Hydration of electrolytes with ionic bond. Solutions

THEORY OF ELECTROLYTIC DISSOCIATION

Solutions of all substances can be divided into two groups: conduct electric current or are not conductors.

You can get acquainted with the features of the dissolution of substances experimentally by studying the electrical conductivity of solutions of these substances using the device shown in the figure.

Observe the following experiment " The study of the electrical conductivity of substances.

To explain the features of aqueous solutions of electrolytes by Swedish scientists S. Arrhenius in 1887 was proposed theory of electrolytic dissociation . Later it was developed by many scientists on the basis of the theory of the structure of atoms and chemical bonding. The current content of this theory can be reduced to the following three propositions:

1. electrolytes when dissolved in water or melted break apart (dissociate) into ions - positively (cations) and negative (anions) charged particles.

ions are in more stable electronic states than atoms. They can be made up of one atom simple ions ( Na + , mg 2+ , BUTl 3+ etc.) - or from several atoms - this is complex ions ( NAbout 3 - ,SO 2- 4 , RO Z-4, etc.).

2. In solutions and melts electrolytes conduct electricity .

Under the influence of an electric current, the ions acquire a directed movement: positively charged ions move towards the cathode, negatively charged ones - towards the anode. Therefore, the first are called cations, the second - anions. The directed movement of ions occurs as a result of their attraction by oppositely charged electrodes.

TESTING SUBSTANCES FOR ELECTRICAL CONDUCTIVITY

SUBSTANCES
ELECTROLYTES	NON-ELECTROLYTES
electrolytesare substances whose aqueous solutions or melts conduct electricity	Non-electrolytesare substances whose aqueous solutions or melts do not conduct electricity
Substances with ionic chemical bond or covalent highly polar chemical bond - acids, salts, bases	Substances with covalent non-polar chemical bond or covalent weakly polar chemical communication
In solutions and melts ions are formed	In solutions and melts ions are not formed

REMINDER

ELECTROLYTES AND NON-ELECTROLYTES

THERMAL EFFECTS DURING DISSOLUTION OF SUBSTANCES IN WATER

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

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

KA ↔K + + A -

Consider the process of dissolving electrolytes in water

In general, the water molecule is not charged. But inside the moleculeH 2 O the hydrogen and oxygen atoms are arranged so that the positive and negative charges are at opposite ends of the molecule (Fig. 1). Therefore, the water molecule is a dipole.

Dissolution of substances with an ionic chemical bond in water

(on the example of sodium chloride - table salt)

Mechanism of electrolytic dissociationNaCl when salt is dissolved in water (Fig. 2), it consists in the sequential elimination of sodium and chlorine ions by polar water molecules. Following the transition of ions Na + and Cl - hydrates of these ions are formed from the crystal into the solution.

Dissolution of substances with a covalent highly polar chemical bond in water

(on the example of hydrochloric acid)

When hydrochloric acid is dissolved in water (in moleculesHCl the bond between atoms is covalent strongly polar), the nature of the chemical bond changes. Under the influence of polar water molecules, a polar covalent bond is converted into an ionic one. The resulting ions remain associated with water molecules - hydrated. If the solvent is non-aqueous, then the ions are called solvated (Fig. 3).

Basic provisions:

Electrolytic dissociation - This is the process of decomposition of the electrolyte into ions when it is dissolved in water or melted.

electrolytes- These are substances that, when dissolved in water or in a molten state, decompose into ions.

ions are atoms or groups of atoms that have a positive ( cations) or negative ( anions) charge.

Ions differ from atoms both in structure and in properties.

Example 1 Let's compare the properties of molecular hydrogen (consists of two neutral hydrogen atoms) with the properties of an ion.

hydrogen atom	hydrogen ion
1 H 0 1 s 1	1 H + 1 s 0

Example 2 Let us compare the properties of atomic and molecular chlorine with the properties of the ion.

Chlorine atom	chloride ion
17 Cl 0 1s 2 2s 2 2p 6 3s 2 3p 5	17 Cl - 1s 2 2s 2 2p 6 3s 2 3p 6
Chlorine atoms have an incomplete external level, so they are very active chemically, they accept electrons and are reduced. That is why gaseous chlorine is poisonous, when inhaled it causes poisoning of the body.	Chlorine ions have a completed external level, so they are chemically inactive and are in a stable electronic state. Chlorine ions are part of table salt, the use of which does not cause poisoning of the body.

