Carbonic acid comes from decomposition carbon dioxide in the aquatic environment. This substance is artificially saturated mineral water. The formula of carbonic acid is H2CO3. Therefore, when you open a bottle of carbonated water, you can see active bubbles. The main production of carbonic acid occurs in water.

The equation

CO2 (g) + H2O CO2. H2O (solution) H2CO3 H+ + HCO3- 2H+ + CO32-.

By itself, carbonic acid is a weak, fragile compound that cannot be isolated in a free state from water.

But it is worth noting the fact that during the decomposition of ammonium bicarbonate, stable compounds of carbonic acid are formed. So strong chemical bonds are formed only during the period when ammonium bicarbonate enters the gas phase of the reaction.

The substance is an interesting object for study. It has been studied by Australian scientists for more than 6 years. In an anhydrous state, this acid resembles transparent crystals, which are highly resistant to low temperatures, but when heated, carbonic acid crystals begin to decompose.

This substance is considered weak in its structure, but at the same time, carbonic acid is stronger than boric acid. The whole secret lies in the number of hydrogen atoms. Carbonic acid contains two hydrogen atoms, so it is considered dibasic, and boric acid is monobasic.

Features of salts of carbonic acid

This acid is considered dibasic, therefore it can create salts of two types:

• . carbonates of carbonic acid - medium salts,
• . bicarbonates are acidic salts.

Carbonates of carbonic acid can act in compounds: Na2CO3, (NH4)2CO3. They are not able to dissolve in the aquatic environment. Acid salts of this substance include: NaHCO3, Ca(HCO3)2 bicarbonates. To obtain bicarbonates, a reaction is carried out in which the main substances are: carbonic acid and sodium.

Salts of carbonic acid have helped mankind in construction, medicine and even cooking. Because they are found in:

• . chalk,
• . food, soda ash and crystalline soda,
• . limestone rock,
• . marble stone,
• . potash.

Bicarbonates and carbonates of an acid can react with acids, during these reactions carbon dioxide may be released. Also, these substances can be interchangeable, they are able to decompose under the influence of temperature.

Reactions of carbonic acid:

2NaHCO3 → Na2CO3 +H2O +CO2
Na2CO3 + H2O + CO2 →2NaHCO3

Chemical properties

This acid in its structure is capable of reacting with many substances.

The properties of carbonic acid are revealed in the reactions:

• . dissociation,
• . with metals
• . with the grounds
• . with basic oxides.

Na2O + CO2 → Na2CO3
2NaOH + CO2 → Na2CO3 + H2O
NaOH + CO2 → NaHCO3

Carbonic acid is a weak electrolyte, since a weak volatile acid cannot act as a powerful electrolyte, unlike, for example, hydrochloric acid. This fact can be seen as a result of adding litmus to a solution of carbonic acid. Color change will be minor. Therefore, it can be argued that carbonic acid can maintain 1 level of dissociation.

Application

This substance can be seen in the composition of carbonated waters. But salts of carbonic acid are widely used:

• . for the construction industry,
• . in the glass production process,
• . in the production of detergents and cleaning products,
• . paper production,
• . for some top dressings and fertilizers for plants,
• . in medicine.

The domestic and world market offers various preparations and chemicals for sale, which include carbonic acid:

• . urea or carbamide,
• . lithium salt of carbonic acid,
• . calcium carbonate (chalk),
• . soda ash (sodium carbonate), etc.

Carbamide is used as a fertilizer for fruit and ornamental plants. Its average price is 30-40 rubles per 1 kg. Finished products are packaged in plastic bags and bags, weighing 1, 5, 25, 50 kg.

The lithium salt of carbonic acid is used in the composition of ceramic products, glass-ceramics. This material is used to produce combustion chambers for jet engines, it is added to glazes, enamels, primers for various metals. Lithium salt is added to primers for processing aluminum, cast iron and steel.

This chemical is added during the glassmaking process. Glasses, to which a lithium salt was added, have an increased light flux permeability. Sometimes the lithium salt of carbonic acid is used in the pyrotechnics manufacturing process.

Manufacturers

The average price of 1 kg of such a substance in Russia is 3900-4000 rubles. The main manufacturer of this substance is the Moscow plant OOO Component-Reaktiv. Also, lithium salt of carbonic acid is produced in the following companies: KurskKhimProm LLC, VitaChem LLC, Ruskhim LLC, Khimpek CJSC.

