Bank of works
Electrolytic dissociation of ion exchange reaction. Chemistry tutor manual. Reference material for taking the test

Electrolytic dissociation of ion exchange reaction. Chemistry tutor manual. Reference material for taking the test

Finished works

DEGREE WORKS

Much has already passed and now you are a graduate, if, of course, you write your thesis on time. But life is such a thing that only now it becomes clear to you that, having ceased to be a student, you will lose all the student joys, many of which you have never tried, putting everything off and putting it off until later. And now, instead of catching up, you're working on your thesis? There is an excellent solution: download the thesis you need from our website - and you will instantly have a lot of free time!
Theses have been successfully defended at leading universities of the Republic of Kazakhstan.
Cost of work from 20,000 tenge

COURSE WORKS

The course project is the first serious practical work. It is with the writing of coursework that preparation for the development of diploma projects begins. If a student learns to correctly present the content of a topic in a course project and format it competently, then in the future he will not have problems either with writing reports or with compiling theses, nor with the implementation of others practical tasks. In order to assist students in writing this type of student work and to clarify questions that arise during its preparation, in fact, this information section was created.
Cost of work from 2,500 tenge

MASTER'S DISSERTATIONS

Currently in higher educational institutions In Kazakhstan and the CIS countries, the level of higher education is very common vocational education, which follows a bachelor's degree - a master's degree. In the master's program, students study with the aim of obtaining a master's degree, which is recognized in most countries of the world more than a bachelor's degree, and is also recognized by foreign employers. The result of master's studies is the defense of a master's thesis.
We will provide you with up-to-date analytical and textual material, the price includes 2 science articles and abstract.
Cost of work from 35,000 tenge

PRACTICE REPORTS

After completing any type of student internship (educational, industrial, pre-graduation), a report is required. This document will be confirmation practical work student and the basis for forming an assessment for practice. Usually, in order to draw up a report on the internship, you need to collect and analyze information about the enterprise, consider the structure and work routine of the organization in which the internship is taking place, draw up a calendar plan and describe your practical activities.
We will help you write a report on your internship, taking into account the specifics of the activities of a particular enterprise.

During the lesson we will study the topic “ Electrolytic dissociation. Ion exchange reactions". Let's consider the theory of electrolytic dissociation and get acquainted with the definition of electrolytes. Let's get acquainted with the physical and chemical theory of solutions. Let's consider, in the light of the theory of electrolytic dissociation, the definition of bases, acids and salts, and also learn how to compose equations for ion exchange reactions and learn about the conditions for their irreversibility.

Topic: Solutions and their concentration, dispersed systems, electrolytic dissociation

Lesson: Electrolytic dissociation. Ion exchange reactions

1. Physical and chemical theory of solutions

Even at the dawn of the study of electrical phenomena, scientists noticed that not only metals, but also solutions can conduct current. But not all of them. So, aqueous solutions table salt and other salts, solutions of strong acids and alkalis conduct current well. Solutions of acetic acid, carbon dioxide and sulfur dioxide conduct it much worse. But solutions of alcohol, sugar and most other organic compounds do not conduct electricity at all.

Electricity- this is the directed movement of free charged particles. In metals, such movement is carried out due to relatively free electrons, electron gas. But not only metals are capable of conducting electric current.

Electrolytes - These are substances whose solutions or melts conduct electric current.

Non-electrolytes - These are substances whose solutions or melts do not conduct electric current.

To describe the electrical conductivity of some solutions, it is necessary to understand what a solution is. TO end of the 19th century centuries, there were 2 main theories of solutions:

· Physical. According to this theory, the solution - it is a purely mechanical mixture of components, and there is no interaction between particles in it. She described the properties of electrolytes well, but had certain difficulties in describing electrolyte solutions.

· Chemical. According to this theory, upon dissolution occurs chemical reaction between the solute and the solvent. This is confirmed by the presence of a thermal effect upon dissolution, as well as a change in color. For example, when white anhydrous copper sulfate is dissolved, a saturated blue solution is formed.

