The value A is expressed in the fractions of the unit or in% and depends on the nature of the electrolyte, solvent, temperature, concentration and composition of the solution.

A solvent played a special role: in some cases, during the transition from aqueous solutions to organic solvents, the degree of dissociation of electrolytes can increase or decrease. In the future, in the absence of special instructions, we assume that the solvent is water.

According to the degree of dissociation, electrolytes are conventionally divided into strong (A\u003e 30%), middle (3% < a < 30%) и weak (A.< 3%).

Silent electrolytes include:

1) some inorganic acids (HCl, HBr, Hi, HNO 3, H 2 SO 4, HCLO 4 and a number of others);

2) alkali hydroxides (Li, Na, k, Rb, Cs) and alkaline earth (CA, SR, BA) metals;

3) Almost all soluble salts.

The high-force electrolytes include Mg (OH) 2, H 3 PO 4, HCOOH, H 2 SO 3, HF and some others.

All carboxylic acids are considered weak electrolytes (except HCOOH) and hydrated forms of aliphatic and aromatic amines. Many inorganic acids are also weak electrolytes (HCN, H 2 S, H 2 CO 3, etc.) and the base (NH 3 ∙ H 2 O).

Despite some coincidences, in general, one should not identify the solubility of the substance with its degree of dissociation. Thus, acetic acid and ethyl alcohol are unlimited soluble in water, but at the same time, the first substance is a weak electricity, and the second is non-election.

Acids and bases

Despite the fact that the concepts of "acid" and "base" are widely used to describe chemical processes, a single approach to the classification of substances from the point of view of their attribution to acids or there are no grounds. Currently existing theories ( ionic theory S. Arrhenius, prollatical theory I. Brensteda and T. Lowry and electronic theory G. Lewisa) have certain limitations and, thus, applicable only in particular cases. Let us dwell on each of these theories.

Arrhenius theory.

In the ion theory of Arrhenius, the concept of "acid" and "base" are closely related to the process of electrolytic dissociation:

Acid is an electrolyte, dissociating in solutions to form H + ions;

The base is the electrolyte, dissociating in solutions with the formation of ions it -;

An ampholite (amphoteric electrolyte) is an electrolyte dissociating in solutions to form both H + ions and ions it is.

For example:

On ⇄ n + + a - nh + + meo n n - ⇄ me (it) n ⇄ me n + + non -

In accordance with ionic theory, acids can be both neutral molecules and ions, for example:

HF ⇄ H + + F -

H 2 PO 4 - ⇄ H + + HPO 4 2 -

NH 4 + ⇄ H + + NH 3

Similar examples can be brought for bases:

Kon k + + he -

- ⇄ Al (OH) 3 + he -

+ ⇄ Fe 2+ + he -

Ampholites include zinc hydroxides, aluminum, chromium and some others, as well as amino acids, proteins, nucleic acids.

In general, the acid-base interaction in the solution is reduced to the neutralization reaction:

H + + it - H 2 O

However, a number of experimental data shows the limited ion theory. So, ammonia, organic amines, oxides of metals type Na 2 O, Cao, anions of weak acids, etc. In the absence of water, the properties of typical bases are exhibiting, although they do not have hydroxide ions.

On the other hand, many oxides (SO 2, SO 3, p 2 O 5, etc.), halides, hydrogen halogenhydrides, without having hydrogen ions, even in the absence of water, the acid properties exhibit, i.e. Neutralize grounds.

In addition, the behavior of the electrolyte in aqueous solution and in the non-aqueous medium can be opposite.

So, CH 3 COOH in water is weak acid:

CH 3 COOH ⇄ CH 3 COO - + H +,

and in the liquid fluoride is the properties of the base:

HF + CH 3 COOH ⇄ CH 3 COOH 2 + + F -

Studies of such types of reactions and in particular reactions occurring in non-aqueous solvents led to the creation of more general theories of acids and bases.

The theory of Brenstead and Lowry.

The further development of the theory of acids and the grounds was the proposed protolytic (proton) theory proposed by I. Brenstened and T. Lowri. In accordance with this theory:

Acid is called any substance, molecules (or ions) which are able to give proton, i.e. be a proton donor;

The base is called any substance, molecules (or ions) of which are capable of attaching a proton, i.e. be a proton acceptor;

Thus, the concept of foundation is significantly expanding, which is confirmed by the following reactions:

He is - + N + N 2

NH 3 + H + NH 4 +

H 2 N-NH 3 + + H + H 3 N + -NH 3 +

According to I. Brensted, T. Loury Acid and the base make up a conjugate pair and are associated with equilibrium:

Acid ⇄ Proton + Base

Since the proton transfer reaction (protolytic reaction) is reversible, and the proton is also transmitted in the reverse process, then the reaction products are each with respect to a friend with acid and base. This can be written in the form of an equilibrium process:

On + in ⇄ VN + + A -,

where is an acid, B - the base, VN + - acid conjugated with the base of B, A is the base associated with acid on.

