, 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, it means there are impurities in it. Distilled water is non-conductive.

The dissociation of weak electrolytes is reversible. The resulting ions reassemble into molecules.

What have we learned?

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

Strong and weak electrolytes

In solutions of some electrolytes, only a portion of the molecules dissociate. To quantitatively characterize the strength of the electrolyte, the concept of the degree of dissociation was introduced. The ratio of the number of molecules dissociated into ions to the total number of molecules of the solute is called the degree of dissociation a.

where C is the concentration of dissociated molecules, mol/l;

C 0 is the initial concentration of the solution, mol/l.

According to the degree of dissociation, all electrolytes are divided into strong and weak. Strong electrolytes include those whose degree of dissociation is more than 30% (a > 0.3). These include:

· strong acids (H 2 SO 4, HNO 3, HCl, HBr, HI);

· soluble hydroxides, except NH 4 OH;

· soluble salts.

Electrolytic dissociation of strong electrolytes is irreversible

HNO 3 ® H + + NO - 3 .

Weak electrolytes have a degree of dissociation less than 2% (a< 0,02). К ним относятся:

· weak inorganic acids (H 2 CO 3, H 2 S, HNO 2, HCN, H 2 SiO 3, etc.) and all organic ones, for example, acetic acid (CH 3 COOH);

· insoluble hydroxides, as well as soluble hydroxide NH 4 OH;

· insoluble salts.

Electrolytes with intermediate values ​​of the degree of dissociation are called electrolytes of medium strength.

The degree of dissociation (a) depends on the following factors:

on the nature of the electrolyte, that is, on the type of chemical bonds; dissociation most easily occurs at the site of the most polar bonds;

from the nature of the solvent - the more polar the latter, the easier the dissociation process occurs in it;

from temperature - increasing temperature enhances dissociation;

on the concentration of the solution - when the solution is diluted, the dissociation also increases.

As an example of the dependence of the degree of dissociation on the nature of chemical bonds, consider the dissociation of sodium hydrogen sulfate (NaHSO 4), the molecule of which contains the following types of bonds: 1-ionic; 2 - polar covalent; 3 - the bond between the sulfur and oxygen atoms is low-polar. Breaking occurs most easily at the site of the ionic bond (1):

Na 1 O 3 O S 3 H 2 O O

1. NaHSO 4 ® Na + + HSO - 4, 2. then at the site of a polar bond of a lesser degree: HSO - 4 ® H + + SO 2 - 4. 3. The acid residue does not dissociate into ions.

The degree of electrolyte dissociation strongly depends on the nature of the solvent. For example, HCl dissociates strongly in water, less strongly in ethanol C 2 H 5 OH, and almost does not dissociate in benzene, in which it practically does not conduct electric current. Solvents with high dielectric constant (e) polarize the solute molecules and form solvated (hydrated) ions with them. At 25 0 C e(H 2 O) = 78.5, e(C 2 H 5 OH) = 24.2, e(C 6 H 6) = 2.27.

In solutions of weak electrolytes, the dissociation process occurs reversibly and, therefore, the laws of chemical equilibrium apply to the equilibrium in solution between molecules and ions. So, for the dissociation of acetic acid

CH 3 COOH « CH 3 COO - + H + .

The equilibrium constant Kc will be determined as

K c = K d = CCH 3 COO - · C H + / CCH 3 COOH.

The equilibrium constant (K c) for the dissociation process is called the dissociation constant (K d). Its value depends on the nature of the electrolyte, solvent and temperature, but it does not depend on the concentration of the electrolyte in the solution. The dissociation constant is an important characteristic of weak electrolytes, since it indicates the strength of their molecules in solution. The smaller the dissociation constant, the weaker the electrolyte dissociates and the more stable its molecules. Considering that the degree of dissociation, in contrast to the dissociation constant, changes with the concentration of the solution, it is necessary to find the relationship between K d and a. If the initial concentration of the solution is taken to be equal to C, and the degree of dissociation corresponding to this concentration is a, then the number of dissociated molecules of acetic acid will be equal to a · C. Since

CCH 3 COO - = C H + = a C,

then the concentration of undissolved molecules of acetic acid will be equal to (C - a · C) or C(1- a · C). From here

K d = aС · a С /(С - a · С) = a 2 С / (1- a). (1)

Equation (1) expresses Ostwald's dilution law. For very weak electrolytes a<<1, то приближенно К @ a 2 С и

a = (K/C). (2)

As can be seen from formula (2), with a decrease in the concentration of the electrolyte solution (when diluted), the degree of dissociation increases.

Weak electrolytes dissociate in stages, for example:

1st stage H 2 CO 3 « H + + HCO - 3,

Stage 2 HCO - 3 « H + + CO 2 - 3 .

Such electrolytes are characterized by several constants, depending on the number of stages of decomposition into ions. For carbonic acid

K 1 = CH + CHCO - 2 / CH 2 CO 3 = 4.45 × 10 -7; K 2 = CH + · CCO 2- 3 / CHCO - 3 = 4.7 × 10 -11.

As can be seen, the decomposition into carbonic acid ions is determined mainly by the first stage, and the second can only appear when the solution is highly diluted.