Chalk is produced for technical and feed purposes. The average price of fodder chalk is 1800 rubles per 1 ton. Packed mainly in 50 kg, 32 kg. Manufacturers: Melovik LLC, MT Resource LLC, Zoovetsnab LLC, Agrokhiminvest LLC.

Soda ash is used for laundry, stain removal and bleaching. The average price for this product in the retail market varies between 16-30 rubles per 1 kg. Producers: Novera LLC, KhimReaktiv CJSC, HimPlus LLC, SpecBurTechnology LLC, SpetsKomplekt LLC, etc.

1. ELECTROLYTES

1.1. Electrolytic dissociation. Degree of dissociation. The strength of electrolytes

According to the theory of electrolytic dissociation, salts, acids, hydroxides, dissolving in water, completely or partially decompose into independent particles - ions.

The process of disintegration of molecules of substances into ions under the action of polar solvent molecules is called electrolytic dissociation. Substances that dissociate into ions in solution are called electrolytes. As a result, the solution acquires the ability to conduct electricity, because mobile carriers of electric charge appear in it. According to this theory, when dissolved in water, electrolytes decompose (dissociate) into positively and negatively charged ions. Positively charged ions are called cations; these include, for example, hydrogen and metal ions. Negatively charged ions are called anions; these include ions of acid residues and hydroxide ions.

For a quantitative characteristic of the dissociation process, the concept of the degree of dissociation is introduced. The degree of dissociation of an electrolyte (α) is the ratio of the number of its molecules decomposed into ions in a given solution ( n ), to the total number of its molecules in solution ( N ), or

α = .

The degree of electrolytic dissociation is usually expressed either in fractions of a unit or as a percentage.

Electrolytes with a degree of dissociation greater than 0.3 (30%) are usually called strong electrolytes, with a degree of dissociation from 0.03 (3%) to 0.3 (30%) - medium, less than 0.03 (3%) - weak electrolytes. So, for a 0.1 M solution CH3COOH α = 0.013 (or 1.3%). Therefore, acetic acid is a weak electrolyte. The degree of dissociation shows what part of the dissolved molecules of a substance has decomposed into ions. The degree of electrolytic dissociation of an electrolyte in aqueous solutions depends on the nature of the electrolyte, its concentration, and temperature.

By their nature, electrolytes can be divided into two large groups: strong and weak. Strong electrolytes dissociate almost completely (α = 1).

Strong electrolytes include:

1) acids (H 2 SO 4, HCl, HNO 3, HBr, HI, HClO 4, H M nO 4);

2) bases - hydroxides of metals of the first group of the main subgroup (alkalis) - LiOH , NaOH , KOH , RbOH , CsOH , as well as hydroxides of alkaline earth metals - Ba (OH) 2, Ca (OH) 2, Sr (OH) 2;.

3) salts soluble in water (see table of solubility).

Weak electrolytes dissociate into ions to a very small extent, in solutions they are mainly in an undissociated state (in molecular form). For weak electrolytes, an equilibrium is established between undissociated molecules and ions.

Weak electrolytes include:

1) inorganic acids ( H 2 CO 3 , H 2 S , HNO 2 , H 2 SO 3 , HCN , H 3 PO 4 , H 2 SiO 3 , HCNS , HClO, etc.);

2) water (H 2 O);

3) ammonium hydroxide ( NH4OH);

4) most organic acids

(for example, acetic CH 3 COOH, formic HCOOH);

5) insoluble and sparingly soluble salts and hydroxides of certain metals (see table of solubility).

Process electrolytic dissociation portrayed using chemical equations. For example, the dissociation of hydrochloric acid (HC l ) is written as follows:

HCl → H + + Cl - .

Bases dissociate to form metal cations and hydroxide ions. For example, the dissociation of KOH

KOH → K + + OH -.

Polybasic acids, as well as bases of polyvalent metals, dissociate in steps. For example,

H 2 CO 3 H + + HCO 3 -,

HCO 3 - H + + CO 3 2–.

The first equilibrium - dissociation along the first stage - is characterized by a constant

.

For dissociation in the second step:

.

In the case of carbonic acid, the dissociation constants have the following values: K I = 4.3× 10 -7 , K II = 5.6 × 10–11 . For stepwise dissociation, always K I> K II > K III >... , because the energy that must be expended to detach an ion is minimal when it is detached from a neutral molecule.

