Topics Unified State Exam codifier: Electrolytic dissociation of electrolytes in aqueous 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 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 bond polar bond (simple substances), all oxides (which are chemically Not interact with water), most organic matter(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. 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 electron pair 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 at the nodes crystal lattice, as a result of which it is destroyed, 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 molecules starting material 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 organic acids- potassium acetate CH 3 COOK, sodium formate HCOONa, etc.)
2. Strong acids: HCl, HI, HBr, HNO 3, H 2 SO 4 (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. degree of dissociation strong electrolytesα is approximately equal to 1 only for unsaturated solutions 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.
The conductivity of substances with electric current or the lack of conductivity can be observed using a simple device.
It consists of carbon rods (electrodes) connected by wires to an electrical network. An electric light is included in the circuit, which indicates the presence or absence of current in the circuit. If you dip the electrodes in a sugar solution, the light bulb does not light up. But it will light up brightly if they are dipped in a sodium chloride solution.
Substances that disintegrate into ions in solutions or melts and therefore conduct electric current are called electrolytes.
Substances that, under the same conditions, do not disintegrate into ions and do not conduct electric current are called nonelectrolytes.
Electrolytes include acids, bases and almost all salts.
Non-electrolytes include most organic compounds, as well as substances whose molecules contain only covalent non-polar or low-polar bonds.
Electrolytes are conductors of the second kind. In a solution or melt, they break up into ions, which is why current flows. Obviously, the more ions in a solution, the better it conducts electric current. Pure water conducts electricity very poorly.
There are strong and weak electrolytes.
Strong electrolytes, when dissolved, completely dissociate into ions.
These include:
1) almost all salts;
2) many mineral acids, for example H 2 SO 4, HNO 3, HCl, HBr, HI, HMnO 4, HClO 3, HClO 4;
3) bases of alkali and alkaline earth metals.
Weak electrolytes When dissolved in water, they only partially dissociate into ions.
These include:
1) almost all organic acids;
2) some mineral acids, for example H 2 CO 3, H 2 S, HNO 2, HClO, H 2 SiO 3;
3) many metal bases (except alkali and alkaline earth metal bases), as well as NH 4 OH, which can be represented as ammonia hydrate NH 3 ∙H 2 O.
Water is a weak electrolyte.
Weak electrolytes cannot produce a high concentration of ions in solution.
Basic principles of the theory of electrolytic dissociation.
The breakdown of electrolytes into ions when dissolved in water is called electrolytic dissociation.
Thus, sodium chloride NaCl, when dissolved in water, completely decomposes into sodium ions Na + and chloride ions Cl -.
Water forms hydrogen ions H + and hydroxide ions OH - only in very small quantities.
To explain the characteristics of aqueous solutions of electrolytes, the Swedish scientist S. Arrhenius proposed the theory of electrolytic dissociation in 1887. Subsequently, it was developed by many scientists on the basis of the doctrine of the structure of atoms and chemical bonds.
The modern content of this theory can be reduced to the following three provisions:
1. Electrolytes, when dissolved in water, break up (dissociate) into ions - positive and negative.
Ions are in more stable electronic states than atoms. They can consist of one atom - these are simple ions (Na +, Mg 2+, Al 3+, etc.) - or of several atoms - these are complex ions (NO 3 -, SO 2- 4, PO Z- 4 etc.).
2. Under the influence of electric current, ions acquire directional movement: positively charged ions move towards the cathode, negatively charged ones - towards the anode. Therefore, the former are called cations, the latter - anions.
The directional movement of ions occurs as a result of their attraction by oppositely charged electrodes.
3. Dissociation is a reversible process: in parallel with the disintegration of molecules into ions (dissociation), the process of combining ions (association) occurs.
Therefore, in the equations of electrolytic dissociation, instead of the equal sign, the reversibility sign is used. For example, the equation for the dissociation of an electrolyte molecule KA into a K + cation and an A - anion is generally written as follows:
KA ↔ K + + A -
The theory of electrolytic dissociation is one of the main theories in inorganic chemistry and is fully consistent with atomic-molecular science and the theory of atomic structure.
Degree of dissociation.
One of the most important concepts of Arrhenius's theory of electrolytic dissociation is the concept of the degree of dissociation.
The degree of dissociation (a) is the ratio of the number of molecules dissociated into ions (n") to the total number of dissolved molecules (n):
The degree of electrolyte dissociation is determined experimentally and is expressed in fractions of a unit or as a percentage. If α = 0, then there is no dissociation, and if α = 1 or 100%, then the electrolyte completely disintegrates into ions. If α = 20%, then this means that out of 100 molecules of a given electrolyte, 20 have broken up into ions.
Different electrolytes have different degrees of dissociation. Experience shows that it depends on the electrolyte concentration and temperature. With a decrease in electrolyte concentration, i.e. When diluted with water, the degree of dissociation always increases. As a rule, the degree of dissociation and temperature increase increase. Based on the degree of dissociation, electrolytes are divided into strong and weak.
Let us consider the shift in equilibrium established between undissociated molecules and ions during the electrolytic dissociation of a weak electrolyte - acetic acid:
CH 3 COOH ↔ CH 3 COO - + H +
When a solution of acetic acid is diluted with water, the equilibrium will shift towards the formation of ions, and the degree of dissociation of the acid increases. On the contrary, when a solution is evaporated, the equilibrium shifts towards the formation of acid molecules - the degree of dissociation decreases.
From this expression it is obvious that α can vary from 0 (no dissociation) to 1 (complete dissociation). The degree of dissociation is often expressed as a percentage. The degree of electrolyte dissociation can only be determined experimentally, for example, by measuring the freezing point of the solution, by measuring the electrical conductivity of the solution, etc.
Dissociation mechanism
Substances with ionic bonds dissociate most easily. As you know, these substances consist of ions. When they dissolve, the water dipoles are oriented around the positive and negative ions. Mutual attractive forces arise between the ions and dipoles of water. As a result, the bond between the ions weakens, and the ions move from the crystal to the solution. In this case, hydrated ions are formed, i.e. ions chemically bonded to water molecules.
