Water is the most common solvent on planet Earth, which largely determines the nature of terrestrial chemistry as a science. Most of chemistry, at its inception as a science, began precisely as the chemistry of aqueous solutions of substances. It is sometimes considered as an ampholyte - both an acid and a base at the same time (H cation + OH anion -). In the absence of foreign substances in water, the concentration of hydroxide ions and hydrogen ions (or hydronium ions) is the same.

Water is a chemically quite active substance. It reacts with many substances of organic and inorganic chemistry.

1) Water reacts with many metals to release hydrogen:

2Na + 2H 2 O \u003d H 2 + 2NaOH (stormy)

2K + 2H 2 O = H 2 + 2KOH(violently)

3Fe + 4H 2 O = 4H 2 + Fe 3 O 4 (only when heated)

Not all, but only sufficiently active metals can participate in redox reactions of this type. Alkali and alkaline earth metals of groups I and II react most easily.

From non-metals for example, carbon and its hydrogen compound (methane) react with water. These substances are much less active than metals, but still able to react with water at high temperatures:

C + H 2 O \u003d H 2 + CO (with strong heating)

CH 4 + 2H 2 O \u003d 4H 2 + CO 2 (with strong heating)

2) Electrolysis. Water decomposes into hydrogen and oxygen under the action of an electric current. It is also a redox reaction, where water is both an oxidizing agent and a reducing agent.

3) Water reacts with many non-metal oxides. Unlike the previous ones, these reactions are not redox, but compound reactions:

SO 2 + H 2 O \u003d H 2 SO 3

SO 3 + H 2 O \u003d H 2 SO 4

CO 2 + H 2 O \u003d H 2 CO 3

4) Some metal oxides can also react with water:

CaO + H 2 O \u003d Ca (OH) 2

Not all metal oxides are capable of reacting with water. Some of them are practically insoluble in water and therefore do not react with water. We have already met with such oxides. These are ZnO, TiO 2 , Cr 2 O 3 , from which, for example, water-resistant paints are prepared. Iron oxides are also insoluble in water and do not react with it.

5) Water forms numerous compounds in which its molecule is completely preserved. These are the so-called hydrates. If the hydrate is crystalline, then it is called crystalline hydrate. For example:

CuSO 4 + 5H 2 O \u003d CuSO 4 * 5H 2 O (crystalline hydrate (copper sulfate))

Here are other examples of hydrate formation:

H 2 SO 4 + H 2 O \u003d H 2 SO 4 * H 2 O (sulfuric acid hydrate)

NaOH + H 2 O \u003d NaOH * H 2 O (caustic soda hydrate)

Compounds that bind water into hydrates and crystalline hydrates are used as desiccants. With their help, for example, remove water vapor from moist atmospheric air.

6) Photosynthesis. A special reaction of water is the synthesis of starch by plants (C 6 H 10 O 5) n and other similar compounds (carbohydrates), occurring with the release of oxygen:

6n CO 2 + 5n H 2 O \u003d (C 6 H 10 O 5) n + 6n O 2 (under the action of light)

7) Hydration reactions in organic chemistry. (Addition of water to hydrocarbon molecules.) For example:

C 2 H 4 + H 2 O \u003d C 2 H 5 OH

Water (hydrogen oxide) is a binary inorganic compound with the chemical formula H 2 O. The water molecule consists of two hydrogen atoms and one oxygen, which are interconnected by a covalent bond.

Hydrogen peroxide.

Physical and chemical properties

The physical and chemical properties of water are determined by the chemical, electronic and spatial structure of H 2 O molecules.

The H and O atoms in the H 2 0 molecule are in their stable oxidation states, respectively +1 and -2; therefore, water does not exhibit pronounced oxidizing or reducing properties. Please note: in metal hydrides, hydrogen is in the -1 oxidation state.

The H 2 O molecule has an angular structure. H-O bonds are very polar. There is an excess negative charge on the O atom, and excess positive charges on the H atoms. In general, the H 2 O molecule is polar, i.e. dipole. This explains the fact that water is a good solvent for ionic and polar substances.

The presence of excess charges on H and O atoms, as well as unshared electron pairs at O ​​atoms, causes the formation of hydrogen bonds between water molecules, as a result of which they are combined into associates. The existence of these associates explains the anomalously high values ​​of mp. etc. kip. water.

Along with the formation of hydrogen bonds, the result of the mutual influence of H 2 O molecules on each other is their self-ionization:
in one molecule, a heterolytic break of the polar O-H bond occurs, and the released proton joins the oxygen atom of another molecule. The resulting hydroxonium ion H 3 O + is essentially a hydrated hydrogen ion H + H 2 O, therefore, the water self-ionization equation is simplified as follows:

H 2 O ↔ H + + OH -

The dissociation constant of water is extremely small:

This indicates that water very slightly dissociates into ions, and therefore the concentration of undissociated H 2 O molecules is almost constant:

In pure water, [H + ] = [OH - ] = 10 -7 mol / l. This means that water is a very weak amphoteric electrolyte that exhibits neither acidic nor basic properties to a noticeable degree.
However, water has a strong ionizing effect on the electrolytes dissolved in it. Under the action of water dipoles, polar covalent bonds in the molecules of solutes turn into ionic ones, the ions are hydrated, the bonds between them are weakened, resulting in electrolytic dissociation. For example:
HCl + H 2 O - H 3 O + + Cl -

(strong electrolyte)

(or excluding hydration: HCl → H + + Cl -)

CH 3 COOH + H 2 O ↔ CH 3 COO - + H + (weak electrolyte)

(or CH 3 COOH ↔ CH 3 COO - + H +)

According to the Bronsted-Lowry theory of acids and bases, in these processes, water exhibits the properties of a base (proton acceptor). According to the same theory, water acts as an acid (proton donor) in reactions, for example, with ammonia and amines:

NH 3 + H 2 O ↔ NH 4 + + OH -

CH 3 NH 2 + H 2 O ↔ CH 3 NH 3 + + OH -

Redox reactions involving water

I. Reactions in which water plays the role of an oxidizing agent

These reactions are possible only with strong reducing agents, which are able to reduce the hydrogen ions that are part of the water molecules to free hydrogen.

1) Interaction with metals

a) Under normal conditions, H 2 O interacts only with alkali. and alkali-earth. metals:

2Na + 2H + 2 O \u003d 2NaOH + H 0 2

Ca + 2H + 2 O \u003d Ca (OH) 2 + H 0 2

b) At high temperatures, H 2 O also reacts with some other metals, for example:

Mg + 2H + 2 O \u003d Mg (OH) 2 + H 0 2

3Fe + 4H + 2 O \u003d Fe 2 O 4 + 4H 0 2

c) Al and Zn displace H 2 from water in the presence of alkalis:

2Al + 6H + 2 O + 2NaOH \u003d 2Na + 3H 0 2

2) Interaction with non-metals having low EO (reactions occur under harsh conditions)

C + H + 2 O \u003d CO + H 0 2 ("water gas")

2P + 6H + 2 O \u003d 2HPO 3 + 5H 0 2

In the presence of alkalis, silicon displaces hydrogen from water:

Si + H + 2 O + 2NaOH \u003d Na 2 SiO 3 + 2H 0 2

3) Interaction with metal hydrides

NaH + H + 2 O \u003d NaOH + H 0 2

CaH 2 + 2H + 2 O \u003d Ca (OH) 2 + 2H 0 2

4) Interaction with carbon monoxide and methane

CO + H + 2 O \u003d CO 2 + H 0 2

2CH 4 + O 2 + 2H + 2 O \u003d 2CO 2 + 6H 0 2

Reactions are used in industry to produce hydrogen.