The truth is between these two extreme points. Namely, both chemical and physical processes occur in solutions.

Rice. 1. Svante Arrhenius

In 1887, the Swedish physical chemist Svante Arrhenius (Fig. 1), studying the electrical conductivity of aqueous solutions, suggested that in such solutions substances disintegrate into charged particles - ions, which can move to electrodes - a negatively charged cathode and a positively charged anode.

This is the reason for the electric current in solutions. This process is called electrolytic dissociation ( literal translation- splitting, decomposition under the influence of electricity). This name also suggests that dissociation occurs under the influence of an electric current. Further research showed that this is not so: ions are only charge carriers in a solution and exist in it regardless of whether current passes through the solution or not. At active participation Svante Arrhenius formulated the theory of electrolytic dissociation, which is often named after this scientist. The main idea of this theory is that electrolytes spontaneously disintegrate into ions under the influence of a solvent. And it is these ions that are charge carriers and are responsible for the electrical conductivity of the solution.

2. Basic principles of the theory of electrolytic dissociation

1. Electrolytes in solutions under the influence of a solvent spontaneously disintegrate into ions. This process is called electrolytic dissociation. Dissociation can also occur when solid electrolytes melt.

2. Ions differ from atoms in composition and properties. In aqueous solutions, ions are in a hydrated state. Ions in the hydrated state differ in properties from ions in the gaseous state of the substance. This is explained as follows: ionic compounds already initially contain cations and anions. When dissolved, a water molecule begins to approach charged ions: positive pole - to negative ion, negative pole - to the positive. The ions are called hydrated (Fig. 2).

3. In solutions or melts of electrolytes, ions move chaotically, but when an electric current is passed, the ions move directionally: cations - towards the cathode, anions - to the anode.

3. Bases, acids, salts in the light of the theory of electrolytic dissociation

In the light of the theory of electrolytic dissociation, bases, acids and salts can be defined as electrolytes.

Grounds- these are electrolytes, as a result of the dissociation of which in aqueous solutions only one type of anion is formed: hydroxide anion: OH-.

NaOH ↔ Na+ + OH−

The dissociation of bases containing several hydroxyl groups occurs in stages:

Ba(OH)2↔ Ba(OH)+ + OH− First stage

Ba(OH)+ ↔ Ba2+ + 2OH− Second stage

Ba(OH)2↔ Ba2+ + 2 OH− Summary equation

Acids - These are electrolytes, as a result of the dissociation of which in aqueous solutions only one type of cations is formed: H+. A hydrogen ion is precisely a hydrated proton and is designated H3O+, but for simplicity we write H+.

HNO3↔ H+ + NO3−

Polybasic acids dissociate stepwise:

H3PO4↔ H+ + H2PO4- First stage

H2PO4- ↔ H+ + HPO42- Second stage

HPO42-↔ H+ + PO43- Third stage

H3PO4↔ 3H+ + PO43-Summary equation

Salts - These are electrolytes that dissociate in aqueous solutions into metal cations and anions of the acid residue.

Na2SO4 ↔ 2Na+ + SO42−

Medium salts - These are electrolytes that dissociate in aqueous solutions into metal cations or ammonium cations and acid residue anions.

Basic salts - These are electrolytes that dissociate in aqueous solutions into metal cations, hydroxide anions and acid residue anions.

Acid salts - These are electrolytes that dissociate in aqueous solutions into metal cations, hydrogen cations and acid residue anions.

Double salts - These are electrolytes that dissociate in aqueous solutions into cations of several metals and anions of an acidic residue.

KAl(SO4)2↔ K+ + Al3+ + 2SO42

Mixed salts - these are electrolytes that dissociate in aqueous solutions into metal cations and anions of several acidic residues

4. Strong and weak electrolytes

Electrolytic dissociation to varying degrees - the process is reversible. But when some compounds are dissolved, the dissociation equilibrium is largely shifted towards the dissociated form. In solutions of such electrolytes, dissociation occurs almost irreversibly. Therefore, when writing dissociation equations for such substances, either an equal sign or a straight arrow is written, indicating that the reaction occurs almost irreversibly. Such substances are called strong electrolytes.