Electrolytes, whose molecules are formed according to the type of polar covalent bond (polar molecules), dissociate similarly. Around each polar molecule of a substance, water dipoles are also oriented, which are attracted by their negative poles to the positive pole of the molecule, and by their positive poles - to the negative pole. As a result of this interaction, the connecting electron cloud (electron pair) is completely shifted towards the atom with higher electronegativity, the polar molecule turns into an ionic one and then hydrated ions are easily formed:
Dissociation of polar molecules can be complete or partial.
Thus, electrolytes are compounds with ionic or polar bonds - salts, acids and bases. And they can dissociate into ions in polar solvents.
Dissociation constant.
Dissociation constant. A more accurate characteristic of electrolyte dissociation is the dissociation constant, which does not depend on the concentration of the solution.
The expression for the dissociation constant can be obtained by writing the equation for the dissociation reaction of the AA electrolyte in general form:
A K → A - + K + .
Since dissociation is a reversible equilibrium process, the law of mass action is applied 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. For example, high value TO for reaction
means that if metallic copper is added to a solution containing silver ions Ag +, then at the moment equilibrium is reached, the concentration of copper ions is much greater than the square of the concentration of silver ions 2. On the contrary, low value TO in reaction
indicates that by the time equilibrium was reached, a negligible amount of silver iodide AgI had dissolved.
Pay special attention to the form of writing expressions for the equilibrium constant. If the concentrations of some reactants do not change significantly during the reaction, then they are not written into the expression for the equilibrium constant (such constants are denoted K 1).
So, for the reaction of copper with silver the expression will be incorrect:
The correct form would be:
This is explained by the fact that the concentrations of metallic copper and silver are introduced into the equilibrium constant. Copper and silver concentrations are determined by their densities and cannot be changed. Therefore, there is no point in taking these concentrations into account when calculating the equilibrium constant.
The expressions for the equilibrium constants when dissolving AgCl and AgI are explained in a similar way
Product of solubility. The dissociation constants of poorly soluble metal salts and hydroxides are called the product of solubility of the corresponding substances (denoted PR).
For the water dissociation reaction
the constant expression will be:
This is explained by the fact that the concentration of water during reactions in aqueous solutions changes very slightly. Therefore, it is assumed that the concentration of [H 2 O] remains constant and is introduced into the equilibrium constant.
Acids, bases and salts from the standpoint of electrolytic dissociation.
Using the theory of electrolytic dissociation, they define and describe the properties of acids, bases and salts.
Acids are electrolytes whose dissociation produces only hydrogen cations as cations.
For example:
НCl ↔ Н + + С l - ;
CH 3 COOH ↔ H + + CH 3 COO -
The dissociation of a polybasic acid occurs mainly through the first step, to a lesser extent through the second, and only to a small extent through the third. Therefore, in an aqueous solution of, for example, phosphoric acid, along with H 3 PO 4 molecules, there are ions (in successively decreasing quantities) H 2 PO 2-4, HPO 2-4 and PO 3-4
N 3 PO 4 ↔ N + + N 2 PO - 4 (first stage)
N 2 PO - 4 ↔ N + + NPO 2- 4 (second stage)
NRO 2- 4 ↔ N+ PO Z- 4 (third stage)
The basicity of an acid is determined by the number of hydrogen cations that are formed during dissociation.
So, HCl, HNO 3 - monobasic acids - one hydrogen cation is formed;
H 2 S, H 2 CO 3, H 2 SO 4 - dibasic,
H 3 PO 4, H 3 AsO 4 are tribasic, since two and three hydrogen cations are formed, respectively.
Of the four hydrogen atoms contained in the acetic acid molecule CH 3 COOH, only one, which is part of the carboxyl group - COOH, is capable of being cleaved off in the form of the H + cation - monobasic acetic acid.
Dibasic and polybasic acids dissociate stepwise (gradually).
Bases are electrolytes whose dissociation produces only hydroxide ions as anions.
For example:
KOH ↔ K + + OH - ;
NH 4 OH ↔ NH + 4 + OH -
Bases that dissolve in water are called alkalis. There are not many of them. These are the bases of alkali and alkaline earth metals: LiOH, NaOH, KOH, RbOH, CsOH, FrOH and Ca(OH) 2, Sr(OH) 2, Ba(OH) 2, Ra(OH) 2, as well as NH 4 OH. Most bases are slightly soluble in water.
The acidity of a base is determined by the number of its hydroxyl groups (hydroxy groups). For example, NH 4 OH is a one-acid base, Ca(OH) 2 is a two-acid base, Fe(OH) 3 is a three-acid base, etc. Two- and polyacid bases dissociate stepwise
Ca(OH) 2 ↔ Ca(OH) + + OH - (first stage)
Ca(OH) + ↔ Ca 2+ + OH - (second stage)
However, there are electrolytes that, upon dissociation, simultaneously form hydrogen cations and hydroxide ions. These electrolytes are called amphoteric or ampholytes. These include water, zinc, aluminum, chromium hydroxides and a number of other substances. Water, for example, dissociates into H + and OH - ions (in small quantities):
H 2 O ↔ H + + OH -
Consequently, it has equally pronounced acidic properties, due to the presence of hydrogen cations H +, and alkaline properties, due to the presence of OH - ions.
The dissociation of amphoteric zinc hydroxide Zn(OH) 2 can be expressed by the equation
2OH - + Zn 2+ + 2H 2 O ↔ Zn(OH) 2 + 2H 2 O ↔ 2- + 2H +
Salts are electrolytes, upon dissociation of which metal cations are formed, as well as ammonium cation (NH 4) and anions of acid residues
For example:
(NH 4) 2 SO 4 ↔ 2NH + 4 + SO 2- 4;
Na 3 PO 4 ↔ 3Na + + PO 3- 4
This is how medium salts dissociate. Acidic and basic salts dissociate stepwise. In acidic salts, metal ions are first eliminated, and then hydrogen cations. For example:
KHSO 4 ↔ K + + HSO - 4
HSO - 4 ↔ H + + SO 2- 4
In basic salts, acid residues are eliminated first, and then hydroxide ions.