II. Reactions in which water acts as a reducing agent

These reactions are possible only with very strong oxidizing agents that are capable of oxidizing oxygen CO CO -2, which is part of water, to free oxygen O 2 or to peroxide anions 2-. In an exceptional case (in reaction with F 2), oxygen is formed with c o. +2.

1) Interaction with fluorine

2F 2 + 2H 2 O -2 \u003d O 0 2 + 4HF

2F 2 + H 2 O -2 \u003d O +2 F 2 + 2HF

2) Interaction with atomic oxygen

H 2 O -2 + O \u003d H 2 O - 2

3) Interaction with chlorine

At high T, a reversible reaction occurs

2Cl 2 + 2H 2 O -2 \u003d O 0 2 + 4HCl

III. Reactions of intramolecular oxidation - reduction of water.

Under the action of an electric current or high temperature, water can be decomposed into hydrogen and oxygen:

2H + 2 O -2 \u003d 2H 0 2 + O 0 2

Thermal decomposition is a reversible process; the degree of thermal decomposition of water is low.

Hydration reactions

I. Hydration of ions. Ions formed during the dissociation of electrolytes in aqueous solutions attach a certain number of water molecules and exist in the form of hydrated ions. Some ions form such strong bonds with water molecules that their hydrates can exist not only in solution, but also in the solid state. This explains the formation of crystalline hydrates such as CuSO4 5H 2 O, FeSO 4 7H 2 O, etc., as well as aqua complexes: CI 3 , Br 4 , etc.

II. Hydration of oxides

III. Hydration of organic compounds containing multiple bonds

Hydrolysis reactions

I. Hydrolysis of salts

Reversible hydrolysis:

a) according to the salt cation

Fe 3+ + H 2 O \u003d FeOH 2+ + H +; (acidic environment. pH

b) by salt anion

CO 3 2- + H 2 O \u003d HCO 3 - + OH -; (alkaline environment. pH > 7)

c) by the cation and by the anion of the salt

NH 4 + + CH 3 COO - + H 2 O \u003d NH 4 OH + CH 3 COOH (environment close to neutral)

Irreversible hydrolysis:

Al 2 S 3 + 6H 2 O \u003d 2Al (OH) 3 ↓ + 3H 2 S

II. Hydrolysis of metal carbides

Al 4 C 3 + 12H 2 O \u003d 4Al (OH) 3 ↓ + 3CH 4 netane

CaC 2 + 2H 2 O \u003d Ca (OH) 2 + C 2 H 2 acetylene

III. Hydrolysis of silicides, nitrides, phosphides

Mg 2 Si + 4H 2 O \u003d 2Mg (OH) 2 ↓ + SiH 4 silane

Ca 3 N 2 + 6H 2 O \u003d ZCa (OH) 2 + 2NH 3 ammonia

Cu 3 P 2 + 6H 2 O \u003d ZCu (OH) 2 + 2PH 3 phosphine

IV. Hydrolysis of halogens

Cl 2 + H 2 O \u003d HCl + HClO

Br 2 + H 2 O \u003d HBr + HBrO

V. Hydrolysis of organic compounds

Classes of organic substances	Hydrolysis products (organic)
Halogenalkanes (alkyl halides)
Aryl halides
Dihaloalkanes	Aldehydes or ketones
Metal alcoholates
Carboxylic acid halides	carboxylic acids
Anhydrides of carboxylic acids	carboxylic acids
Esters of carboxylic acids	Carboxylic acids and alcohols
	Glycerin and higher carboxylic acids
Di- and polysaccharides	Monosaccharides
Peptides and proteins	α-Amino acids
Nucleic acids

Everyone should know the properties of water - since they largely determine our life and ourselves as such ...

Chemical and physical properties of water in a liquid state - terms, definitions and comments

Strictly speaking, in this article we will briefly consider not onlychemical and physical properties of liquid water,but also the properties inherent in it in general as such.

You can read more about the properties of water in the solid state in our article - PROPERTIES OF WATER IN A SOLID STATE (read →).

Water- a super-significant substance for our planet. Without it, life on Earth is impossible; not a single geological process takes place without it. The great scientist and thinker Vladimir Ivanovich Vernadsky wrote in his works that there is no such component, the value of which could "compare with it in terms of its influence on the course of the main, most formidable geological processes." Water is present not only in the body of all living beings of our planet, but also in all substances on Earth - in minerals, in rocks ... The study of the unique properties of water constantly reveals more and more secrets to us, sets us new mysteries and throws new challenges.

Anomalous properties of water

Many physical and chemical properties of water surprise and fall out of the general rules and patterns and are abnormal, for example:

• In accordance with the laws established by the principle of similarity, within the framework of such sciences as chemistry and physics, we might expect that:
  • water will boil at minus 70°С, and freeze at minus 90°С;
  • water it will not drip from the tip of the tap, but pour in a thin stream;
  • ice will sink rather than float on the surface;
  • in glass water more than a few grains of sugar would not dissolve.
• Surface water has a negative electrical potential;
• When heated from 0°C to 4°C (3.98°C to be exact), water contracts;
• The surprisingly high heat capacity of water liquid state;

As noted above, in this material we list the main physical and chemical properties of water and make brief comments on some of them.

Physical properties of water

PHYSICAL PROPERTIES are properties that appear outside of chemical reactions.

Purity

The purity of water depends on the presence of impurities, bacteria, salts of heavy metals in it ..., to get acquainted with the interpretation of the term CLEAR WATER according to our website, you need to read the article PURE WATER (read →) .

Color

Color water– depends on the chemical composition and mechanical impurities

For example, let's take the definition of "Colors of the Sea", given by the "Great Soviet Encyclopedia".

The color of the sea. The color perceived by the eye when the observer looks at the surface of the sea. The color of the sea depends on the color of sea water, the color of the sky, the number and nature of clouds, the height of the Sun above the horizon, and other reasons.

The concept of the color of the sea should be distinguished from the concept of the color of sea water. The color of sea water is understood as the color perceived by the eye when viewing sea water vertically over a white background. Only an insignificant part of the light rays falling on it is reflected from the sea surface, the rest of them penetrate deep into, where they are absorbed and scattered by water molecules, particles of suspended solids and tiny gas bubbles. The scattered rays reflected and emerging from the sea create the C. m. Water molecules scatter the blue and green rays most of all. Suspended particles scatter all rays almost equally. Therefore, sea water with a small amount of suspensions seems blue-green (the color of the open parts of the oceans), and with a significant amount of suspensions - yellowish-green (for example, Baltic). The theoretical side of the doctrine of the C. m. was developed by V. V. Shuleikin and C. V. Raman.

Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978

Smell

Smell water– Pure water is usually odorless.

Transparency

Transparency water- depends on the mineral substances dissolved in it and the content of mechanical impurities, organic substances and colloids:

TRANSPARENCY OF WATER - the ability of water to transmit light. Usually measured by the Secchi disk. It depends mainly on the concentration of organic and inorganic substances suspended and dissolved in water. It can sharply decrease as a result of anthropogenic pollution and eutrophication of water bodies.

