Weak electrolytes

Weak electrolytes- substances that partially dissociate into ions. Solutions weak electrolytes Along with ions, they contain undissociated molecules. Weak electrolytes cannot produce a high concentration of ions in solution. Weak electrolytes include:

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

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

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

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

The concentrations of ions in solutions of weak electrolytes are qualitatively characterized by the degree and dissociation constant.

The degree of dissociation is expressed in fractions of a unit or as a percentage (a = 0.3 is the conventional boundary for dividing into strong and weak electrolytes).

The degree of dissociation depends on the concentration of the weak electrolyte solution. When diluted with water, the degree of dissociation always increases, because the number of solvent molecules (H 2 O) per solute molecule increases. According to Le Chatelier’s principle, the equilibrium of electrolytic dissociation in this case should shift in the direction of the formation of products, i.e. hydrated ions.

The degree of electrolytic dissociation depends on the temperature of the solution. Typically, as the temperature increases, the degree of dissociation increases, because bonds in molecules are activated, they become more mobile and are easier to ionize. The concentration of ions in a weak electrolyte solution can be calculated by knowing the degree of dissociation a and initial concentration of the substance c in solution.

HAn = H + + An - .

The equilibrium constant K p of this reaction is the dissociation constant K d:

K d = . / . (10.11)

If we express the equilibrium concentrations in terms of the concentration of the weak electrolyte C and its degree of dissociation α, we obtain:

K d = C. α. S. α/S. (1-α) = C. α 2 /1-α. (10.12)

This relationship is called Ostwald's dilution law. For very weak electrolytes at α<<1 это уравнение упрощается:

K d = C. α 2. (10.13)

This allows us to conclude that with infinite dilution the degree of dissociation α tends to unity.

Protolytic equilibrium in water:

At a constant temperature in dilute solutions, the concentration of water in water is constant and equal to 55.5, ( )

, (10.15)

where K in is the ionic product of water.

Then =10 -7. In practice, due to the convenience of measurement and recording, the value used is the hydrogen index, (criterion) of the strength of an acid or base. Similarly .

From equation (11.15): . At pH=7 – the solution reaction is neutral, at pH<7 – кислая, а при pH>7 – alkaline.

Under normal conditions (0°C):

, Then

Figure 10.4 - pH of various substances and systems

10.7 Strong electrolyte solutions

Strong electrolytes 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, HClO 4, H 2 SO 4, HNO 3) and strong bases (LiOH, NaOH, KOH, RbOH, CsOH, Ba(OH) 2, Sr(OH) 2, Ca(OH) 2).

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

The fundamental difference between strong electrolytes and weak ones is that the dissociation equilibrium of strong electrolytes is completely shifted to the right:

H 2 SO 4 = H + + HSO 4 - ,

and therefore the equilibrium (dissociation) constant turns out to be an uncertain quantity. The decrease in electrical conductivity with increasing concentration of a strong electrolyte is due to the electrostatic interaction of ions.

The Dutch scientist Petrus Josephus Wilhelmus Debye and the German scientist Erich Hückel, having proposed a model that formed the basis of the theory of strong electrolytes, postulated:

1) the electrolyte completely dissociates, but in relatively dilute solutions (C M = 0.01 mol. l -1);

2) each ion is surrounded by a shell of ions of the opposite sign. In turn, each of these ions is solvated. This environment is called an ionic atmosphere. During the electrolytic interaction of ions of opposite signs, it is necessary to take into account the influence of the ionic atmosphere. When a cation moves in an electrostatic field, the ionic atmosphere is deformed; it thickens in front of him and thins out behind him. This asymmetry of the ionic atmosphere has a more inhibiting effect on the movement of the cation, the higher the concentration of electrolytes and the greater the charge of the ions. In these systems the concept of concentration becomes ambiguous and must be replaced by activity. For a binary single-charge electrolyte KatAn = Kat + + An - the activities of the cation (a +) and anion (a -) are respectively equal

a + = γ + . C + , a - = γ - . C - , (10.16)

where C + and C - are the analytical concentrations of the cation and anion, respectively;

γ + and γ - are their activity coefficients.