Examples.

1) in the reaction:

HCl + OH - ⇄ Cl - + H 2 O,

HCl and H 2 O - acids, cl - and oh - - appropriate bases conjugate with them;

2) in the reaction:

HSO 4 - + H 2 O ⇄ SO 4 2 - + H 3 O +,

HSO 4 - and H 3 O + - acids, SO 4 2 - and H 2 O - bases;

3) in the reaction:

NH 4 + + NH 2 - ⇄ 2NH 3,

NH 4 + - acid, NH 2 - base, and NH 3 acts as acid (one molecule) and bases (other molecule), i.e. Demonstrates signs of amphoterity - the ability to show the properties of acid and base.

Water has this ability:

2N 2 O ⇄ H 3 O + + He -

Here, one molecule H 2 o joins the proton (base), forming a conjugate acid - ion hydroxonium H 3 O +, the other gives a proton (acid), forming a conjugate base. This process is called autoprotolize.

From the above examples, it can be seen that, unlike the representations of Arrhenius, in the theory of Brensman and Loury, the reactions of acids do not lead to mutual neutralization, and are accompanied by the formation of new acids and grounds.

It should also be noted that the protolytic theory considers the concept of "acid" and "base" not as a property, but as a function that the considered compound in the protolytic reaction is performed. The same compound may react in some conditions as an acid, in others - as a base. Thus, in an aqueous solution of CH 3, the coxy exhibits the properties of the acid, and 100% H 2 SO 4 - the base.

However, despite its advantages, a protolytic theory, as well as the theory of Arrhenius, is not applicable to substances that are not containing hydrogen atoms, but, at the same time, manifests the function of the acid: boron halides, aluminum, silicon, tin.

Lewis theory.

Another approach to the classification of substances from the point of view of their attribution to acids and the grounds was the electronic theory of Lewis. As part of the electronic theory:

acid is called a particle (molecule or ion) capable of attaching an electron pair (electron acceptor);

the base is called a particle (molecule or ion) capable of giveing \u200b\u200ban electron pair (electron donor).

According to Lewis's ideas, acid and base interact with each other with the formation of donor-acceptor communication. As a result of the addition of pair of electrons at an electronic deficit atom, a complete electronic configuration occurs - electrons octet. For example:

Similarly, a reaction between neutral molecules can be represented:

The neutralization reaction in the terms of the Lewis theory is considered as the addition of the electron pair of hydroxide ion to the hydrogen ion providing a free orbital to accommodate this pair:

Thus, the proton itself, easily connecting the electronic pair, from the point of view of the Lewis theory, performs the function of the acid. In this regard, the acids according to Brenets can be considered as reaction products between Lewis acids and bases. So, HCl is a product of neutralization of acid H + base CL -, and ion H 3 O + is formed as a result of neutralization of acid H + base H 2 O.

The reactions between acids and the bases of Lewis also illustrate the following examples:

Lewis bases also include halide ions, ammonia, aliphatic and aromatic amines, oxygen-containing organic compounds of type R 2 CO, (where R is an organic radical).

Lewis acids include boron halides, aluminum, silicon, tin and other elements.

Obviously, in the theory of Lewis, the concept of "acid" includes a wider range of chemical compounds. This is explained by the fact that in Lewis, the assignment of the substance to the class of acid is due to the sole to the structure of its molecule, which determine the electron-acceptor properties, and is not necessarily associated with the presence of hydrogen atoms. Lewis acids that do not contain hydrogen atoms are called apronic.

Standards Solving Tasks

1. Write the Al 2 electrolytic dissociation equation (SO 4) 3 in water.

Aluminum sulfate is a strong electrolyte and in an aqueous solution is subjected to full decay on the ions. Dissociation equation:

Al 2 (SO 4) 3 + (2x + 3Y) H 2 O 2 3+ + 3 2 -,

or (excluding ion hydration process):

Al 2 (SO 4) 3 2AL 3+ + 3SO 4 2 -.

2. What is the HCO 3 ion - from the standpoint of the theory of Brensteda Loury?

Depending on the conditions of the HCO 3 ion - can give protons:

HCO 3 - + OH - CO 3 2 - + H 2 O (1),

so attach the protons:

HCO 3 - + H 3 O + H 2 CO 3 + H 2 O (2).

Thus, in the first case, the HCO 3 ion is acid, in the second - base, i.e. is an ampholite.

3. Determine than from the position of Lewis theory is AG + ion in the reaction:

AG + + 2NH 3 +

In the process of forming chemical bonds, which flows through a donor-acceptor mechanism, AG + ion, having a free orbital, is an electronic pairs acceptor, and thus exhibits Lewis acid properties.