Mg(OH)Cl ↔ Mg(OH) + + Cl -
As is known, when dissolving, even without stirring due to diffusion, the solution gradually becomes homogeneous, i.e. its concentration in all parts becomes the same.
Let's take the case when the solution is separated from the pure solvent by a semi-permeable partition (parchment, collodion film, cellophane, etc.), as shown in Fig. 15. Such partitions allow solvent molecules to pass through quite easily, but do not allow the dissolved substance to pass through. The process of equalizing concentrations on both sides of the partition is complicated. The solute cannot pass through the partition into the solvent. Only penetration of solvent molecules through the partition into the solution is possible. Thus, it will gradually decrease due to dilution with a solvent.
The process of penetration of a solvent into a solution through a semi-permeable partition is called osmosis. The higher, the more pronounced osmosis.
Osmosis also occurs when solutions of different concentrations are separated by a semi-permeable partition. As the solvent penetrates through the semi-permeable partition into the solution, with higher concentrations, the volume of the latter increases. Therefore, if you place a solution in a vessel made of a semi-permeable membrane, attaching a vertical tube to it, as shown in Fig. 15, and then lower this vessel into the solvent, due to the increase in volume, the solution will rise up the tube. The resulting column of liquid will create a certain amount of pressure, which at some point will cause osmosis to stop. The force that balances the pressure of this column of liquid from inside the solution is called osmotic pressure. The magnitude of osmotic pressure is measured by the external pressure at which osmosis stops.
Rice. 15. A device for observing the phenomenon of osmosis. 1 - vessel with water; 2 - semi-permeable membrane; 3 - tube for observing the resulting osmotic pressure; 4 - solution.
The walls of plant and animal cells are semi-permeable partitions containing protoplasm. What is constantly maintained in them determines the elasticity of cells and tissues.
■ 62. Under what conditions does osmosis occur?
63. What is ?
64. What is the importance of osmosis for plant and animal organisms?
Electrolytic dissociation theory
At the turn of the 18th and 19th centuries, when electric current began to be used to study the properties of substances, attention was drawn to the fact that some conduct electric current in an aqueous solution, while others do not conduct it. Subsequently, the aqueous solutions of which conduct electric current were called electrolytes. These included alkalis, acids, and salts. Substances whose solutions did not conduct electric current were called non-electrolytes (sugar, alcohol, benzene and other organic substances).
Nowadays, when the types of chemical bonds have become known, it has become possible to explain such differences in the behavior of substances. The phenomenon of electrical conductivity of substances in aqueous solutions depends on the type of chemical bond in the molecules of both the solute and the solvent.
The water molecule, as we have already said, is a dipole (see pp. 32-34). If a substance is dissolved in water, the molecule of which has an ionic type of bond and therefore its crystal lattice is also ionic, the water dipoles are oriented towards positive ions with their negative poles, and towards negative ions - with their positive poles (Fig. 16.a). Between the ions and the dipoles of water, the forces of electrostatic attraction increase and peculiar bonds arise, which ultimately dismember the ionic crystal lattice into individual ions surrounded by the dipoles of water,
therefore they are called hydrated ions. Approximately the same thing happens if a substance with polar molecules, for example chloride, is dissolved in water (see Fig. 16, b). At the same time, if the molecules of the solute are built according to a covalent non-polar type of bond, then no ions are formed in the solution, since non-polar molecules do not experience the same influence from water molecules as ionic and polar molecules. Basically, the molecules of most organic substances are built according to the covalent nonpolar type. Therefore, organic substances, as a rule, are not electrolytes!
Rice. 16. Scheme of dissociation of sodium chloride in water (a) and dissociation of polar HCl molecules in water (b)
Thus, only substances whose molecules are built according to the ionic, or polar, type of bond between atoms in the molecule can be electrolytes. In addition, the solvent molecules must also have a polar structure and e. Only under such conditions can we expect the molecules to decompose into ions.
The breakdown of electrolyte molecules into ions under the action of a solvent is called electrolytic dissociation.
Write down the definition of electrolytic dissociation in your notebook.
The word "dissociation" means "reversible disintegration." If the electrolyte solution is evaporated, then we will again receive the same electrolyte in the same quantity as before dissolution, since the reverse process will occur - molarization.
■ 65. How does an electrolyte differ from a non-electrolyte in terms of the type of chemical bond and behavior in solution?
66. Why is it necessary for the process of electrolytic dissociation that the solvent have dipole molecules, and the electrolyte-ionic or polar nature of the chemical bond?
67. Why cannot substances with non-polar molecules be electrolytes?
68. Formulate what electrolytic dissociation is. Learn the definition by heart.
60. How does the process of molarization differ from dissociation?
The dissociation of electrolytes in solution was first explained in 1887 by the Swedish scientist Arrennus. He formulated the main provisions of the theory, which he called the theory of electrolytic dissociation,
The main provisions of this theory are as follows.
1 All substances whose solutions conduct electric current (electrolytes), when dissolved, disintegrate into positively and negatively charged particles - ions.
2. If a direct electric current is passed through a solution, then positively charged ions will move to the negative pole - the cathode, which is why they are called cations. Negatively charged ions will move towards the positive pole - the anode, which is why they are called anions. The total charge of the cations in the solution is equal to the total charge of the anions, so the solution is always electrically neutral.
3. Ions and atoms of the same elements are very different from each other in properties. For example, copper ions have a blue color, which is due to copper sulfate, and free copper ions are a red metal. Sodium atoms react with water, releasing it and forming an alkali, while sodium ions practically do not react with water.