(10.17)

It is impossible to determine the activity of each ion separately; therefore, for single-charge electrolytes, geometric mean values of the activities are used.

and activity coefficients.

Electrolyte dissociation is quantitatively characterized by the degree of dissociation. Dissociation degree a–this is the ratio of the number of molecules dissociated into ions N diss.,to the total number of molecules of dissolved electrolyte N :

a =

a– the fraction of electrolyte molecules that have broken up into ions.

The degree of electrolyte dissociation depends on many factors: the nature of the electrolyte, the nature of the solvent, the concentration of the solution, and temperature.

Based on their ability to dissociate, electrolytes are conventionally divided into strong and weak. Electrolytes that exist in solution only in the form of ions are usually called strong . Electrolytes, which in a dissolved state are partly in the form of molecules and partly in the form of ions, are called weak .

Strong electrolytes include almost all salts, some acids: H 2 SO 4, HNO 3, HCl, HI, HClO 4, hydroxides of alkali and alkaline earth metals (see appendix, table 6).

The process of dissociation of strong electrolytes continues to completion:

HNO 3 = H + + NO 3 - , NaOH = Na + + OH - ,

and equal signs are placed in the dissociation equations.

In relation to strong electrolytes, the concept of “degree of dissociation” is conditional. " Apparent degree of dissociation (a each) below the true one (see appendix, table 6). With increasing concentration of a strong electrolyte in a solution, the interaction of oppositely charged ions increases. When sufficiently close to each other, they form associates. The ions in them are separated by layers of polar water molecules surrounding each ion. This affects the decrease in the electrical conductivity of the solution, i.e. the effect of incomplete dissociation is created.

To take this effect into account, an activity coefficient g was introduced, which decreases with increasing concentration of the solution, varying from 0 to 1. To quantitatively describe the properties of solutions of strong electrolytes, a quantity called activity (a).

The activity of an ion is understood as its effective concentration, according to which it acts in chemical reactions.

Ion activity ( a) is equal to its molar concentration ( WITH), multiplied by the activity coefficient (g):

A = g WITH.

Using activity instead of concentration allows one to apply to solutions the laws established for ideal solutions.

Weak electrolytes include some mineral acids (HNO 2, H 2 SO 3, H 2 S, H 2 SiO 3, HCN, H 3 PO 4) and most organic acids (CH 3 COOH, H 2 C 2 O 4, etc.) , ammonium hydroxide NH 4 OH and all bases that are slightly soluble in water, organic amines.

The dissociation of weak electrolytes is reversible. In solutions of weak electrolytes, an equilibrium is established between ions and undissociated molecules. In the corresponding dissociation equations, the reversibility sign (“”) is placed. For example, the dissociation equation for weak acetic acid is written as follows:

CH 3 COOH « CH 3 COO - + H + .

In a solution of a weak binary electrolyte ( CA) the following equilibrium is established, characterized by an equilibrium constant called the dissociation constant TO d:

KA « K + + A - ,

If 1 liter of solution is dissolved WITH moles of electrolyte CA and the degree of dissociation is a, which means dissociated aС moles of electrolyte and each ion was formed aС moles. In the undissociated state remains ( WITH – aС) moles CA.

KA « K + + A - .

C – aС aС aС

Then the dissociation constant will be equal to:

Since the dissociation constant does not depend on concentration, the derived relation expresses the dependence of the degree of dissociation of a weak binary electrolyte on its concentration. From equation (6.1) it is clear that a decrease in the concentration of a weak electrolyte in a solution leads to an increase in the degree of its dissociation. Equation (6.1) expresses Ostwald's dilution law .

For very weak electrolytes (at a<<1), уравнение Оствальда можно записать следующим образом:

TO d a 2 C, or a" (6.2)

The dissociation constant for each electrolyte is constant at a given temperature, it does not depend on the concentration of the solution and characterizes the ability of the electrolyte to disintegrate into ions. The higher the Kd, the more the electrolyte dissociates into ions. The dissociation constants of weak electrolytes are tabulated (see appendix, table 3).

