Home Trees and shrubs Carbonic acid is a strong or weak electrolyte. Strong and weak electrolytes, their characteristics

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Carbonic acid is a strong or weak electrolyte. Strong and weak electrolytes, their characteristics

Strong electrolytes, when dissolved in water, almost completely dissociate into ions, regardless of their concentration in solution.

Therefore, in the equations of dissociation of strong electrolytes, an equal sign (=) is put.

Strong electrolytes include:

Soluble salts;

Many do not organic acids: HNO3, H2SO4, HCl, HBr, HI;

Bases formed by alkali metals (LiOH, NaOH, KOH, etc.) and alkaline earth metals (Ca (OH) 2, Sr (OH) 2, Ba (OH) 2).

Weak electrolytes in aqueous solutions only partially (reversibly) dissociate into ions.

Therefore, in the dissociation equations weak electrolytes put a reversibility sign (⇄).

Weak electrolytes include:

Almost all organic acids and water;

Some inorganic acids: H2S, H3PO4, H2CO3, HNO2, H2SiO3, etc .;

Insoluble metal hydroxides: Mg (OH) 2, Fe (OH) 2, Zn (OH) 2, etc.

Ionic reaction equations

Ionic reaction equations
Chemical reactions in solutions of electrolytes (acids, bases and salts) proceed with the participation of ions. The final solution may remain clear (the products are highly soluble in water), but one of the products will be a weak electrolyte; in other cases, precipitation or gas evolution will be observed.

For reactions in solutions with the participation of ions, not only a molecular equation is made up, but also a complete ionic and a short ionic one.
In ionic equations at the suggestion of the French chemist K. -L. Berthollet (1801), all strong well-soluble electrolytes are written in the form of ion formulas, and precipitates, gases and weak electrolytes are written in the form molecular formulas... The formation of precipitation is marked with an arrow down (↓), the formation of gases is indicated with an arrow up (). An example of writing the reaction equation according to Berthollet's rule:

a) molecular equation
Na2CO3 + H2SO4 = Na2SO4 + CO2 + H2O
b) complete ionic equation
2Na + + CO32− + 2H + + SO42− = 2Na + + SO42− + CO2 + H2O
(CO2 - gas, H2O - weak electrolyte)
c) a short ionic equation
CO32− + 2H + = CO2 + H2O

Usually, when writing, they are limited to a short ionic equation, with reagent solids denoted by an index (t), gaseous reagents - by an index (g). Examples:

1) Cu (OH) 2 (s) + 2HNO3 = Cu (NO3) 2 + 2H2O
Cu (OH) 2 (t) + 2H + = Cu2 + + 2H2O
Cu (OH) 2 is practically insoluble in water
2) BaS + H2SO4 = BaSO4 ↓ + H2S
Ba2 + + S2− + 2H + + SO42− = BaSO4 ↓ + H2S
(full and short ionic equations are the same)
3) CaCO3 (t) + CO2 (g) + H2O = Ca (HCO3) 2
CaCO3 (t) + CO2 (g) + H2O = Ca2 + + 2HCO3−
(most acidic salts are readily soluble in water).

If not involved in the reaction strong electrolytes, the ionic form of the equation is absent:

Mg (OH) 2 (t) + 2HF (p) = MgF2 ↓ + 2H2O

TICKET number 23

Hydrolysis of salts

Salt hydrolysis is the interaction of salt ions with water with the formation of low-dissociating particles.

Hydrolysis, literally, is decomposition by water. Giving such a definition of the salt hydrolysis reaction, we emphasize that salts in solution are in the form of ions, and that driving force reaction is the formation of low-dissociating particles ( general rule for many reactions in solution).

Hydrolysis occurs only in those cases when the ions formed as a result of the electrolytic dissociation of the salt - cation, anion, or both together - are able to form weakly dissociating compounds with water ions, and this, in turn, occurs when the cation is highly polarizing ( weak base cation), and the anion readily polarizes (weak acid anion). This changes the pH of the medium. If the cation forms a strong base, and the anion forms a strong acid, then they do not undergo hydrolysis.