4. Determine the ionic force of the solution in one liter of which is 0.1 mol KCl and 0.1 mol Na 2 SO 4.

The dissociation of the represented electrolytes occurs in accordance with the equations:

Na 2 SO 4 2NA + + SO 4 2 -

Hence: C (k +) \u003d C (CL -) \u003d C (KCl) \u003d 0.1 mol / l;

C (Na +) \u003d 2 × C (Na 2 SO 4) \u003d 0.2 mol / l;

C (SO 4 2 -) \u003d C (Na 2 SO 4) \u003d 0.1 mol / l.

The ionic power force is calculated by the formula:

5. Determine the concentration of CUSO 4 in the solution of this electrolyte with I. \u003d 0.6 mol / l.

The dissociation CUSO 4 flows through the equation:

CUSO 4 CU 2+ + SO 4 2 -

Come with (Cuso 4) for x. mol / l, then, in accordance with the reaction equation, C (Cu 2+) \u003d C (SO 4 2 -) \u003d x. mol / l In this case, the expression for calculating the ion power will be viewed:

6. Determine the coefficient of activity of ion K + in an aqueous solution of KCl with C (KCl) \u003d 0.001 mol / l.

which in this case will take the form:

.

Ion power of the solution will find by the formula:

7. Determine the activity coefficient of FE 2+ ion in an aqueous solution, the ionic strength of which is equal to 1.

In accordance with the law of Debai Hyukkel:

hence:

8. Determine the dissociation constant HA, if in a solution of this acid with a concentration of 0.1 mol / l a \u003d 24%.

By the magnitude of the dissociation, it can be determined that this acid is an electrolyte of the middle force. Therefore, to calculate the constant of the dissociation of acid, we use the law of breeding of ostelald in its full form:

9. Determine the electrolyte concentration, if A \u003d 10%, K. d \u003d 10 - 4.

From the law of breeding Ostvalda:

10. The degree of dissociation of mono-axial acid HA does not exceed 1%. (HA) \u003d 6.4 × 10 - 7. Determine the degree of dissociation Ha in its solution with a concentration of 0.01 mol / l.

By magnitude of the degree of dissociation, it can be determined that this acid is a weak electrolyte. This allows the use of an approximate formula of the Redemption Law:

11. The degree of discociation of electrolyte in its solution with a concentration of 0.001 mol / l is 0.009. Determine the dissociation constant of this electrolyte.

From the condition of the problem, it can be seen that this electrolyte is weak (a \u003d 0.9%). Therefore:

12. (HNO 2) \u003d 3.35. Compare HNO 2 strength with single-major acid HA, the degree of dissociation of which in solution with C (HA) \u003d 0.15 mol / l is equal to 15%.

Calculate (HA) using the full form of the Ostvald equation:

As (ha)< (HNO 2), то кислота HA является более сильной кислотой по сравнению с HNO 2 .

13. There are two KCL solutions containing other ions. It is known that the ion power of the first solution ( I. 1) equal to 1, and the second ( I. 2) is 10 - 2 amounts. Compare activity coefficients f.(K +) in these solutions and conclude how the properties of these solutions differ from the properties of infinitely diluted KCL solutions.

The activity coefficients of K + ions will calculate using the Debye-Hyukkel law:

The coefficient of activity f. - This is a measure of deviation in the behavior of the electrolyte solution of this concentration from its behavior with infinite mortar.

As f. 1 \u003d 0.316 deflected more than 1 than f. 2 \u003d 0.891, in a solution with a greater ionic force, a greater deflection is observed in the behavior of the KCL solution from its behavior with infinite dilution.

Questions for self-control

1. What is electrolytic dissociation?

2. What substances are called electrolytes and non-electrolytes? Give examples.

3. What is the degree of dissociation?

4. From what factors depends the degree of dissociation?

5. What electrolytes are considered strong? What are the average power? What are weak? Give examples.

6. What is a dissociation constant? What depends on and what does dissociation constant do not depend?

7. How are the constant and the degree of dissociation in the binary solutions of medium and weak electrolytes?

8. Why are the solutions of strong electrolytes in their behavior detect deviations from ideality?

9. What is the essence of the term "apparent degree of dissociation"?

10. What is the activity of ion? What is the asset coefficient?

11. How is the magnitude of the activity coefficient with dilution (concentration) of a solution of strong electrolyte? What is the limit value of the activity coefficient with infinite mortar breeding?

12. What is the ion power of the solution?

13. How do you calculate the activity coefficient? Word Debai Hyukkel law.

14. What is the essence of the ionic theory of acids and the foundations (the theory of Arrhenius)?

15. What is the principal difference between the protolytic theory of acids and the grounds (theory of Brenstead and Lowri) from the theory of Arrhenius?

16. How treats electronic theory (Lewis theory) The concept of "acid" and "base"? Give examples.

Task options for self solutions

Option number 1

1. Write the FE 2 (SO 4) 3 electrolytic dissociation equation (SO 4) 3.

On + H 2 O ⇄ H 3 O + + A -.

Option number 2.