Medium (normal) salts, soluble in water, dissociate with the formation of positively charged metal ions and negatively charged ions of the acid residue

Ca(NO 3) 2 → Ca 2+ + 2NO 3 -

Al 2 (SO 4) 3 → 2Al 3+ + 3SO 4 2–.

Acid salts (hydrosalts) - electrolytes containing hydrogen in the anion, capable of splitting off in the form of a hydrogen ion H +. Acid salts are considered as a product obtained from polybasic acids in which not all hydrogen atoms are replaced by a metal. The dissociation of acid salts occurs in stages, for example:

KHCO3 → K + + HCO 3 - (first stage)

Acids: HCl HBr HI HClO 4 HMnO 4 H 2 SO 4 HNO 3

Bases: hydroxides formed by s-elements of group 1 and s-elements of group 11, starting with Ca

NaOH KOH Ca(OH) 2 Sr(OH) 2 Ba(OH) 2

Salts are almost everything.

acids from the point of view of the theory of dissociation, these are electrolytes that dissociate with the formation of a hydrogen cation and an anion of an acid residue. The presence of hydrogen cations in acid solutions causes them sour taste, the ability to change the color of the indicator, to have an irritating and even inflammatory effect.

Acids, depending on the strength, dissociate in different ways.

Strong acids dissociate immediately and irreversibly:

Weak electrolytes dissociate stepwise and reversibly

CH 3 COOH \u003d CH 3 COO - + H +

H 2 CO 3 \u003d H + + HCO 3 -

HCO 3 - \u003d H + + CO 3 2-

The dissociation of weak kmlots, as a reversible process, is characterized by the dissociation constant

TO dis. CH 3 COOH = (CH 3 COO -) * (H +)

For carbonic acid, as a dibasic acid, the presence of

TO dis 1 st H 2 CO 3 = (HCO 3 -) * (H +)

TO dis.2st H 2 CO 3 = (CO 3 2-) * (H +)

The dissociation constant, like any constant of a reversible process, is a constant value for each electrolyte (it depends on the nature of the substance) and depends on the temperature of the solution. The smaller the dissociation constant, the weaker the electrolyte. (To dis. - the value is constant and can be found in the reference table)

Grounds - These are electrolytes that dissociate to form a metal cation and a hydroxide anion. Strong bases dissociate immediately and irreversibly:

KOH K + + OH -

Weak electrolytes dissociate stepwise and reversible

Mg(OH) 2 MgOH + + OH -

MgOH + Mg 2+ + OH -

salt- strong elpetrolites, therefore, in solution immediately and completely decompose into metal cations and anions of the acid residue.

Al 2 (SO 4) 3 2Al 3+ + 3SO 4 2-

Na 3 PO 4 3Na + + PO 4 3-

Acid salts dissociate first into a metal cation and an anion of the acid residue

NaHCO 3 Na + + HCO 3 -

And then the acid residue dissociates as an acid

HCO 3 - H + + CO 3 2-

The concept of hydrogen index (ph)

The most commonly used solvent is water. Although water belongs to weak, but electrolytes, it dissociates in solution

H 2 O \u003d H + + OH -

Like any reversible process, it is characterized by its dissociation constant

TO dis. = (H +) * (OH -)

It has been experimentally proven that out of 10,000,000 molecules, only one decomposes into ions. Therefore, the water concentration is taken as a constant value and the following expression is obtained

Kdis * (H 2 O) \u003d Kw \u003d (H +) * (OH -) \u003d const \u003d 10 -14 (this value is called the ionic product of water)

Because this value is constant, then it is used to calculate the concentration of H + or OH ions -

For example, (OH -) \u003d 10 -3 determine (H +) \u003d?

(H+)= K w= 10 -14 =10 -11

- (OH -) \u003d 10 -1 (H +) \u003d 10 -13 ph \u003d 13

- (OH -) = 10 -5 (H +) = 10 -9 ph = 9

- (OH -) = 10 -7 (H +) = 10 -7 ph = 7

- (OH -) \u003d 10 -10 (H +) \u003d 10 -4 ph \u003d 4

- (OH -) \u003d 10 -14 (H +) \u003d 10 0 \u003d 1 ph \u003d 1

All subsequent calculations are made similarly to the first. It is inconvenient to use fractional notation for concentrations, so the concept is introduced pH value ( its values ​​are given in the far right column)

(H +)= 10 -6 ph=6, (H +) = 10 -11 ph=11

ELECTROLYTES Substances whose solutions or melts conduct electricity.