Chlorine ions are colorless, non-toxic, colorless and odorless, as can be seen when examining the same solution of sodium chloride, and it itself is greenish-yellow
poisonous gas with a characteristic pungent odor.
Write down the main provisions of the theory in your notebook.
In order to distinguish an atom from an ion when writing, the sign of the charge and its magnitude are indicated at the top right of the ion. For example: the sodium atom is Na, and the sodium ion is Na + (read: “singly charged sodium cation”); the copper atom is Cu, and the copper ion is Cu 2+ (read: “doubly charged copper cation”); the aluminum atom is Al, and the aluminum ion is Al 3+ (read: “three-charged aluminum cation”), the sulfur atom is S, and the sulfur ion is S 2-; (read: “doubly charged sulfur anion”), the chlorine atom Cl, and the chlorine ion Cl -, etc.
■ 70. What are ions?
71. How do ions differ from neutral atoms?
72. Which ions are called cations, which anions and why?
73. How to distinguish an ion from a neutral atom in writing (give examples)?
74. Name the following ions: Fe 2+, Fe 3+, K +, Br -.
Dissociation of bases, acids and salts
We have already said that only compounds whose molecules are built according to an ionic or polar type of bond can decompose into ions, considering this using the example of NaCl and HCl. As for non-polar molecules, they do not disintegrate into ions in aqueous solutions.
However, there are often substances in whose molecules both types of bonds are observed, for example, in a molecule of sodium hydroxide NaOH, the metal is bound to hydroxyl by an ionic bond, and to oxygen by a covalent bond. In the sulfuric acid molecule H2SO4, hydrogen is connected to the acidic residue by a polar bond, and to oxygen by a covalent nonpolar bond. In the aluminum nitrate molecule, Al(NO 3) 3 is connected to the acid residue by an ionic bond, and the nitrogen atoms are connected to the oxygen atoms by a covalent bond. In such cases, the breakdown of the molecule into ions occurs at the site of an ionic or polar bond. Covalent bonds remain undissociated.
From the above it follows that ions can be not only individual atoms, but also groups of atoms. For example, hydroxyl upon dissociation forms one anion OH-, which is called hydroxyl ion. The acid residue SO 4 forms a doubly charged anion - sulfate ion. The charge of each ion is determined by its valence.
Now we can consider into which ions different classes of inorganic substances dissociate. Like chemical reaction equations, dissociation equations can also be written. For example, the decomposition into sodium hydroxide ions is written as follows:
NaOH = Na + + OH -
Sometimes, instead of the equal sign in such equations, the reversibility sign ⇄ is used to show that dissociation is a reversible process and can occur in the opposite direction when the solvent is removed.
Calcium hydroxide dissociates as follows:
Ca(OH) 2 = Ca 2+ + 2OH -
(the index indicating the number of hydroxyl groups becomes a coefficient).
To check the correctness of the entry, the total positive charge of the cations and the total negative charge of the anions should be calculated. They must be equal in absolute value. In this case, the sum of positive charges is +2, and negative charges are -2. From the above, a definition of bases arises in the light of the theory of electrolytic dissociation.
Bases are those electrolytes that dissociate in solution to form only a metal cation and hydroxyl anions.
Write the definition of bases in your notebook.
■ 75. Write the dissociation equations for the following bases, first checking with the solubility table whether they are electrolytes: barium hydroxide, iron hydroxide, potassium hydroxide, strontium hydroxide, zinc hydroxide, lithium hydroxide.
The decomposition into acid ions occurs where a polar bond takes place, i.e., between the hydrogen atom and the acid residue.
For example, nitric acid is expressed by the equation:
HNO 3 = H + + NO 3 —
For two or more basic acids, dissociation occurs in steps, for example, for H 2 CO 3:
Н 2 СО 3 ⇄ Н + + НСО з — (first stage) НСО 3 ⇄ Н + + CO 2 3 — (second stage)
Stepwise dissociation is sometimes depicted as continuous equality.
H 2 CO 3 ⇄ H + + HCO 3 - ⇄ 2H + + CO 2 3 -
With stepwise dissociation, the decay in steps is greatly reduced, and at the last step it is usually very small.
Thus, acids are electrolytes that dissociate in solutions to form only hydrogen ions as cations.
Write down the definition of acids in your notebook.
■ 76. Write the dissociation equations for the following acids: sulfuric, phosphoric, hydrogen sulfide, sulfurous, hydrochloric. In the case of two or more basic acids, write the equations in steps.
The nature of the dissociation of bases and acids depends on the radius and charge of the ion forming the base or acid.
The radius of the Na + ion is greater than the radius of the H + ion, therefore the electron shells of oxygen attract the hydrogen nucleus more strongly than the sodium nucleus. Therefore, during dissociation, the Na-OH bond should break faster. The larger the radius of the hydroxide-forming ion with the same charge, the easier the dissociation occurs.
In the same subgroup, a metal hydroxide with a greater charge of the atomic nucleus and, therefore, with a large ionic radius will dissociate more strongly.
■ 77. Using D.I. Mendeleev’s periodic table of elements, indicate which of the bases will dissociate more strongly: Mg(OH) 2 or Sr(OH) 2. Why?
In the case of close values of the radii of the ions forming the hydroxide (or acid), the nature of the dissociation depends on the magnitude of its charge. So, since the charge of the silicon ion in silicic acid H 2 SiO 3 is Si(+4), and the charge of the ion
chlorine in perchloric acid HClO 4 - Cl (+7), then the latter is stronger. The more positive an ion is, the more it repels the positive hydrogen ion. Acid-type dissociation occurs.
The amphoteric nature of beryllium (II period) is explained by a peculiar balance between the repulsive forces of the hydrogen ion and its attraction by the beryllium ion.
■ 78. Why in the III period of D.I. Mendeleev’s periodic table does magnesium hydroxide exhibit basic properties, aluminum hydroxide - amphoteric, but forms an acid? Explain this by comparing the charges and radii of magnesium, aluminum, and sulfur ions.