Ecological encyclopedic dictionary. - Chisinau I.I. Grandpa. 1989

TRANSPARENCY OF WATER - the ability of water to transmit light rays. It depends on the thickness of the water layer passed by the rays, the presence of suspended impurities, dissolved substances, etc. In water, red and yellow rays are absorbed more strongly, violet rays penetrate deeper. According to the degree of transparency, in order of decreasing it, waters are distinguished:

• transparent;
• slightly opalescent;
• opalescent;
• slightly cloudy;
• cloudy;
• very cloudy.

Dictionary of hydrogeology and engineering geology. - M.: Gostoptekhizdat. 1961

Taste

The taste of water depends on the composition of the substances dissolved in it.

Dictionary of hydrogeology and engineering geology

The taste of water is a property of water that depends on the salts and gases dissolved in it. There are tables of palpable concentration of salts dissolved in water (in mg / l), for example, the following table (according to Staff).

Temperature

Melting point of water:

MELTING POINT - The temperature at which a substance changes from solid to liquid. The melting point of a solid is equal to the freezing point of a liquid, for example, the melting point of ice, 0°C, is equal to the freezing point of water.

Boiling point of water : 99.974°C

Scientific and technical encyclopedic dictionary

BOILING POINT, the temperature at which a substance passes from one state (phase) to another, i.e. from liquid to vapor or gas. The boiling point increases as the external pressure increases and decreases as it decreases. It is usually measured at a standard pressure of 1 atmosphere (760 mm Hg). The boiling point of water at a standard pressure is 100 °C.

Scientific and technical encyclopedic dictionary.

Triple point of water

Triple point of water: 0.01 °C, 611.73 Pa;

Scientific and technical encyclopedic dictionary

TRIPLE POINT, temperature and pressure at which all three states of matter (solid, liquid, gaseous) can exist simultaneously. For water, the triple point is at a temperature of 273.16 K and a pressure of 610 Pa.

Scientific and technical encyclopedic dictionary.

Surface tension of water

Surface tension of water - determines the strength of adhesion of water molecules to each other, for example, how this or that water is absorbed by the human body depends on this parameter.

Adhesion and cohesion of water

Adhesion and cohesion are properties that determine the "stickiness of water" to other materials. Adhesion determines the "stickiness" of water to other substances, and cohesion is the stickiness of water molecules in relation to each other.

Capillarity

Capillarity is the property of water that allows water to rise vertically in porous materials. This property is realized through other properties of water, such as surface tension, adhesion and cohesion.

Hardness of water

Water hardness - determined by the amount of salt content, read more in the materials HARD WATER - WHAT IS IT (read →) and WATER MINERALIZATION (read →).

Marine vocabulary

WATER HARDNESS (Stiffness of Water) - a property of water, bled by the content of alkaline earth metal salts dissolved in it, ch. arr. calcium and magnesium (in the form of bicarbonate salts - bicarbonates), and salts of strong mineral acids - sulfuric and hydrochloric. The hardness of water is measured in special units, the so-called. degrees of hardness. The degree of hardness is the weight content of calcium oxide (CaO), equal to 0.01 g in 1 liter of water. Hard water is unsuitable for feeding boilers, as it contributes to the strong formation of scale on their walls, which can cause burnout of the boiler tubes. Boilers of large capacities and especially high pressures must be fed with completely purified water (condensate from steam engines and turbines, purified by filters from oil impurities, as well as distillate prepared in special evaporators).

Samoilov K.I. Marine Dictionary. - M.-L.: State Naval Publishing House of the NKVMF of the USSR, 1941

Scientific and technical encyclopedic dictionary

HARDNESS OF WATER, the inability of water to form foam with soap due to salts dissolved in it, mainly calcium and magnesium.

Scale in boilers and pipes is formed due to the presence of dissolved calcium carbonate in water, which enters the water upon contact with limestone. In hot or boiling water, calcium carbonate precipitates as hard lime deposits on surfaces inside boilers. Calcium carbonate also prevents soap from lathering. The ion-exchange container (3) is filled with granules coated with sodium-containing materials. with which the water comes into contact. Sodium ions, being more active, replace calcium ions. Since sodium salts remain soluble even when boiled, scale does not form.

Scientific and technical encyclopedic dictionary.

Water structure

under the structure water refers to a certain arrangement of water molecules in relation to each other. This concept is actively used in the theory of structured water- read our article STRUCTURED WATER - BASIC CONCEPTS (read →).

Water mineralization

Mineralization water:

Ecological Encyclopedic Dictionary

MINERALIZATION OF WATER - saturation of water inorganic. (mineral) substances present in it in the form of ions and colloids; the total amount of inorganic salts contained mainly in fresh water, the degree of mineralization is usually expressed in mg / l or g / l (sometimes in g / kg).

Ecological encyclopedic dictionary. - Chisinau: Main edition of the Moldavian Soviet Encyclopedia. I.I. Grandpa. 1989

Viscosity of water

Viscosity of water - characterizes the internal resistance of liquid particles to its movement:

Geological dictionary

The viscosity of water (liquid) is a property of a liquid that causes the appearance of a friction force during movement. It is a factor that transfers motion from layers of water moving at a high speed to layers with a lower speed. The viscosity of water depends on the temperature and concentration of the solution. Physically, it is estimated by the coefficient. viscosity, which is included in a number of formulas for the movement of water.

Geological dictionary: in 2 volumes. - M.: Nedra. Edited by K. N. Paffengolts et al. 1978

There are two types of viscosity water:

• Dynamic viscosity of water - 0.00101 Pa s (at 20°C).
• The kinematic viscosity of water is 0.01012 cm 2 /s (at 20°C).

Critical point of water

critical point water called its state at a certain ratio of pressure and temperature, when its properties are the same in the gaseous and liquid state (gaseous and liquid phase).

Critical point of water: 374°C, 22.064 MPa.

The dielectric constant

Dielectric constant, in general, is a coefficient showing how much the force of interaction between two charges in a vacuum is greater than in a certain medium.

In the case of water, this figure is unusually high and for static electric fields is 81.

Heat capacity of water

Heat capacity water- water has a surprisingly high heat capacity:

Ecological dictionary

Heat capacity is the property of substances to absorb heat. It is expressed as the amount of heat absorbed by a substance when it is heated by 1°C. The heat capacity of water is about 1 cal/g, or 4.2 J/g. The heat capacity of the soil (at 14.5-15.5°C) ranges (from sandy to peaty soils) from 0.5 to 0.6 cal (or 2.1-2.5 J) per unit volume and from 0.2 up to 0.5 cal (or 0.8-2.1 J) per unit mass (g).

Ecological dictionary. - Alma-Ata: "Science". B.A. Bykov. 1983

Scientific and technical encyclopedic dictionary

SPECIFIC HEAT CAPACITY (symbol c), the heat required to raise the temperature of 1 kg of a substance by 1K. It is measured in J / K.kg (where J is JOUL). Substances with high specific heat, such as water, require more energy to raise the temperature than substances with low specific heat.

Scientific and technical encyclopedic dictionary.

Thermal conductivity of water

The thermal conductivity of a substance refers to its ability to conduct heat from its hotter parts to its colder parts.

Heat transfer in water occurs either at the molecular level, that is, it is transferred by molecules water, or due to the movement / movement of any volumes of water - turbulent thermal conductivity.

The thermal conductivity of water depends on temperature and pressure.

Fluidity

The fluidity of substances is understood as their ability to change their shape under the influence of constant stress or constant pressure.

The fluidity of liquids is also determined by the mobility of their particles, which at rest are unable to perceive shear stresses.

Inductance

Inductance determines the magnetic properties of closed electric current circuits. Water, with the exception of some cases, conducts electric current, and therefore has a certain inductance.

