Modern electrolytic dissociation theory. Theory of electrolytic dissociation

It is well known that solutions can acquire some qualities that are not observed by any of the components taken individually. Thus, the aqueous NaCl solution is well conducted by electric current, while neither clean water, nor dry salt with electrical conductivity possess. In this regard, all dissolved substances are made to divide into two types:

1) substances whose solutions have electrical conductivity called electrolyte;

2) substances whose solutions do not possess electrical water, called non-electroliths.

Neelectricates include oxides, gases, most organic compounds (hydrocarbons, alcohols, aldehydes, ketones, etc.).

The electrolytes include most inorganic and some organic acids, bases and salts.

The appearance of electrical conductivity in the solutions of electrolytes explained S. Arrhenius, which in 1887 suggested theory of electrolytic dissociation:

The electrolytic dissociation is the process of decay of electrolyte per ions under the action of solvent molecules.

The main cause of electrolytic dissociation is the process of solvation (hydration) ions. Due to the solvation, the return process is hampered reconganization ions, also called association or polyarization.

In this regard, some provisions can be formulated:

1) dissociations are subjected to substances with ion or close to the ionic type of chemical bond;

2) the dissociation process is stronger in polar solvent and weaker (if possible) in a non-polar solvent;

3) the dissociation process is the stronger than the above the dielectric permeability of the solvent.

IN general The electrolytic dissociation process in water can be represented as follows:

KT N An M  ( x.  y.) H 2 O ⇄ N m +  m n ,

where KT M + is a positively charged ion ( cation);

An N  is a negatively charged ion ( anion).

Values x. and y.reflecting the amount of water molecules in hydrate shells vary widely depending on the nature and concentration of ions, temperature, pressure, etc. In this regard, it is more convenient to use simplified electroly fat dissociation equations, i.e. excluding hydration:

NaCl Na +  Cl ;

Cuso 4 Cu 2+  SO 4 2 ;

K 3 PO 4 3K +  PO 4 3 .

However, it should be borne in mind that with acid dissociation in aqueous solutions, no free H + ions, but rather stable hydroxonium ions H 3 O + are formed, therefore the acid dissociation equation (for example, HCl) should look like this:

HCl  H 2 O H 3 O +  Cl .

However, in the chemical literature, a form of recording is more common, reflecting only the process of electrolyte decay without taking into account the effect of hydration. In the future, we will also use simplifying terminology.

Strong and weak electrolytes

The quantitative characteristic of the electrolytic dissociation process is the degree of dissociation.

The degree of dissociation Called the ratio of the number of electrolyte, sprinkled by ions (n.), to the total number of electrolyte (n. 0 ):

The value of  is expressed in the fractions of the unit or% and depends on the nature of the electrolyte, solvent, temperature, concentration and composition of the solution.

A solvent played a special role: in some cases, during the transition from aqueous solutions to organic solvents, the degree of dissociation of electrolytes can increase or decrease. In the future, in the absence of special instructions, we assume that the solvent is water.

According to the degree of dissociation, electrolytes are conventionally divided into strong ( > 30%), middle (3% <  < 30%) и weak ( < 3%).

Silent electrolytes include:

1) some inorganic acids (HCl, HBr, Hi, HNO 3, H 2 SO 4, HCLO 4 and a number of others);

2) alkali hydroxides (Li, Na, k, Rb, Cs) and alkaline earth (CA, SR, BA) metals;

3) Almost all soluble salts.

The high-force electrolytes include Mg (OH) 2, H 3 PO 4, HCOOH, H 2 SO 3, HF and some others.

Last electrolytes consider everything carboxylic acids (except HCOOH) and hydrated forms of aliphatic and aromatic amines. Many inorganic acids are also weak electrolytes (HCN, H 2 S, H 2 CO 3, etc.) and the base (NH 3 ∙ H 2 O).

Despite some coincidences, in general, one should not identify the solubility of the substance with its degree of dissociation. Thus, acetic acid and ethanol Unlimited soluble in water, but at the same time, the first substance is a weak electricity, and the second  non-election.

Electrolytes - Substances, aqueous solutions and melts of which are carried out electric current. These substances have ionic and covalent strong-polar communications. Electrolytes are acids, bases, salts. The behavior of electrolytes in the solution explains the theory of electrolytic dissociation, formulated Svante Arrhenius In 1887:

Substances whose solutions are electrolytes, when dissolved disintegrates on particles (ions) carrying positive and negative charges.

The process of decay of electrolyte on ions is called electrolytic dissociation. Under the action of electrical stress, positively charged ions move to the cathode, and negatively charged to the anode.