Since in salt molecules there is an ionic bond between the metal atoms and the acidic residue, the salts dissociate, respectively, with the formation of metal cations and anions of the acidic residue, for example:
Al 2 (SO 4) 3 = 2Al 3+ + 3SO 2 4 -
Based on this, salts are electrolytes that, upon dissociation, form metal ions as cations and ions of an acidic residue as anions.
■ 79. Write the dissociation equations for the following intermediate salts: sodium phosphate, magnesium nitrate, aluminum chloride, potassium silicate, sodium carbonate, potassium sulfide, copper (II) nitrate, iron (III) chloride.
The dissociation of acidic, basic and other salts proceeds somewhat differently, as will be discussed below.
Degree of dissociation
Electrolytic dissociation is a reversible process. Consequently, simultaneously with the formation of ions, the opposite process occurs - the combination of ions into molecules. A balance is established between them. The more dilute the solution, the more complete the dissociation occurs. The completeness of dissociation is judged by the degree of dissociation, denoted by the letter α.
is the ratio of the number of dissociated molecules n to the total number of molecules N of the dissolved substance, expressed as a percentage:
Write down the formula and determination of the degree of dissociation in your notebook.
In other words, it shows what percentage of dissolved molecules have broken up into ions.
Depending on the degree of dissociation, electrolytes are distinguished between strong and weak. The more, the stronger the electrolyte.
Based on the amount of decomposition into ions, electrolytes are distinguished as strong, medium, and weak.
Strong electrolytes, for example HNO 3, HCl, H 2 SO 4, caustic alkalis and all salts dissociate almost completely (100%), However, strong electrolytes also include those with α > 30%, i.e. more 30% of the molecules broke up into ions. Average electrolytes, for example H 3 PO 4 and H 2 SO 3, have a degree of dissociation ranging from 2 to 30%. Weak electrolytes, for example NH 4 OH, H 2 CO 3, H 2 S, dissociate poorly: α< 2%.
Comparison of the degree of dissociation of different electrolytes is carried out in solutions of the same concentration (most often 0.1 N), since the degree of dissociation strongly depends on the concentration of the solution.
The degree of dissociation is influenced by the nature of the solute itself, the solvent, and a number of other external influences. Thus, when they say “strong acid” or “strong base”, they mean the degree of dissociation of the substance in solution. In this case, we are talking about these substances as electrolytes. The degree of dissociation of a substance determines its behavior in a chemical reaction and the course of the reaction itself.
■ 80. What characterizes the degree of dissociation α?
81. Draw a table in your notebook:
Based on the text you read, give at least two examples in each column. 82. What do the expressions “strong acid” and “weak base” mean?
Exchange reactions between electrolytes.Ionic equations
Since electrolytes in solutions disintegrate into ions, electrolyte reactions must occur between the ions.
The interaction of ions in a solution is called an ionic reaction.
Write the wording in your notebook.
Both exchange and redox reactions can occur with the participation of ions. Consider the exchange reactions of electrolytes in solution, for example the interaction between two salts:
NaCl + AgNO 3 = AgCl↓ + NaNO 3
and how strong electrolytes dissociate into ions:
NaCl ⇄ Na + + Cl —
AgNO 3 ⇄ Ag + + NO 3 —
therefore, the left side of the equality can be written as follows: Na + + Cl - + Ag + + NO 3 - =
Let's consider the substances obtained as a result of the reaction: AgCl is an insoluble substance, so it will not dissociate into ions, and NaNO 3 is a soluble salt, it dissociates perfectly into ions according to the scheme
NaNO 3 ⇄ Na + + NO 3 —
NaNO 3 is a strong electrolyte, so the right side of the equation is written like this:
... = Na + + NO 3 - + AgCl The equation as a whole will have the following form:
Na + + Cl - + Ag + + NO 3 - = Na + + NO 3 - + AgCl
This equation is called the complete ionic equation. Reducing similar terms in this equation, we obtain the abbreviated ionic equation
Ag + + Cl - = AgCl
So, the sequence of composing the ionic equation.
1. Write in ionic form the formulas of the starting products (those that dissociate).
2. Write the formulas of the resulting products (those that dissociate) in ionic form.
3. Check whether the total number of positive and negative charges of ions coincides in absolute value on the left side of the equation, and then on the right.
4. Check whether the number of ions of the same name on the left and right sides of the equation is the same (taking into account the atoms that are part of the non-dissociating substance).
This completes the compilation of the complete ionic equation.
Write down the sequence of composing the ionic equation in your notebook.
5. To compile an abbreviated ionic equation, you should find similar terms with the same signs on the left and right sides of the equation and exclude them from the equation, and then write down the resulting abbreviated ionic equation.
The given abbreviated ionic equation expresses the essence of not only this reaction. Let's write several reaction equations, for example:
1) HCl + AgNO 3 = AgCl↓ + HNO 3
H + + Cl - + Ag + + NO 3 - = H + + NO 3 - + AgCl↓
Ag + + Cl - = AgCl
2) BaCl 2 + 2AgNO 3 = Ba(NO 3) 2 + 2AgCl↓
Ba 2+ + 2Cl - + 2Ag + + 2NO 3 - = Ba 2+ + 2NO 3 - + 2AgCl↓
Ag + + Cl - = AgCl
3)AlCl 3 + 3AgNO 3 = Al(NO 3) 3 + 3AgCl↓
Al 3+ + 3Cl - + 3Ag + + 3NO 3 - = Al 3+ + 3NO 3 - + 3AgCl
Ag + + Cl - = AgCl
In all the examples given, the abbreviated ionic equation is the same. This circumstance plays a very important role in analytical chemistry for qualitative analysis.