Density of water

Density water- is determined by the ratio of its mass to volume at a certain temperature. Read more in our material - WHAT IS THE DENSITY OF WATER (read →) .

Water compressibility

Water compressibility– is very small and depends on the salinity of the water and pressure. For example, for distilled water, it is 0.0000490. Under natural conditions, water is practically incompressible, but in industrial production for technical purposes, water is highly compressed. For example, for cutting hard materials, including such as metals.

Electrical conductivity of water

The electrical conductivity of water - largely depends on the amount of salts dissolved in them.

Radioactivity

Water radioactivity- depends on the content of radon in it, the emanation of radium.

Physical and chemical properties of water

Dictionary of hydrogeology and engineering geology

PHYSICAL AND CHEMICAL PROPERTIES OF WATER - parameters that determine the physical and chemical characteristics of natural waters. These include indicators of hydrogen ion concentration (pH) and redox potential (Eh).

Dictionary of hydrogeology and engineering geology. - M.: Gostoptekhizdat. Compiled by: A. A. Makkaveev, editor O. K. Lange. 1961

Solubility

Different sources classify this property in different ways - some refer it to the physical, others to the chemical properties of the substance. Therefore, at this stage, we attributed it to the physicochemical properties of water, which is confirmed by one of the definitions of solubility given below.

Big Encyclopedic Dictionary

SOLUBILITY - the ability of a substance in a mixture with one or more other substances to form solutions. A measure of the solubility of a substance in a given solvent is the concentration of its saturated solution at a given temperature and pressure. The solubility of gases depends on temperature and pressure, the solubility of liquid and solid bodies practically does not depend on pressure.

Big Encyclopedic Dictionary. 2000

Directory of road terms

Solubility is the property of a material (substances) to form homogeneous systems having the same chemical composition and physical properties.

Directory of road terms, M. 2005

general chemistry

Solubility - the property of gaseous, liquid and solid substances to go into a dissolved state; expressed by the equilibrium mass ratio of solute and solvent at a given temperature.

General chemistry: A. V. Zholnin textbook; ed. V. A. Popkova, A. V. Zholnina. 2012

Physical Encyclopedia

Solubility - the ability of a substance to form solutions with other substances. It is quantitatively characterized by the concentration of a substance in a saturated solution. Solubility is determined by physical. and chem. the affinity of the molecules of the solvent and the solute, a cut is characterized by the so-called. energy of interchange of solution molecules. As a rule, solubility is high if the molecules of the solute and the solvent have similar properties ("like dissolves like").

The dependence of solubility on temperature and pressure is established using the Le Chatelier-Brown principle. Solubility increases with increasing pressure and passes through a maximum at high pressures; The solubility of gases in liquids decreases with increasing temperature, while in metals it increases.

Physical encyclopedia. In 5 volumes. - M.: Soviet Encyclopedia. Editor-in-Chief A. M. Prokhorov. 1988

Acid-base balance (pH of water)

The acid-base balance of water is determined by the pH indicator, the value of which can vary from 0 to 14. A value of 7 - determines the acid-base balance of water as neutral, if less than 7 - acidic water, more than 7 - alkaline water.

Redox potential of water

The redox potential of water (ORP) is the ability of water to enter into biochemical reactions.

Chemical properties of water

CHEMICAL PROPERTIES OF A SUBSTANCE are properties that appear as a result of chemical reactions.

Below are the Chemical properties of water according to the textbook “Fundamentals of Chemistry. Internet textbook" by A. V. Manuylov, V. I. Rodionov.

Interaction of water with metals

When water interacts with most metals, a reaction occurs with the release of hydrogen:

• 2Na + 2H2O = H2 + 2NaOH (violently);
• 2K + 2H2O = H2 + 2KOH (violently);
• 3Fe + 4H2O = 4H2 + Fe3O4 (only when heated).

Not all, but only sufficiently active metals can participate in redox reactions of this type. Alkali and alkaline earth metals of groups I and II react most easily.

When water interacts with noble metals such as gold, platinum..., there is no reaction.

Interaction water with non-metals

Among non-metals, for example, carbon and its hydrogen compound (methane) react with water. These substances are much less active than metals, but still able to react with water at high temperatures:

• C + H2O = H2 + CO (with strong heating);
• CH4 + 2H2O = 4H2 + CO2 (with strong heating).

Interaction water with electric current

When exposed to an electric current, water decomposes into hydrogen and oxygen. It is also a redox reaction, where water is both an oxidizing agent and a reducing agent.

Interaction of water with non-metal oxides

Water reacts with many non-metal oxides and some metal oxides. These are not redox reactions, but compound reactions:

• SO2 + H2O = H2SO3 (sulphurous acid);
• SO3 + H2O = H2SO4 (sulfuric acid);
• CO2 + H2O = H2CO3 (carbonic acid).

Interaction of water with metal oxides

Some metal oxides can also react with water.

We have already seen examples of such reactions:

CaO + H2O = Ca(OH)2 (calcium hydroxide (slaked lime).

Not all metal oxides are capable of reacting with water. Some of them are practically insoluble in water and therefore do not react with water. For example: ZnO, TiO2, Cr2O3, from which, for example, water-resistant paints are prepared. Iron oxides are also insoluble in water and do not react with it.

Hydrates and crystalline hydrates

Water forms compounds, hydrates and crystalline hydrates, in which the water molecule is completely preserved. .

For example:

• CuSO4 + 5H2O = CuSO4.5H2O;
• CuSO4 - white substance (anhydrous copper sulfate);
• CuSO4.5H2O - crystalline hydrate (copper sulfate), blue crystals.

Other examples of hydrate formation:

• H2SO4 + H2O = H2SO4.H2O (sulfuric acid hydrate);
• NaOH + H2O = NaOH.H2O (caustic soda hydrate).

Compounds that bind water into hydrates and crystalline hydrates are used as desiccants. With their help, for example, remove water vapor from moist atmospheric air.

Biosynthesis

Water is involved in bio-synthesis as a result of which oxygen is formed:

6n CO 2 + 5n H 2 O \u003d (C 6 H 10 O 5) n + 6n O 2 (under the action of light)

Conclusion

We see that the properties of water are diverse and cover almost all aspects of life on Earth. As one of the scientists formulated … it is necessary to study water in a complex way, and not in the context of its individual manifestations.

In preparing the material, information from books was used- Yu. P. Rassadkina “Ordinary and extraordinary water”, Yu. Ya. Fialkov “Unusual properties of ordinary solutions”, Textbook “Fundamentals of Chemistry. Internet textbook" by A. V. Manuylov, V. I. Rodionov and others.

Hydrogen oxide (H 2 O), much better known to all of us under the name "water", without exaggeration, is the main liquid in the life of organisms on Earth, because all chemical and biological reactions take place either with the participation of water or in solutions.

Water is the second most important substance for the human body after air. A person can live without water for no more than 7-8 days.

Pure water in nature can exist in three states of aggregation: in solid - in the form of ice, in liquid, actually water, in gaseous - in the form of steam. No other substance in nature can boast of such a variety of aggregate states.

Physical properties of water

• at n.o. - it is a colorless, odorless and tasteless liquid;
• water has a high heat capacity and low electrical conductivity;
• melting point 0°C;
• boiling point 100°C;
• the maximum density of water at 4°C is 1 g/cm 3 ;
• water is a good solvent.