Ions charged positively called cations, and denially charged ions - anions. Cations are positively charged metal ions, hydrogen ion, NH 4 +, anions -Cyl remnants and hydroxide ion. The magnitude of the ion charge coincides with the valence of an atom or acid residue, and the number of positive charges is equal to the number of negative. Therefore, the solution as a whole is electronically. The electrolytic dissociation process is represented as follows:

NaCl ↔ Na + + CL~

H 2 SO 4 ↔ 2H + + SO 4 2-

The theory of Arrhenius explained many phenomena associated with the properties of the solutions of electrolytes, but did not answer the question: why some substances are electrolytes, and others are not, and also a role in the formation of ions the solvent is played.

2 . Dissociation mechanism

The theory of the dissociation process was developed by I.A. Heel (1891).

Imagine that an ion crystal, such as NaCl, is entered into water. Each ion located on the surface of the crystal forms an electric field around itself. Near Na + is created a positive sign field, near the Cl - the electrostatic field of the negative sign is given. The influence of these fields extends to a certain distance from the crystal. In solution, the crystal from all sides surround randomly moving water molecules. Finding into the field of electrical charged ions, they change their movement: in the immediate vicinity of the crystal, they are oriented in such a way that they are directed by a positively charged pole to a negatively charged CL dipoles, and to a positively charged Na + ion - a negatively charged pole ( Fig. 1). Such a phenomenon is called the orientation of polar molecules in the electrostatic field. There are Coulomb forces of attraction between ions and dipoles of water. As a result of ion-dipole interaction, energy is distinguished, which contributes to the rupture of ionic bonds in the crystal and transformation of the ion from the crystal into the solution. Separated by each other ions immediately after breaking the relationship between them are closely surrounded by polar water molecules and become completely hydrated. The phenomenon of the interaction of ions with water molecules, resulting in the formation of a hydrate shell, is called hydration ions.

Fig. 1. Dissociation of ionic compounds

Hydrated ions that have opposite charges can interact with each other. But since the ions are moving in solution together with hydrate shells, the strength of their interaction is significantly reduced, and they are capable of independent existence.

When the polar compounds is dissolved, the orientation of the dipoles of water around the dissolved molecules occurs, causing even greater polarization of them. Polar covalent bond between atoms goes into ionic. The total electron pair shifts to one of the atoms (Fig. 2).

Fig. 2. Dissociation of molecules with polar covalent bond

For example, in HCl, the electron pair shifts to the chlorine atom, which turns into a hydrated chlorine ion, and the proton with a water molecule forms a complex positively charged particle H 3 O + - hydroxony ion.

HCl + XH 2 O ↔ H 3 O + + CL - ∙ YH 2 O

Thus, the electrolytes can only be compounds with ionic or polar covalent bond. Electrolytes can dissociate only in polar solvents.

Municipal budgetary general Education

Maharkhangelsky district

"Ivanovo Middle comprehensive school»

Open chemistry lesson in grade 8 on the topic

« The main provisions of the theory of electrolytic dissociation ».

Chemistry teacher Trohin S.N.

D. Second Ivan, 2015

The main provisions of the theory of electrolytic dissociation.

Objectives lesson:

Educational -

formulate the main provisions of the theory of electrolytic dissociation;

summarize information about ions;

secure the ability to record the dissociation process using chemical signs and formulas.

Educational - To bring up a desire to learn actively, with interest, instilling conscious discipline, clarity and organization in operation.

Developing - Develop the ability of students based on theoretical knowledge to compare, analyze, summarize, logically arguing, draw conclusions, develop oral speech.

Teaching methods: explanation, conversation, setting and decision educational problems, independent individual work.

Means of education: Multimedia projector, computer, solubility table acid, bases and salts in water, training exercises.

Type of lesson : Lesson studying a new material.

During the classes:

I. .Organizing time.

II. Actualization of the material passed: checking homework.

Check homework. You have on the tables there are sheets with tasks. Write your surname and name in the upper right corner. We proceed to perform the task. The task is 5 minutes.

Exercise 1.

Check your knowledge. Extract definitions.

Substances, the solutions of which are conducted by electric current, are called ... (electrolytes)

The process of decay of electrolyte per ions is called ... (electrolytic dissociation)

Substances whose solutions do not conduct an electric current, called ... (non-electrolytes)

The ratio of the number of particles encountered on ions to the total number of dissolved particles is called ... (degree of electrolytic dissociation)

Task 2.

Check your knowledge. Complete the scheme.

Task 3.

Check your knowledge. Fill the table.

Electrolytes.