There may be cases when the reaction results in the formation of a (lowly dissociating substance)
Ca(OH) 2 + 2HCl = CaCl 2 + 2H 2 O
Ca 2+ + 2ON — + 2H + + 2Сl — = Ca 2+ + 2Сl — + 2Н 2 O
H + + OH - = H 2 O
or gas is released
Na 2 CO 3 + 2HNO 3 = 2NaNO 3 + H 2 O + CO2
2Na + + CO 2 3 - + 2H + + 2NO 3 - = 2Na + + 2NO 3 - + H 2 O + CO 2 ↓
2H + + CO 2 3 - = H 2 O + CO 2
As is known, there are conditions for the exchange reactions to proceed to the end: 1) if a precipitate is formed, 2) if gas is released, and 3) if . All these conditions from the standpoint of the theory of electrolytic dissociation can be formulated as follows: exchange reactions proceed to completion if the reaction results in the formation of non-dissociating or low-dissociating substances.
In cases where both resulting substances dissociate well, the reaction is reversible, for example:
2КCl + Na 2 SO 4 ⇄ 2NaCl + K 2 SO 4
Tasks No. 7 with solutions.
Let's look at assignments No. 7 from the OGE for 2016.
Tasks with solutions.
Task No. 1.
Only potassium cations and phosphate anions are formed during the dissociation of a substance whose formula is
1. KHPO4
2. Ca3(PO4)2
3. KH2PO4
4. K3PO4
Explanation: if during dissociation only potassium cations and phosphate ions are formed, then only these ions are part of the desired substance. Let's confirm with the dissociation equation:
K3PO4 → 3K+ + PO4³‾
The correct answer is 4.
Task No. 2.
Electrolytes include each of the substances whose formulas are
1. N2O, KOH, Na2CO3
2. Cu(NO3)2, HCl, Na2SO4
3. Ba(OH)2, NH3xH2O, H2SiO3
4. CaCl2, Cu(OH)2, SO2
Explanation: electrolytes are substances that conduct electric current due to dissociation into ions in solutions and melts. Therefore, electrolytes are soluble substances.
The correct answer is 2.
Tasks No. 3.
Upon complete dissociation of sodium sulfide, ions are formed
1. Na+ and HS‾
2. Na+ and SO3²‾
3. Na+ and S²‾
4. Na+ and SO4²‾
Explanation: let's write the dissociation equation for sodium sulfide
Na2S → 2Na+ + S²‾
Hence, the correct answer is 3.
Tasks No. 4.
In the list of ions
A. Nitrate ion
B. Ammonium ion
B. Hydroxide ion
D. Hydrogen ion
D. Phosphate ion
E. Magnesium ion
cations are:
1. BGD 2. BGE 3. AGE 4. HGE
Explanation: cations are positive species, such as metal ions or hydrogen ions. Of the above, these are ammonium ion, hydrogen ion and magnesium ion. The correct answer is 2.
Tasks No. 5.
Are the following statements about the electrolytic dissociation of salts correct?
A. All salts upon dissociation form metal cations, hydrogen cations and anions of acid residues
B. During the process of dissociation, salts form metal cations and anions of acid residues
1. Only A is correct
2. Only B is correct
3. Both judgments are correct
4. Both judgments are wrong.
Explanation: only acid salts upon dissociation form hydrogen cations, therefore, A is incorrect, but B is correct. Here's an example:
NaCl → Na+ + Cl‾
The correct answer is 2.
Tasks No. 6.
The same number of moles of cations and anions is formed upon complete dissociation in an aqueous solution of 1 mol
1. KNO3
2.CaCl2
3. Ba(NO3)2
4. Al2(SO4)3
Explanation: in this equation we can either write the dissociation equations and look at the resulting coefficients, or look at the indices in the formulas of the given salts. Only the KNO3 molecule has the same number of moles:
KNO3 → K+ + NO3‾
The correct answer is 1.
Task No. 7.
Chloride ions are formed during the dissociation of a substance whose formula is
1. KClO3
2. AlCl3
3. NaClO
4. Cl2O7
Explanation: Among the given substances, chloride ions are found only in the aluminum chloride molecule - AlCl3. Let us present the dissociation equation for this salt:
AlCl3 → Al3+ + 3Cl‾
The correct answer is 2.
Task No. 8.
Hydrogen ions are formed during the dissociation of a substance whose formula is
1. H2SiO3
2.NH3xH2O
3. HBr
4. NaOH
Explanation: Hydrogen ions are included, among those listed, only in HBr: HBr → H+ + Br‾
(H2SiO3 in solution dissociates into H2O and SiO2)
The correct answer is 3.
Task No. 9.
In the list of substances:
A. Sulfuric acid
B. Oxygen
B. Potassium hydroxide
G. Glucose
D. Sodium sulfate
E. Ethyl alcohol
electrolytes include:
1. WHERE 2. ABG 3. WDE 4. AED
Explanation: Electrolytes are strong acids, bases or salts. Among those listed are sulfuric acid (H2SO4), potassium hydroxide (KOH), sodium sulfate (Na2SO4). The correct answer is 4.
Task No. 10.
During the process of dissociation, phosphate ions form each of the substances, the formulas of which are
1. H3PO4, (NH4)3PO4, Cu3(PO4)2
2. Mg3(PO4)2, Na3PO4, AlPO4
3. Na3PO4, Ca3(PO4)2, FePO4
4. K3PO4, H3PO4, Na3PO4
Explanation: as in the previous task, here we need to know that electrolytes are strong acids or soluble salts, as, for example, in No. 4:
K3PO4 → 3K+ + PO4³‾
H3PO4 → 3H+ + PO4³‾
Na3PO4 → 3Na+ + PO4³‾
The correct answer is 4.
Tasks for independent solution.
