In aqueous solutions, hydroxides and salts conduct electric current as a result of decomposition into charged particles - ions. This process is called electrolytic dissociation, A Substances that break down into ions in an aqueous solution are called electrolytes.
Acids from the point of view of the theory of electrolytic dissociation substances are called, decomposing in aqueous solutions into positively charged hydrogen ions (H +) and negatively charged ions of the acid residue. For example, hydrochloric acid dissociates according to the equation
HCl↔H + + Cl - .
Positively charged ions are called cations, and negatively charged ones are called anions. Thus, during the dissociation of hydrochloric acid, a hydrogen cation and a chlorine anion are formed ( chloride ion).
The number of hydrogen ions formed upon complete dissociation of an acid molecule is called the basicity of the acid. Thus, hydrochloric acid is monobasic, and sulfuric acid – dibasic, because upon its dissociation, two hydrogen ions are formed:
Electrolytes can dissociate (break apart into ions) completely, and such substances are called strong electrolytes. Electrolytes that dissociate partially are called weak or medium. Sulfuric, nitric and hydrochloric acids are strong electrolytes (strong acids), while carbonic acid is a weak acid (weak electrolyte).
From the point of view of the theory of electrolytic dissociation, bases are substances that disintegrate in aqueous solutions into positively charged metal ions and negatively charged hydroxide ions (OH -). For example, sodium hydroxide dissociates according to the equation
NaOH↔Na + + OH - .
The number of hydroxide ions formed upon complete dissociation of a base molecule is called the acidity of the base. So, sodium hydroxide is monoacid, and calcium hydroxide – diacid:
Ca(OH) 2 ↔ Ca 2+ + 2OH - .
Bases, like all electrolytes, can be strong, weak or medium strength. Sodium, potassium and calcium hydroxides are strong bases, while ammonium hydroxide is a weak base.
Amphoteric hydroxides can dissociate as acids and bases:
Zn(OH) 2 ↔ Zn 2+ + 2OH - ;
Zn(OH) 2 ↔ 2H + + ZnO 2 2- .
In reactions with acids, amphoteric hydroxides dissociate as bases, and in reactions with bases, as acids.
Medium salts in aqueous solutions dissociate into metal cations and anions of the acid residue:
Acidic salts can dissociate partially:
or completely with the formation of, in addition to the metal ion, also a hydrogen cation:
Accordingly, basic salts can also dissociate partially or completely:
CaOHCl ↔ CaOH + + + Cl - ;
CaOHCl ↔ Ca 2+ + OH - + Cl - .
Most salts are strong electrolytes.
1.11. Control questions
1. Which elements of the 3rd period of PSEM belong to metals? Answer: sodium, magnesium, aluminum.
2. Which elements of the 4th group of the main subgroup belong to non-metals, semi-metals, metals?
Answer: carbon, silicon are non-metals, germanium is a semi-metal, tin and lead are metals.
3. Write the reaction equations for the interaction of zinc (CO + 2), phosphorus (CO + 5) and titanium (CO + 4) with oxygen.
4. Write the reaction equations for the interaction of calcium, phosphorus (CO +3, and potassium with chlorine.
5. Write the reaction equations for the interaction of phosphorus(V) oxide and magnesium oxide with water.
6. Write the equation for the reaction between magnesium oxide and carbon monoxide (IV).
7. Write the reaction equations for the interaction of aluminum oxide with sodium oxide and sulfur oxide (VI).
8. Write the equation for the reaction between hydrochloric acid and aluminum.
9. Write the equation for the reaction between nitric acid and aluminum oxide.
10. Write the reaction equations for the interaction of sulfuric acid with potassium hydroxide and iron (II) hydroxide.
11. Write the equation for the reaction between ammonium hydroxide and hydrochloric acid.
12. Write the reaction equations for the interaction of sodium hydroxide with sulfur oxide (VI) and zinc oxide (zinc oxidation state +2).
13. Write the equation for the reaction between iron (III) sulfate and sodium hydroxide, knowing that iron (III) hydroxide is insoluble in water.
14. Write the equation for the reaction between hydrochloric acid and sodium sulfite (Na 2 SO 3), knowing that sulfurous acid is unstable and decomposes into water and gaseous sulfur oxide (IV).
15. Write the equation for the reaction between sodium sulfate and calcium chloride, knowing that calcium sulfate is insoluble in water.
16. Write the equations for the electrolytic dissociation of nitric acid, potassium hydroxide and magnesium chloride.
17. Write the equations for the electrolytic dissociation of carbonic acid, magnesium hydroxide and aluminum sulfate.
18. Write the equations for the electrolytic dissociation of phosphoric acid (H 3 PO 4), barium hydroxide (Ba(OH) 2) and magnesium nitrate.
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 above 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 whose formulas 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 during 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 correct?
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: A.S. Koroshchenko. and Kuptsova A.A.