1. Write the equation of electrolytic dissociation CUCL 2.

2. Determine than from the position of the Lewis theory is ion S 2 - in the reaction:

2AG + + S 2 - ⇄ AG 2 S.

3. Calculate the molar concentration of the electrolyte in the solution, if a \u003d 0.75%, and \u003d 10 - 5.

Option number 3.

1. Write the Na 2 SO 4 electrolytic dissociation equation.

2. Determine than from the standpoint of the Lewis theory is CN ion - in the reaction:

Fe 3 + + 6cn - ⇄ 3 -.

3. The ion power of the CaCl 2 solution is 0.3 mol / l. Calculate C (CACL 2).

Option number 4.

1. Write an electrolytic dissociation equation Ca (OH) 2.

2. Determine than from the standpoint of the theory of Brensman is the H 2 O molecule in the reaction:

H 3 O + ⇄ H + + H 2 O.

3. The ion power of the solution K 2 SO 4 is 1.2 mol / l. Calculate with (K 2 SO 4).

Option number 5.

1. Write the electrolytic dissociation equation K 2 SO 3.

NH 4 + + H 2 O ⇄ NH 3 + H 3 O +.

3. (CH 3 COOH) \u003d 4.74. Compare the power of CH 3 COOH with the power of mono-block acid Ha, the degree of dissociation of which in solution with C (HA) \u003d 3.6 × 10 - 5 mol / l is equal to 10%.

Option number 6.

1. Write the electrolytic dissociation equation K 2 S.

2. Determine than from the standpoint of Lewis theory is ALBR 3 molecule in the reaction:

Br - + Albr 3 ⇄ -.

Option number 7.

1. Write an electrolytic dissociation equation Fe (NO 3) 2.

2. Determine than from the position of the Lewis theory is the CL ion - in the reaction:

Cl - + AlCl 3 ⇄ -.

Option number 8.

1. Write the equation of electrolytic dissociation K 2 MNO 4.

2. Determine than from the standpoint of the Brenestian theory is the HSO 3 ion - in the reaction:

HSO 3 - + OH - ⇄ SO 3 2 - + H 2 O.

Option number 9.

1. Write the equation of electrolytic dissociation Al 2 (SO 4) 3.

2. Determine than from the standpoint of Lewis theory is the CO 3+ ion in the reaction:

CO 3+ + 6NO 2 - ⇄ 3 -.

3. In 1 liter of solution contains 0.348 g k 2 SO 4 and 0.17 g Nano 3. Determine the ionic strength of this solution.

Option number 10.

1. Write the equation of electrolytic dissociation Ca (NO 3) 2.

2. Determine than from the standpoint of the theory of Brensman is the H 2 O molecule in the reaction:

B + H 2 O ⇄ OH - + BH +.

3. Calculate the electrolyte concentration in the solution, if a \u003d 5%, and \u003d 10 - 5.

Option number 11.

1. Write the electrolytic dissociation equation KMNO 4.

2. Determine than from the position of the Lewis theory is the Cu 2+ ion in the reaction:

Cu 2+ + 4NH 3 ⇄ 2 +.

3. Calculate the activity coefficient of Cu 2+ ion in the CUSO 4 C solution (Cuso 4) \u003d 0.016 mol / l.

Option number 12.

1. Write the equation of electrolytic dissociation Na 2 CO 3.

2. Determine than from the standpoint of the theory of Brensman is the H 2 O molecule in the reaction:

K + + XH 2 O ⇄ +.

3. There are two NaCl solutions containing other electrolytes. The values \u200b\u200bof the ionic strength of these solutions are respectively equal: I. 1 \u003d 0.1 mol / l, I. 2 \u003d 0.01 mol / l. Compare activity coefficients f.(Na +) in these solutions.

Option number 13.

1. Write the Al (NO 3) 3 electrolytic dissociation equation.

2. Determine than from the positions of the Lewis theory is the RNH 2 molecule in the reaction:

RNH 2 + H 3 O + ⇄ RNH 3 + + H 2 O.

3. Compare the coefficients of the activity of cations in a solution containing FESO 4 and KNO 3, provided that the concentrations of electrolytes are, respectively, 0.3 and 0.1 mol / l.

Option number 14.

1. Write the electrolytic dissociation equation K 3 PO 4.

2. Determine than from the standpoint of the theory of Brenstead is ion H 3 O + in the reaction:

HSO 3 - + H 3 O + ⇄ H 2 SO 3 + H 2 O.

Option number 15.

1. Write the electrolytic dissociation equation K 2 SO 4.

2. Determine than from the position of the Lewis theory is PB (OH) 2 in the reaction:

PB (OH) 2 + 2OH - ⇄ 2 -.

Option number 16.

1. Write the electrolytic dissociation equation Ni (NO 3) 2.

2. Determine than from the standpoint of the theory of Brenstead is the hydroxonium ion (H 3 O +) in the reaction:

2H 3 O + + S 2 - ⇄ H 2 S + 2H 2 O.