NON-ELECTROLYTES Substances whose solutions or melts do not conduct electricity.

Dissociation- decomposition of compounds into ions.

Degree of dissociation is the ratio of the number of molecules dissociated into ions to the total number of molecules in the solution.

STRONG ELECTROLYTES when dissolved in water, they almost completely dissociate into ions.

When writing the equations of dissociation of strong electrolytes put an equal sign.

Strong electrolytes include:

Soluble salts ( see solubility table);

Many inorganic acids: HNO 3, H 2 SO 4, HClO 3, HClO 4, HMnO 4, HCl, HBr, HI ( Look acids-strong electrolytes in the solubility table);

Bases of alkali (LiOH, NaOH, KOH) and alkaline earth (Ca (OH) 2, Sr (OH) 2, Ba (OH) 2) metals ( see strong electrolyte bases in the solubility table).

WEAK ELECTROLYTES in aqueous solutions only partially (reversibly) dissociate into ions.

When writing the dissociation equations for weak electrolytes, the sign of reversibility is put.

Weak electrolytes include:

Almost all organic acids and water (H 2 O);

Some inorganic acids: H 2 S, H 3 PO 4, HClO 4, H 2 CO 3, HNO 2, H 2 SiO 3 ( Look acids-weak electrolytes in the solubility table);

Insoluble metal hydroxides (Mg (OH) 2, Fe (OH) 2, Zn (OH) 2) ( see basescweak electrolytes in the solubility table).

The degree of electrolytic dissociation is influenced by a number of factors:

the nature of the solvent and electrolyte: strong electrolytes are substances with ionic and covalent strongly polar bonds; good ionizing ability, i.e. the ability to cause dissociation of substances, have solvents with a high dielectric constant, the molecules of which are polar (for example, water);

temperature: since dissociation is an endothermic process, an increase in temperature increases the value of α;

concentration: when the solution is diluted, the degree of dissociation increases, and with increasing concentration, it decreases;

stage of the dissociation process: each subsequent stage is less effective than the previous one, approximately 1000–10,000 times; for example, for phosphoric acid α 1 > α 2 > α 3:

H3PO4⇄Н++H2PO−4 (first stage, α 1),

H2PO−4⇄H++HPO2−4 (second stage, α 2),

НPO2−4⇄Н++PO3−4 (third stage, α 3).

For this reason, in a solution of this acid, the concentration of hydrogen ions is the highest, and the concentration of PO3−4 phosphate ions is the lowest.

1. Solubility and the degree of dissociation of a substance are not related to each other. For example, a weak electrolyte is acetic acid, which is highly (unrestrictedly) soluble in water.

2. A solution of a weak electrolyte contains less than others those ions that are formed at the last stage of electrolytic dissociation

The degree of electrolytic dissociation is also affected by addition of other electrolytes: e.g. degree of dissociation of formic acid

HCOOH ⇄ HCOO − + H+

decreases if a little sodium formate is added to the solution. This salt dissociates to form formate ions HCOO − :

HCOONa → HCOO − + Na +

As a result, the concentration of HCOO– ions in the solution increases, and according to the Le Chatelier principle, an increase in the concentration of formate ions shifts the equilibrium of the formic acid dissociation process to the left, i.e. the degree of dissociation decreases.

Ostwald dilution law- ratio expressing the dependence of the equivalent electrical conductivity of a dilute solution of a binary weak electrolyte on the concentration of the solution:

Here, is the dissociation constant of the electrolyte, is the concentration, and are the values ​​of the equivalent electrical conductivity at concentration and at infinite dilution, respectively. The ratio is a consequence of the law of mass action and equality

where is the degree of dissociation.

The Ostwald dilution law was developed by W. Ostwald in 1888 and confirmed by him experimentally. The experimental establishment of the correctness of the Ostwald dilution law was of great importance for substantiating the theory of electrolytic dissociation.