Neelectrics

Soluble salts

Organic substances

Alkalis

Simple substances

Acid

Insoluble oxides

Insoluble salts, acids, bases

II. I. . Introductory conversation: message theme, explanation of the goals and objectives of the lesson.

Today, we will introduce the main provisions of the theory of electrolytic dissociation. This topic is a continuation of the previous lesson. Therefore, today the goal of our lesson will summarize information about ions, consolidate the ability to record the dissociation process using chemical signs and formulas, formulate the main provisions of the electrolytic dissociation theory

IV. Studying a new material.

The history of the opening of the theory of electrolytic dissociation.

Swedish scientist Svante Arrhenius Studying the electrical conductivity of solutions of various substances, came to the conclusion that the cause of electrical conductivity is the presence of ions in the ion solution, which is formed when the electrolyte is dissolved in water. This process was called electrolytic dissociation. In 1887, Arrhenius formulated the main provisions of the theory of electrolytic dissociation. Consider the main provisions of the theory of electrolytic dissociation (in the abbreviated version of the TED).

The main provisions of the theory of the TED.

1. When dissolved in water, electrolytes dissociate (disintegrate) on positive and negative ions.

For example: NaCl \u003d Na + + Cl. -

Ions are one of the forms of existence. chemical element. Ions differ from atoms by the number of electrons, i.e. Electric charge. Atoms are neutral particles, ions have a charge (positive or negative). These two circumstances and determine the difference in their properties.

Consequently, ions are positive or negatively charged particles in which atoms or groups of atoms are converted as a result of the return or addition of electrons. This transformation process can be represented as a schema.

We will analyze the difference in the properties of atoms and ions on the example of everyone famous substance - Craw salt. 1 Electron is a lot to change the properties, therefore the properties of the ions are completely similar to the properties of atoms that form them. Metal sodium is a very reactive substance that is even stored under the kerosene layer, otherwise sodium will start interacting with the components ambient. Sodium vigorously interacts with water, while forming alkali and hydrogen at the same time, while positive sodium ions such products do not form. Chlorine has a yellow-green color and a sharp smell, poisonous, and chlorine ions are colorless, neyovites are devoid of smell. Nobody will come to mind to use in food metal sodium and chlorine gaseous chlorine, while without sodium chloride consisting of sodium and chlorine ions, it is impossible to prepare food. These two particles are distinguished by only one electron.

The word "ion" translated from Greek means "wandering". In the solutions of the ions randomly move ("Walk") in various directions. The composition of the ions are divided into simple - CL - , Na. + Sophisticated - NH 4 + , So. 4 -.

2. The cause of electrolyte dissociation in aqueous solution is its hydration, i.e. Electrolyte interaction with water molecules and gap chemical bond in him.

As a result of the interaction of electrolyte with water molecules, hydrated, i.e. associated with water molecules, ions.

Consequently, according to the presence of an aqueous membrane, the ions are divided into hydrated (in solutions and crystallohydrates) and nonhyracted (in anhydrous salts). For example: crystallohydrates - Glumber Salt, copper sipop; Anhydrous salts - copper sulfate, sodium nitrate. The properties of hydrated and nonhyracted ions differ as you were able to make sure the example of copper ions.

Ions (for the presence of aquatic shell)

hydrated
in solutions and crystallohydrates: CUSO 4 * 5h. 2 O, Na. 2 SO. 4 * 10h. 2 O.

nonhyracted
in anhydrous saline: Cu 2+ SO. 4 2- , Na. + No. 3 -

3. Under action electric current Positively charged ions are moving towards the negative pole of the current source - cathode, so they are called cations, and negatively charged ions move to the positive pole of the current source - anoma, so they are called anions.

Consequently, there is another classification of ions - by the sign of their charge.

Ions
* Cations (positively charged particles)
* Anions (negatively charged particles)

In electrolyte solutions, the amount of charge charges of the cations is equal to the sum of the charges of anions, as a result of which these solutions are electronically.

4.Electronic dissociation is the process of reversible for weak electrolytes. Along with the dissociation process (the disintegration of the electrolyte per ions) proceeds and the reverse process - the association (ions). Therefore, in the electrolytic dissociation equations, instead of a sign of equality, a sign of reversibility is put, for example:

Hno. 2 ↔ H. + + No. 2-

5. Not all electrolytes are equally dissociated on ions.

The degree of dissociation depends on the nature of the electrolyte and its concentration.

According to the dissociation, electrolytes are divided into weak and strong.

6. Chemical properties The solutions of the electrolytes are determined by the properties of the ions they form during dissociation.

According to the nature of the ions generated during dissociation, three types of electrolytes are distinguished: acid, bases and salts.

V. Flowstone material.

Let us now try to task using the information received. When performing a task, pay attention to whether the substance is an electrolyte.

THE TASK.

HCL

Hno. 3

H. 2 SiO. 3

Based on the compiled schemes, try to define acids from the point of view of the TED.