Salts, their properties, hydrolysis

8th grade student B of school No. 182

Petrova Polina

Chemistry teacher:

Kharina Ekaterina Alekseevna

MOSCOW 2009

In everyday life, we are accustomed to dealing with only one salt - table salt, i.e. sodium chloride NaCl. However, in chemistry, a whole class of compounds is called salts. Salts can be considered as products of the replacement of hydrogen in an acid with a metal. Table salt, for example, can be obtained from hydrochloric acid by a substitution reaction:

2Na + 2HCl = 2NaCl + H2.

acid salt

If you take aluminum instead of sodium, another salt is formed - aluminum chloride:

2Al + 6HCl = 2AlCl3 + 3H2

Salts- These are complex substances consisting of metal atoms and acidic residues. They are the products of complete or partial replacement of hydrogen in an acid with a metal or a hydroxyl group in a base with an acid residue. For example, if in sulfuric acid H 2 SO 4 we replace one hydrogen atom with potassium, we get the salt KHSO 4, and if two - K 2 SO 4.

There are several types of salts.

Types of salts	Definition	Examples of salts
Average	The product of complete replacement of acid hydrogen with metal. They contain neither H atoms nor OH groups.	Na 2 SO 4 sodium sulfate CuCl 2 copper (II) chloride Ca 3 (PO 4) 2 calcium phosphate Na 2 CO 3 sodium carbonate (soda ash)
Sour	A product of incomplete replacement of acid hydrogen by metal. Contain hydrogen atoms. (They are formed only by polybasic acids)	CaHPO 4 calcium hydrogen phosphate Ca(H 2 PO 4) 2 calcium dihydrogen phosphate NaHCO 3 sodium bicarbonate (baking soda)
Basic	The product of incomplete replacement of the hydroxyl groups of a base with an acidic residue. Includes OH groups. (Formed only by polyacid bases)	Cu(OH)Cl copper (II) hydroxychloride Ca 5 (PO 4) 3 (OH) calcium hydroxyphosphate (CuOH) 2 CO 3 copper (II) hydroxycarbonate (malachite)
Mixed	Salts of two acids	Ca(OCl)Cl – bleach
Double	Salts of two metals	K 2 NaPO 4 – dipotassium sodium orthophosphate
Crystalline hydrates	Contains water of crystallization. When heated, they dehydrate - they lose water, turning into anhydrous salt.	CuSO4. 5H 2 O – pentahydrate copper(II) sulfate (copper sulfate) Na 2 CO 3. 10H 2 O – sodium carbonate decahydrate (soda)

Methods for obtaining salts.

1. Salts can be obtained by acting with acids on metals, basic oxides and bases:

Zn + 2HCl ZnCl 2 + H 2

zinc chloride

3H 2 SO 4 + Fe 2 O 3 Fe 2 (SO 4) 3 + 3H 2 O

iron(III) sulfate

3HNO 3 + Cr(OH) 3 Cr(NO 3) 3 + 3H 2 O

chromium(III) nitrate

2. Salts are formed by the reaction of acidic oxides with alkalis, as well as acidic oxides with basic oxides:

N 2 O 5 + Ca(OH) 2 Ca(NO 3) 2 + H 2 O

calcium nitrate

SiO 2 + CaO CaSiO 3

calcium silicate

3. Salts can be obtained by reacting salts with acids, alkalis, metals, non-volatile acid oxides and other salts. Such reactions occur under the conditions of evolution of gas, precipitation of a precipitate, evolution of an oxide of a weaker acid, or evolution of a volatile oxide.

Ca 3 (PO4) 2 + 3H 2 SO 4 3CaSO 4 + 2H 3 PO 4

calcium orthophosphate calcium sulfate

Fe 2 (SO 4) 3 + 6NaOH 2Fe(OH) 3 + 3Na 2 SO 4

iron (III) sulfate sodium sulfate

CuSO 4 + Fe FeSO 4 + Cu

copper (II) sulfate iron (II) sulfate

CaCO 3 + SiO 2 CaSiO 3 + CO 2

calcium carbonate calcium silicate

Al 2 (SO 4) 3 + 3BaCl 2 3BaSO 4 + 2AlCl 3

sulfate chloride sulfate chloride

aluminum barium barium aluminum

4. Salts of oxygen-free acids are formed by the interaction of metals with non-metals:

2Fe + 3Cl 2 2FeCl 3

iron(III) chloride

Physical properties.