Aqueous solutions of some substances are conductors of electric current. These substances are classified as electrolytes. Electrolytes are acids, bases and salts, melts of some substances.
DEFINITION
The process of decomposition of electrolytes into ions in aqueous solutions and melts under the influence of electric current is called electrolytic dissociation.
Solutions of some substances in water do not conduct electricity. Such substances are called non-electrolytes. These include many organic compounds, such as sugars and alcohols.
Electrolytic dissociation theory
The theory of electrolytic dissociation was formulated by the Swedish scientist S. Arrhenius (1887). The main provisions of the theory of S. Arrhenius:
— electrolytes, when dissolved in water, break up (dissociate) into positively and negatively charged ions;
— under the influence of electric current, positively charged ions move to the cathode (cations), and negatively charged ones move to the anode (anions);
— dissociation is a reversible process
KA ↔ K + + A −
The mechanism of electrolytic dissociation is the ion-dipole interaction between ions and water dipoles (Fig. 1).
Rice. 1. Electrolytic dissociation of sodium chloride solution
Substances with ionic bonds dissociate most easily. Dissociation occurs similarly in molecules formed according to the type of polar covalent bond (the nature of the interaction is dipole-dipole).
Dissociation of acids, bases, salts
When acids dissociate, hydrogen ions (H +) are always formed, or more precisely hydronium (H 3 O +), which are responsible for the properties of acids (sour taste, action of indicators, interaction with bases, etc.).
HNO 3 ↔ H + + NO 3 −
When bases dissociate, hydrogen hydroxide ions (OH −) are always formed, which are responsible for the properties of bases (changes in the color of indicators, interaction with acids, etc.).
NaOH ↔ Na + + OH −
Salts are electrolytes, upon dissociation of which metal cations (or ammonium cation NH 4 +) and anions of acid residues are formed.
CaCl 2 ↔ Ca 2+ + 2Cl −
Polybasic acids and bases dissociate stepwise.
H 2 SO 4 ↔ H + + HSO 4 − (I stage)
HSO 4 − ↔ H + + SO 4 2- (II stage)
Ca(OH) 2 ↔ + + OH − (I stage)
+ ↔ Ca 2+ + OH −
Degree of dissociation
Electrolytes are divided into weak and strong solutions. To characterize this measure, there is the concept and value of the degree of dissociation (). The degree of dissociation is the ratio of the number of molecules dissociated into ions to the total number of molecules. often expressed in %.
Weak electrolytes include substances whose degree of dissociation in a decimolar solution (0.1 mol/l) is less than 3%. Strong electrolytes include substances whose degree of dissociation in a decimolar solution (0.1 mol/l) is greater than 3%. Solutions of strong electrolytes do not contain undissociated molecules, and the process of association (combination) leads to the formation of hydrated ions and ion pairs.
The degree of dissociation is particularly influenced by the nature of the solvent, the nature of the dissolved substance, temperature (for strong electrolytes, with increasing temperature, the degree of dissociation decreases, and for weak electrolytes, it passes through a maximum in the temperature range of 60 o C), the concentration of solutions, and the introduction of ions of the same name into the solution.
Amphoteric electrolytes
There are electrolytes that, upon dissociation, form both H + and OH − ions. Such electrolytes are called amphoteric, for example: Be(OH) 2, Zn(OH) 2, Sn(OH) 2, Al(OH) 3, Cr(OH) 3, etc.
H + +RO − ↔ ROH ↔ R + +OH −
Ionic reaction equations
Reactions in aqueous solutions of electrolytes are reactions between ions - ionic reactions, which are written using ionic equations in molecular, full ionic and abbreviated ionic forms. For example:
BaCl 2 + Na 2 SO 4 = BaSO 4 ↓ + 2NaCl (molecular form)
Ba 2+ + 2 Cl − + 2 Na+ + SO 4 2- = BaSO 4 ↓ + 2 Na + + 2 Cl− (full ionic form)
Ba 2+ + SO 4 2- = BaSO 4 ↓ (short ionic form)
pH value
Water is a weak electrolyte, so the dissociation process occurs to an insignificant extent.
H 2 O ↔ H + + OH −
The law of mass action can be applied to any equilibrium and the expression for the equilibrium constant can be written:
K = /
The equilibrium concentration of water is a constant value, therefore.
K = = KW
It is convenient to express the acidity (basicity) of an aqueous solution through the decimal logarithm of the molar concentration of hydrogen ions, taken with the opposite sign. This value is called the pH value.
What substances are formed when iron reacts with dilute sulfuric acid?
