Electrolytes – substances whose aqueous solutions and melts conduct electric current. These substances have ionic and covalent highly polar bonds. Electrolytes are acids, bases, and salts. The behavior of electrolytes in solution is explained by the theory of electrolytic dissociation, formulated Svante Arrhenius in 1887:
Substances whose solutions are electrolytes, when dissolved, disintegrate into particles (ions) carrying positive and negative charges.
The process of electrolyte breaking down into ions is called electrolytic dissociation. Under the influence of electrical voltage, positively charged ions move towards the cathode, and negatively charged ones move towards the anode.
Ions that are positively charged are called cations, and negatively charged ions – anions. Cations are positively charged metal ions, hydrogen ion, NH 4 +, anions - acid residues and hydroxide ion. The amount of charge on the ion coincides with the valence of the atom or acidic residue, and the number of positive charges is equal to the number of negative ones. Therefore, the solution as a whole is electrically neutral. The process of electrolytic dissociation is depicted as follows:
NaCl ↔ Na + + Cl‾
H 2 SO 4 ↔ 2H + + SO 4 2–
Arrhenius's theory explained many phenomena associated with the properties of electrolyte solutions, but did not answer the question: why some substances are electrolytes and others are not, and also what role the solvent plays in the formation of ions.
2 . Dissociation mechanism
The theory of the dissociation process was developed by I.A. Heels (1891).
Let's imagine that an ionic crystal, for example NaCl, is added to water. Each ion located on the surface of the crystal forms around itself electric field. Near Na + a field of a positive sign is created, near Cl – an electrostatic field of a negative sign is given. The influence of these fields extends over a certain distance from the crystal. In solution, the crystal is surrounded on all sides by randomly moving water molecules. When they enter the field of action of electrically charged ions, they change their movement: in the immediate vicinity of the crystal they are oriented in such a way that the water dipoles are directed towards the negatively charged Cl – ion with a positively charged pole, and towards the positively charged Na + ion – with a negatively charged pole ( Fig. 1). This phenomenon is called the orientation of polar molecules in an electrostatic field. Coulomb attractive forces act between the ions and water dipoles. As a result of ion-dipole interaction, energy is released, which contributes to the breaking of ionic bonds in the crystal and the transfer of the ion from the crystal to the solution. The ions separated from each other, immediately after breaking the bond between them, are closely surrounded by polar water molecules and become completely hydrated. The phenomenon of interaction of ions with water molecules, resulting in the formation of a hydration shell, is called hydration of ions.
Rice. 1. Dissociation of ionic compounds
Hydrated ions that have opposite charges can interact with each other. But since the ions move in the solution together with the hydration shells, the force of their interaction is significantly reduced, and they are capable of independent existence.
When polar compounds are dissolved, water dipoles are oriented around the dissolved molecules, causing them to become even more polarized. A polar covalent bond between atoms becomes ionic. The shared electron pair moves to one of the atoms (Fig. 2).
Rice. 2. Dissociation of molecules with polar covalent bonds
For example, in HCl, an 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 + - hydronium ion.
HCl + xH 2 O ↔ H 3 O + + Cl – ∙yH 2 O
Thus, electrolytes can only be compounds with ionic or polar covalent bonds. Electrolytes can only dissociate in polar solvents.
Basic principles of the theory of electrolytic dissociation. Lesson-lecture using multimedia presentation
Pinaeva Galina Ivanovna, teacher of chemistry and biology
Sections: Chemistry teaching
Lesson objectives:
Educational –
formulate the main provisions of the theory of electrolytic dissociation;
summarize information about ions;
consolidate the ability to write down the dissociation process using chemical symbols and formulas.
Educational – cultivate a desire to learn actively, with interest, instill conscious discipline, clarity and organization in work.
Developmental – develop students' skills based on theoretical knowledge compare, analyze, generalize, reason logically, draw conclusions, develop oral speech.
Teaching methods: explanation, conversation, comparison, formulation and solution educational problems, chemical experiment(video), independent individual work.