Weak are called electrolytes in which dissociation occurs insignificantly. When writing, use the reversibility sign. Table 1.

To quantify the strength of the electrolyte, the concept electrolytic degreedissociation.

The strength of an electrolyte can also be characterized using chemical equilibrium constants dissociation. It's called the dissociation constant.

Factors influencing the degree of electrolytic dissociation:

Nature of the electrolyte

Concentration of electrolyte in solution

· Temperature

As the temperature increases and the solution is diluted, the degree of electrolytic dissociation increases. Therefore, it is possible to evaluate the strength of an electrolyte only by comparing them under the same conditions. The standard is t = 180C and c = 0.1 mol/l.

5. Ion exchange reactions

The essence of the reaction in electrolyte solutions is expressed by the ionic equation. It takes into account the fact that in one solution electrolytes are present in the form of ions. And weak electrolytes and non-dissociable substances are written in a form that dissociates into ions. The solubility of an electrolyte in water cannot be used as a criterion for its strength. Many water-insoluble salts are strong electrolytes, but the concentration of ions in the solution is very low precisely because of their low solubility. That is why, when writing reaction equations involving such substances, it is customary to write them in non-dissociated form .

Reactions in electrolyte solutions proceed in the direction of ion binding.

There are several forms of ion binding:

1. Formation of sediment

2. Gas release

3. Formation of a weak electrolyte.

· 1. Formation of sediment:

BaCl2 + Na2CO3 → BaCO3↓ + 2NaCl.

Ba2++2Cl - + 2Na++CO32-→ BaCO3↓ + 2Na++2Cl- complete ionic equation

Ba2+ + CO32-→ BaCO3↓ reduced ionic equation.

The abbreviated ionic equation shows that when any soluble compound containing the Ba2+ ion reacts with a compound containing the carbonate anion CO32-, the result is an insoluble precipitate of BaCO3↓.

· 2. Gas release:

Na2CO3 +H2SO4 → Na2SO4 + H2O + CO2&

Electrolytes and non-electrolytes

It is known from physics lessons 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 to the explanation of electrolytic dissociation chemical theory D.I. Mendeleev and proved that when an electrolyte dissolves, chemical reaction dissolved substance with water, which leads to the formation of 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 decaying into ions (N g) to total number 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.

According to 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 are transformed chemical elements as a result of the donation or gain 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 rupture chemical bond in him.

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. Chemical properties electrolyte solutions are determined by the properties of the ions 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. General properties 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 acid residues.

According to the theory of electrolytic dissociation all reactions in aqueous solutions of electrolytes are reactions between ions. This is due to high speed 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

Even at the dawn of the study of electrical phenomena, scientists noticed that not only metals, but also solutions can conduct current. But not all of them. Thus, aqueous solutions of table salt and other salts, solutions of strong acids and alkalis conduct current well. Solutions of acetic acid, carbon dioxide and sulfur dioxide conduct it much worse. But solutions of alcohol, sugar and most other organic compounds do not conduct electricity at all.

Electric current is the directed movement of free charged particles . In metals, such movement is carried out due to relatively free electrons, electron gas. But not only metals are capable of conducting electric current.

Electrolytes are substances whose solutions or melts conduct electric current.

Nonelectrolytes are substances whose solutions or melts do not conduct electric current.

To describe the electrical conductivity of some solutions, it is necessary to understand what a solution is. By the end of the 19th century, there were 2 main theories of solutions

· Physical. According to this theory, a solution is a purely mechanical mixture of components and there is no interaction between particles in it. She described the properties of electrolytes well, but had certain difficulties in describing electrolyte solutions.

· Chemical. According to this theory, during dissolution, a chemical reaction occurs between the solute and the solvent. This is confirmed by the presence of a thermal effect during dissolution, as well as a change in color. For example, when white anhydrous copper sulfate is dissolved, a saturated blue solution is formed.