1) iron(III) sulfate, water and sulfur(IV) oxide
2) iron(II) sulfate and hydrogen
3) iron(III) sulfite and hydrogen
4) iron(II) sulfide and hydrogen
Calcium oxide interacts with
6. A solution of hydrochloric acid reacts with each of two substances:
1) AgNO 3 and Cu(OH) 2
3) MgO and HBr
Reacts with sodium hydroxide solution
Both oxygen and hydrogen react with
NH 3 + O 2 → H 2 O + NO
The coefficient in front of the reducing agent formula is equal to
10. Ethylene is characterized by the following:
1) the molecule contains 2 carbon atoms and 6 hydrogen atoms
2) the molecule has a flat shape
3) bond angle is 120°
4) does not add hydrogen and chlorine
5) upon hydration forms acetic acid
CuSO 4 + Nal -> Cul + Na 2 SO 4 + I 2 .
(9th grade)
Option 11
2) Cl 2 + H 2 = 2HCl
3) Cl 2 + 2KI = 2Кl + I 2
2. The abbreviated ionic equation H + + OH - = H 2 O corresponds to the reaction
1) KOH + H 2 SO 4 →
2) NH 4 OH + H 2 SO 4 →
3) Fe(OH) 2 + H 2 SO 4 →
4) Ba(OH) 2 + H 2 SO 4 →
3. Weak electrolytes do not include:
Magnesium is easily soluble in
1) distilled water
2) ammonia water
3) HCl solution
4) Na 2 CO 3 solution
Which of the following substances reacts with phosphorus(V) oxide?
3) carbon monoxide (IV)
4) carbon monoxide (II)
When silver reacts with concentrated nitric acid, it predominantly forms
1) silver(I) nitrate, hydrogen, water
2) substances do not interact
3) nitrogen oxide(IV), silver(I) nitrate, water
4) nitrogen oxide(IV), silver(I) nitrite, water
A chemical reaction is possible between
1) Zn and CuCl 2
2) NaOH and K 3 PO 4
4) HCl and Ba(NO 3)
The interaction between sodium and oxygen is predominantly expressed by the equation
1) 4Na + O 2 = 2Na 2 O
3) 2Na + O 2 = Na 2 O 2
4) Na + O 2 = NaO 2
In the equation of the redox reaction
Al + H 2 O → Al(OH) 3 + H 2
The coefficient in front of the oxidizing formula is equal to
10. Stearic acid:
1) has the formula C 17 H 35 COOH
2) can interact with glycerin
3) mainly included in vegetable fats
4) dissolves well in water
5) interacts with sodium chloride
Using the electron balance method, create an equation for the reaction
H 2 S + FeCl 3 → S + HCl + FeCl 2.
Identify the oxidizing agent and the reducing agent.
Final test in chemistry
(9th grade)
Option 12
Redox reactions include interactions between
1) sodium oxide and water
2) carbon monoxide (IV) and calcium oxide
3) iron and copper(II) chloride
4) sulfuric acid and barium nitrate
Molecular reaction equation
CuO + 2HNO 3 = Cu(NO 3) 2 + H 2 O
The corresponding abbreviated ionic equation is
1) CuO + 2HNO 3 = Cu 2+ + 2NO - + H 2 O
2) CuO + 2H + + 2NO 3 − = Cu 2+ + 2NO 3 - + H 2 O
3) CuO + 2H + = Cu 2+ + H 2 O
4) O 2- + 2H + = H 2 O
3. Electrolytes include each of two substances:
1) potassium hydroxide (solution) and sodium acetate (solution)
2) iron(III) oxide and acetic acid (solution)
3) barium chloride (solution) and fructose (solution)
4) ethanol (solution) and calcium carbonate
Zinc reacts quickly with an aqueous solution
Forms acid when reacting with water
Magnesium hydroxide reacts with
1) copper(II) hydroxide
2) calcium oxide
3) potassium chloride
4) phosphoric acid
Which diagram corresponds to a practically impossible reaction?
1) Cu + Fe(NO 3) 2 →
2) Mg + РbСl 2 →
3) Fe + CuSO 4 →
4) Cl 2 + NaBr →
8. They don't respond together
1) chlorine and hydrogen
2) oxygen and calcium
3) nitrogen and water
4) iron and sulfur
In the equation for the reaction of complete combustion of hydrogen sulfide in oxygen, the coefficient in front of the oxidizer formula is equal to
10. The following reactions are characteristic of ethyne:
1) hydrogenation and hydration
2) hydration and isomerization
3) replacement of hydrogen atoms with halogen and oxygen
4) addition of halogens and nitrogen
5) polymerization and oxidation
Using the electron balance method, create an equation for the reaction
HI + HNO 3 (conc.) → HIO 3 + NO 2 + H 2 O.
Identify the oxidizing agent and the reducing agent.
Final test in chemistry
(9th grade)
Option 13
A redox reaction is not
1) 2Cl 2 + 2H 2 O = 4HCl + O 2
2) Cl 2 + H 2 = 2HCl
3) Cl 2 + 2KI = 2Кl + I 2
4) HCl + AgNO 3 = AgCl + HNO 3
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., associated with 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) 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, i.e. associated with water molecules, are formed. 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 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 the solutions are mixed, a mixture of ions is formed.
Reference material for taking the test:
Mendeleev table
Solubility table
