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 have highest value 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. 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 single 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 are part of 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 blue vitriol and water, which must be taken to prepare 10 kg of a 5% solution of copper sulfate.
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.

Water- the most widespread compound on our planet. It covers 4/5 of the entire surface of the Earth. This is the only unique compound that can be in 3 different states of aggregation: ice, water and steam.

Water plays an important role not only in industry, but also in the life of every person. It is known that without food a person can live a month, but without water he will not live even a week.

Pure water does not exist in nature, there are always impurities. To clean these contaminants, the process of distillation, distillation is used, so you can often find the phrase "distilled water",

Water does not have a smell, color and taste.

Chemical properties of water.

Water is a chemical compound. The bond is covalent.

Water serves as an excellent solvent for many substances due to its significant dipole moment. The process of dissolution is called hydration, and those substances that enter into hydration reactions are most often electrolytes (conduct electric current).

1. Acid-base reactions. Water is amphoteric, so it can react with acids and bases:

BaO + H 2 O \u003d Ba (OH) 2,

N 2 O 5 + H 2 O \u003d 2HNO 3.

2. Water reacts with almost all salts, forming hydrates:

CaCl 2 + 6 H 2 O = CaCl2 6H2 O.

3. Water oxidizes metals in the voltage series to tin. with alkali metals ( Na, Li, K) react violently:

2 K + H 2 O =2 KOH + H 2 .

With less active metals, water reacts when heated:

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

Water (hydrogen oxide)- a chemical substance in the form of a transparent liquid that has no color (in a small volume), odor and taste. Chemical formula: H 2 O. In the solid state it is called ice, snow or hoarfrost, and in the gaseous state it is called water vapor. About 71% of the Earth's surface is covered with water (oceans, seas, lakes, rivers, ice). Under natural conditions, it always contains dissolved substances (salts, gases).

It is of key importance in the creation and maintenance of life on Earth, in the chemical structure of living organisms, in the formation of climate and weather. It is the most important nutrient for all living beings on planet Earth.

Physical Properties

Under normal atmospheric conditions, it retains a liquid state of aggregation, while similar hydrogen compounds are gases. This is due to the special characteristics of the constituent atoms of the molecule and the presence of bonds between them. The hydrogen atoms are attached to the oxygen atom at an angle of 104.45°, and this configuration is strictly conserved. Due to the large difference in the electronegativity of hydrogen and oxygen atoms, electron clouds are strongly shifted towards oxygen. For this reason, the water molecule is an active dipole, where the oxygen side is negative and the hydrogen side is positive. As a result, water molecules are attracted by their opposite poles, and form polar bonds, which require a lot of energy to break. As part of each molecule, the hydrogen ion (proton) has no internal electron layers and is small in size, as a result of which it can penetrate into the electron shell of a negatively polarized oxygen atom of a neighboring molecule, forming a hydrogen bond with another molecule. Each molecule is connected to four others by hydrogen bonds - two of them form an oxygen atom and two hydrogen atoms. The combination of these bonds between water molecules - polar and hydrogen - determines its very high boiling point and specific heat of vaporization. As a result of these connections, a pressure of 15-20 thousand atmospheres arises in the aquatic environment, which explains the reason for the difficulty of compressing water, so with an increase in atmospheric pressure by 1 bar, water is compressed by 0.00005 of its initial volume.

Water also has the highest surface tension among liquids, second only to mercury. The relatively high viscosity of water is due to the fact that hydrogen bonds prevent water molecules from moving at different speeds.

For similar reasons, water is a good solvent for polar substances. Each solute molecule is surrounded by water molecules, and the positively charged parts of the solute molecule attract oxygen atoms, and the negatively charged parts attract hydrogen atoms. Because the water molecule is small, many water molecules can surround each solute molecule. This property of water is used by living beings. In a living cell and in the intercellular space, solutions of various substances in water interact. Water is necessary for the life of all unicellular and multicellular living beings on Earth without exception.

Chemical properties

Water is a chemically quite active substance. Strongly polar water molecules solvate ions and molecules, form hydrates and crystalline hydrates. Solvolysis, and in particular hydrolysis, occurs in animate and inanimate nature, and is widely used in the chemical industry.