3. The ionic force of a solution containing only Na 3 PO 4 is 1.2 mol / l. Determine the concentration of Na 3 PO 4.

Option number 17.

1. Write an electrolytic dissociation equation (NH 4) 2 SO 4.

2. Determine than from the standpoint of the theory of Brenstead is ion NH 4 + in the reaction:

NH 4 + + OH - ⇄ NH 3 + H 2 O.

3. The ionic force of a solution containing at the same time Ki and Na 2 SO 4 is 0.4 mol / l. C (ki) \u003d 0.1 mol / l. Determine concentration of Na 2 SO 4.

Option number 18.

1. Write an electrolytic dissociation equation CR 2 (SO 4) 3.

2. Determine than from the standpoint of the theory of Brenstead is a protein molecule in the reaction:

Information block

Scale PH

Table 3. The relationship of the concentrations of H + and OH ions -.

Standards Solving Tasks

1. The concentration of hydrogen ions in the solution is 10 - 3 mol / l. Calculate pH, POH and [it -] in this solution. Determine the solution medium.

Note.Recruitments are used for computing: LG10 A. = a.; 10 LG. a. = but.

The environment of the solution with pH \u003d 3 is acidic, since pH< 7.

2. Calculate the pH of the hydrochloric acid solution with a molar concentration of 0.002 mol / l.

Since in the diluted solution of NS1 "1, and in a solution of monosocond acid C (k-you) \u003d c (to-you), we can write:

3. To 10 ml of acetic acid solution with C (CH 3 of the Soam) \u003d 0.01 mol / l added 90 ml of water. Find the difference in the values \u200b\u200bof the PN solution before and after dilution, if (CH 3 of the coxy) \u003d 1.85 × 10 - 5.

1) in the initial solution of weak mono large acid CH 3 of the Soam:

Hence:

2) Adding to 10 ml of an acid solution 90 ml of water corresponds to a 10-fold dilution of the solution. Therefore.

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

Dissociation process

Electrolytic dissociation - the process of decaying molecules to 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 decay of covalent polar bonds under the action of water molecules. Polar molecules are delayed with aqueous molecules. In solids, ionic ties are destroyed during the heating process. High temperatures cause ion oscillations in the nodes of the crystal lattice.

Fig. 1. Dissociation process.

Substances that easily disintegrate on ions in solutions or in melts and, therefore, electric current is called electrolytes. Non-electrolytes do not conduct electricity, because Do not disintegrate on cations and anions.

Depending on the dissociation, strong and weak electrolytes differ. Strong dissolve in water, i.e. Fully, without the possibility of recovery disintegrate into ions. Weak electrolytes disintegrate 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 molecules in the total concentration of substances. It is expressed by the formula α \u003d n / n.

Fig. 2. The degree of dissociation.

Weak electrolytes

List of weak electrolytes:

• diluted 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 - 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.

Fig. 3. Solubility table.

The dissociation reaction is recorded using an ion equation:

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

Multi-axis acids dissociate stepwise:

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

Insoluble grounds are also collected in stages:

• Fe (OH) 3 ↔ FE (OH) 2 + + OH -;
• Fe (OH) 2 + ↔ Feoh 2+ + Oh -;
• Feoh 2+ ↔ Fe 3+ + Oh -.

Water belongs to weak electrolytes. Water practically does not conduct an electric current, because Weakly disintegrated into hydrogen cations and anions of the Gyroxide ion. The formed ions are collected in water molecules:

H 2 O ↔ H + + OH -.

If the water easily carries out electricity, it means that there are impurities in it. Distilled water is non-electroprip.

The dissociation of weak electrolytes is reversible. Formed ions are collected in the molecule.

What did we know?

Weak electrolytes include substances that partially decaying ions are positive cations and negative anions. Therefore, such substances are poorly conducted by electric current. These include weak and diluted acids, insoluble bases, low-soluble salts. The weakest electrolyte is water. The dissociation of weak electrolytes is a reversible reaction.

Strong and weak electrolytes

Acids, bases and salts in aqueous solutions are dissociated - disintegrate into ions. This process can be reversible or irreversible.

With irreversible dissociation in solutions, all substance or almost everything falls into ions. This is characteristic of strong electrolytes (Fig. 10.1, and, p. 56). Some acids and all soluble salts and bases (alkaline and alkaline earth hydroxides) (Scheme 5, p. 56) include soluble electrolytes.

Fig. 10.1. Comparison of the number of ions in solutions with the same initial amount of electrolyte: A - chloride acid (strong electrolyte); b - nitrite acid

(weak electrolyte)

Scheme 5. Classification of electrolytes by force

When the dissociation is reversible, two opposite process flows: simultaneously with the decay of the substance on ions (dissociation) there is a reverse process of combining ions in the substance molecules (association). Due to this, a part of the substance in the solution exists in the form of ions, and part - in the form of molecules (Fig. 10.1, b). Electrolytes,

which when dissolved in water, disintegrate only partially, is called weak electrolytes. These include water, many acids, as well as insoluble hydroxides and salts (scheme 5).