Electrolytic dissociation of water. Hydrogen indicator pH Water is a weak amphoteric electrolyte: H2O H+ + OH- or, more precisely: 2H2O \u003d H3O + + OH- The dissociation constant of water at 25 ° C is: can be considered constant and equal to 55.55 mol / l (water density 1000 g / l, mass 1 l 1000 g, amount of water substance 1000g: 18g / mol \u003d 55.55 mol, C \u003d 55.55 mol: 1 l \u003d 55 .55 mol/l). Then This value is constant at a given temperature (25 ° C), it is called the ion product of water KW: The dissociation of water is an endothermic process, therefore, with an increase in temperature, in accordance with the Le Chatelier principle, dissociation increases, the ion product increases and reaches a value of 10-13 at 100 ° C. In pure water at 25°C, the concentrations of hydrogen and hydroxyl ions are equal to each other: = = 10-7 mol/l Solutions in which the concentrations of hydrogen and hydroxyl ions are equal to each other are called neutral. If acid is added to pure water, the concentration of hydrogen ions will increase and become more than 10-7 mol / l, the medium will become acidic, while the concentration of hydroxyl ions will instantly change so that the ion product of water retains its value of 10-14. The same thing will happen when alkali is added to pure water. The concentrations of hydrogen and hydroxyl ions are related to each other through the ion product, therefore, knowing the concentration of one of the ions, it is easy to calculate the concentration of the other. For example, if = 10-3 mol/l, then = KW/ = 10-14/10-3 = 10-11 mol/l, or if = 10-2 mol/l, then = KW/ = 10-14 /10-2 = 10-12 mol/l. Thus, the concentration of hydrogen or hydroxyl ions can serve as a quantitative characteristic of the acidity or alkalinity of the medium. In practice, it is not the concentrations of hydrogen or hydroxyl ions that are used, but the hydrogen pH or hydroxyl pOH indicators. The hydrogen index pH is equal to the negative decimal logarithm of the concentration of hydrogen ions: pH = - lg The hydroxyl index pOH is equal to the negative decimal logarithm of the concentration of hydroxyl ions: pOH = - lg It is easy to show by pronouncing the ionic product of water that pH + pOH = 14 the medium is neutral, if less than 7 - acidic, and the lower the pH, the higher the concentration of hydrogen ions. pH greater than 7 - alkaline environment, the higher the pH, the higher the concentration of hydroxyl ions.

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§ 6.3. Strong and weak electrolytes

The material in this section is partly familiar to you from previously studied school chemistry courses and from the previous section. Let's briefly review what you know and get acquainted with the new material.

In the previous section, we discussed the behavior in aqueous solutions of some salts and organic substances that completely decompose into ions in aqueous solution.
There is a number of simple but undeniable evidence that some substances in aqueous solutions decompose into particles. Thus, aqueous solutions of sulfuric H 2 SO 4 , nitric HNO 3 , chlorine HClO 4 , hydrochloric (hydrochloric) HCl, acetic CH 3 COOH and other acids have a sour taste. In the formulas of acids, the common particle is the hydrogen atom, and it can be assumed that (in the form of an ion) it is the cause of the same taste of all these so different substances.
The hydrogen ions formed during dissociation in an aqueous solution give the solution a sour taste, which is why such substances are called acids. In nature, only hydrogen ions taste sour. They create a so-called acidic (acidic) environment in an aqueous solution.

Remember, when you say "hydrogen chloride", you mean the gaseous and crystalline state of this substance, but for an aqueous solution, you should say "hydrogen chloride solution", "hydrochloric acid" or use common name"hydrochloric acid", although the composition of a substance in any state is expressed by the same formula - HCl.

Aqueous solutions of hydroxides of lithium (LiOH), sodium (NaOH), potassium (KOH), barium (Ba (OH) 2), calcium (Ca (OH) 2) and other metals have the same unpleasant bitter-soapy taste and cause on the skin of the hands sliding feeling. Apparently, OH– hydroxide ions, which are part of such compounds, are responsible for this property.
Hydrochloric HCl, hydrobromic HBr and hydroiodic HI acids react with zinc in the same way, despite their different composition, since it is not the acid that actually reacts with zinc:

Zn + 2HCl = ZnCl 2 + H2,

and hydrogen ions:

Zn + 2H + = Zn 2+ + H 2,

and hydrogen gas and zinc ions are formed.
The mixing of some salt solutions, for example, potassium chloride KCl and sodium nitrate NaNO 3, is not accompanied by a noticeable thermal effect, although after evaporation of the solution, a mixture of crystals of four substances is formed: the original ones - potassium chloride and sodium nitrate - and new ones - potassium nitrate KNO 3 and sodium chloride NaCl . It can be assumed that in the solution, the two initial salts completely decompose into ions, which, when it is evaporated, form four crystalline substances:

Comparing this information with the electrical conductivity of aqueous solutions of acids, hydroxides and salts and with a number of other provisions, S.A. Arrhenius in 1887 put forward the hypothesis of electrolytic dissociation, according to which the molecules of acids, hydroxides and salts, when dissolved in water, dissociate into ions.
The study of electrolysis products allows you to assign positive or negative charges to ions. Obviously, if an acid, for example, nitric HNO 3, dissociates, suppose, into two ions and hydrogen is released during the electrolysis of an aqueous solution at the cathode (negatively charged electrode), then, therefore, there are positively charged hydrogen ions H + in the solution. Then the dissociation equation should be written as follows:

HNO 3 \u003d H + +.

Electrolytic dissociation- complete or partial decomposition of the compound when it is dissolved in water into ions as a result of interaction with a water molecule (or other solvent).
electrolytes- acids, bases or salts, aqueous solutions of which conduct an electric current as a result of dissociation.
Substances that do not dissociate into ions in an aqueous solution and whose solutions do not conduct electricity are called non-electrolytes.
The dissociation of electrolytes is quantified degree of dissociation- the ratio of the number of "molecules" (formula units) decomposed into ions to the total number of "molecules" of the solute. The degree of dissociation is denoted by the Greek letter . For example, if out of every 100 "molecules" of a solute, 80 decompose into ions, then the degree of dissociation of the solute is: \u003d 80/100 \u003d 0.8, or 80%.
According to the ability to dissociate (or, as they say, “by strength”), electrolytes are divided into strong, medium And weak. According to the degree of dissociation, strong electrolytes include those for whose solutions > 30%, weak ones -< 3%, к средним – 3% 30%. Сила электролита – величина, зависящая от концентрации вещества, температуры, природы растворителя и др.
In the case of aqueous solutions, strong electrolytes(> 30%) belong to the following groups of compounds.
1 . Many inorganic acids, such as hydrochloric HCl, nitric HNO 3 , sulfuric H 2 SO 4 in dilute solutions. The strongest inorganic acid is perchloric HClO 4.
The strength of non-oxygen acids increases in a series of compounds of the same type when moving down the subgroup of acid-forming elements:

HCl-HBr-HI.

Hydrofluoric (hydrofluoric) acid HF dissolves glass, but this does not at all indicate its strength. This acid from oxygen-free halogen-containing acids belongs to acids of medium strength due to the high energy of the H–F bond, the ability of HF molecules to unite (associate) due to strong hydrogen bonds, the interaction of F ions with HF molecules (hydrogen bonds) with the formation of ions and other more complex particles. As a result, the concentration of hydrogen ions in an aqueous solution of this acid is significantly reduced, so hydrofluoric acid is considered to be of medium strength.
Hydrogen fluoride reacts with silicon dioxide, which is part of the glass, according to the equation:

SiO 2 + 4HF \u003d SiF 4 + 2H 2 O.

Hydrofluoric acid must not be stored in glass vessels. For this, vessels made of lead, some plastics and glass are used, the walls of which are covered from the inside with a thick layer of paraffin. If hydrogen fluoride gas is used to “etch” the glass, the glass surface becomes matte, which is used to apply inscriptions and various patterns on the glass. "Etching" glass with an aqueous solution of hydrofluoric acid results in corrosion of the glass surface, which remains transparent. On sale is usually a 40% solution of hydrofluoric acid.

The strength of the same type of oxygen acids changes in the opposite direction, for example, iodic acid HIO 4 is weaker than perchloric acid HClO 4.
If an element forms several oxygen acids, then the acid in which the acid-forming element has the highest valence has the greatest strength. So, in the series of acids HClO (hypochlorous) - HClO 2 (chloric) - HClO 3 (chloric) - HClO 4 (chloric) the latter is the strongest.

One volume of water dissolves about two volumes of chlorine. Chlorine (about half of it) interacts with water:

Cl 2 + H 2 O \u003d HCl + HClO.

Hydrochloric acid is strong; there are practically no HCl molecules in its aqueous solution. The correct equation for the reaction is:

Cl 2 + H 2 O \u003d H + + Cl - + HClO - 25 kJ / mol.