Remember!

1. Ions differ from atoms and molecules in structure and properties;

2. A common and characteristic feature of ions is the presence of electric charges;

3. Solutions and melts of electrolytes conduct electric current due to the presence of ions in them.

Lecture. Theory of electrolytic dissociation.

Electrolytes, non-electrolytes. electrolytic dissociation.

The reason for the deviation from the laws of van't Hoff and Raoult was first established in 1887 by the Swedish scientist Svante Arrhenius, who proposed the theory of electrolytic dissociation, which is based on two postulates:

Substances whose solutions are electrolytes (i.e., they conduct an electric current), when dissolved, they decompose into particles (ions), which are formed as a result of the dissociation of the solute. In this case, the number of particles increases. Ions that are positively charged are called cations , because under the influence of an electric field, they move towards the cathode. Negatively charged ions - anions , because under the influence of an electric field move towards the anode. Electrolytes include salts, acids and bases.

Al(NO3)3 ® Al ³ + + NO3ֿ

Electrolytes do not completely dissociate. The ability of a substance to dissociate is characterized by the value of the degree of electrolytic dissociation - a. The degree of electrolytic dissociation is the ratio of the amount of electrolyte substance decomposed into ions to the total amount of dissolved electrolyte.

a = ionized / N dissolved

n is the number of molecules decomposed into ions

N is the total number of molecules in solution

a- degree of electrolytic dissociation

The value of a can vary from 0 to 1, often a is expressed as a percentage (from 0 to 100%). The degree of dissociation shows what part of the dissolved amount of electrolyte under given conditions is in solution in the form of hydrated ions.

The causes of electrolytic dissociation are due to:

the nature of chemical bonds in compounds (electrolytes include substances with an ionic or covalent highly polar bond)

The nature of the solvent: the water molecule is polar, i.e. is a dipole

In this way, electrolytic dissociation called the process of decay or polar compounds into ions under the action of polar solvent molecules.

The mechanism of electrolytic dissociation.

The theory of Arrtsius was significantly developed by Russian scientists I.A. Kablukov and V.A. Kistyakovsky, they proved that when the electrolyte is dissolved, the chemical interaction of the dissolved substance with water occurs, which leads to the formation of hydrates, and then they dissociate into ions, i.e. hydrated ions in solution.

The easiest way is the dissociation of a substance with an ionic bond. The sequence of processes occurring during the dissociation of substances with an ionic bond (salts, alkalis) will be as follows:

orientation of water dipole molecules near crystal ions

hydration (interaction) of water molecules with ions of the surface layer of the crystal

dissociation (decay) of the electrolyte crystal into hydrated ions.

Taking into account the hydration of ions, the dissociation equation looks like this:

NaCl + X H2O ® Na + n H2O + Cl - n H2O

Since the composition of hydrated ions is not always constant, the equation is written in abbreviated form:

NaCl ® Na + + Cl -

Similarly, the process of dissociation of substances with a polar bond occurs, the sequence of ongoing processes is as follows:

orientation of water molecules around the poles of an electrolyte molecule

hydration (interaction) of water molecules with electrolyte molecules

ionization of electrolyte molecules (transformation of a covalent polar bond into an ionic one)

dissociation (decay) of electrolyte molecules into hydrated ions.

HCl + H2O ® H3O + + Cl -

HCl ® H + + Cl -

In the process of dissociation, the hydrogen ion does not occur in a free form, only in the form of the hydronium ion H3O + .

The main reason for dissociation is the polarization interaction of polar solvent molecules with molecules of the solute. For example, a water molecule is polar, its dipole moment μ = 1.84 D, i.e. it has a strong polarizing effect. Depending on the structure of the dissolved substance in the anhydrous state, its dissociation proceeds differently. The two most typical cases are:

Rice. 4.8 Dissolution of a substance with an ionic crystal lattice

1. Ionic solute (NaCl, KCl, etc.). Crystals of such substances already consist of ions. When they are dissolved, polar water molecules (dipoles) will be oriented towards the ions with their opposite ends. Forces of mutual attraction arise between the ions and dipoles of water (ion-dipole interaction), as a result, the bond between the ions weakens, and they pass into solution in a hydrated form (Fig. 4.8). In the case under consideration, dissociation of molecules occurs simultaneously with dissolution. Substances with an ionic bond dissociate most easily.