Extract definition

Acids are electrolytes that dissociate on the cations ... and anions ...

Acid These are electrolytes, which for dissociation form hydrogen cations and an anions of the acid residue.

For example:

HCl \u003d H. + + Cl. -
Hno. 3 \u003d H. + + No. 3 -

For multi-axis acids, stepped dissociation proceeds. For example, for phosphoric acid H3PO4:

1st Step - Education Digidrophosphate - ions:

H. 3 PO 4 ↔ H. + + H. 2 PO 4 -

2nd Stage - Education Hydrophosphate - ions:

H. 2 PO 4 - ↔ H. + + HPO. 4 2-

It should be borne in mind that the dissociation of electrolytes over the second stage is much weaker than in the first. Dissociation for the third stage under normal conditions is almost not occurring.

All acids combine that they necessarily form hydrogen cations. Therefore, it is logical to assume that the general characteristic properties of acids - sour taste, change in the color of the indicators, etc. - due to the cations of hydrogen.

Perform the following task based on the main positions of the TED.

THE TASK.

Make possible equations of electrolytic dissociation of substances in aqueous solutions.

Naoh.

Koh.

Fe (OH) 2

Name the data class substances.

Based on the compiled schemes, try to define the bases from the point of view of the TED.

Extract definition

The bases are electrolytes that dissociate on the cations ... and anions ...

Grounds these are electrolytes, which for dissociation form metal cations and hydroxide anions.

For example:

NaOH \u003d Na. + + Oh. -
Koh \u003d K. + + Oh. -

Multi-acid bases dissociate stepwise, mainly at the first stage. For example, barium hydroxide BA (OH) 2:

1st stage - the formation of hydroxy ions:

BA (OH) 2 ↔ Oh. - + Baoh. +

2nd Stage - Baria ions formation:

Baoh + ↔ BA 2+ + Oh. -

Everything general properties The grounds are soaps to the touch, changing the color of the indicators, etc. - due to the general for all bases by hydroxide ions he - .

Perform the following task.

THE TASK.

Make possible equations of electrolytic dissociation of substances in aqueous solutions.

NaCl.

KNO. 3

BASO. 4

Name the data class substances.

Based on the compiled schemes, try to define the salts from the point of view of the TED.

Extract definition

Salts are electrolytes that dissociate on the cations ... and anions ...

Soli these are electrolytes, which for dissociation form metal cations (or ammonium NH 4 ) and anions of acid residues.

For example:

K. 3 PO 4 \u003d 3K. + + PO. 4 3-
NH 4 CL \u003d NH 4 + + Cl. -

It is obvious that the properties of salts are defined both metal cations and an anions of the acid residue. So, ammonium salts have both general properties due to NH ions 4 + and specialific, due to various anions. Similarly, the general properties of sulfate sulfate - are determined by SO ions 4 2- , and various - different cations. In contrast to polypic acids and bases containing several hydroxide ions, such salts like K 2 SO. 4 , Al 2 (So. 4 ) 3 etc., dissociate completely completely, not step.

And now let's do more difficult taskBased on the material studied in the lesson.

Check your knowledge

Using the solubility table, provide examples of three substances that form sulfate ions in solutions. Record the equation of electrolytic dissociation of these substances.

For example:

H. 2 SO. 4 ↔ H. + + So. 4 -
HSO. 4 ↔ H. + + So. 4 2-

VI Did notide lesson.

VII .Homework.

§36, the provisions of the TED record in the notebook, learn by heart;

Definitions of acids, bases, salts to learn by heart;

Task number 5 (writing).

\u003e\u003e Chemistry: the main provisions of the electrolytic dissociation theory summarizing the information on electrolytic dissociation in the form of the main provisions of the current generally accepted theory. They are as follows.
As a result of this interaction, hydrate-baths are formed, that is, associated with water molecules, ions.

Consequently, according to the presence of an aqueous membrane, the ions are divided into hydrated (in solutions and crystallohydrates) and non-hydrated (in anhydrous salts).

The properties of the hydraidonous and non-abstructing ions differ as you could already make sure the example of copper ions.

When dissolved in water electrolytes dissociate(Oxt) on positive and negative ions.

The properties of ions are absolutely not similar to the properties of atoms that form them. Ions - ZTO is one of the forms of the existence of the chemical element. For example, sodium metal atoms vigorously interact with water, forming alkali and hydrogen n, while sodium ions such products do not form. Chlorine has yellow-green and sharp points, poisonous, and chlorine ions are colorless, neyondites are devoid of smell. Nobody will come to mind to use in food metal sodium and chlorine gaseous chlorine, while without sodium chloride consisting of sodium and chlorine ions, it is impossible to prepare food.

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