1.Hydrolysis of a salt of a weak base and strong acid passes through the cation, while a weak base or basic salt may form and the pH of the solution will decrease

2.Hydrolysis of a salt of a weak acid and a strong base passes through the anion, while a weak acid or acid salt and the pH of the solution will increase

3.Hydrolysis of a salt of a weak base and a weak acid usually passes completely with the formation of a weak acid and a weak base; In this case, the pH of the solution slightly differs from 7 and is determined by the relative strength of the acid and base

4.Hydrolysis of the salt of a strong base and a strong acid does not proceed

Question 24 Classification of oxides

Oxides are called complex substances, whose molecules include oxygen atoms in the oxidation state - 2 and some other element.

Oxides can be obtained by direct interaction of oxygen with another element, and indirectly (for example, by decomposition of salts, bases, acids). Under normal conditions, oxides are in solid, liquid and gaseous state, this type of compound is quite common in nature. Oxides are contained in Earth crust... Rust, sand, water, carbon dioxide Are oxides.

Salt-forming oxides For example,

CuO + 2HCl → CuCl 2 + H 2 O.

CuO + SO 3 → CuSO 4.

Salt-forming oxides Are such oxides that, as a result chemical reactions form salts. These are oxides of metals and non-metals, which, when interacting with water, form the corresponding acids, and when interacting with bases, they form the corresponding acidic and normal salts. For example, copper oxide (CuO) is a salt-forming oxide, because, for example, when it interacts with hydrochloric acid(HCl) salt forms:

CuO + 2HCl → CuCl 2 + H 2 O.

Other salts can be obtained as a result of chemical reactions:

CuO + SO 3 → CuSO 4.

Non-salt-forming oxides such oxides are called which do not form salts. An example is CO, N 2 O, NO.

Weak electrolytes

Weak electrolytes- substances that partially dissociate into ions. Solutions of weak electrolytes, along with ions, contain undissociated molecules. Weak electrolytes cannot give 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 Ca 3 (PO 4) 2, poorly soluble in water; Cu (OH) 2; Al (OH) 3; NH 4 OH;

They conduct poorly (or hardly conduct) electric current.

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

The degree of dissociation is expressed in fractions of a unit or as a percentage (a = 0.3 is the conditional border of division into strong and weak electrolytes).

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 (H 2 O) per solute molecule increases. According to Le Chatelier's principle, the equilibrium of electrolytic dissociation in this case should shift in the direction of product formation, i.e. hydrated ions.

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

HAn = H + + An -.

The equilibrium constant K p of this reaction is the dissociation constant K d:

K d =. /. (10.11)

If we express the equilibrium concentrations through the concentration of a weak electrolyte C and its degree of dissociation α, we get:

K d = C. α. S. α / S. (1-α) = C. α 2/1-α. (10.12)

This attitude is called Ostwald dilution law... For very weak electrolytes at α<<1 это уравнение упрощается:

K d = C. α 2. (10.13)

This allows us to conclude that with infinite dilution, the degree of dissociation α tends to unity.

Protolytic equilibrium in water:

At constant temperature in dilute solutions, the concentration of water in water is constant and equal to 55.5, ( )

, (10.15)

where K in is the ionic product of water.

Then = 10 -7. In practice, due to the convenience of measuring and recording, a quantity is used - the pH, (criterion) of the strength of an acid or base. Similarly .

From equation (11.15): . At pH = 7 - the reaction of the solution is neutral, at pH<7 – кислая, а при pH>7 - alkaline.

Under normal conditions (0 ° C):

, then

Figure 10.4 - pH of various substances and systems

10.7 Strong electrolyte solutions

Strong electrolytes are substances that, when dissolved in water, almost completely decompose into ions. As a rule, strong electrolytes include substances with ionic or strongly polar bonds: all readily soluble salts, strong acids (HCl, HBr, HI, HClO 4, H 2 SO 4, HNO 3) and strong bases (LiOH, NaOH, KOH, RbOH, CsOH, Ba (OH) 2, Sr (OH) 2, Ca (OH) 2).

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

The fundamental difference between strong and weak electrolytes is that the dissociation balance of strong electrolytes is completely shifted to the right:

H 2 SO 4 = H + + HSO 4 -,

and therefore the constant of equilibrium (dissociation) turns out to be an indefinite quantity. A decrease in electrical conductivity with an increase in the concentration of a strong electrolyte is due to the electrostatic interaction of ions.