The resulting solution is called chlorine water.
Hypochlorous acid is a fast-acting oxidizing agent, so it is used to bleach fabrics.

2 . Hydroxides of elements of the main subgroups of groups I and II periodic system: LiOH, NaOH, KOH, Ca (OH) 2, etc. When moving down the subgroup, as the metallic properties of the element increase, the strength of hydroxides increases. Soluble hydroxides of the main subgroup of group I elements are classified as alkalis.

Bases soluble in water are called alkalis. These also include the hydroxides of the elements of the main subgroup of group II (alkaline earth metals) and ammonium hydroxide (an aqueous solution of ammonia). Sometimes alkalis are those hydroxides that create a high concentration of hydroxide ions in an aqueous solution. In outdated literature, you can find among the alkalis potassium carbonates K 2 CO 3 (potash) and sodium Na 2 CO 3 (soda), sodium bicarbonate NaHCO 3 (baking soda), borax Na 2 B 4 O 7, sodium hydrosulfides NaHS and potassium KHS etc.

Calcium hydroxide Ca (OH) 2 as a strong electrolyte dissociates into one step:

Ca (OH) 2 \u003d Ca 2+ + 2OH -.

3 . Almost all salts. Salt, if it is a strong electrolyte, dissociates into one step, for example ferric chloride:

FeCl 3 \u003d Fe 3+ + 3Cl -.

In the case of aqueous solutions, weak electrolytes ( < 3%) относят перечисленные ниже соединения.

1 . Water H 2 O is the most important electrolyte.

2 . Some inorganic and almost all organic acids: H 2 S (hydrosulfide), H 2 SO 3 (sulphurous), H 2 CO 3 (carbonic), HCN (hydrocyanic), H 3 PO 4 (phosphoric, orthophosphoric), H 2 SiO 3 (silicon), H 3 BO 3 (boric, orthoboric), CH 3 COOH (acetic), etc.
Note that carbonic acid does not exist in the formula H 2 CO 3. When carbon dioxide CO 2 is dissolved in water, its hydrate CO 2 H 2 O is formed, which we write for convenience of calculations by the formula H 2 CO 3, and the equation for the dissociation reaction looks like this:

The dissociation of weak carbonic acid proceeds in two steps. The resulting bicarbonate ion also behaves like a weak electrolyte.
Other polybasic acids dissociate in the same way: H 3 PO 4 (phosphoric), H 2 SiO 3 (silicon), H 3 BO 3 (boric). In an aqueous solution, dissociation practically passes only through the first stage. How to carry out dissociation along the last step?
3 . Hydroxides of many elements, such as Al (OH) 3, Cu (OH) 2, Fe (OH) 2, Fe (OH) 3, etc.
All these hydroxides dissociate in an aqueous solution in steps, for example, iron hydroxide
Fe(OH)3:

In an aqueous solution, dissociation proceeds practically only through the first stage. How to shift the equilibrium towards the formation of Fe 3+ ions?
The basic properties of hydroxides of the same element increase with a decrease in the valency of the element. Thus, the basic properties of iron dihydroxide Fe (OH) 2 are more pronounced than those of Fe (OH) 3 trihydroxide. This statement is equivalent to the fact that the acidic properties of Fe(OH) 3 are stronger than that of Fe(OH) 2 .
4 . Ammonium hydroxide NH 4 OH.
When gaseous ammonia NH 3 is dissolved in water, a solution is obtained that conducts electricity very poorly and has a bitter-soapy taste. The solution medium is basic or alkaline. This behavior of ammonia is explained as follows. When ammonia is dissolved in water, ammonia hydrate NH 3 H 2 O is formed, to which we conditionally attribute the formula of the non-existent ammonium hydroxide NH 4 OH, assuming that this compound dissociates with the formation ammonium ion and hydroxide ion OH -:

NH 4 OH \u003d + OH -.

5 . Some salts: zinc chloride ZnCl 2, iron thiocyanate Fe (NCS) 3, mercury cyanide Hg (CN) 2, etc. These salts dissociate in steps.

For electrolytes of medium strength, some include phosphoric acid H 3 PO 4. We will consider phosphoric acid as a weak electrolyte and write down the three steps of its dissociation. Sulfuric acid in concentrated solutions behaves like an electrolyte of medium strength, and in very concentrated solutions it behaves like a weak electrolyte. We will further consider sulfuric acid strong electrolyte and write the equation of its dissociation in one step.