Salts are solids of various colors. Their solubility in water varies. All salts of nitric and acetic acids, as well as sodium and potassium salts, are soluble. The solubility of other salts in water can be found in the solubility table.

Chemical properties.

1) Salts react with metals.

Since these reactions occur in aqueous solutions, Li, Na, K, Ca, Ba and other active metals that react with water under normal conditions cannot be used for experiments, or reactions cannot be carried out in a melt.

CuSO 4 + Zn ZnSO 4 + Cu

Pb(NO 3) 2 + Zn Zn(NO 3) 2 + Pb

2) Salts react with acids. These reactions occur when a stronger acid displaces a weaker one, releasing gas or precipitating.

When carrying out these reactions, they usually take dry salt and act with concentrated acid.

BaCl 2 + H 2 SO 4 BaSO 4 + 2HCl

Na 2 SiO 3 + 2HCl 2NaCl + H 2 SiO 3

3) Salts react with alkalis in aqueous solutions.

This is a method of obtaining insoluble bases and alkalis.

FeCl 3 (p-p) + 3NaOH(p-p) Fe(OH) 3 + 3NaCl

CuSO 4 (p-p) + 2NaOH (p-p) Na 2 SO 4 + Cu(OH) 2

Na 2 SO 4 + Ba(OH) 2 BaSO 4 + 2NaOH

4) Salts react with salts.

The reactions take place in solutions and are used to obtain practically insoluble salts.

AgNO 3 + KBr AgBr + KNO 3

CaCl 2 + Na 2 CO 3 CaCO 3 + 2NaCl

5) Some salts decompose when heated.

A typical example of such a reaction is the firing of limestone, the main component of which is calcium carbonate:

CaCO 3 CaO + CO2 calcium carbonate

1. Some salts are capable of crystallizing to form crystalline hydrates.

Copper (II) sulfate CuSO 4 is a white crystalline substance. When it is dissolved in water, it heats up and a blue solution is formed. The release of heat and color changes are signs of a chemical reaction. When the solution is evaporated, crystalline hydrate CuSO 4 is released. 5H 2 O (copper sulfate). The formation of this substance indicates that copper (II) sulfate reacts with water:

CuSO 4 + 5H 2 O CuSO 4 . 5H 2 O + Q

white blue-blue

Use of salts.

Most salts are widely used in industry and in everyday life. For example, sodium chloride NaCl, or table salt, is indispensable in cooking. In industry, sodium chloride is used to produce sodium hydroxide, soda NaHCO 3, chlorine, sodium. Salts of nitric and orthophosphoric acids are mainly mineral fertilizers. For example, potassium nitrate KNO 3 is potassium nitrate. It is also part of gunpowder and other pyrotechnic mixtures. Salts are used to obtain metals, acids, and in glass production. Many plant protection products from diseases, pests, and some medicinal substances also belong to the class of salts. Potassium permanganate KMnO 4 is often called potassium permanganate. Limestone and gypsum – CaSO 4 – are used as building materials. 2H 2 O, which is also used in medicine.

Solutions and solubility.

As stated earlier, solubility is an important property of salts. Solubility is the ability of a substance to form with another substance a homogeneous, stable system of variable composition, consisting of two or more components.

Solutions- These are homogeneous systems consisting of solvent molecules and solute particles.

So, for example, a solution of table salt consists of a solvent - water, a dissolved substance - Na +, Cl - ions.

Ions(from Greek ión - going), electrically charged particles formed by the loss or gain of electrons (or other charged particles) by atoms or groups of atoms. The concept and term “ion” was introduced in 1834 by M. Faraday, who, while studying the effect of electric current on aqueous solutions of acids, alkalis and salts, suggested that the electrical conductivity of such solutions is due to the movement of ions. Faraday called positively charged ions moving in solution towards the negative pole (cathode) cations, and negatively charged ions moving towards the positive pole (anode) - anions.

Based on the degree of solubility in water, substances are divided into three groups:

1) Highly soluble;

2) Slightly soluble;

3) Practically insoluble.

Many salts are highly soluble in water. When deciding the solubility of other salts in water, you will have to use the solubility table.

It is well known that some substances, when dissolved or molten, conduct electric current, while others do not conduct current under the same conditions.