The structure of the water molecule

The water molecule consists of one oxygen atom, which is connected to two hydrogen atoms, while the O-H bonds form an angle of 104.5 °, while the common electron pairs are shifted to the oxygen atom, which is more electronegative compared to hydrogen atoms, therefore, by a partial negative charge is formed on the oxygen atom, respectively, on the hydrogen atoms - a positive one. Thus, the water molecule can be considered as a dipole.

Water molecules can form hydrogen bonds with each other, attracted by oppositely charged parts (hydrogen bonds are shown in the figure by a dotted line):

The formation of hydrogen bonds explains the high density of water, its boiling and melting points.

The number of hydrogen bonds depends on the temperature - the higher the temperature, the smaller the number of bonds formed: in water vapor there are only its individual molecules; in the liquid state, associates (H 2 O) n are formed; in the crystalline state, each water molecule is connected to neighboring molecules by four hydrogen bonds.

Chemical properties of water

Water "willingly" reacts with other substances:

• water reacts with alkali and alkaline earth metals at n.o.: 2Na + 2H 2 O \u003d 2NaOH + H 2
• with less active metals and non-metals, water reacts only at high temperatures: 3Fe + 4H 2 O \u003d FeO → Fe 2 O 3 + 4H 2 C + 2H 2 O → CO 2 + 2H 2
• with basic oxides at n.o. water reacts to form bases: CaO + H 2 O \u003d Ca (OH) 2
• with acid oxides at n.o.s. water reacts to form acids: CO 2 + H 2 O \u003d H 2 CO 3
• water is the main participant in hydrolysis reactions (for more details, see Hydrolysis of salts);
• water participates in hydration reactions, adding to organic substances with double and triple bonds.

Solubility of substances in water

• highly soluble substances - more than 1 g of the substance dissolves in 100 g of water at n.o.s.;
• poorly soluble substances - 0.01-1 g of the substance dissolves in 100 g of water;
• practically insoluble substances - less than 0.01 g of the substance dissolves in 100 g of water.

Completely insoluble substances do not exist in nature.

11.1. physical dissolution

If any substance enters the water, it can:
a) dissolve in water, that is, mix with it at the atomic-molecular level;
b) enter into a chemical reaction with water;
c) do not dissolve and do not react.
What determines the result of the interaction of a substance with water? Naturally, from the characteristics of the substance and from the characteristics of water.
Let's start with dissolution and consider what characteristics of water and substances interacting with it are of the greatest importance in these processes.
Place in two test tubes a small portion of naphthalene C 10 H 8 . Pour water into one of the test tubes, and C 7 H 16 heptane into the other (gasoline can be used instead of pure heptane). Naphthalene will dissolve in heptane, but not in water. Let's check whether naphthalene really dissolved in heptane or reacted with it. To do this, put a few drops of the solution on the glass and wait until the heptane evaporates - colorless lamellar crystals form on the glass. The fact that this is naphthalene can be seen by the characteristic smell.

One of the differences between heptane and water is that its molecules are non-polar, while water molecules are polar. In addition, there are hydrogen bonds between water molecules, but there are none between heptane molecules.

To dissolve naphthalene in heptane, it is required to break weak intermolecular bonds between naphthalene molecules and weak intermolecular bonds between heptane molecules. When dissolved, equally weak intermolecular bonds are formed between the molecules of naphthalene and heptane. The thermal effect of such a process is practically zero.
Why does naphthalene dissolve in heptane? Only due to the entropy factor (disorder grows in the naphthalene-heptane system).

To dissolve naphthalene in water, it is necessary, in addition to weak bonds between its molecules, to break hydrogen bonds between water molecules. In this case, hydrogen bonds between the molecules of naphthalene and water are not formed. The process turns out to be endothermic and so energetically unfavorable that the entropy factor cannot help here.
And if instead of naphthalene we take another substance whose molecules are capable of forming hydrogen bonds with water molecules, will such a substance dissolve in water?
If there are no other obstacles, then there will be. For example, you know that sugar (sucrose C 12 H 22 O 11) is perfectly soluble in water. Looking at the structural formula of sucrose, you will see that there are –O–H groups in its molecule that can form hydrogen bonds with water molecules.
Make sure experimentally that sucrose is poorly soluble in heptane, and try to explain on your own why the properties of naphthalene and sucrose differ so much.
The dissolution of naphthalene in heptane and sucrose in water is called physical dissolution.

Only molecular substances can physically dissolve.

The other components of the solution are called solutes.

The regularities we have revealed also apply to cases of dissolution in water (and in most other solvents) of liquid and gaseous substances. If all the substances that form the solution were in the same state of aggregation before dissolution, then the solvent is usually called the substance that is more in the solution. The exception to this rule is water: it is usually called a solvent, even if it is less than the solute.
The reason for the physical dissolution of a substance in water can be not only the formation of hydrogen bonds between the molecules of the dissolved substance and water, but also the formation of other types of intermolecular bonds. This happens primarily in the case of dissolution in water of gaseous substances (for example, carbon dioxide or chlorine), in which the molecules are not bound to each other at all, as well as some liquids with very weak intermolecular bonds (for example, bromine). The gain in energy is achieved here due to the orientation of dipoles (water molecules) around polar molecules or polar bonds in the solute, and in the case of chlorine or bromine, it is caused by the tendency to attach electrons to the atoms of chlorine and bromine, which is also preserved in the molecules of these simple substances (more details - in § 11.4).
In all these cases, the substances are much less soluble in water than in the formation of hydrogen bonds.
If the solvent is removed from the solution (for example, as you did in the case of a solution of naphthalene in heptane), then the solute will stand out in a chemically unchanged form.

PHYSICAL DISSOLVE, SOLVENT.
1. Explain why heptane is insoluble in water
2. Tell me the sign of the heat effect of dissolving ethyl alcohol (ethanol) in water.
3. Why is ammonia well soluble in water, and oxygen is bad?
4. Which substance is better soluble in water - ammonia or phosphine (PH 3)?
5. Explain the reason for the better solubility of ozone in water than oxygen.
6. Determine the mass fraction of glucose (grape sugar, C 6 H 12 O 6) in an aqueous solution, if 120 ml of water and 30 g of glucose were used to prepare it (take the density of water to be 1 g / ml). What is the concentration of glucose in this solution if the density of the solution is 1.15 g/ml?
7. How much sugar (sucrose) can be isolated from 250 g of syrup with a mass fraction of water equal to 35%?

1. Experiments on the dissolution of various substances in various solvents.
2. Preparation of solutions.

11.2. Chemical dissolution

In the first paragraph, we considered cases of dissolution of substances in which the chemical bonds remained unchanged. But this is not always the case.
Place a few crystals of sodium chloride in a test tube and add water. After a while, the crystals will dissolve. What happened?
Sodium chloride is a non-molecular substance. The NaCl crystal is composed of Na and Cl ions. When such a crystal enters the water, these ions pass into it. In this case, ionic bonds in the crystal and hydrogen bonds between water molecules are broken. The ions that enter the water interact with the water molecules. In the case of chloride ions, this interaction is limited by the electrostatic attraction of dipole water molecules to the anion, and in the case of sodium cations, it approaches in nature the donor-acceptor interaction. Somehow, the ions are covered hydration shell(Fig. 11.1).

In the form of a reaction equation, this can be written as follows:

NaCl cr + ( n + m)H 2 O = + A

or abbreviated , where the index aq means that the ion hydrated. Such an equation is called ionic equation.

You can also write down the "molecular" equation of this process: (this name has been preserved since it was assumed that all substances consist of molecules)

Hydrated ions are weaker attracted to each other, and the energy of thermal motion is sufficient to prevent these ions from sticking together into a crystal.