The Dutch scientist Petrus Josephus Wilhelmus Debye and the German scientist Erich Hückel, proposing a model that formed the basis of the theory of strong electrolytes, postulated:

1) the electrolyte completely dissociates, but in relatively dilute solutions (C M = 0.01 mol l -1);

2) each ion is surrounded by a shell of ions of the opposite sign. In turn, each of these ions is solvated. This environment is called the ionic atmosphere. In the electrolytic interaction of ions of opposite signs, it is necessary to take into account the influence of the ionic atmosphere. When a cation moves in an electrostatic field, the ionic atmosphere is deformed; it thickens in front of him and thinns behind him. This asymmetry of the ionic atmosphere has the more inhibiting effect on the movement of the cation, the higher the concentration of electrolytes and the greater the charge of the ions. In these systems, the concept of concentration becomes ambiguous and must be replaced by activity. For a binary single-charged electrolyte KatAn = Kat + + An - the activities of the cation (a +) and anion (a -), respectively, are

a + = γ +. C +, a - = γ -. C -, (10.16)

where C + and C - are the analytical concentrations of the cation and anion, respectively;

γ + and γ - are their activity coefficients.

(10.17)

It is impossible to determine the activity of each ion separately, therefore, for single-charged electrolytes, they use the geometric mean values of the activities I

and activity coefficients.

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 Ca 3 (PO 4) 2, poorly soluble in water; Cu (OH) 2; Al (OH) 3; NH 4 OH;

They conduct poorly (or hardly conduct) electric current.

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

The degree of dissociation is expressed in fractions of a unit or as a percentage (a = 0.3 is the conditional border of division into strong and weak electrolytes).

HAn = H + + An -.

The equilibrium constant K p of this reaction is the dissociation constant K d:

K d =. /. (10.11)

If we express the equilibrium concentrations through the concentration of a weak electrolyte C and its degree of dissociation α, we get:

K d = C. α. S. α / S. (1-α) = C. α 2/1-α. (10.12)

This attitude is called Ostwald dilution law... For very weak electrolytes at α<<1 это уравнение упрощается:

K d = C. α 2. (10.13)

This allows us to conclude that with infinite dilution, the degree of dissociation α tends to unity.

Protolytic equilibrium in water:

At constant temperature in dilute solutions, the concentration of water in water is constant and equal to 55.5, ( )

, (10.15)

where K in is the ionic product of water.

Then = 10 -7. In practice, due to the convenience of measuring and recording, a quantity is used - the pH, (criterion) of the strength of an acid or base. Similarly .

From equation (11.15): . At pH = 7 - the reaction of the solution is neutral, at pH<7 – кислая, а при pH>7 - alkaline.

Under normal conditions (0 ° C):

, then

Figure 10.4 - pH of various substances and systems

10.7 Strong electrolyte solutions

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

The fundamental difference between strong and weak electrolytes is that the dissociation balance of strong electrolytes is completely shifted to the right:

H 2 SO 4 = H + + HSO 4 -,

The Dutch scientist Petrus Josephus Wilhelmus Debye and the German scientist Erich Hückel, proposing a model that formed the basis of the theory of strong electrolytes, postulated:

1) the electrolyte completely dissociates, but in relatively dilute solutions (C M = 0.01 mol l -1);

a + = γ +. C +, a - = γ -. C -, (10.16)

where C + and C - are the analytical concentrations of the cation and anion, respectively;

γ + and γ - are their activity coefficients.

(10.17)

It is impossible to determine the activity of each ion separately, therefore, for single-charged electrolytes, they use the geometric mean values of the activities I

and activity coefficients:

The Debye-Hückel activity coefficient depends at least on temperature, solvent dielectric constant (ε) and ionic strength (I); the latter serves as a measure of the intensity of the electric field produced by ions in a solution.

For a given electrolyte, the ionic strength is expressed by the Debye-Hückel equation:

The ionic strength, in turn, is

where C is the analytical concentration;

z is the charge of the cation or anion.