The truth is between these two extremes. Namely , both chemical and physical processes occur in solutions.

In 1887, the Swedish physical chemist Svante Arrhenius, studying the electrical conductivity of aqueous solutions, suggested that in such solutions substances disintegrate into charged particles - ions, which can move to electrodes - a negatively charged cathode and a positively charged anode.

This is the reason for the electric current in solutions. This process is called electrolytic dissociation(literal translation – splitting, decomposition under the influence of electricity). This name also suggests that dissociation occurs under the influence of an electric current. Further research showed that this is not the case: ions are onlycharge carriers in solution and exist in it regardless of whether it passes throughcurrent solution or not. With the active participation of Svante Arrhenius, the theory of electrolytic dissociation was formulated, which is often named after this scientist. The main idea of this theory is that electrolytes spontaneously disintegrate into ions under the influence of a solvent. And it is these ions that are charge carriers and are responsible for the electrical conductivity of the solution.

1. Electrolytes in solutions under the influence of a solvent spontaneously disintegrate into ions. This process is called electrolytic dissociation. Dissociation can also occur when solid electrolytes melt.

2. Ions differ from atoms in composition and properties. In aqueous solutions, ions are in a hydrated state. Ions in the hydrated state differ in properties from ions in the gaseous state of the substance. This is explained as follows: ionic compounds already initially contain cations and anions. When dissolved, the water molecule begins to approach charged ions - the positive pole to the negative ion, the negative pole to the positive ion. The ions are called hydrated.

3. In solutions or melts of electrolytes, ions move chaotically, but when an electric current is passed, the ions move directionally: cations - towards the cathode, anions - towards the anode.

In the light of the theory of electrolytic dissociation, bases, acids and salts can be defined as electrolytes.

Grounds– these are electrolytes, as a result of the dissociation of which in aqueous solutions, only one type of anion is formed: hydroxide anion: OH -

NaOH ↔ Na + + OH −

The dissociation of bases containing several hydroxyl groups occurs in steps.

Ba(OH) 2 ↔ Ba(OH) + + OH − First stage

Ba(OH) + ↔ Ba 2+ + 2OH − Second stage

Ba(OH) 2 ↔ Ba 2+ + 2 OH − Summary equation

Acids- these are electrolytes, as a result of the dissociation of which in aqueous solutions, only one type of cations is formed: H +. The hydrogen ion is called a hydrated proton and is designated H 3 O +, but for simplicity we write H +

HNO 3 ↔ H + + NO 3 −

Polybasic acids dissociate stepwise

H 3 PO 4 ↔ H + + H 2 PO 4 - First stage:

H 2 PO 4 - ↔ H + + HPO 4 2- Second stage:

HPO 4 2- ↔ H + + PO 4 3- Third stage:

H 3 PO 4 ↔ 3H + + PO 4 3- Summary equation

Salts- these are electrolytes that dislocate in aqueous solutions into metal cations and anions of the acid residue.
Na 2 SO 4 ↔ 2Na + + SO 4 2−

Medium salts These are electrolytes that dissociate in aqueous solutions into metal cations or ammonium cations and anions of the acid residue.

Basic salts- these are electrolytes that dissociate in aqueous solutions into metal cations, hydroxide anions and acid residue anions.

Acid salts These are electrolytes that dissociate in aqueous solutions into metal cations, hydrogen cations and acid residue anions.

Double salts- these are electrolytes that dissociate in aqueous solutions into cations of several metals and anions of an acid residue.

KAl (SO 4) 2 ↔ K + + Al 3+ + 2SO 4 2

Mixed salts- these are electrolytes that dissociate in aqueous solutions into metal cations and anions of several acidic residues

Electrolytic dissociation is, to one degree or another, a reversible process. But when some compounds are dissolved, the dissociation equilibrium is largely shifted towards the dissociated form. In solutions of such electrolytes, dissociation occurs almost irreversibly. Therefore, when writing dissociation equations for such substances, either an equal sign or a straight arrow is written, indicating that the reaction occurs almost irreversibly. Such substances are called strong electrolytes.