In practice, the presence of ions in a solution can be easily confirmed by studying the electrical conductivity of sodium chloride, water, and the resulting solution. You already know that sodium chloride crystals do not conduct electric current, because although they contain charged particles - ions, they are "fixed" in the crystal and cannot move. Water conducts electric current very poorly, because although oxonium ions and hydroxide ions are formed in it due to autoprotolysis, they are very few. A solution of sodium chloride, on the contrary, conducts electricity well, because there are many ions in it, and they can move freely, including under the influence of an electric voltage.
Energy must be expended to break ionic bonds in a crystal and hydrogen bonds in water. When ions are hydrated, energy is released. If the energy costs for bond breaking exceed the energy released during ion hydration, then dissolution endothermic, and if vice versa, then - exothermic.
Sodium chloride dissolves in water with almost zero thermal effect, therefore, the dissolution of this salt occurs only due to an increase in entropy. But usually dissolution is accompanied by a noticeable release of heat (Na 2 CO 3, CaCl 2, NaOH, etc.) or its absorption (KNO 3, NH 4 Cl, etc.), for example:

When water is evaporated from solutions obtained by chemical dissolution, solutes are again released from them in a chemically unchanged form.

Chemical dissolution- dissolution, in which chemical bonds are broken.

In both physical and chemical dissolution, a solution of the substance that we dissolved is formed, for example, a solution of sugar in water or a solution of sodium chloride in water. In other words, the solute can be separated from the solution when the water is removed.

HYDRATION SHELL, HYDRATION, CHEMICAL DISSOLUTION.
Give three examples of substances well known to you a) soluble in water or reacting with it, b) insoluble in water and not reacting with it.
2. What is a solvent and what is a dissolved substance (or substances) in the following solutions: a) soapy water, b) table vinegar, c) vodka d) hydrochloric acid, e) motorcycle fuel, f) pharmacy "hydrogen peroxide", g) sparkling water, i) "brilliant green", j) cologne?
In case of difficulty, consult with the parents.
3. List the ways in which a solvent can be removed from a liquid solution.
4. How do you understand the expression "in a chemically unchanged form" in the last paragraph of the first paragraph of this chapter? What changes can occur to the substance as a result of its dissolution and subsequent separation from the solution?
5. It is known that fats are insoluble in water, but dissolve well in gasoline. Based on this, what can be said about the structure of fat molecules?
6. Write down the equations of chemical dissolution in water of the following ionic substances:
a) silver nitrate, b) calcium hydroxide, c) cesium iodide, d) potassium carbonate, e) sodium nitrite, f) ammonium sulfate.
7. Write down the equations of crystallization of substances from the solutions listed in task 6 when water is removed.
8. How do solutions obtained by physical dissolution of substances differ from solutions obtained by chemical dissolution? What do these solutions have in common?
9. Determine the mass of the salt that must be dissolved in 300 ml of water to obtain a solution with a mass fraction of this salt equal to 0.1. The density of water is 1 g/ml, and the density of the solution is 1.05 g/ml. What is the concentration of salt in the resulting solution if its formula weight is 101 Days?
10. How much water and barium nitrate do you need to take to prepare 0.5 l of a 0.1 M solution of this substance (solution density 1.02 g / ml)?
Experiments on the dissolution of ionic substances in water.

11.3. saturated solutions. Solubility

Any portion of sodium chloride (or other similar substance) placed in water would always dissolve completely if, apart from the process of dissolution

the reverse process would not proceed - the process of crystallization of the initial substance from the solution:

At the moment the crystal is placed in water, the rate of the crystallization process is zero, but as the concentration of ions in the solution increases, it increases and at some point becomes equal to the rate of dissolution. A state of equilibrium occurs:

the resulting solution is called saturated.

As such a characteristic, the mass fraction of the dissolved substance, its concentration, or another physical quantity characterizing the composition of the solution can be used.
By solubility in a given solvent, all substances are divided into soluble, slightly soluble and practically insoluble. Usually practically insoluble substances are called simply insoluble. For the conditional boundary between soluble and poorly soluble substances, a solubility equal to 1 g in 100 g of H 2 O ( w 1%), and beyond the conditional boundary between poorly soluble and insoluble substances - a solubility equal to 0.1 g in 100 g H 2 O ( w 0,1%).
The solubility of a substance depends on temperature. Since solubility is a characteristic of equilibrium, its change with temperature changes occurs in full accordance with the Le Chatelier principle, that is, with an exothermic dissolution of a substance, its solubility decreases with increasing temperature, and with an endothermic one it increases.
Solutions in which, under the same conditions, the solute is less than in saturated ones, are called unsaturated.

SATURATED SOLUTION; UNSATURATED SOLUTION; SOLUBILITY OF THE SUBSTANCE; SOLUBLE, SLOWLY SOLUBLE AND INSOLUTION SUBSTANCES.

1. Write down the equilibrium equations in the system saturated solution - sediment for a) potassium carbonate, b) silver nitrate and c) calcium hydroxide.
2. Determine the mass fraction of potassium nitrate in an aqueous solution of this salt saturated at 20 ° C, if, when preparing such a solution, 100 g of potassium nitrate was added to 200 g of water, and at the same time, after the preparation of the solution, 36.8 g of potassium nitrate did not dissolve.
3. Is it possible to prepare an aqueous solution of potassium chromate K 2 CrO 4 at 20 ° C with a mass fraction of the dissolved substance equal to 45%, if at this temperature no more than 63.9 g of this salt is dissolved in 100 g of water.
4. The mass fraction of potassium bromide in a saturated aqueous solution at 0 ° C is 34.5%, and at 80 ° C - 48.8%. Determine the mass of potassium bromide released when 250 g of an aqueous solution of this salt saturated at 80°C is cooled to 0 ° C.
5. The mass fraction of calcium hydroxide in a saturated aqueous solution at 20 ° C is 0.12%. How many liters of a solution of calcium hydroxide (lime water) saturated at this temperature can be obtained with 100 g of calcium hydroxide? Take the density of the solution equal to 1 g/ml.
6. At 25 °C, the mass fraction of barium sulfate in a saturated aqueous solution is 2.33 10 -2%. Determine the minimum volume of water required to completely dissolve 1 g of this salt.
preparation of saturated solutions.

11.4. Chemical reactions of substances with water

Many substances, when in contact with water, enter into chemical reactions with it. As a result of such an interaction with an excess of water, as with dissolution, a solution is obtained. But if water is removed from this solution, we will not get the original substance.

What products are formed in the chemical reaction of a substance with water? It depends on the type of chemical bond in the substance; if the bonds are covalent, then on the degree of polarity of these bonds. In addition, other factors also influence, some of which we will get acquainted with.

a) Compounds with ionic bond

Most ionic compounds either chemically dissolve in water or do not. Ionic hydrides and oxides stand apart, that is, compounds containing the same elements as water itself, and some other substances. Let us consider the behavior of ionic oxides in contact with water using calcium oxide as an example.
Calcium oxide, being an ionic substance, could chemically dissolve in water. In this case, calcium ions and oxide ions would pass into the solution. But a doubly charged anion is not the most stable valence state of the oxygen atom (if only because the affinity energy for the second electron is always negative, and the radius of the oxide ion is relatively small). Therefore, oxygen atoms tend to lower their formal charge. In the presence of water, this is possible. Oxide ions found on the surface of the crystal interact with water molecules. This reaction can be represented as a diagram showing its mechanism ( mechanism diagram).