For a single-charged electrolyte, the ionic strength coincides with the concentration. Thus, NaCl and Na 2 SO 4 at the same concentration will have different ionic strengths. Comparison of the properties of solutions of strong electrolytes can be carried out only when the ionic strengths are the same; even small impurities dramatically change the properties of the electrolyte.

Figure 10.5 - Dependency

Depending on the degree of dissociation, electrolytes are distinguished between strong and weak. K is the dissociation constant, which depends on the temperature and nature of the electrolyte and solvent, but does not depend on the concentration of the electrolyte. Reactions between ions in electrolyte solutions go almost to the end towards the formation of precipitates, gases and weak electrolytes.

Electrolyte is a substance that conducts an electric current due to dissociation into ions, which occurs in solutions and melts, or the movement of ions in the crystal lattices of solid electrolytes. Examples of electrolytes include aqueous solutions of acids, salts and bases and some crystals (eg silver iodide, zirconium dioxide).

How to identify strong and weak electrolytes

At the same time, the processes of association of ions into molecules take place in the electrolyte. To quantitatively characterize electrolytic dissociation, the concept of the degree of dissociation was introduced. Most often, they mean an aqueous solution containing certain ions (for example, "absorption of electrolytes" in the intestine). Multicomponent solution for metal electrodeposition, etching, etc. (technical term, for example, gilding electrolyte).

The main object of research and development in electroplating is electrolytes for surface treatment and coating. In the chemical etching of metals, the name of the electrolyte is determined by the name of the basic acids or alkalis that help dissolve the metal. This is how the group name of electrolytes is formed. Sometimes the difference (especially in the value of polarizability) between electrolytes of different groups is leveled by the additives contained in the electrolytes.

Electrolytes and electrolytic dissociation

Therefore, such a name cannot be a classification (that is, a group name), but should serve as an additional subgroup name of an electrolyte. If the electrolyte density in all cells of the battery is normal or close to normal (1.25-1.28 g / cm3), and the NRC is not lower than 12.5 V, then it is necessary to check for an open circuit inside the battery. If the density of the electrolyte in all cells is low, the battery should be charged until the density stabilizes.

In engineering [edit edit wiki text]

During the transition from one state to another, the voltage and density indicators of the electrolyte change linearly within certain limits (Fig. 4 and Table 1). The deeper the battery discharge, the lower the density of the electrolyte. Accordingly, the volume of the electrolyte contains the amount of sulfuric acid required for the full use of the active substance of the plates in the reaction.

Ionic conductivity is inherent in many chemical compounds with an ionic structure, for example, salts in the solid or molten state, as well as many aqueous and non-aqueous solutions. Electrolytic dissociation is understood as the disintegration of electrolyte molecules in solution with the formation of positively and negatively charged ions - cations and anions. The degree of dissociation is often expressed as a percentage. This is due to the fact that the concentrations of metallic copper and silver are introduced into the equilibrium constant.

This is explained by the fact that the concentration of water during the reactions in aqueous solutions changes very little. Therefore, it is assumed that the concentration remains constant and is entered into the equilibrium constant. Since electrolytes in solutions form ions, so-called ionic reaction equations are often used to reflect the essence of reactions.

The term electrolyte is widely used in biology and medicine. The process of decomposition of molecules in a solution or molten electrolyte into ions is called electrolytic dissociation. Therefore, a certain fraction of the molecules of the substance is dissociated in electrolytes. There is no clear boundary between these two groups; the same substance can exhibit the properties of a strong electrolyte in one solvent, and a weak one in the other.

Electrolytic dissociation theory proposed by the Swedish scientist S. Arrhenius in 1887.

Electrolytic dissociation- This is the disintegration of electrolyte molecules with the formation of positively charged (cations) and negatively charged (anions) ions in solution.

For example, acetic acid dissociates like this in an aqueous solution:

CH 3 COOH⇄H + + CH 3 COO -.

Dissociation refers to reversible processes. But different electrolytes dissociate in different ways. The degree depends on the nature of the electrolyte, its concentration, the nature of the solvent, external conditions (temperature, pressure).

Dissociation degree α - the ratio of the number of molecules decayed into ions to the total number of molecules:

α = v´ (x) / v (x).