Substances that disintegrate into ions in solutions or melts and therefore conduct electric current are called electrolytes.

Substances that, under the same conditions, do not disintegrate into ions and do not conduct electric current are called non-electrolytes.

Electrolytes include acids, bases and almost all salts. Electrolytes themselves do not conduct electricity. In solutions and melts, they break up into ions, which is why current flows.

The breakdown of electrolytes into ions when dissolved in water is called electrolytic dissociation. Its content boils down to the following three provisions:

1) Electrolytes, when dissolved in water, break up (dissociate) into ions - positive and negative.

2) Under the influence of an electric current, ions acquire directional movement: positively charged ions move towards the cathode and are called cations, and negatively charged ions move towards the anode and are called anions.

3) Dissociation is a reversible process: in parallel with the disintegration of molecules into ions (dissociation), the process of combining ions (association) occurs.

reversibility

Strong and weak electrolytes.

To quantitatively characterize the ability of an electrolyte to disintegrate into ions, the concept of the degree of dissociation (α), t . E. The ratio of the number of molecules disintegrated into ions to the total number of molecules. For example, α = 1 indicates that the electrolyte has completely disintegrated into ions, and α = 0.2 means that only every fifth of its molecules has dissociated. When a concentrated solution is diluted, as well as when heated, its electrical conductivity increases, as the degree of dissociation increases.

Depending on the value of α, electrolytes are conventionally divided into strong (dissociate almost completely, (α 0.95)) medium strength (0.95

Strong electrolytes are many mineral acids (HCl, HBr, HI, H 2 SO 4, HNO 3, etc.), alkalis (NaOH, KOH, Ca(OH) 2, etc.), and almost all salts. Weak ones include solutions of some mineral acids (H 2 S, H 2 SO 3, H 2 CO 3, HCN, HClO), many organic acids (for example, acetic acid CH 3 COOH), an aqueous solution of ammonia (NH 3. 2 O), water, some mercury salts (HgCl 2). Electrolytes of medium strength often include hydrofluoric HF, orthophosphoric H 3 PO 4 and nitrous HNO 2 acids.

Hydrolysis of salts.

The term "hydrolysis" comes from the Greek words hidor (water) and lysis (decomposition). Hydrolysis is usually understood as an exchange reaction between a substance and water. Hydrolytic processes are extremely common in the nature around us (both living and nonliving), and are also widely used by humans in modern production and household technologies.

Salt hydrolysis is the reaction of interaction between the ions that make up the salt and water, which leads to the formation of a weak electrolyte and is accompanied by a change in the solution environment.

Three types of salts undergo hydrolysis:

a) salts formed by a weak base and a strong acid (CuCl 2, NH 4 Cl, Fe 2 (SO 4) 3 - hydrolysis of the cation occurs)

NH 4 + + H 2 O NH 3 + H 3 O +

NH 4 Cl + H 2 O NH 3 . H2O + HCl

The reaction of the medium is acidic.

b) salts formed by a strong base and a weak acid (K 2 CO 3, Na 2 S - hydrolysis occurs at the anion)

SiO 3 2- + 2H 2 O H 2 SiO 3 + 2OH -

K 2 SiO 3 +2H 2 O H 2 SiO 3 +2KOH

The reaction of the medium is alkaline.

c) salts formed by a weak base and a weak acid (NH 4) 2 CO 3, Fe 2 (CO 3) 3 - hydrolysis occurs at the cation and at the anion.

2NH 4 + + CO 3 2- + 2H 2 O 2NH 3. H2O + H2CO3

(NH 4) 2 CO 3 + H 2 O 2NH 3. H2O + H2CO3

Often the reaction of the environment is neutral.

d) salts formed by a strong base and a strong acid (NaCl, Ba(NO 3) 2) are not subject to hydrolysis.