The total equilibrium of H 2 CO 3 « 2H + + CO 2 - 3 corresponds to the total dissociation constant

K d = C 2 n + · CCO 2- 3 / CH 2 CO 3.

The quantities K 1 and K 2 are related to each other by the relation

K d = K 1 · K 2.

The bases of polyvalent metals dissociate in a similar stepwise manner. For example, two stages of dissociation of copper hydroxide

Cu(OH) 2 « CuOH + + OH - ,

CuOH + « Cu 2+ + OH -

correspond to the dissociation constants

K 1 = СCuOH + · СОН - / СCu(OH) 2 and К 2 = Сcu 2+ · СОН - / СCuOH + .

Since strong electrolytes are completely dissociated in solution, the very term dissociation constant for them has no meaning.

Dissociation of different classes of electrolytes

From the point of view of the theory of electrolytic dissociation acid is a substance whose dissociation produces only the hydrated hydrogen ion H3O (or simply H+) as a cation.

The basis is a substance that, in an aqueous solution, forms hydroxide ions OH - and no other anions - as an anion.

According to Brønsted theory, an acid is a proton donor and a base is a proton acceptor.

The strength of bases, like the strength of acids, depends on the value of the dissociation constant. The larger the dissociation constant, the stronger the electrolyte.

There are hydroxides that can interact and form salts not only with acids, but also with bases. Such hydroxides are called amphoteric. These include Be(OH) 2 , Zn(OH) 2 , Sn(OH) 2 , Pb(OH) 2 , Cr(OH) 3 , Al(OH) 3. Their properties are due to the fact that they weakly dissociate as acids and as bases

H + + RO - « ROH « R + + OH -.

This equilibrium is explained by the fact that the bond strength between the metal and oxygen differs slightly from the bond strength between oxygen and hydrogen. Therefore, when beryllium hydroxide reacts with hydrochloric acid, beryllium chloride is obtained

Be(OH) 2 + HCl = BeCl 2 + 2H 2 O,

and when interacting with sodium hydroxide - sodium beryllate

Be(OH) 2 + 2NaOH = Na 2 BeO 2 + 2H 2 O.

Salts can be defined as electrolytes that dissociate in solution to form cations other than hydrogen cations and anions other than hydroxide ions.

Medium salts, obtained by completely replacing the hydrogen ions of the corresponding acids with metal cations (or NH + 4), dissociate completely Na 2 SO 4 « 2Na + + SO 2- 4.

Acid salts dissociate step by step

1 stage NaHSO 4 « Na + + HSO - 4 ,

2nd stage HSO - 4 « H + + SO 2- 4 .

The degree of dissociation in the 1st step is greater than in the 2nd step, and the weaker the acid, the lower the degree of dissociation in the 2nd step.

Basic salts obtained by incomplete replacement of hydroxide ions with acid residues, also dissociate in stages:

1st stage (CuОH) 2 SO 4 « 2 CuОH + + SO 2- 4,

Stage 2 CuОH + « Cu 2+ + OH - .

Basic salts of weak bases dissociate mainly in the 1st step.

Complex salts, containing a complex complex ion that retains its stability upon dissolution, dissociate into a complex ion and outer sphere ions

K 3 « 3K + + 3 - ,

SO 4 « 2+ + SO 2 - 4 .

At the center of the complex ion is a complexing atom. This role is usually performed by metal ions. Polar molecules or ions, and sometimes both together, are located (coordinated) near the complexing agents; they are called ligands. The complexing agent together with the ligands constitutes the inner sphere of the complex. Ions located far from the complexing agent, less tightly bound to it, are located in the external environment of the complex compound. The inner sphere is usually enclosed in square brackets. The number indicating the number of ligands in the inner sphere is called coordination. Chemical bonds between complex and simple ions are relatively easily broken during the process of electrolytic dissociation. Bonds leading to the formation of complex ions are called donor-acceptor bonds.

Outer sphere ions are easily split off from the complex ion. This dissociation is called primary. Reversible disintegration of the inner sphere is much more difficult and is called secondary dissociation

Cl « + + Cl - - primary dissociation,

+ « Ag + +2 NH 3 - secondary dissociation.

secondary dissociation, like the dissociation of a weak electrolyte, is characterized by an instability constant

K nest. = × 2 / [ + ] = 6.8 × 10 -8 .

The instability constants (K inst.) of various electrolytes is a measure of the stability of the complex. The less K nest. , the more stable the complex.

So, among similar compounds:

-	+	+	+
K nest = 1.3×10 -3	K nest =6.8×10 -8	K nest =1×10 -13	K nest =1×10 -21

The stability of the complex increases upon transition from - to +.

The values ​​of the instability constant are given in chemistry reference books. Using these values, it is possible to predict the course of reactions between complex compounds, with a strong difference in instability constants, the reaction will go towards the formation of a complex with a lower instability constant.

A complex salt with a low-stable complex ion is called double salt. Double salts, unlike complex salts, dissociate into all the ions included in their composition. For example:

KAl(SO 4) 2 « K + + Al 3+ + 2SO 2- 4,

NH 4 Fe(SO 4) 2 « NH 4 + + Fe 3+ + 2SO 2- 4.

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