In the dissociation equations of weak electrolytes, instead of a conventional arrow record a bidirectional arrow (reversibility sign):

The power of the electrolytes can be explained by the polarity of the chemical bond, which is broken upon dissociation. The more polar communication, the easier the water molecules, it turns into ionic, therefore, the stronger the electrolyte. In salts and hydroxides, the polarity of communication is the largest, since there is an ionic connection between the ionic elements and hydroxide ions, therefore all soluble salts and bases are strong electrolytes. In oxygen-containing acids during dissociation, the connection of O - H is broken, the polarity of which depends on the qualitative and quantitative composition of the acid residue. The force of most oxygen-containing acids can be determined if the usual acid formula is written as E (OH) M O n. If this formula is N< 2 — кислота слабая, если n >2 - Strong.

The dependence of acids from the composition of the acid residue

The degree of dissociation

The power of the electrolytes quantitatively characterizes the degree of electrolytic dissociation A, showing the share of molecules of substances that broke up in the solution on ions.

The degree of dissociation A is equal to the ratio of the number of m molecules n or the amount of the substance N, which has been impaved on the ions, to the total number of molecules N 0 or the amount of dissolved substance N 0:

The degree of dissociation can be expressed not only in the fractions of the unit, but also in percent:

The value A may vary from 0 (there is no dissociation) to 1, or 100% (complete dissociation). The better the electrolyte decays, the greater the value of the degree of dissociation.

According to the value of the degree of electrolytic dissociation, electrolytes are often separated by no two, but into three groups: strong, weak and electrolytes of the middle force. Those strong electrolytes consider the degree of dissociation of which more than 30%, and weak with a degree of less than 3%. Electrolytes with intermediate values \u200b\u200bA - from 3% to 30% - called medium power electrolytes. For this classification, acids are considered: HF, HNO 2, H 3 PO 4, H 2 SO 3 and some others. The two recent acids are medium power electrolytes only at the first stage of dissociation, and in others it is weak electrolytes.

The degree of dissociation is the variable value. It depends not only on the nature of the electrolyte, but also on its concentration in solution. This dependence first identified and explored Wilhelm Ostvald. Today, it is called the law of reducing Ostvald: when the solution is diluted with water, as well as with increasing temperature, the degree of dissociation increases.

Calculation of the degree of dissociation

Example. In one liter of water dissolved hydrogen fluoride with the amount of substance 5 mol. The resulting solution contains 0.06 mol hydrogen ions. Determine the degree of dissociation of fluoride acid (as a percentage).

We write the fluoride acid dissociation equation:

During dissociation from one acid molecule, one hydrogen ion is formed. If the solution contains 0.06 mol H + ions, this means that the predsissorate-Valo 0.06 mole of the hydrogen fluoride molecules. Consequently, the degree of dissociation is:

An outstanding German physico-chemist, the Nobel Prize winner in 1909 Chemistry. Born in Riga, studied at the University of Derpta, where he began teaching and scientific activities. At 35, he moved to Leipzig, where he was headed by the Physics and Chemical Institute. He studied the laws of chemical equilibrium, the properties of solutions, discovered the law of breeding called by his name, developed the foundations of the theory of acid-base catalysis, a lot of time paid the history of chemistry. He founded the world's first department of physical chemistry and the first physico-chemical magazine. In personal life possessed strange habits: he felt disgust for the haircut, and with his secretary communicated exclusively with the help of a bicycle call.

Key idea

Dissociation of weak electrolytes - reversible process, and strong -

irreversible.

Control questions

116. Give the definition of strong and weak electrolytes.

117. Give examples of strong and weak electrolytes.

118. What size is used for the quantitative characteristic of the power of the electrolyte? Is it constant in any solutions? How can I increase the degree of electrolyte dissociation?

Tasks for mastering the material

119. Give one example of salt, acid and bases that are: a) with a strong electrolyte; b) weak electrolyte.

120. Give an example of a substance: a) a two-axis acid, which in the first stage is an electrolyte of the middle force, and on the second - weak electrolyte; b) two-axis acid, which on both stages is a weak electrolyte.

121. In some acid at the first stage, the dissociation degree is 100%, and in the second - 15%. What acid can it be?

122. What kind of particles are larger in the sulfide hydrogen solution: molecules H 2 S, H + ions, S 2 ions or HS ions -?

123. From the above list of substances separately, write out the formula: a) strong electrolytes; b) weak electrolytes.

NaCl, HCl, NaOH, Nano 3, HNO 3, HNO 2, H 2 SO 4, BA (OH) 2, H 2 S, K 2 S, PB (NO 3) 2.

124. Make the equation of dissociation of strontium nitrate, Mercury (11) chloride, calcium carbonate, calcium hydroxide, sulfide acid. In what cases does dissociation be reversible?