For a better understanding of what is happening, we conditionally divide this process into stages:
1. The water molecule turns to the oxide ion with a hydrogen atom (oppositely charged).
2. The oxide ion is divided with the hydrogen atom by an unshared pair of electrons; a covalent bond is formed between them (it is formed by the donor-acceptor mechanism).
3. At the hydrogen atom in a single valence orbital (1 s) turns out to be four electrons (two "old" and two "new"), which contradicts the Pauli principle. Therefore, the hydrogen atom donates a pair of bond electrons ("old" electrons) to the oxygen atom, which is part of the water molecule, especially since this pair of electrons was already largely displaced to the oxygen atom. The bond between the hydrogen atom and the oxygen atom is broken.
4. Due to the formation of a bond by the donor-acceptor mechanism, the formal charge on the former oxide ion becomes equal to –1 e; on the oxygen atom, which was previously part of the water molecule, a charge appears, also equal to -1 e. Thus, two hydroxide ions are formed.
5. Calcium ions, now not bound by an ionic bond with oxide ions, go into solution and are hydrated:

The positive charge of calcium ions seems to be "smeared" over the entire hydrated ion.
6. The resulting hydroxide ions are also hydrated:

The negative charge of the hydroxide ion is also "washed out".
The overall ionic equation for the reaction of calcium oxide with water
CaO cr + H 2 O Ca 2 aq+ 2OH aq .

Calcium ions and hydroxide ions appear in the solution in a ratio of 1:2. The same would happen if calcium hydroxide was dissolved in water. Indeed, by evaporating the water and drying the residue, we can obtain crystalline calcium hydroxide from this solution (but by no means an oxide!). Therefore, the equation for this reaction is often written as follows:

CaO cr + H 2 O \u003d Ca (OH) 2p

and called " molecular"the equation of this reaction. In both equations, letter indices are sometimes not given, which often makes it very difficult to understand the ongoing processes, or even simply misleads. At the same time, the absence of letter indices in the equations is permissible, for example, when solving calculation problems
In addition to calcium oxide, the following oxides also interact with water: Li 2 O, Na 2 O, K 2 O, Rb 2 O, Cs 2 O, SrO, BaO - that is, oxides of those metals that themselves react with water. All these oxides are basic oxides. Other ionic oxides do not react with water.
Ionic hydrides, for example, sodium hydride NaH, react with water in exactly the same way. The sodium ion is only hydrated, and the hydride ion reacts with a water molecule:

As a result, sodium hydroxide remains in the solution.
The ionic equation for this reaction

NaH cr + H 2 O = Na aq+OH aq+H2,

and the "molecular" equation is NaH cr + H 2 O = NaOH p + H 2.

b) Substances with a metallic bond

As an example, consider the interaction of sodium with water.

In the diagrams, a half-arrow curve means the transfer or movement of one electron

The sodium atom tends to donate its only valence electron. Once in the water, it easily gives it to the hydrogen atom of the water molecule (there is a significant + on it) and turns into a sodium cation (Na). The hydrogen atom, having received an electron, becomes neutral (H · ) and can no longer hold a pair of electrons that binds it to an oxygen atom (remember the Pauli principle). This pair of electrons completely passes to the oxygen atom (in the water molecule it was already shifted towards it, but only partially). The oxygen atom acquires a formal charge A, the bond between the hydrogen and oxygen atoms breaks, and a hydroxide ion (О–Н) is formed.
The fate of the resulting particles is different: the sodium ion interacts with other water molecules and, naturally, is hydrated

just like the sodium ion, the hydroxide ion is hydrated, and the hydrogen atom, "waiting" for the appearance of another similar hydrogen atom, forms a hydrogen molecule 2H with it · \u003d H 2.
Due to the non-polarity of its molecules, hydrogen is practically insoluble in water and is released from solution in the form of a gas. The ionic equation for this reaction

2Na cr + 2H 2 O = 2Na aq+ 2OH aq+H2

a "molecular" –

2Na cr + 2H 2 O \u003d 2NaOH p + H 2

Just like sodium, Li, K, Rb, Cs, Ca, Sr, Ba react violently with water at room temperature. When heated, Mg reacts with it, as well as some other metals.

c) Substances with covalent bonds

Of the substances with covalent bonds with water, only those substances can react
a) the bonds in which are highly polar, which gives these substances some resemblance to ionic compounds, or
b) which include atoms that have a very high tendency to attach electrons.
Thus, they do not react with water and are insoluble in it (or very slightly soluble):
a) diamond, graphite, silicon, red phosphorus and other simple non-molecular substances;
b) silicon dioxide, silicon carbide and other complex non-molecular substances;
c) methane, heptane and other molecular substances with low polarity bonds;
d) hydrogen, sulfur, white phosphorus and other simple molecular substances, the atoms of which are not very inclined to accept electrons, as well as nitrogen, the molecules of which are very strong.
Of greatest importance is the interaction with water of molecular oxides, hydrides and hydroxides, and of simple substances - halogens.
How molecular oxides react with water, we will look at the example of sulfur trioxide:

At the expense of one of the lone pairs of electrons of the oxygen atom, the water molecule attacks the positively charged sulfur atom (+) and joins it with the O–S bond, and a formal charge B arises on the oxygen atom. Having received extra electrons, the sulfur atom ceases to hold an electron pair of one of -bonds, which completely passes to the corresponding oxygen atom, on which a formal charge A arises due to this. Then the lone pair of electrons of this oxygen atom is accepted by one of the hydrogen atoms that were part of the water molecule, which thus passes from one oxygen atom to another . As a result, a molecule of sulfuric acid is formed. Reaction equation:

SO 3 + H 2 O \u003d H 2 SO 4.

Similarly, but somewhat more difficultly, N 2 O 5 , P 4 O 10 and some other molecular oxides react with water. All of them are acid oxides.
N 2 O 5 + H 2 O \u003d 2HNO 3;
P 4 O 10 + 6H 2 O \u003d 4H 3 PO 4.

In all these reactions, acids are formed, which, in the presence of an excess of water, react with it. But, before considering the mechanism of these reactions, let's see how hydrogen chloride, a molecular substance with strongly polar covalent bonds between hydrogen and chlorine atoms, reacts with water:

A polar hydrogen chloride molecule, once in water, orients itself as shown in the diagram (opposite charges of dipoles attract). The rarefied electron shell due to polarization (1 s-EO) of a hydrogen atom accepts a lone pair sp 3-hybrid electrons of the oxygen atom, and hydrogen joins the water molecule, completely giving the chlorine atom a pair of electrons that bound these atoms in the hydrogen chloride molecule. As a result, the chlorine atom turns into a chloride ion, and the water molecule into an oxonium ion. Reaction equation:

HCl g + H 2 O \u003d H 3 O aq+Cl aq .

At low temperatures, crystalline oxonium chloride (H 3 O) Cl ( t pl = –15 °C).

The interaction of HCl and H 2 O can be imagined in another way:

that is, as a result of the transfer of a proton from a hydrogen chloride molecule to a water molecule. Therefore, it is an acid-base reaction.
Similarly, nitric acid interacts with water

which can also be represented as a proton transfer:

Acids, in the molecules of which there are several hydroxyls (OH-groups), react with water in several stages (stepwise). An example is sulfuric acid.