Water reacts at room temperature:

• with active metals (sodium, potassium, calcium, barium, etc.);
• with halogens (fluorine, chlorine) and interhalogen compounds;
• with salts formed by a weak acid and a weak base, causing their complete hydrolysis;
• with anhydrides and halides of carboxylic and inorganic acids;
• with active organometallic compounds (diethylzinc, Grignard reagents, methyl sodium, etc.);
• with carbides, nitrides, phosphides, silicides, hydrides of active metals (calcium, sodium, lithium, etc.);
• with many salts, forming hydrates;
• with boranes, silanes;
• with ketenes, carbon suboxide;
• with noble gas fluorides.

Water reacts when heated:

• with iron, magnesium;
• with coal, methane;
• with some alkyl halides.

Water reacts in the presence of a catalyst:

• with amides, esters of carboxylic acids;
• with acetylene and other alkynes;
• with alkenes;
• with nitriles.

Water and sports

Athletes need to drink fluids, but exactly how much water should they consume?

The amount of water or other fluids you need before, during, and after exercise depends largely on the intensity and duration of the exercise. But there are other factors as well, such as air temperature, humidity, altitude, and even your own physiology. All of these can affect how much water you need during your workout.

How much water should be consumed daily?

If you exercise regularly, then you will probably need to drink half to a full ounce of water (or other liquid) for every pound of body weight per day.

To determine the base water demand range, use the following formula:

Low end of range = body weight (kg) x 0.5 = (fluid ounces/day)
Upper range limit = body weight (kg) x 1 = (fluid ounces/day)

When to drink water while exercising?

Start your day with a big glass of water every morning, whether you're going to be exercising or relaxing. During training days, the following schedule applies, which is effective for most athletes:

1. Before exercise
   Drink two to three cups of water two hours before your workout. Weigh yourself just before you start exercising.
2. During a workout
   Drink one cup of water every 15 minutes.
3. After exercise
   Weigh yourself immediately after your workout.
   Drink two to three cups of water for every pound of body weight you lose during exercise.

How much water should be consumed during strength training?

If your workout is longer than 90 minutes at moderate to high intensity, you need to consume more than just water. You need to replenish your glycogen stores with simple carbohydrates. Sports drinks are the most in a simple way obtaining the required energy. For longer workouts, choose drinks between 60 and 100 calories per eight ounces and consume eight to ten grams every 15 to 30 minutes.

For those who are in extreme conditions electrolytes will need to be replaced within three, four, or five hours. Complex sports drinks and specialty foods will help provide your body with the calories and electrolytes it needs to keep going.

• On average, the body of plants and animals contains more than 50% water.
• The composition of the Earth's mantle contains 10-12 times more water than the amount of water in the oceans.
• With an average depth of 3.6 km, the World Ocean covers about 71% of the planet's surface and contains 97.6% of the world's known free water reserves.
• If there were no depressions and bulges on the Earth, water would cover the entire Earth, and its thickness would be 3 km.
• If all the glaciers melted, then the water level on Earth would rise by 64 m and about 1/8 of the land surface would be flooded with water.
• Sea water, with its usual salinity of 35 ‰, freezes at a temperature of −1.91 ° ​​C.
• Sometimes water freezes at a positive temperature.
• Under certain conditions (inside nanotubes), water molecules form a new state in which they retain the ability to flow even at temperatures close to absolute zero.
• Water reflects 5% of the sun's rays, while snow reflects about 85%. Only 2% of sunlight penetrates under the ocean ice.
• The blue color of clear ocean water is due to the selective absorption and scattering of light in the water.
• With the help of drops of water from taps, you can create a voltage of up to 10 kilovolts, the experiment is called the "Kelvin Dropper".
• There is the following saying using the formula of water - H2O: "My boots of that - pass H2O". Instead of boots, other holey shoes may also be involved in the saying.
• Water is one of the few substances in nature that expand during the transition from a liquid phase to a solid one (in addition to water, bismuth, gallium, germanium and some compounds and mixtures have this property).
• Water and water vapor burn in an atmosphere of fluorine. Mixtures of water vapor with fluorine within explosive concentrations are explosive. As a result of this reaction, hydrogen fluoride and elemental oxygen are formed.