1. Hydrogen ions and acid residue are formed in the process of electrolytic dissociation:
1. Water
2. Nitric acid
3. Silicic acid
4. Potassium nitrate
2. Electrolytes are each of the substances whose formulas are:
1. KOH, H2O(dist), CaCl2
2. BaSO4, Al(NO3)3, H2SO4
3. BaCl2, H2SO4, LiOH
4. H2SiO3, AgCl, HCl
3. Are the following statements about electrolytes true?
A. Nitric and sulfuric acids are strong electrolytes
B. Hydrogen sulfide in an aqueous solution completely disintegrates into ions
1. Only A is correct
2. Only B is correct
3. Both judgments are correct
4. Both judgments are wrong.
4. Each of two substances is an electrolyte
1. Copper(II) sulfide and ethanol
2. Hydrochloric acid and potassium sulfate
3. Mercury (II) oxide and calcium sulfate
4. Magnesium carbonate and nitric oxide (I)
5. In an aqueous solution, it dissociates stepwise
1. Copper(II) nitrate
2. Nitric acid
3. Sulfuric acid
4. Sodium hydroxide
6. Are the following statements about electrolytes true?
A. Beryllium hydroxide and iron(III) hydroxide are strong electrolytes.
B. Silver nitrate in an aqueous solution completely disintegrates into ions
1. Only A is correct
2. Only B is correct
3. Both judgments are correct
4. Both judgments are wrong.
7. Sulfate ions are formed during the dissociation process
1. Potassium sulfide
2. Hydrogen sulfide acid
3. Copper sulfide
4. Barium sulfate
8. The general chemical properties of sodium hydroxide and barium hydroxide are determined by
1. The presence of sodium and barium ions in their solutions
2. Their good solubility in water
3. The presence of three elements in their composition
4. The presence of hydroxide ions in their solutions
9. The cation is
1. Sulfate ion
2. Sodium ion
3. Sulfide ion
4. Sulfite ion
10. Anion is
1. Calcium ion
2. Silicate ion
3. Magnesium ion
4. Ammonium ion
The tasks provided were taken from the collection for preparation for the Unified State Exam in Chemistry, authors: Koroshchenko A.S. and Kuptsova A.A.
Electrolytes and non-electrolytes
From physics lessons we know that solutions of some substances are capable of conducting electric current, while others are not.
Substances whose solutions conduct electric current are called electrolytes.
Substances whose solutions do not conduct electric current are called non-electrolytes. For example, solutions of sugar, alcohol, glucose and some other substances do not conduct electricity.
Electrolytic dissociation and association
Why do electrolyte solutions conduct electric current?
The Swedish scientist S. Arrhenius, studying the electrical conductivity of various substances, came to the conclusion in 1877 that the cause of electrical conductivity is the presence in solution ions, which are formed when an electrolyte is dissolved in water.
The process of electrolyte breaking down into ions is called electrolytic dissociation.
S. Arrhenius, who adhered to the physical theory of solutions, did not take into account the interaction of the electrolyte with water and believed that there were free ions in solutions. In contrast, Russian chemists I.A. Kablukov and V.A. Kistyakovsky applied the chemical theory of D.I. Mendeleev to explain electrolytic dissociation and proved that when an electrolyte is dissolved, a chemical interaction of the dissolved substance with water occurs, which leads to the formation hydrates, and then they dissociate into ions. They believed that solutions contained not free, not “naked” ions, but hydrated ones, that is, “dressed in a coat” of water molecules.
Water molecules are dipoles(two poles), since the hydrogen atoms are located at an angle of 104.5°, due to which the molecule has an angular shape. The water molecule is shown schematically below.
As a rule, substances dissociate most easily with ionic bond and, accordingly, with an ionic crystal lattice, since they already consist of ready-made ions. When they dissolve, the water dipoles are oriented with oppositely charged ends around the positive and negative ions of the electrolyte.
Mutual attractive forces arise between electrolyte ions and water dipoles. As a result, the bond between the ions weakens, and the ions move from the crystal to the solution. It is obvious that the sequence of processes occurring during the dissociation of substances with ionic bonds (salts and alkalis) will be as follows:
1) orientation of water molecules (dipoles) near the ions of the crystal;
2) hydration (interaction) of water molecules with ions of the surface layer of the crystal;
3) dissociation (decay) of the electrolyte crystal into hydrated ions.
Simplified processes can be reflected using the following equation:
Electrolytes whose molecules have a covalent bond (for example, molecules of hydrogen chloride HCl, see below) dissociate similarly; only in this case, under the influence of water dipoles, the transformation of a covalent polar bond into an ionic one occurs; The sequence of processes occurring in this case will be as follows:
1) orientation of water molecules around the poles of electrolyte molecules;
2) hydration (interaction) of water molecules with electrolyte molecules;
3) ionization of electrolyte molecules (conversion of a covalent polar bond into an ionic one);
4) dissociation (decay) of electrolyte molecules into hydrated ions.
In a simplified way, the process of dissociation of hydrochloric acid can be reflected using the following equation:
It should be taken into account that in electrolyte solutions, chaotically moving hydrated ions can collide and recombine with each other. This reverse process is called association. Association in solutions occurs in parallel with dissociation, therefore the reversibility sign is put in the reaction equations.
The properties of hydrated ions differ from those of non-hydrated ions. For example, the unhydrated copper ion Cu 2+ is white in anhydrous crystals of copper (II) sulfate and has a blue color when hydrated, i.e., bound to water molecules Cu 2+ nH 2 O. Hydrated ions have both constant and variable number of water molecules.
Degree of electrolytic dissociation
In electrolyte solutions, along with ions, there are also molecules. Therefore, electrolyte solutions are characterized degree of dissociation, which is denoted by the Greek letter a (“alpha”).
This is the ratio of the number of particles broken up into ions (N g) to the total number of dissolved particles (N p).
The degree of electrolyte dissociation is determined experimentally and is expressed in fractions or percentages. If a = 0, then there is no dissociation, and if a = 1, or 100%, then the electrolyte completely disintegrates into ions. Different electrolytes have different degrees of dissociation, i.e. the degree of dissociation depends on the nature of the electrolyte. It also depends on the concentration: as the solution is diluted, the degree of dissociation increases.