In some cases, hydrolysis proceeds irreversibly (as they say, it goes to the end). So, when mixing solutions of sodium carbonate and copper sulfate, a blue precipitate of hydrated basic salt precipitates, which, when heated, loses part of the water of crystallization and acquires a green color - it turns into anhydrous basic copper carbonate - malachite:

2CuSO 4 + 2Na 2 CO 3 + H 2 O (CuOH) 2 CO 3 + 2Na 2 SO 4 + CO 2

When mixing solutions of sodium sulfide and aluminum chloride, hydrolysis also proceeds to completion:

2AlCl 3 + 3Na 2 S + 6H 2 O 2Al(OH) 3 + 3H 2 S + 6NaCl

Therefore, Al 2 S 3 cannot be isolated from an aqueous solution. This salt is obtained from simple substances.

Electrolytes are substances, alloys of substances or solutions that have the ability to electrolytically conduct galvanic current. It is possible to determine which electrolytes a substance belongs to using the theory of electrolytic dissociation.

Instructions

1. The essence of this theory is that when melted (dissolved in water), virtually all electrolytes are decomposed into ions, which can be both positively and negatively charged (which is called electrolytic dissociation). Under the influence of electric current, negative ones (anions, “-”) move towards the anode (+), and positively charged ones (cations, “+”) move towards the cathode (-). Electrolytic dissociation is a reversible process (the reverse process is called “molarization”).

2. The degree of (a) electrolytic dissociation depends on the nature of the electrolyte itself, the solvent, and their concentration. This is the ratio of the number of molecules (n) that broke up into ions to the total number of molecules introduced into the solution (N). You get: a = n / N

3. Thus, powerful electrolytes are substances that completely disintegrate into ions when dissolved in water. Strong electrolytes, as usual, include substances with highly polar or ionic bonds: these are salts that are highly soluble, strong acids (HCl, HI, HBr, HClO4, HNO3, H2SO4), as well as powerful bases (KOH, NaOH, RbOH, Ba (OH)2, CsOH, Sr(OH)2, LiOH, Ca(OH)2). In a strong electrolyte, the substance dissolved in it is mostly in the form of ions (anions and cations); There are actually no molecules that are undissociated.

4. Weak electrolytes are substances that only partially dissociate into ions. Weak electrolytes, together with ions in solution, contain undissociated molecules. Weak electrolytes do not provide a strong concentration of ions in solution. Weak ones include: - organic acids (approximately all) (C2H5COOH, CH3COOH, etc.); - some of the inorganic acids (H2S, H2CO3, etc.); - virtually all salts, sparingly soluble in water, ammonium hydroxide, as well as all bases (Ca3(PO4)2; Cu(OH)2; Al(OH)3; NH4OH); - water. They actually do not conduct electric current, or they conduct, but poorly.

A strong base is an inorganic chemical compound formed by the hydroxyl group -OH and an alkaline (elements of group I of the periodic table: Li, K, Na, RB, Cs) or alkaline earth metal (elements of group II Ba, Ca). Written in the form of the formulas LiOH, KOH, NaOH, RbOH, CsOH, Ca(OH)?, Ba(OH)?.

You will need

evaporation cup
burner
indicators
metal rod
N?RO?

Instructions

1. Powerful bases exhibit chemical properties characteristic of all hydroxides. The presence of alkalis in a solution is determined by a change in the color of the indicator. Add methyl orange, phenolphthalein or omit the litmus paper to the sample with the test solution. Methyl orange produces a yellow color, phenolphthalein produces a purple color, and litmus paper turns blue. The stronger the base, the more saturated the color of the indicator.

2. If you need to find out which alkalis are presented to you, then conduct a good review of the solutions. Particularly common powerful bases are lithium, potassium, sodium, barium and calcium hydroxides. Bases react with acids (neutralization reactions) to form salt and water. In this case, it is possible to isolate Ca(OH)?, Ba(OH)? and LiOH. When interacting with orthophosphoric acid, insoluble precipitates are formed. The remaining hydroxides will not produce precipitation, because all K and Na salts are soluble.3 Ca(OH) ? + 2 N?RO? –? Ca?(PO?)??+ 6 H?O3 Ba(OH) ? +2 N?RO? –? Ba?(PO?)??+ 6 H?O3 LiOH + H?PO? –? Li?PO?? + 3 H?О Strain them and dry them. Add the dried sediment to the burner flame. By changing the color of the flame, it is possible to accurately determine the ions of lithium, calcium and barium. Accordingly, you will determine which hydroxide is which. Lithium salts color the burner flame a carmine-scarlet color. Barium salts are green, and calcium salts are red.

