Sulfurous acid - unstable dibasic inorganic acid of medium strength. Corresponds to the oxidation state of sulfur +4. Chemical formula $\backslash mathsf(H\_2SO\_3)$.

Chemical properties

Acid medium strength:

$\backslash mathsf(H\_2SO\_3\; \backslash rightleftarrows\; H^+\; +\; HSO\_3^-)$ $\backslash mathsf(HSO\_3^-\; \backslash rightleftarrows\; H^+\; +\; SO\_3^(2-))$

Exists only in dilute aqueous solutions (not isolated in a free state):

$\backslash mathsf(SO\_2+H\_2O\; \backslash rightleftarrows\; H\_2SO\_3\; \backslash rightleftarrows\; H^++HSO\_3^-\; \backslash rightleftarrows\; 2H^+\; +\; SO\_3^(2-))$

Solutions of H 2 SO 3 always have a sharp, specific odor of SO 2 not chemically bound with water.

$\backslash mathsf(H\_2SO\_3\; +\; NaOH\; \backslash longrightarrow\; NaHSO\_3\; +\; H\_2O)$ $\backslash mathsf(H\_2SO\_3\; +\; 2NaOH\; \backslash longrightarrow\; Na\_2SO\_3\; +\; 2H\_2O)$

Like sulfur dioxide, sulfurous acid and its salts are strong reducing agents:

$\backslash mathsf(H\_2SO\_3\; +\; Br\_2\; +\; H\_2O\; \backslash longrightarrow\; H\_2SO\_4\; +\; 2HBr)$

When interacting with even stronger reducing agents, it can play the role of an oxidizing agent:

$\backslash mathsf(H\_2SO\_3\; +\; 2H\_2S\; \backslash longrightarrow\; 3S\; \backslash downarrow\; +\; 3H\_2O)$

Qualitative reaction to sulfite ions - discoloration of a solution of potassium permanganate:

$\backslash mathsf(5SO\_3^(2-)\; +\; 6H^(+)\; +\; 2MnO\_4^(-)\; \backslash longrightarrow\; 5SO\_4^(2-)\; +\; 2Mn^(2+)\; +\; 3H\_2O)$

Application

Sulfurous acid and its salts are used as reducing agents for bleaching wool, silk and other materials that cannot withstand bleaching with strong oxidizing agents (chlorine). Sulfurous acid is used in canning fruits and vegetables. Calcium hydrosulfite (sulfite liquor, Ca(HSO 3) 2) is used to process wood into so-called sulfite cellulose (a solution of calcium hydrosulfite dissolves lignin, a substance that binds cellulose fibers, as a result of which the fibers are separated from each other; wood treated in this way is used for receipt of paper).

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Literature

• Chemical Encyclopedia / Editorial Board: Knunyants I.L. and others. - M.: Soviet encyclopedia, 1995. - T. 4 (Pol-Three). - 639 p. - ISBN 5-82270-092-4.

Excerpt characterizing sulfurous acid

- Now, now. -Are you ready, mom?
- Just pin the current.
“Don’t do it without me,” Natasha shouted, “you won’t be able to!”
- Yes, ten.
It was decided to be at the ball at half past ten, and Natasha still had to get dressed and stop by the Tauride Garden.
Having finished her hair, Natasha, in a short skirt, from which her ballroom shoes were visible, and in her mother’s blouse, ran up to Sonya, examined her and then ran to her mother. Turning her head, she pinned the current, and, barely having time to kiss her gray hair, again ran to the girls who were hemming her skirt.
The issue was Natasha's skirt, which was too long; Two girls were hemming it, hastily biting the threads. The third, with pins in her lips and teeth, ran from the Countess to Sonya; the fourth held her entire smoky dress on her raised hand.
- Mavrusha, rather, my dear!
- Give me a thimble from there, young lady.
- Soon, finally? - said the count, entering from behind the door. - Here's some perfume for you. Peronskaya is already tired of waiting.
“It’s ready, young lady,” said the maid, lifting the hemmed smoky dress with two fingers and blowing and shaking something, expressing with this gesture an awareness of the airiness and purity of what she was holding.
Natasha began to put on her dress.
“Now, now, don’t go, dad,” she shouted to her father, who opened the door, still from under the haze of her skirt, which covered her entire face. Sonya slammed the door. A minute later the count was let in. He was in a blue tailcoat, stockings and shoes, perfumed and oiled.
- Oh, dad, you are so good, dear! – Natasha said, standing in the middle of the room and straightening the folds of the haze.
“Excuse me, young lady, allow me,” said the girl, standing on her knees, pulling off her dress and turning the pins from one side of her mouth to the other with her tongue.
- Your will! – Sonya cried out with despair in her voice, looking at Natasha’s dress, “your will, it’s long again!”
Natasha moved away to look around in the dressing table. The dress was long.
“By God, madam, nothing is long,” said Mavrusha, crawling on the floor behind the young lady.
“Well, it’s long, so we’ll sweep it up, we’ll sweep it up in a minute,” said the determined Dunyasha, taking out a needle from the handkerchief on her chest and getting back to work on the floor.
At this time, the countess entered shyly, with quiet steps, in her current and velvet dress.
- Ooh! my beauty! - the count shouted, - better than all of you!... - He wanted to hug her, but she pulled away, blushing, so as not to crumple.
“Mom, more on the side of the current,” Natasha said. “I’ll cut it,” and she rushed forward, and the girls who were hemming, did not have time to rush after her, tore off a piece of smoke.
- My God! What is this? It's not my fault...
“I’ll sweep it all away, it won’t be visible,” Dunyasha said.
- Beauty, it’s mine! - said the nanny who came in from behind the door. - And Sonyushka, what a beauty!...
At a quarter past ten they finally got into the carriages and drove off. But we still had to stop by the Tauride Garden.
Peronskaya was already ready. Despite her old age and ugliness, she did exactly the same thing as the Rostovs, although not with such haste (this was a common thing for her), but her old, ugly body was also perfumed, washed, powdered, and the ears were also carefully washed , and even, and just like the Rostovs, the old maid enthusiastically admired her mistress’s outfit when she yellow dress With a code she went out into the living room. Peronskaya praised the Rostovs' toilets.
The Rostovs praised her taste and dress, and, taking care of her hair and dresses, at eleven o'clock they settled into their carriages and drove off.