Weak are called electrolytes in which dissociation occurs insignificantly. When writing, use the reversibility sign.Table 1.

To quantify the strength of the electrolyte, the concept electrolytic degree dissociation .

The strength of an electrolyte can also be characterized using chemical equilibrium constants dissociation. This is called the dissociation constant.

Factors influencing the degree of electrolytic dissociation:

Nature of the electrolyte

Concentration of electrolyte in solution

· Temperature

As the temperature increases and the solution is diluted, the degree of electrolytic dissociation increases. Therefore, the strength of an electrolyte can only be assessed by comparing them under the same conditions. The standard is t=18 0 C and c=0.1 mol/l.

STRONG ELECTROLYTES

WEAK ELECTROLYTES

The degree of dissociation at 18 0 C in solutions with an electrolyte concentration of 0.1 mol/l is close to 100%. They dissociate almost irreversibly.

The degree of dissociation at 18 0 C in solutions with an electrolyte concentration of 0.1 mol/l is significantly less than 100%. Dissociation is irreversible.

· Some inorganic acids (HNO 3, HClO 4, HI, HCl, HBr, H 2 SO 4)

· Metal hydroxides, except groups IA and IIA, ammonia solution

· Many inorganic acids (H 2 S, HCN, HClO, HNO 2)

Organic acids (HCOOH, CH 3 COOH)

Reactions in electrolyte solutions proceed in the direction of ion binding.

There are several forms of ion binding.

1. Formation of sediment

2. Gas release

3. Formation of a weak electrolyte.

· 1. Formation of sediment:

BaCl 2 + Na 2 CO 3 → BaCO 3 ↓ + 2NaCl.

Ba 2+ +2Cl - + 2Na + + CO 3 2- →BaCO 3 ↓ + 2Na + +2Cl - complete ionic equation

Ba 2+ + CO 3 2- → BaCO 3 ↓ shortened ionic equation.

The abbreviated ionic equation shows that when any soluble compound containing the Ba 2+ ion reacts with a compound containing the carbonate anion CO 3 2, the result is an insoluble precipitate BaCO 3 ↓.

· 2. Gas release.

Na 2 CO 3 +H 2 SO 4 → Na 2 SO 4 + H 2 O + CO 2

2Na + + CO 3 2- +2H + + SO 4 2 - → 2Na + + SO 4 2 - + H 2 O + CO 2 complete ionic equation

2H + + CO 3 2- → H 2 O + CO 2 abbreviated ionic equation.

· 3. Formation of a weak electrolyte

KOH + HBr → KBr + H2O

K + + OH - + H + + Br - → K + + Br - + H 2 O complete ionic equation

OH - + H + → H 2 O abbreviated ionic equation.

Considering these examples, we were convinced that all reactions in electrolyte solutions occur in the direction of ion binding.

ELECTROLYTIC DISSOCIATION
ELECTROLYTES AND NON-ELECTROLYTES

Electrolytic dissociation theory

(S. Arrhenius, 1887)

When dissolved in water (or melted), electrolytes break down into positively and negatively charged ions (subject to electrolytic dissociation).

Under the influence of electric current, cations (+) move towards the cathode (-), and anions (-) move towards the anode (+).

Electrolytic dissociation is a reversible process ( backlash called molarization).

Degree of electrolytic dissociation ( a) depends on the nature of the electrolyte and solvent, temperature and concentration. It shows the ratio of the number of molecules broken up into ions ( n) to the total number of molecules introduced into the solution ( N).

a = n / N 0< a < 1

Mechanism of electrolytic dissociation of ionic substances

When dissolving compounds with ionic bonds ( e.g. NaCl) the hydration process begins with the orientation of water dipoles around all the protrusions and faces of the salt crystals.