Learning Tools: multimedia projector, computer, table of solubility of acids, bases and salts in water, training exercises, educational literature: “Chemistry. 8th grade”, authors – O.S. Gabrielyan - M.: Bustard, 2008.
Lesson progress
I. Organizational moment.
II. Introductory conversation: presentation of the topic, explanation of the goals and objectives of the lesson.
(2 min) /slide 1, 2/
The topic of the lesson today is “Basic principles of the theory of electrolytic dissociation.” This topic is a continuation of the previous lesson. Therefore, today the goal of our lesson will be to summarize information about ions, consolidate the ability to write down the dissociation process using chemical symbols and formulas, and formulate the basic principles of the theory of electrolytic dissociation
III. Updating the material covered: checking homework.
Let's check homework. You have worksheets on your desks. Write your first and last name in the upper right corner. Let's start the task. To complete the task – 5 minutes.
Task 1/slide 3/
Test your knowledge. Complete the definitions.
Substances whose solutions conduct electric current are called... (electrolytes)
The process of electrolyte decomposition into ions is called ... (electrolytic dissociation)
Substances whose solutions do not conduct electric current are called ... (non-electrolytes)
The ratio of the number of particles decaying into ions to total number dissolved particles are called ... (degree of electrolytic dissociation)
Task 2 /slide 4/
Test your knowledge. Complete the diagram.
Task 3 /slide 5/
Test your knowledge. Fill out the table.
| ELECTROLYTES | NON-ELECTROLYTES |
| Soluble salts | Organic matter |
| Simple substances |
|
| Insoluble oxides |
|
| Insoluble salts, acids, bases |
Task 4/slide 6/
You have 3 minutes to answer.
Using the diagram on the screen, tell us about the sequence of processes that occur during dissociation
A) substances with ionic bonds
orientation of molecules - water dipoles near crystal ions;
hydration (interaction) of water molecules with oppositely charged ions of the surface layer of the crystal;
dissociation (decay) of an electrolyte crystal into hydrated ions.
B) substances with polar covalent bonds
orientation of water molecules around the poles of an electrolyte molecule;
hydration (interaction) of water molecules with electrolyte molecules;
ionization of electrolyte molecules (conversion of a covalent polar bond into an ionic one);
dissociation (decay) of electrolyte molecules into hydrated ions.
IV. Learning new material.
History of the discovery of the theory of electrolytic dissociation. /slide 7/
Swedish scientist Svante Arrhenius studying the electrical conductivity of solutions various substances, came to the conclusion that the cause of electrical conductivity is the presence in the solution of ions that are formed when the electrolyte is dissolved in water. This process is called electrolytic dissociation. In 1887, Arrhenius formulated the basic principles of the theory of electrolytic dissociation. Let us consider the main provisions of the theory of electrolytic dissociation (in its abbreviated version, TED). /slide 8/
Basic provisions of the theory of TED
1. When dissolved in water, electrolytes dissociate (break up) into positive and negative ions.
For example: NaCl = Na + + Cl -
Ions are one of the forms of existence chemical element. Ions differ from atoms in the number of electrons, i.e. electric charge. Atoms are neutral particles, ions have a charge (positive or negative). These two circumstances determine the difference in their properties.
/slide 9/
Consequently, ions are positively or negatively charged particles into which atoms or groups of atoms are transformed as a result of the loss or addition of electrons. This transformation process can be represented in the form of a diagram.
Let us examine the difference in the properties of atoms and ions using the example of a well-known substance - table salt. 1 electron is a lot to change properties, so the properties of ions are completely different from the properties of the atoms that formed them. Metallic sodium is a very reactive substance, which is even stored under a layer of kerosene, otherwise the sodium will begin to interact with the components environment. Sodium reacts vigorously with water, forming alkali and hydrogen, while positive sodium ions do not form such products. Chlorine has a yellow-green color and a pungent odor, and is poisonous, while chlorine ions are colorless, non-toxic, and odorless. No one would think of using metallic sodium and chlorine gas in food, while without sodium chloride, consisting of sodium and chlorine ions, cooking is impossible. These two particles differ in only one electron.