2. A solute with a polar covalent bond (for example, HCl, H 2 SO 4 , H 2 S, etc.). Here, too, water dipoles are oriented in a corresponding way around each polar molecule of the substance with the formation of hydrates. As a result of such a dipole-dipole interaction, the binding electron cloud (electron pair) will almost completely shift to an atom with a higher electronegativity, while the polar molecule turns into an ionic one (the stage of ionization of the molecule) and then decays into ions, which pass into solution in a hydrated form (Fig. 4.9). Dissociation can be complete or partial - it all depends on the degree of polarity of the bonds in the molecule.

ionization dissociation

Rice. 4.9 Dissolution of a substance with a polar covalent bond

The difference between the cases considered is that in the case of an ionic bond, the ions existed in the crystal, while in the case of a polar bond, they are formed in the process of dissolution. Compounds containing both ionic and polar bonds dissociate first along ionic and then along covalent polar bonds. For example, sodium hydrosulfate NaHSO 4 completely dissociates over the Na-O bond, partially - over the H-O bond, and practically does not dissociate over the low-polarity bonds of sulfur with oxygen.

In this way , upon dissolution, only compounds with ionic and covalent polar bonds dissociate, and only in polar solvents.

Degree of dissociation. Strong and weak electrolytes

The quantitative characteristic of electrolytic dissociation is the degree of dissociation of the electrolyte in solution. This characteristic was introduced by Arrhenius. Degree of dissociation – α - this is the ratio of the number of molecules N, decomposed into ions, to the total number of molecules of the dissolved electrolyte N 0:

α is expressed in fractions of a unit or in%.

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

When dissolved in water strong electrolytes dissociate almost completely, the process of dissociation in them is irreversible. For strong electrolytes, the degree of dissociation in solutions is equal to one (α=1) and almost does not depend on the concentration of the solution. In the dissociation equations for strong electrolytes, the sign “=” or “ ” is put. For example, the dissociation equation for the strong electrolyte sodium sulfate is

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

Strong electrolytes in aqueous solutions include almost all salts, bases of alkali and alkaline earth metals, acids: H 2 SO 4 , HNO 3 , HCl, HBr, HI, HСlO 4 , HClO 3 , HBrO 4 , HBrO 3 , HIO 3 , H 2 SeO 4 , HMnO 4 , H 2 MnO 4 etc.

To the weak electrolytes include electrolytes, the degree of dissociation of which in solutions is less than unity (α<1) и она уменьшается с ростом концентрации.

The process of dissociation of weak electrolytes proceeds reversibly until an equilibrium is established in the system between the undecayed molecules of the solute and its ions. In the equations of dissociation of weak electrolytes put the sign of "reversibility". For example, the dissociation equation for a weak electrolyte of ammonium hydroxide has the form

NH 4 + OH NH 4 + + OH -

Weak electrolytes include water, almost all organic acids (formic, acetic, benzoic, etc.), a number of inorganic acids (H 2 SO 3, HNO 2, H 2 CO 3, H 3 AsO 4, H 3 AsO 3, H 3 BO 3, H 3 PO 4, H 2 SiO 3, H 2 S, H 2 Se, H 2 Te, HF, HCN, HCNS), bases of p-, d-, f- elements (Al (OH) 3 , Cu (OH) 2, Fe (OH) 2, etc.), ammonium hydroxide, magnesium and beryllium hydroxides, some salts (CdI 2, CdCl 2, HgCl 2, Hg (CN) 2, Fe (CNS) 3 etc.).

Depending on the degree of dissociation, electrolytes are distinguished strong and weak. Electrolytes with a degree of dissociation of more than 30% are usually called strong electrolytes, with a degree of dissociation from 3 to 30% - medium, less than 3% - weak electrolytes.

numerical the value of the degree of electrolytic dissociation depends on various factors:

1 . The nature of the solvent.