3. The remaining alkalis form soluble orthophosphates.3 NaOH + H?PO?–? Na?PO? + 3 H?O3 KOH + H?PO?–? K?RO? + 3 H?ОIt is necessary to evaporate the water to a dry residue. Place the evaporated salts on a metal rod one by one into the burner flame. Where sodium salt is located, the flame will turn clear yellow, and potassium orthophosphate will turn pink-violet. Thus, having the smallest set of equipment and reagents, you have identified all the powerful bases given to you.

An electrolyte is a substance that in its solid state is a dielectric, that is, it does not conduct electric current, but when dissolved or molten it becomes a conductor. Why does such a sharp change in properties occur? The fact is that electrolyte molecules in solutions or melts dissociate into positively charged and negatively charged ions, as a result of which these substances in such an aggregate state are capable of conducting electric current. Many salts, acids, and bases have electrolytic properties.

Instructions

1. Is that all electrolytes identical in strength, that is, they are excellent conductors of current? No, because many substances in solutions or melts dissociate only to a small extent. Consequently electrolytes are divided into strong, medium strength and weak.

2. What substances are considered powerful electrolytes? Such substances in solutions or melts of which virtually 100% of the molecules undergo dissociation, regardless of the concentration of the solution. The list of strong electrolytes includes an absolute variety of soluble alkalis, salts and some acids, such as hydrochloric, bromide, iodide, nitric, etc.

3. How are they different from them? electrolytes medium strength? The fact that they dissociate to a much lesser extent (from 3% to 30% of molecules disintegrate into ions). Typical representatives of such electrolytes are sulfuric and phosphoric acids.

4. How do weak compounds behave in solutions or melts? electrolytes? Firstly, they dissociate to a very small extent (no more than 3% of the total number of molecules), and secondly, their dissociation is the more clumsy and leisurely, the higher the saturation of the solution. Such electrolytes include, say, ammonia (ammonium hydroxide), many organic and inorganic acids (including hydrofluoric acid - HF) and, of course, water, which is familiar to us all. Because only a pitifully small fraction of its molecules breaks down into hydrogen ions and hydroxyl ions.

5. Remember that the degree of dissociation and, accordingly, the strength of the electrolyte depend on many factors: the nature of the electrolyte itself, the solvent, and temperature. Consequently, this distribution itself is to a certain extent arbitrary. In tea, the same substance can, under different conditions, be both a powerful electrolyte and a weak one. To assess the strength of the electrolyte, a special value was introduced - the dissociation constant, determined on the basis of the law of mass action. But it is applicable only to weak electrolytes; powerful electrolytes do not obey the law of mass action.

Salts- these are chemical substances consisting of a cation, that is, a positively charged ion, a metal and a negatively charged anion - an acid residue. There are many types of salts: typical, acidic, basic, double, mixed, hydrated, complex. This depends on the cation and anion compositions. How is it possible to determine base salt?

Instructions

1. Let's imagine you have four identical containers with burning solutions. You know that these are solutions of lithium carbonate, sodium carbonate, potassium carbonate and barium carbonate. Your task: determine what salt is contained in the entire container.

2. Recall the physical and chemical properties of compounds of these metals. Lithium, sodium, potassium are alkali metals of the first group, their properties are very similar, activity increases from lithium to potassium. Barium is a group 2 alkaline earth metal. Its carbonic salt dissolves perfectly in hot water, but dissolves poorly in cold water. Stop! This is the first chance to immediately determine which container contains barium carbonate.

3. Cool the containers, say by placing them in a container with ice. Three solutions will remain clear, but the fourth will quickly become cloudy and a white precipitate will begin to form. This is where the barium salt is found. Set this container aside.

4. You can quickly determine barium carbonate using another method. Alternately, pour a little of the solution into another container with a solution of some sulfate salt (say, sodium sulfate). Only barium ions, binding with sulfate ions, instantly form a dense white precipitate.

5. It turns out that you have identified barium carbonate. But how do you differentiate between the 3 alkali metal salts? This is quite easy to do, you will need porcelain evaporation cups and an alcohol lamp.