The word “ion” translated from Greek means “wanderer.” In solutions, ions move randomly (“travel”) in different directions. According to their composition, ions are divided into simple - Cl -, Na + complex - NH 4 +, SO 4 -.
Basic provisions of the theory of TED
2. The reason for the dissociation of an electrolyte in an aqueous solution is its hydration, i.e. interaction of the electrolyte with water molecules and breaking of the chemical bond in it.
As a result of the interaction of the electrolyte with water molecules, hydrated, i.e., are formed. ions associated with water molecules.
Consequently, according to the presence of an aqueous shell, ions are divided into hydrated (in solutions and crystalline hydrates) and non-hydrated (in anhydrous salts). For example: crystal hydrates - Gluber's salt, copper sulfate; anhydrous salts - copper sulfate, sodium nitrate. The properties of hydrated and unhydrated ions are different, as you can see from the example of copper ions.
IONS (based on the presence of a water shell)
hydrated
in solutions and crystalline hydrates: CuSO 4 *5H 2 O, Na 2 SO 4 *10H 2 O
unhydrated
in anhydrous salts: Cu 2+ SO 4 2-, Na + NO 3 -
Basic provisions of the TED
3. Under the influence electric current Positively charged ions move to the negative pole of the current source - the cathode, which is why they are called cations, and negatively charged ions move to the positive pole of the current source - the anode, which is why they are called anions.
Consequently, there is another classification of ions - according to the sign of their charge.
IONS
*cations (positively charged particles)
*anions (negatively charged particles)
In electrolyte solutions, the sum of the charges of the cations is equal to the sum of the charges of the anions, as a result of which these solutions are electrically neutral.
Basic provisions of the TED
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:
HNO 2 ↔ H + + NO 2-
/slide 17/
Basic provisions of the TED
5. Not all electrolytes dissociate into ions to the same extent.
The degree of dissociation depends on the nature of the electrolyte and its concentration.
Based on the degree of dissociation, electrolytes are divided into weak and strong.
Basic provisions of the TED
6. Chemical properties electrolyte solutions are determined by the properties of the ions they form during dissociation.
Based on the nature of the ions formed during the dissociation of electrolytes, three types of electrolytes are distinguished: acids, bases and salts.
Let's now try to complete the task using the information received. When completing the task, pay attention to whether the substance is an electrolyte.
Based on the diagrams compiled, try to define acids from the point of view of TED.
ADD DEFINITION
Acids are electrolytes that dissociate into cations... and anions...
ACIDS- These are electrolytes that, upon dissociation, form hydrogen cations and anions of an acidic residue.
For example:
HCl = H + + Cl -
HNO 3 = H + + NO 3 -
For polybasic acids, stepwise dissociation occurs. For example, for phosphoric acid H3PO4:
1st stage – formation of dihydrogen phosphate ions:
H 3 PO 4 ↔ H + + H 2 PO 4 -
2nd stage – formation of hydrogen phosphate ions:
H 2 PO 4 - ↔ H + + HPO 4 2-
It should be taken into account that the dissociation of electrolytes in the second stage is much weaker than in the first. Dissociation in the third step at normal conditions almost never happens.
All acids have in common the fact that upon dissociation they necessarily form hydrogen cations. Therefore, it is logical to assume that the general characteristic properties of acids - sour taste, changes in the color of indicators, etc. - are caused precisely by hydrogen cations.
Let's complete the following task based on the main provisions of the TED.
Write down possible equations for the electrolytic dissociation of substances in aqueous solutions.
Name the class of these substances.
Based on the diagrams compiled, try to define the grounds from the point of view of TED.
ADD DEFINITION
Bases are electrolytes that dissociate into cations... and anions...
BASES- These are electrolytes that, upon dissociation, form metal cations and hydroxide anions.