The second proton is split off much more difficult than the first, so the second stage of this process is reversible. By comparing the magnitude and distribution of charges in a sulfuric acid molecule and in a hydrosulfate ion, try to explain this phenomenon yourself.
Upon cooling, individual substances can be isolated from sulfuric acid solutions: (H 3 O) HSO 4 (t pl \u003d 8.5 ° С) and (H 3 O) 2 SO 4 (t pl \u003d - 40 ° С).
Anions formed from acid molecules after the abstraction of one or more protons are called acidic residues.
Of the simple molecular substances, only F 2 , Cl 2 , Br 2 and, to an extremely small extent, I 2 react with water under normal conditions. Fluorine reacts violently with water, completely oxidizing it:

2F 2 + H 2 O \u003d 2HF + OF 2.

Other reactions also take place.
Much more important is the reaction of chlorine with water. Possessing a high propensity to attach electrons (the molar energy of the electron affinity of the chlorine atom is 349 kJ/mol), chlorine atoms partially retain it in the molecule as well (the molar energy of the electron affinity of the chlorine molecule is 230 kJ/mol). Therefore, when dissolving, chlorine molecules are hydrated, attracting oxygen atoms of water molecules to themselves. At some of these oxygen atoms, chlorine atoms can accept a lone pair of electrons. The following is shown in the mechanism diagram:

The overall equation for this reaction

Cl 2 + 2H 2 O \u003d HClO + H 3 O + Cl.

But the reaction is reversible, so an equilibrium is established:

Cl 2 + 2H 2 O HClO + H 3 O + Cl.

The resulting solution is called "chlorine water". Due to the presence of hypochlorous acid in it, it has strong oxidizing properties and is used as a bleaching and disinfectant.
Remembering that Cl and H 3 O are formed during the interaction ("dissolution") of hydrogen chloride in water, we can write the "molecular" equation:

Cl 2 + H 2 O HClO p + HCl p.

Bromine reacts similarly with water, only the equilibrium in this case is strongly shifted to the left. Iodine practically does not react with water.

To imagine the extent to which chlorine and bromine physically dissolve in water, and to what extent they react with it, we use the quantitative characteristics of solubility and chemical equilibrium.

The molar fraction of chlorine in an aqueous solution saturated at 20 ° C and atmospheric pressure is 0.0018, that is, for every 1000 water molecules there are approximately 2 molecules of chlorine. For comparison, in a nitrogen solution saturated under the same conditions, the mole fraction of nitrogen is 0.000012, that is, one nitrogen molecule accounts for approximately 100,000 water molecules. And to obtain a solution of hydrogen chloride saturated under the same conditions, for every 100 molecules of water, you need to take about 35 molecules of hydrogen chloride. From this we can conclude that chlorine, although soluble in water, is insignificant. The solubility of bromine is slightly higher - about 4 molecules per 1000 molecules of water.

5. Give the reaction equations that make it possible to carry out the following transformations:

11.5. Crystal hydrates

With the chemical dissolution of ionic substances, hydration of the ions passing into the solution occurs. Both cations and anions are hydrated. As a rule, hydrated cations are stronger than anions, and hydrated simple cations are stronger than complex ones. This is due to the fact that simple cations have free valence orbitals, which can partially accept unshared electron pairs of oxygen atoms that make up water molecules.
When trying to isolate the initial substance from the solution by removing water, it often fails to obtain it. For example, if we dissolve colorless copper sulfate CuSO 4 in water, we get a blue solution, which is given to it by hydrated copper ions:

After evaporation of the solution (removal of water) and cooling, blue crystals will stand out from it, having the composition CuSO 4 5H 2 O (the point between the formulas of copper sulfate and water means that for each formula unit of copper sulfate there is the number of water molecules indicated in the formula). The original copper sulfate can be obtained from this compound by heating it to 250 ° C. In this case, the reaction occurs:

CuSO 4 5H 2 O \u003d CuSO 4 + 5H 2 O.

A study of the structure of CuSO 4 5H 2 O crystals showed that in its formula unit four water molecules are associated with a copper atom, and the fifth one with sulfate ions. Thus, the formula of this substance is SO 4 H 2 O, and it is called tetraaquacopper(II) sulfate monohydrate, or simply "copper sulfate".
Four water molecules bound to a copper atom are the remainder of the hydration shell of the Cu 2 ion aq, and the fifth water molecule is the remainder of the hydration shell of the sulfate ion.
A similar structure has the compound SO 4 H 2 O - hexaaqua iron sulfate monohydrate (II), or "iron vitriol".
Other examples:
Cl is hexaaquacalcium chloride;
Cl 2 - hexaaquamagnesium chloride.
These and similar substances are called crystalline hydrates, and the water they contain water of crystallization.
Often the structure of the crystalline hydrate is unknown, or it cannot be expressed by conventional formulas. In these cases, the "dotted formulas" mentioned above and simplified names are used for crystalline hydrates, for example:
CuSO 4 5H 2 O - copper sulfate pentahydrate;
Na 2 CO 3 10H 2 O - sodium carbonate decahydrate;
AlCl 3 6H 2 O - aluminum chloride hexahydrate.

When crystalline hydrates are formed from the starting materials and water, the O-H bonds do not break in water molecules.

If the water of crystallization is held in the crystal hydrate by weak intermolecular bonds, then it is easily removed when heated:
Na 2 CO 3 10H 2 O \u003d Na 2 CO 3 + 10H 2 O (at 120 ° C);
K 2 SO 3 2H 2 O \u003d K 2 SO 3 + 2H 2 O (at 200 ° C);
CaCl 2 6H 2 O \u003d CaCl 2 + 6H 2 O (at 250 ° C).

If, in a crystalline hydrate, the bonds between water molecules and other particles are close to chemical, then such a crystalline hydrate either dehydrates (loses water) at a higher temperature, for example:
Al 2 (SO 4) 3 18H 2 O \u003d Al 2 (SO 4) 3 + 18H 2 O (at 420 ° C);
СoSO 4 7H 2 O \u003d CoSO 4 + 7H 2 O (at 410 ° C);

or, when heated, decomposes to form other chemicals, such as:
2 (FeCl 3 6H 2 O) \u003d Fe 2 O 3 + 6HCl + 9H 2 O (above 250 ° C);
2 (AlCl 3 6H 2 O) \u003d Al 2 O 3 + 6HCl + 9H 2 O (200 - 450 ° C).

Thus, the interaction of anhydrous substances forming crystalline hydrates with water can be either a chemical dissolution or a chemical reaction.

CRYSTAL HYDRATES
Determine the mass fraction of water in a) copper sulfate pentahydrate, b) sodium hydroxide dihydrate, c) KAl (SO 4) 2 12H 2 O (potassium alum).
2. Determine the composition of magnesium sulfate crystalline hydrate if the mass fraction of water in it is 51.2%. 3. What is the mass of water released during the calcination of sodium sulfate decahydrate (Na 2 SO 4 10H 2 O) weighing 644 g?
4. How much anhydrous calcium chloride can be obtained by calcining 329 g of calcium chloride hexahydrate?
5. Calcium sulfate dihydrate CaSO 4 2H 2 O loses 3/4 of its water when heated to 150 ° C. Make a formula for the resulting crystalline hydrate (alabaster) and write down the equation for the transformation of gypsum into alabaster.
6. Determine the mass of copper sulfate and water that must be taken to prepare 10 kg of a 5% copper sulfate solution.
7. Determine the mass fraction of iron (II) sulfate in the solution obtained by mixing 100 g of ferrous sulfate (FeSO 4 7H 2 O) with 9900 g of water.
Obtaining and decomposition of crystalline hydrates.