Based on the degree of electrolytic dissociation, electrolytes are divided into strong and weak.
Strong electrolytes- these are electrolytes that, when dissolved in water, almost completely dissociate into ions. For such electrolytes, the degree of dissociation tends to unity.
Strong electrolytes include:
1) all soluble salts;
2) strong acids, for example: H 2 SO 4, HCl, HNO 3;
3) all alkalis, for example: NaOH, KOH.
Weak electrolytes- these are electrolytes that, when dissolved in water, almost do not dissociate into ions. For such electrolytes, the degree of dissociation tends to zero.
Weak electrolytes include:
1) weak acids - H 2 S, H 2 CO 3, HNO 2;
2) an aqueous solution of ammonia NH 3 H 2 O;
4) some salts.
Dissociation constant
In solutions of weak electrolytes, due to their incomplete dissociation, dynamic equilibrium between undissociated molecules and ions. For example, for acetic acid:
You can apply the law of mass action to this equilibrium and write down the expression for the equilibrium constant:
The equilibrium constant characterizing the process of dissociation of a weak electrolyte is called dissociation constant.
The dissociation constant characterizes the ability of an electrolyte (acid, base, water) dissociate into ions. The larger the constant, the easier the electrolyte breaks down into ions, therefore, the stronger it is. The values of dissociation constants for weak electrolytes are given in reference books.
Basic principles of the theory of electrolytic dissociation
1. When dissolved in water, electrolytes dissociate (break up) into positive and negative ions.
Ions is one of the forms of existence of a chemical element. For example, sodium metal atoms Na 0 vigorously interact with water, forming alkali (NaOH) and hydrogen H 2, while sodium ions Na + do not form such products. Chlorine Cl 2 has a yellow-green color and a pungent odor, and is poisonous, while chlorine ions Cl are colorless, non-toxic, and odorless.
Ions- these are positively or negatively charged particles into which atoms or groups of atoms of one or more chemical elements are transformed as a result of the donation or addition of electrons.
In solutions, ions move randomly in different directions.
According to their composition, ions are divided into simple- Cl - , Na + and complex- NH 4 + , SO 2 - .
2. The reason for the dissociation of an electrolyte in aqueous solutions is its hydration, i.e., the interaction of the electrolyte with water molecules and the breaking of the chemical bond in it.
As a result of this interaction, hydrated ions are formed, i.e. associated with water molecules. Consequently, according to the presence of a water shell, ions are divided into hydrated(in solutions and crystalline hydrates) and unhydrated(in anhydrous salts).
3. Under the influence of an electric current, positively charged ions move to the negative pole of the current source - the cathode and are therefore called cations, and negatively charged ions move to the positive pole of the current source - the anode and are therefore called anions.
Consequently, there is another classification of ions - according to the sign of their charge.
The sum of the charges of cations (H +, Na +, NH 4 +, Cu 2+) is equal to the sum of the charges of anions (Cl -, OH -, SO 4 2-), as a result of which electrolyte solutions (HCl, (NH 4) 2 SO 4, NaOH, CuSO 4) remain electrically neutral.
4. Electrolytic dissociation is a reversible process for weak electrolytes.
Along with the dissociation process (decomposition of the electrolyte into ions), the reverse process also occurs - association(combination of ions). Therefore, in the equations of electrolytic dissociation, instead of the equal sign, the reversibility sign is used, for example:
5. Not all electrolytes dissociate into ions to the same extent.
Depends on the nature of the electrolyte and its concentration. The chemical properties of electrolyte solutions are determined by the properties of the ions that they form during dissociation.
The properties of weak electrolyte solutions are determined by the molecules and ions formed during the dissociation process, which are in dynamic equilibrium with each other.
The smell of acetic acid is due to the presence of CH 3 COOH molecules, the sour taste and color change of indicators are associated with the presence of H + ions in the solution.
The properties of solutions of strong electrolytes are determined by the properties of the ions that are formed during their dissociation.
For example, the general properties of acids, such as sour taste, changes in the color of indicators, etc., are due to the presence of hydrogen cations (more precisely, oxonium ions H 3 O +) in their solutions. The general properties of alkalis, such as soapiness to the touch, changes in the color of indicators, etc., are associated with the presence of hydroxide ions OH - in their solutions, and the properties of salts are associated with their decomposition in solution into metal (or ammonium) cations and anions of acidic residues.
According to the theory of electrolytic dissociation all reactions in aqueous solutions of electrolytes are reactions between ions. This accounts for the high speed of many chemical reactions in electrolyte solutions.
Reactions occurring between ions are called ionic reactions, and the equations of these reactions are ionic equations.
Ion exchange reactions in aqueous solutions can occur:
1. Irreversible, to the end.
2. Reversible, that is, to flow simultaneously in two opposite directions. Exchange reactions between strong electrolytes in solutions proceed to completion or are practically irreversible when the ions combine with each other to form substances:
a) insoluble;
b) low dissociating (weak electrolytes);
c) gaseous.
Here are some examples of molecular and abbreviated ionic equations:
The reaction is irreversible, because one of its products is an insoluble substance.
The neutralization reaction is irreversible, because a low-dissociating substance is formed - water.
The reaction is irreversible, because CO 2 gas and a low-dissociating substance - water - are formed.
If among the starting substances and among the reaction products there are weak electrolytes or poorly soluble substances, then such reactions are reversible, that is, they do not proceed to completion.
In reversible reactions, the equilibrium shifts towards the formation of the least soluble or least dissociated substances.
For example:
The equilibrium shifts towards the formation of a weaker electrolyte - H 2 O. However, such a reaction will not proceed to completion: undissociated molecules of acetic acid and hydroxide ions remain in the solution.
If the starting substances are strong electrolytes, which upon interaction do not form insoluble or slightly dissociating substances or gases, then such reactions do not occur: when mixing solutions, a mixture of ions is formed.
Reference material for taking the test:
Periodic table
Solubility table