For example:
NaOH = Na + + OH -
KOH = K + + OH -
Polyacid bases dissociate stepwise, mainly in the first step. For example, barium hydroxide Ba (OH)2:
1st stage – formation of hydroxo ions:
Ba (OH) 2 ↔ OH - + BaOH +
2nd stage – formation of barium ions:
BaOH+ ↔ Ba 2+ + OH -
All general properties bases - soapiness to the touch, change in color of indicators, etc. - are caused by hydroxide ions OH - common to all bases.
Let's complete the following task.
Write down possible equations for the electrolytic dissociation of substances in aqueous solutions.
Name the class of these substances.
Based on the diagrams compiled, try to define salts from the point of view of TED.
ADD DEFINITION
Salts are electrolytes that dissociate into cations... and anions...
SALT- these are electrolytes that, upon dissociation, form metal cations (or ammonium NH 4) and anions of acidic residues.
For example:
K 3 PO 4 = 3K + + PO 4 3-
NH 4 Cl = NH 4 + + Cl -
It is obvious that the properties of salts are determined by both metal cations and anions of the acid residue. Thus, ammonium salts have both general properties due to NH 4 + ions and specific properties due to various anions. Similarly, the general properties of sulfates - salts of sulfuric acid - are determined by SO 4 2- ions, and different ones - by different cations. Unlike polybasic acids and bases containing several hydroxide ions, salts such as K 2 SO 4, Al 2 (SO 4) 3, etc., dissociate completely at once, and not stepwise.
Now let's do a more difficult task, based on all the material we learned in class.
TEST YOUR KNOWLEDGE
Using the solubility table, give examples of three substances that form sulfate ions in solutions. Write down the equations for the electrolytic dissociation of these substances.
For example:
H 2 SO 4 ↔ H + + SO 4 -
HSO 4 ↔ H + + SO 4 2-
At the end of the lesson, I bring to your attention a video recording of an experiment showing the decomposition of a copper chloride solution into ions under the influence of an electric current.
We open our diaries and write down our homework.
§36, write down the provisions of the TED in a notebook, learn by heart;
Learn the definitions of acids, bases, and salts by heart;
Task No. 5, page 203 (written).
Electrolyte substances, when dissolved in water, disintegrate into charged particles - ions. The opposite phenomenon is molarization, or association. The formation of ions is explained by the theory of electrolytic dissociation (Arrhenius, 1887). On the decay mechanism chemical compounds during melting and dissolution, the characteristics of the types of chemical bonds, the structure and nature of the solvent influence.
Electrolytes and non-conductors
In solutions and melts, crystal lattices and molecules are destroyed—electrolytic dissociation (ED). The decomposition of substances is accompanied by the formation of ions, the appearance of such properties as electrical conductivity. Not every compound is capable of dissociating, but only substances that initially consist of ions or highly polar particles. The presence of free ions explains the ability of electrolytes to conduct current. Bases, salts, many inorganic and some organic acids have this ability. Nonconductors consist of low-polarity or unpolarized molecules. They do not break down into ions, being non-electrolytes (many organic compounds). Charge carriers are positive and negative ions (cations and anions).
The role of S. Arrhenius and other chemists in the study of dissociation
The theory of electrolytic dissociation was substantiated in 1887 by a scientist from Sweden S. Arrhenius. But the first extensive studies of the properties of solutions were carried out by the Russian scientist M. Lomonosov. T. Grothus and M. Faraday, R. Lenz contributed to the study of charged particles arising during the dissolution of substances. Arrhenius proved that many inorganic and some organic compounds are electrolytes. The Swedish scientist explained the electrical conductivity of solutions by the breakdown of the substance into ions. Arrhenius's theory of electrolytic dissociation did not attach importance to the direct participation of water molecules in this process. Russian scientists Mendeleev, Kablukov, Konovalov and others believed that solvation occurs - the interaction of a solvent and a dissolved substance. When it comes to water systems, then the name “hydration” is used. This is a complex physicochemical process, as evidenced by the formation of hydrates, thermal phenomena, changes in the color of the substance and the appearance of sediment.