This is due to the dielectric constant of the solvent ε. As follows from Coulomb's law, the force of electrostatic attraction of two oppositely charged particles depends not only on the magnitude of their charges, the distance between them, but also on the nature of the medium in which the charged particles interact, i.e. from ε:

For example, at 298 K ε(H 2 O) = 78.25, and ε(C 6 H 6) = 2.27. Such salts as KCl, LiCl, NaCl, etc., are completely dissociated into ions in water, i.e. behave like strong electrolytes; in benzene, these salts dissociate only partially; are weak electrolytes. Thus, the same substances may exhibit different dissociation capacities depending on the nature of the solvent.

2 . Temperature.

For strong electrolytes, the degree of dissociation decreases with increasing temperature, for weak electrolytes, when the temperature rises to 60°C, α increases, and then begins to decrease.

3 . solution concentration.

If we consider dissociation as an equilibrium chemical process, then, in accordance with Le Chatelier's principle, the addition of a solvent (dilution with water), as a rule, increases the number of pro-dissociated molecules, which leads to an increase in α. The process of formation of molecules from ions as a result of dilution becomes more difficult: for the formation of a molecule, a collision of ions must occur, the probability of which decreases with dilution.

4 . Presence of similar ions.

The addition of ions of the same name reduces the degree of dissociation, which is also consistent with Le Chatelier's principle. For example, in a solution of weak nitrous acid, during electrolytic dissociation, an equilibrium is established between undissociated molecules and ions:

HNO 2 H + + NO 2 -.

With the introduction of nitrite ions NO 2 ˉ into a solution of nitrous acid (by adding a solution of potassium nitrite KNO 2), the equilibrium will shift to the left, therefore, the degree of dissociation α will decrease. A similar effect will be given by the introduction of H + ions into the solution.

It should be noted that the concepts of "strong electrolyte" and "good solubility" should not be confused. For example, the solubility of CH 3 COOH in H 2 O is unlimited, but acetic acid is a weak electrolyte (α = 0.014 in a 0.1 M solution). On the other hand, ВаSO 4 is a sparingly soluble salt (at 20 ° C, the solubility is less than 1 mg in 100 g of H 2 O), but it belongs to strong electrolytes, since all molecules that have passed into solution decompose into Ba 2+ and SO 4 ions 2 - .

Dissociation constant

A more accurate characteristic of electrolyte dissociation is dissociation constant, which does not depend on the concentration of the solution.

The expression for the dissociation constant can be obtained by writing the reaction equation for the dissociation of the AK electrolyte in a general form:

AK A - + K + .

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

where 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.

The range of equilibrium constants for different reactions is very large - from 10 -16 to 10 15 .

The dissociation of substances consisting of more than two ions occurs in steps. For a reaction of the form

A n K m nA – m + mK + n

the dissociation constant has the form

For example, sulfurous acid dissociates in steps:

H 2 SO 3 H + + HSO 3 -

HSO 3 – H + + SO 3 2–

Each dissociation step is described by its own constant:

At the same time, it is clear that

In the stepwise dissociation of substances, the decomposition in the next step always occurs to a lesser extent than in the previous one. In other words:

K d1 > K d2 >…

If the concentration of an electrolyte decomposing into two ions is From to, and the degree of its dissociation is equal to α, then the concentration of the resulting ions will be C to α, and the concentration of undissociated molecules is C in (1–α). The expression for a constant takes the following form:

This equation expresses Ostwald's dilution law . It allows one to calculate the degree of dissociation at various electrolyte concentrations if its dissociation constant is known. For weak electrolytes α<<1, тогда (1–α) → 1. Уравнение в этом случае принимает вид:

This equation clearly shows that the degree of dissociation increases with dilution of the solution.

In aqueous solutions, strong electrolytes are usually completely dissociated, so the number of ions in them is greater than in solutions of weak electrolytes of the same concentration. In this case, the forces of interionic attraction and repulsion are quite large. In such solutions, the ions are not completely free, their movement is constrained by mutual attraction to each other. Due to this attraction, each ion is, as it were, surrounded by a spherical swarm of oppositely charged ions, called the "ionic atmosphere".