125. In an aqueous sodium sulfate solution contains 0.3 mol ions. What mass of this salt was used to prepare such a solution?

126. In the solution of hydrogen fluoride, 1 liter contains 2 g of this acid, and the amount of substance of hydrogen ions is 0.008 mol. What is the amount of fluoride ion substances in this solution?

127. In three tubes, the same volumes of chloride, fluoride and sulfide acid solutions are contained. In all test tubes of the amount of substance, the acids are equal. But in the first test tube, the amount of substance of hydrogen ions is 3. 10 -7 mol, in the second - 8. 10 -5 mol, and in the third - 0.001 mol. Which tube contains each acid?

128. The first test tube contains an electrolyte solution, the degree of dissociation of which is 89%, in the second - electrolyte with a dissociation of 8% o, and in the third - 0.2% of. Bring two examples of electrolytes of different classes of compounds that may be contained in these test tubes.

129 *. In additional sources, find information on the dependence of the power of electrolytes from the nature of substances. Set the relationship between the structure of substances, the nature of the chemical elements that form them and the power of electrolytes.

This is the material of the textbook

Strong and weak electrolytes

Only part of the molecules dissociate in solutions of some electrolytes. For the quantitative characteristic of the electrolyte force, the concept of dissociation degree was introduced. The ratio of the number of molecules dissociated by ions, to the total number of solute molecules is called the degree of dissociation a.

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

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

By the magnitude of the dissociation, all electrolytes are divided into strong and weak. The strong electrolytes belongs to the degree of dissociation of which more than 30% (A\u003e 0.3). These include:

· Strong acids (H 2 SO 4, HNO 3, HCl, HBr, Hi);

· Soluble hydroxides, except NH 4 OH;

· Salties soluble.

Electrolytic dissociation of strong electrolytes proceeds irreversible

HNO 3 ® H + + NO - 3.

Weak electrolytes have a dissociation degree 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, for example, acetic acid (CH 3 COOH);

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

· Insoluble salts.

The electrolytes with intermediate values \u200b\u200bof the dissociation are called medium power electrolytes.

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

from the nature of electrolyte, that is, on the type of chemical bonds; dissociation is most easily occurring at the place of the most polar bonds;

from the nature of the solvent - the larger the latter, the easier it goes in it the dissociation process;

from temperature - temperature increase enhances dissociation;

from the concentration of the solution - when diluting the solution dissociation also increases.

As an example of the dependence of the degree of dissociation on the nature of chemical bonds, we consider dissociation of sodium hydrosulfate (NaHSO 4), in the molecule of which the following types of links are available: 1-ion; 2 - polar covalent; 3 - the relationship between sulfur and oxygen atoms is low-polar. The most easily breaks at the place of ion connection (1):

Na 1 O 3 O S 3 H 2 O O

1. NaHSO 4 ® Na + + HSO - 4, 2. Then, at the place of polar communication, a lesser extent: HSO - 4 ® H + + SO 2 - 4. 3. The acid residue on the ions does not dissociate.

The degree of dissociation of electrolyte strongly depends on the nature of the solvent. For example, HCl dissociates strongly in water, weaker in ethanol C 2 H 5 OH, almost not dissociates in benzene in which it practically does not conduct electric current. Solvents with high dielectric constant (E) polarize the molecules of the dissolved substance and form solvated (hydrated) ions with them. At 25 0 С E (H 2 O) \u003d 78.5, E (C 2 H 5 OH) \u003d 24.2, E (C 6 H 6) \u003d 2.27.

In solutions of weak electrolytes, the dissociation process is reversible and, therefore, the laws of chemical equilibrium are applied to equilibrium in the solution between molecules and ions. So, for acetic acid dissociation

CH 3 COOH "CH 3 COO - + H +.

Equilibrium constant to C will be determined as

K \u003d K d \u003d СХ 3 coo - · with H + / SCH 3 COOH.

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

СХ 3 coo - \u003d with h + \u003d a · s,

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

K D \u003d AS · A C / (C - A · C) \u003d A 2 C / (1- A). (one)

Equation (1) expresses the law of dilution of ostelald. For very weak electrolytes a<<1, то приближенно К @ a 2 С и

a \u003d (k / s). (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 are dissociated by steps, for example:

1 step H 2 CO 3 "H + + NSO - 3,

2 Stage NSO - 3 "H + + CO 2 - 3.

Such electrolytes are characterized by several constants - depending on the number of decay steps on ions. For coalic acid

K 1 \u003d CH + · SNO - 2 / CH 2 C 3 \u003d 4.45 × 10 -7; K 2 \u003d CH + · CSO 2-3 / SNSO - 3 \u003d 4.7 × 10 -11.

As can be seen, the decay on the ions of coalic acid is determined mainly by the first stage, and the second can only manifest themselves with a large solo dilution.