Basic provisions of the theory of electrolytic dissociation (ED)
Many scientists worked to clarify the theory of S. Arrhenius. It required its improvement taking into account modern data on the structure of the atom and chemical bonds. The main provisions of TED are formulated, which differ from the classical theses of the late 19th century:
The phenomena that occur must be taken into account when drawing up equations: apply a special sign for a reversible process, count the negative and positive charges: they must be the same in total.
Mechanism of ED of ionic substances
The modern theory of electrolytic dissociation takes into account the structure of electrolyte substances and solvents. When dissolved, the bonds between oppositely charged particles in ionic crystals are destroyed under the influence of polar water molecules. They literally “pull” ions from the total mass into the solution. The decomposition is accompanied by the formation of a solvate shell (in water, a hydration shell) around the ions. In addition to water, ketones and lower alcohols have increased dielectric constant. During the dissociation of sodium chloride into Na + and Cl - ions, the initial stage is recorded, which is accompanied by the orientation of water dipoles relative to the surface ions in the crystal. On final stage hydrated ions are released and diffuse into the liquid.
The mechanism of ED compounds with covalent highly polar bonds
Solvent molecules affect the elements of the crystal structure of nonionic substances. For example, the effect of water dipoles on hydrochloric acid leads to a change in the type of bond in the molecule from polar covalent to ionic. The substance dissociates, and hydrated hydrogen and chlorine ions enter the solution. This example proves the importance of those processes that occur between the particles of the solvent and the dissolved compound. It is this interaction that leads to the formation of electrolyte ions.
The theory of electrolytic dissociation and the main classes of inorganic compounds
In light of the basic principles of TED, an acid can be called an electrolyte, during the decay of which only the H + proton can be detected from the positive ions. The dissociation of the base is accompanied by the formation or release from the crystal lattice of only the OH - anion and the metal cation. When dissolved, a normal salt produces a positive metal ion and a negative acid residue. The main salt is distinguished by the presence of two types of anions: an OH group and an acid residue. An acid salt contains only hydrogen and metal cations.
Electrolyte Power
To characterize the state of a substance in solution, it is used physical quantity— degree of dissociation (α). Its value is found from the ratio of the number of decayed molecules to the total number in the solution. The depth of dissociation is determined different conditions. The dielectric properties of the solvent and the structure of the dissolved compound are important. Typically, the degree of dissociation decreases with increasing concentration and increases with increasing temperature. Often the degree of dissociation of a particular substance is expressed in fractions of unity.
Classification of electrolytes
The theory of electrolytic dissociation in late XIX century did not contain provisions on the interaction of ions in solution. The influence of water molecules on the distribution of cations and anions seemed unimportant to Arrhenius. Arrhenius' ideas about strong and weak electrolytes were formal. Based on the classical provisions, it is possible to obtain the value α = 0.75-0.95 for strong electrolytes. Experiments have proven the irreversibility of their dissociation (α →1). Soluble salts, sulfuric and hydrochloric acids, and alkalis almost completely disintegrate into ions. Sulfurous, nitrogenous, hydrofluoric, and orthophosphoric acids partially dissociate. Weak electrolytes are considered to be silicon, acetic, hydrogen sulfide and carbonic acids, ammonium hydroxide, and insoluble bases. Water is also considered a weak electrolyte. A small part of the H 2 O molecules dissociates, and at the same time molarization of the ions occurs.
Let us summarize the information about electrolytic dissociation in the form of the main provisions of the now generally accepted theory. It is as follows.
Ions are one of the forms of existence of a chemical element. The properties of ions are completely different from the properties of the atoms that formed them. 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. It would never occur to anyone to use metallic sodium and chlorine gas in food, while without sodium chloride, which consists of sodium and chlorine ions, cooking is impossible. Let us remind you:
The word ion in Greek means “wanderer.” In solutions, ions move randomly (“travel”) in different directions.
According to their composition, ions are divided into simple - C1 -, Na + and complex -.
As a result of the interaction of the electrolyte with water molecules, hydrated ions are formed, that is, associated with water molecules.