The degree can vary from 0 to 1 (from no dissociation to complete completion). It is indicated as a percentage. Determined experimentally. The dissociation of the electrolyte leads to an increase in the number of particles in the solution. The degree of dissociation indicates the strength of the electrolyte.

Distinguish strong and weak electrolytes.

Strong electrolytes- these are electrolytes, the degree of dissociation of which exceeds 30%.

Medium strength electrolytes- these are those, the degree of dissociation of which divides in the range from 3% to 30%.

Weak electrolytes- the degree of dissociation in an aqueous 0.1 M solution is less than 3%.

Examples of weak and strong electrolytes.

Strong electrolytes in dilute solutions completely decompose into ions, i.e. α = 1. But experiments show that dissociation cannot be equal to 1, it has an approximate value, but not 1. This is not a true dissociation, but an apparent one.

For example, let some connection α = 0.7. Those. according to the Arrhenius theory, 30% of non-dissociated molecules "float" in solution. And 70% formed free ions. And the electrostatic theory gives a different definition to this concept: if α = 0.7, then all molecules are dissociated into ions, but the ions are only 70% free, and the remaining 30% are bound by electrostatic interactions.

Apparent degree of dissociation.

The degree of dissociation depends not only on the nature of the solvent and the solute, but also on the concentration of the solution and temperature.

The dissociation equation can be represented as follows:

AK ⇄ A- + K +.

And the degree of dissociation can be expressed as follows:

With an increase in the concentration of the solution, the degree of dissociation of the electrolyte decreases. Those. the value of the degree for a particular electrolyte is not a constant value.

Since dissociation is a reversible process, the reaction rate equations can be written as follows:

If dissociation is in equilibrium, then the rates are equal and as a result we obtain equilibrium constant(dissociation constant):

K depends on the nature of the solvent and on temperature, but does not depend on the concentration of solutions. It can be seen from the equation that the more undissociated molecules, the lower the value of the dissociation constant of the electrolyte.

Polybasic acids dissociate stepwise, and each step has its own value of the dissociation constant.

If a polybasic acid dissociates, then the first proton is most easily split off, and with an increase in the charge of the anion, the attraction increases, and therefore the proton is split off much more difficult. For example,

The dissociation constants of phosphoric acid at each stage should be very different:

I - stage:

II - stage:

III - stage:

At the first stage, orthophosphoric acid is an acid of medium strength, and at the second stage it is weak, at the third stage it is very weak.

Examples of equilibrium constants for some electrolyte solutions.

Let's consider an example:

If metallic copper is added to a solution containing silver ions, then at the moment of equilibrium, the concentration of copper ions must be greater than the concentration of silver.

But the constant has a low value:

AgCl⇄Ag + + Cl -.

This suggests that very little silver chloride had dissolved by the time equilibrium was reached.

The concentration of metallic copper and silver are entered into the equilibrium constant.

Ionic product of water.

The following table has data:

This constant is called ionic product of water which depends only on temperature. According to dissociation, there is one hydroxide ion per 1 H + ion. In pure water, the concentration of these ions is the same: [ H + ] = [OH - ].

Hence, [ H + ] = [OH-] = = 10-7 mol / l.

If you add a foreign substance to the water, for example, hydrochloric acid, then the concentration of hydrogen ions will increase, but the ionic product of water does not depend on the concentration.

And if you add alkali, then the concentration of ions will increase, and the amount of hydrogen will decrease.

Concentration and interrelated: the more one value, the less the other.

The acidity of the solution (pH).

The acidity of solutions is usually expressed by the concentration of ions H +. In acidic environments NS<10 -7 моль/л, в нейтральных - NS= 10 -7 mol / l, in alkaline - NS> 10 -7 mol / l.
The acidity of a solution is expressed through the negative logarithm of the concentration of hydrogen ions, calling it NS.

pH = -lg[ H + ].

The relationship between the constant and the degree of dissociation.

Consider an example of acetic acid dissociation:

Let's find the constant:

Molar concentration C = 1 /V, substitute it into the equation and get:

These equations are breeding law V. Ostwald, according to which the dissociation constant of the electrolyte does not depend on the dilution of the solution.