Orienting around ions crystal lattice, water molecules form either hydrogen or donor-acceptor bonds with them. This process releases a large amount of energy, which is called hydration energy.

The energy of hydration, the magnitude of which is comparable to the energy of the crystal lattice, is used to destroy the crystal lattice. In this case, the hydrated ions pass layer by layer into the solvent and, mixing with its molecules, form a solution.

Mechanism of electrolytic dissociation of polar substances

Substances whose molecules are formed according to the polar type dissociate similarly. covalent bond(polar molecules). Around each polar molecule of matter ( for example HCl), The water dipoles are oriented in a certain way. As a result of interaction with water dipoles, the polar molecule becomes even more polarized and turns into an ionic molecule, then free hydrated ions are easily formed.

Electrolytes and non-electrolytes

The electrolytic dissociation of substances, which occurs with the formation of free ions, explains the electrical conductivity of solutions.

The process of electrolytic dissociation is usually written down in the form of a diagram, without revealing its mechanism and omitting the solvent ( H2O), although he is a major participant.

CaCl 2 « Ca 2+ + 2Cl -

KAl(SO 4 ) 2 « K + + Al 3+ + 2SO 4 2-

HNO 3 « H + + NO 3 -

Ba(OH) 2 « Ba 2+ + 2OH -

From the electrical neutrality of molecules it follows that the total charge of cations and anions should be equal to zero.

For example, for

Al 2 (SO 4 ) 3 –– 2 (+3) + 3 (-2) = +6 - 6 = 0

KCr(SO 4 ) 2 –– 1 (+1) + 3 (+3) + 2 (-2) = +1 + 3 - 4 = 0

Strong electrolytes

These are substances that, when dissolved in water, almost completely disintegrate into ions. As a rule, strong electrolytes include substances with ionic or highly polar bonds: all highly soluble salts, strong acids ( HCl, HBr, HI, HClO4, H2SO4, HNO3) and strong reasons ( LiOH, NaOH, KOH, RbOH, CsOH, Ba (OH) 2, Sr (OH) 2, Ca (OH) 2).

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

Weak electrolytes

Substances that partially dissociate into ions. Solutions of weak electrolytes contain undissociated molecules along with ions. Weak electrolytes cannot produce a high concentration of ions in solution.

Weak electrolytes include:

almost all organic acids ( CH 3 COOH, C 2 H 5 COOH, etc.);

some inorganic acids ( H 2 CO 3, H 2 S, etc.);

almost all salts, bases and ammonium hydroxide that are slightly soluble in water(Ca 3 (PO 4) 2; Cu (OH) 2; Al (OH) 3; NH 4 OH);

water.

They conduct electricity poorly (or almost not at all).

СH 3 COOH « CH 3 COO - + H +

Cu(OH) 2 «[CuOH] + + OH - (first stage)

[CuOH] + « Cu 2+ + OH - (second stage)

H 2 CO 3 « H + + HCO - (first stage)

HCO 3 - « H + + CO 3 2- (second stage)

Non-electrolytes

Substances whose aqueous solutions and melts do not conduct electric current. They contain covalent non-polar or low-polar bonds that do not break down into ions.

Electric current is not conducted by gases, solids (non-metals), organic compounds(sucrose, gasoline, alcohol).

IONIC REACTIONS. HYDROLYSIS

Ionic reactions in solution

Ion exchange reactions are reactions between ions formed as a result of the dissociation of electrolytes.

Rules for composing ionic reaction equations

Water-insoluble compounds ( simple substances, oxides, some acids, bases and salts) do not dissociate.

Reactions use solutions of substances, so even slightly soluble substances are found in solutions in the form of ions.

If a slightly soluble substance is formed as a result of a reaction, then when writing the ionic equation it is considered insoluble.

Sum electric charges ions on the left and right sides of the equation should be the same.

The procedure for composing ionic reaction equations

Write down molecular equation reactions

MgCl 2 + 2AgNO 3 ® 2AgCl + Mg(NO 3 ) 2

Determine the solubility of each substance using the solubility table