REMEMBER!!!

alkali metals - this is group I, A - the main subgroup - Li, Na, K , rb, Cs , Fr

alkaline earth metals - this is group II, A - the main subgroup (Be, Mg do not belong) - Ca, sr, Ba, Ra

n I

Foundations Me(OH) n

OH - hydroxyl group, with valence (I)

alkalis are water-soluble bases (see SOLUBILITY TABLE)

I n

acids are complex substances general formula H n (KO)

(KO) - acid residue

V-VII

acid oxide – non-Me x O y and Me x O y

I, II

Basic oxides – Me x O y

I. Interaction of water with metals.

Depending on the activity of the metal, the reaction proceeds at various conditions and different products are formed.

1). Interaction with the most active metals standing in the periodic system in I A and I I A groups (alkali and alkaline earth metals) and aluminum . In the activity series, these metals are located up to aluminum (inclusive)

The reaction proceeds under normal conditions, with the formation of alkali and hydrogen.

I I

2Li + 2 H 2 O \u003d 2 Li OH + H 2

HOH hydroxide

lithium

I II

Ba + 2 H 2 O \u003d Ba (OH) 2 + H 2

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

hydroxide

aluminum

OH is a hydroxo group, it is always monovalent

CONCLUSION - active metals - Li, Na, K , rb, Cs , fr, Ca, sr, Ba, Ra+ Al - react like this

Me + H 2 O \u003d Me (OH) n + H 2( R. substitution) Base

2) Interaction with less active metals, which are located in the activity series from aluminum to hydrogen.

The reaction proceeds only with vaporous water, i.e. when heated.

In this case, an oxide of this metal and hydrogen are formed.

I II I

Fe + H 2 O \u003d FeO + H 2 (a substitution reaction takes place)

oxide

gland

Ni + H 2 O \u003d NiO + H 2

(The valence of a metal can be easily determined by the activity series of metals, there is a value above their symbol, for example +2, which means that the valency of this metal is 2).

CONCLUSION - metals of medium activity, standing in the series of activity up to (Н 2) - be, mg, Fe, Pb, cr, Ni, Mn, Zn - react like this

3) Metals in the activity series after hydrogen do not react with water.

Cu + H 2 O = no reaction

I I. Interaction with oxides (basic and acidic)

Only those oxides interact with water, which, when interacting with water, give a water-soluble product (acid or alkali).

one). Interaction with basic oxides.

Only basic oxides of active metals interact with water, which are located in groups I A and I I A, except for Be and Mg (alumina does not react, because it is amphoteric). The reaction proceeds under normal conditions, and only alkali is formed.

I II

Na 2 O + H 2 O \u003d 2 NaOHBaO + H 2 O \u003d Ba (OH) 2 (compound reaction proceeds)

2) Interaction of acid oxides with water.

All acidic oxides react with water. The only exception is SiO 2 .

This produces acids. In all acids, hydrogen is in the first place, so the reaction equation is written as follows:

SO 3 + H 2 O \u003d H 2 SO 4 P 2 O 5 + H 2 O \u003d 2 HPO 3

SO 3 cold

+H2O P2O5

H2SO4 + H2O

H2P2O6

P 2 O 5 +3 H 2 O \u003d 2 H 3 PO 4

hot

P2O5

+ H 6 O 3

H 6 P 2 O 8

note that, depending on the temperature of the water, when interacting with P 2 O 5, different products are formed.

IVWater interaction cnon-metals

Examples: Cl 2 + H 2 O \u003d HCl + HClO

C + H 2 O \u003d CO + H 2

coal gas

Si + 2H 2 O \u003d SiO 2 + 2H 2.

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 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 there is a heterolytic break of the polar O-N connections, 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:

AT clean water[H +] \u003d [OH -] \u003d 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 influence 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. The 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 strong ties 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