Total equilibrium H 2 CO 3 "2H + + CO 2 - 3 corresponds to the total constant of dissociation

K d \u003d C 2 H + · CSO 2-3 / CH 2 CO 3.

Values \u200b\u200bto 1 and K 2 are associated with each other ratio

K d \u003d K 1 · K 2.

Similarly, the bases of multivalent metals are dissociated. For example, two stages of the dissociation of copper hydroxide

Cu (OH) 2 "Cuoh + + Oh -,

Cuoh + "Cu 2+ + Oh -

answer constants of dissociation

K 1 \u003d CuOH + · Sleep - / CCU (OH) 2 and K 2 \u003d CCU 2+ · Sleep - / Cuoh +.

Since strong electrolytes dissociated in the solution, the term of the dissociation constant itself is deprived of the content.

Dissociation of various electrolyte classes

From the point of view of the theory of electrolytic dissociation acid a substance is called, with dissociation of which only hydrated hydrogen ion H 3 O (or simply H +) is formed as a cation.

Baseit is called a substance that in aqueous solution as an anion forms hydroxide ions it - and no other anions.

According to the theory of Brencented, the acid is the proton donor, and the base is the protons acceptor.

The base force as the power of acids depends on the magnitude of the dissociation constant. The greater the dissociation constant, the stronger the electrolyte.

There are hydroxides capable of entering into cooperation and form salts not only with acids, but also with the grounds. 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. The properties are due to the fact that they are dissociated by the type of acids in a weak degree and by the type of base

H + + RO - « Roh. « R + + ON -.

This equilibrium is explained by the fact that the strength of the connection between metal and oxygen is slightly different from the strength of the connection between oxygen and hydrogen. Therefore, in the interaction of hydroxide beryllium with hydrochloric acid, it turns out of beryllium chloride

BE (OH) 2 + HCl \u003d BECL 2 + 2H 2 O,

and when interacting with sodium hydroxide - Beryllate sodium

Be (OH) 2 + 2NAOH \u003d Na 2 BEO 2 + 2H 2 O.

Sololi. It can be determined as electrolytes that dissociate in the solution to form cations other than hydrogen cations and anions other than hydroxide ions.

Middle Salts, The resulting replacement of hydrogen ions of appropriate acids on metal cations (eitherNH + 4) is dissociated by completely Na 2 SO 4 "2NA + + SO 2-4.

Sour salts dissociate along steps

1 Step NaHSO 4 "Na + + HSO - 4 ,

2 Step HSO. - 4 "H + + SO 2- 4.

The degree of dissociation in the 1st stage is greater than in the 2nd stage, and the less acid, the less the degree of dissociation in the 2nd stage.

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

1 step (Cuoh) 2 SO 4 "2 Cuoh + + SO 2-4,

2 Step Cuoh + "Cu 2+ + Oh -.

The main salts of weak grounds are dissociated mainly in the 1st stage.

Complex salts, Containing a complex complex ion that preserves its stability during dissolution, dissociate on a complex ion and ions of the external sphere

K 3 "3K + + 3 -,

SO 4 "2+ + SO 2 - 4.

In the center of the complex ion there is an atom - a complex of consumer. This role is usually performed by metal ions. Near complexing agents are located (coordinated) polar molecules or ions, and sometimes those and others together, they are called ligands.The complexing agent together with ligands constitutes the inner sphere of the complex. Ions far away from the complexing agent are less firmly related to it, are in the external environment of the complex compound. The inner sphere usually concludes square brackets. The number indicating the number of ligands in the inner sphere is called coordination. Chemical bonds between complex and simple ions in the process of electrolytic dissociation are relatively easy to break. Communications leading to the formation of complex ions received the name of donor-acceptor ties.

The ions of the external sphere are easily cleaved from the complex ion. This dissociation is called primary. The reversible decay of the internal sphere occurs much more difficult and is called the secondary dissociation.

Cl "+ + Cl - - primary dissociation,

+ "AG + +2 NH 3 - Secondary dissociation.

secondary dissociation, as the dissociation of weak electrolyte, is characterized by a constant of insufficiency

To Nest. \u003d × 2 / [+] \u003d 6.8 × 10 -8.

The constants of the unstoppacy (to nonsense) of various electrolytes is a measure of the sustainability of the complex. The smaller to NAST. , the more stable complex.

So, among the same type of connections:

-	+	+	+
K NAST \u003d 1.3 × 10 -3	K NAST \u003d 6.8 × 10 -8	K NAST \u003d 1 × 10 -13	K NAST \u003d 1 × 10 -21

the stability of the complex increases when moving from - to +.

The values \u200b\u200bof the instability constant lead in reference books in chemistry. With the help of these values, it is possible to predict the reactions between complex compounds with a strong difference between the constants of the inconvenience, the reaction will go towards the formation of a complex with a smaller constant of the insidacity.

Complex salt with a small-resistant complex ion called double Sali.. Double salts, in contrast to complex, dissociate on all 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.