6. Pour a small amount of the entire solution into a separate porcelain cup and evaporate the water over the fire of a spirit lamp. Small crystals form. Place them in the flame of an alcohol lamp or a Bunsen burner - supported by steel tweezers or a porcelain spoon. Your task is to notice the color of the blazing “tongue” of flame. If it is a lithium salt, the color will be clear red. Sodium will color the flame intense yellow, and potassium will color the flame purple-violet. By the way, if barium salt had been tested in the same way, the color of the flame should have been green.

Helpful advice
One famous chemist in his youth exposed the greedy hostess of a boarding house in much the same way. He sprinkled the remains of the half-eaten dish with lithium chloride, a substance that is certainly harmless in small quantities. The next day, at lunch, a slice of meat from the dish served to the table was burned in front of a spectroscope - and the boarding house residents saw a clear red stripe. The hostess was preparing food from yesterday's leftovers.

Note!
True, pure water conducts electricity very poorly, it still has measurable electrical conductivity, explained by the fact that water dissociates slightly into hydroxide ions and hydrogen ions.

Helpful advice
Many electrolytes are hostile substances, so when working with them, be extremely careful and follow safety regulations.

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:

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

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

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

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

The concentrations of ions in solutions of weak electrolytes are qualitatively characterized by the degree and dissociation constant.

The degree of dissociation is expressed in fractions of a unit or as a percentage (a = 0.3 is the conventional boundary for dividing into strong and weak electrolytes).

HAn = H + + An - .

The equilibrium constant K p of this reaction is the dissociation constant K d:

K d = . / . (10.11)

If we express the equilibrium concentrations in terms of the concentration of the weak electrolyte C and its degree of dissociation α, we obtain:

K d = C. α. S. α/S. (1-α) = C. α 2 /1-α. (10.12)

This relationship is called Ostwald's dilution law. For very weak electrolytes at α<<1 это уравнение упрощается:

K d = C. α 2. (10.13)

This allows us to conclude that with infinite dilution the degree of dissociation α tends to unity.

Protolytic equilibrium in water:

At a constant temperature in dilute solutions, the concentration of water in water is constant and equal to 55.5, ( )

, (10.15)

where K in is the ionic product of water.

Then =10 -7. In practice, due to the convenience of measurement and recording, the value used is the hydrogen index, (criterion) of the strength of an acid or base. Similarly .

From equation (11.15): . At pH=7 – the solution reaction is neutral, at pH<7 – кислая, а при pH>7 – alkaline.

Under normal conditions (0°C):

, Then

Figure 10.4 - pH of various substances and systems

10.7 Strong electrolyte solutions

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

The fundamental difference between strong electrolytes and weak ones is that the dissociation equilibrium of strong electrolytes is completely shifted to the right:

H 2 SO 4 = H + + HSO 4 - ,

The Dutch scientist Petrus Josephus Wilhelmus Debye and the German scientist Erich Hückel, having proposed a model that formed the basis of the theory of strong electrolytes, postulated:

1) the electrolyte completely dissociates, but in relatively dilute solutions (C M = 0.01 mol. l -1);

a + = γ + . C + , a - = γ - . C - , (10.16)

where C + and C - are the analytical concentrations of the cation and anion, respectively;

γ + and γ - are their activity coefficients.

(10.17)

It is impossible to determine the activity of each ion separately; therefore, for single-charge electrolytes, geometric mean values of the activities are used.

and activity coefficients:

The Debye-Hückel activity coefficient depends at least on temperature, dielectric constant of the solvent (ε), and ionic strength (I); the latter serves as a measure of the intensity of the electric field created by the ions in the solution.

For a given electrolyte, ionic strength is expressed by the Debye-Hückel equation:

The ionic strength in turn is equal to

where C is the analytical concentration;

z is the charge of the cation or anion.

For a singly charged electrolyte, the ionic strength coincides with the concentration. Thus, NaCl and Na 2 SO 4 at the same concentrations will have different ionic strengths. Comparison of the properties of solutions of strong electrolytes can only be carried out when the ionic strengths are the same; even small impurities dramatically change the properties of the electrolyte.

Figure 10.5 - Dependency

Solutions. Ellytic dissociation theory

Weak electrolytes

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