Consequently, according to the presence of an aqueous shell, ions are divided into hydrated (in solutions and crystalline hydrates) and non-hydrated (in anhydrous salts).
The properties of hydrated and non-hydrated ions differ, as you can already see from the example of copper ions.
Consequently, there is another classification of ions - according to the sign of their charge.
In electrolyte solutions, the sum of the charges of the cations is equal to the sum of the charges of the anions, as a result of which these solutions are electrically neutral.
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 of weak electrolytes, instead of the equal sign, the reversibility sign is put, for example:
The degree of dissociation depends on the nature of the electrolyte and its concentration. Based on the degree of dissociation, electrolytes are divided into strong and weak.
Based on the nature of the ions formed during the dissociation of electrolytes, three types of electrolytes are distinguished: acids, bases and salts.For polybasic acids, stepwise dissociation occurs. For example, for phosphoric acid H 3 P0 4:
1st stage - formation of dihydrogen phosphate ions:
2nd stage - formation of hydrogen phosphate ions:
It should be taken into account that the dissociation of electrolytes in the second stage is much weaker than in the first. Dissociation by the third step almost does not occur under normal conditions.
All acids have in common the fact that upon dissociation they necessarily form hydrogen cations. Therefore, it is logical to assume that the general characteristic properties of acids - sour taste, changes in the color of indicators, etc. - are caused precisely by hydrogen cations.
All common properties of bases - soapiness to the touch, change in color of indicators, etc. - are due to the hydroxide ions OH - common to all bases.
It is obvious that the properties of salts are determined by both metal cations and anions of the acid residue. Thus, ammonium salts have both general properties due to ions and specific properties due to various anions. Similarly, the general properties of sulfates - salts of sulfuric acid - are determined by ions, and the different ones - by different cations. Unlike polybasic acids and bases containing several hydroxide ions, salts such as K 2 SO 4,
A1 2 (SO 4) 3, etc., dissociate immediately completely, and not stepwise:
Key words and phrases
- Basic principles of the theory of electrolytic dissociation.
- Ions are simple and complex, hydrated and non-hydrated, cations and anions.
- Acids, bases and salts in the light of the theory of electrolytic dissociation.
Working with a computer
- Refer to the electronic application. Study the lesson material and complete the assigned tasks.
- Find email addresses on the Internet that can serve as additional sources that reveal the content of keywords and phrases in the paragraph. Offer your help to the teacher in preparing a new lesson - make a report on the key words and phrases of the next paragraph.
Questions and tasks
Basic provisions of the TED(the basic principles of the theory of electrolytic dissociation were formulated by S. Arrhenius in 1887):
1. Electrolyte molecules, when dissolved in water or melted, break down into ions.
2 .In a solution or melt of electrolytes, ions move chaotically. When electric current is passed through a solution or melt, positively charged ions move towards the negatively charged electrode (cathode), and negatively charged ions move towards the positively charged electrode (anode).
3 . Ions differ from atoms both in structure and properties.
4 .Dissociation of many electrolytes is a reversible process. Two opposite processes occur simultaneously: the disintegration of molecules into ions (ionization) and the combination of ions into molecules (molarization).
Electrolytes– these are substances whose solutions or melts conduct electric current.
Electrolytes that almost completely dissociate into ions (ionize) are called strong, and electrolytes that do not completely ionize are called weak.
For quantitative characteristics completeness of dissociation, the concept of degree of dissociation is introduced.
Degree of dissociation ( ) - the ratio of the number of molecules disintegrated into ions (n) to the total number of dissolved molecules (N): n / N
The degree of dissociation is expressed as a percentage or fraction of a unit.
According to the degree of dissociation, electrolytes are conventionally divided into:
Strong – α > 0.3 (30%);
Weak – α< 0,03 (3%);
Medium strength – (3%) 0,03 < α < 0,3 (30%)
The degree of dissociation is determined experimentally by measuring the deviation of the colligative properties of electrolyte solutions (usually boiling and freezing points) from the theoretical dependencies: Δt= iKCm
The change in the freezing point or boiling point of solutions can be calculated after introducing a coefficient that takes into account the increase in the concentration of kinetically independent particles caused by the dissociation of some molecules into ions. The empirical coefficient proposed by Van't Hoffi(isotonic coefficient)shows the degree of deviation of the colligial properties of electrolyte solutions from solutions of non-volatile non-electrolytes. Coefficient valueifor solutions of a given electrolyte increases as it is diluted, tending in the limit to an integer number equal to the number of ions arising during the dissociation of the formula unit of the electrolyte.
Electrolytic dissociation can be characterized quantitatively as an equilibrium reversible processdissociation constant: Kd= (K+)+ (A-)/(KA)The equation is valid for dilute solutions of weak electrolytes. The more the electrolyte dissociates, the sicker you get, constantKd. In contrast to the degree of dissociation, the constantKd depends only on the nature of the solvent, electrolyte and temperature, but does not depend on the concentration of the solution. The equilibrium can be shifted by adding a strong electrolyte that has the same ion.
Between constantKdand the degree of dissociation α there is a relationship. The relationship between the degree of dissociationα , concentrationWITH and dissociation constantTO D electrolyte is expressedOstwald's law of breeding :
Where WITH
O
– acid concentration before dissociation,α
– degree of acid dissociation in solution.
For acetic acidTO
D
= 1,85
∙
10
-5
.
For very weak electrolyteα<<1
, and then the valueα
in the denominator can be neglected (Ostwald's dilution law
):
TO
D
≈ C
O
α
2
or
Ostwald's dilution law - a relationship expressing the dependence of the equivalent electrical conductivity of a dilute solution of a binary weak electrolyte on the concentration of the solution:
Here K is the electrolyte dissociation constant, c is the concentration, λ and λ ∞ - values of equivalent electrical conductivity, respectively, at concentration c and at infinite dilution. The relationship is a consequence of the law of mass action and equality
where α is the degree of dissociation.
We can assume that in all processes in electrolyte solutions only “active ions", i.e. ions, not currently participating in interionic interactions. In this regard, to assess concentrated effects, a quantity calledactivity(a) is the effective concentration according to which the electrolyte participates in various processes.
Activityis related to the true concentration of the solute by the relation:
a =fC, where a is the activity of the electrolyte, mol/l; C – electrolyte concentration, mol/l;f– activity coefficient (<1) (безразмерный).
Activity factorexpresses the deviation of a solution with concentration C from the behavior of a solution at infinite dilution, i.e. in the absence of interionic interactions.
In dilute solutions, the nature of the ions has little effect on the values of the activity coefficient, since interionic interactions are determined only by the charges of the ions and their concentration.
A quantitative characteristic of interionic electrostatic interactions is the ionic strength of the solution.
Ionic strength of solutionThey call a value equal to half the sum of the product of the concentrations of all ions in the solution and the square of their charge:
I= 0,5 ∑ CiZi2, whereCi – molar concentration of ioniin solution;Zi– ion chargei.
Heterogeneous processes– these are processes at the interface. Heterogeneous processes primarily include processes associated with the formation and dissolution of poorly soluble substances of the ionic type. When such substances (strong electrolytes) come into contact with water, some of the ions go into solution and a dynamic equilibrium is established between the hydrated electrolyte ions in the aqueous solution and the crystals of the solid phase - heterogeneous equilibrium. A solution in equilibrium with the solid phase is called saturated.
The thermodynamic condition for the occurrence of equilibrium in the system is the constancy of the Gibbs energy ΔG=0, and the kinetic condition is the equality of the rates of dissolution and crystallization processes.
Reversible dissolution processes occur at the interface, regardless of the amount of crystalline substance, because its concentration (and activity) in the solid phase remains constant. Heterogeneous equilibrium constantKscalledsolubility constant.
The lower the activity (concentration) of ions in the solution, the lower the valueKS and, therefore, the lower the solubility.