Since the morning of that day, Natasha had not had a minute of freedom, and not once had time to think about what lay ahead of her.
In the damp, cold air, in the cramped and incomplete darkness of the swaying carriage, for the first time she vividly imagined what awaited her there, at the ball, in the illuminated halls - music, flowers, dancing, the sovereign, all the brilliant youth of St. Petersburg. What awaited her was so beautiful that she did not even believe that it would happen: it was so incongruous with the impression of the cold, crampedness and darkness of the carriage. She understood everything that awaited her only when, having walked along the red cloth of the entrance, she entered the entryway, took off her fur coat and walked next to Sonya in front of her mother between the flowers along the illuminated stairs. Only then did she remember how she had to behave at the ball and tried to adopt the majestic manner that she considered necessary for a girl at the ball. But fortunately for her, she felt that her eyes were running wild: she saw nothing clearly, her pulse beat a hundred times a minute, and the blood began to pound at her heart. She could not accept the manner that would make her funny, and she walked, frozen with excitement and trying with all her might to hide it. And this was the very manner that suited her most of all. In front and behind them, talking just as quietly and also in ball gowns, guests entered. The mirrors on the stairs reflected ladies in white, blue, pink dresses, with diamonds and pearls on open hands and necks.

Sulfur compounds (1U). Sulfurous acid

In tetrahalides SHal 4, oxohalides SOI Ial 2 and dioxide S0 2, sulfurous acid 1I 2 S0 3, sulfur exhibits an oxidation state of +4. In all these compounds, as well as in their corresponding anionic complexes, the sulfur atom has an unshared pair of electrons. Based on the number of a-bonding and non-bonding electrons, the shape of the molecules of these compounds changes from a distorted tetrahedron (SHal 4) to an angular shape (S0 9) through a trigonal pyramid shape (SOHal 2 and SO3). S(IV) compounds have acidic properties, which manifests themselves in reactions with water:

Sulfur oxide(1U) S0 2, or sulfur dioxide, is formed by burning sulfur in air or oxygen, as well as by calcining sulfides, such as pyrite:

Pyrite oxidation underlies the industrial method for producing S0 2. The S0 2 molecule is built similarly to the Oe molecule and has the structure isosceles triangle with a sulfur atom at the top. Length S-O connection is 0.143 nm, and the bond angle is 119.5°:

The sulfur atom is in the 5/? 2-hybridization. The p-orbital is oriented perpendicular to the plane of the molecule and is not involved in hybridization (Fig. 25.2). Due to this and other similarly oriented p-orbitals of oxygen atoms, a three-center n-bond is formed.

Rice. 25.2.

Under normal conditions, sulfur oxide (1U) is a colorless gas with a characteristic pungent odor. Let's dissolve well in water. Aqueous solutions have an acidic reaction, since S0 2, interacting with water, forms sulfurous acid H 2 S0 3. The reaction is reversible:

A characteristic feature of S0 2 is its redox duality. This is explained by the fact that in SO. ; sulfur has an oxidation state of +4, and therefore it can, by donating two electrons, be oxidized to S(VI), and by receiving four electrons, reduced to S. The manifestation of these and other properties depends on the nature of the reacting component. Thus, with strong oxidizing agents, S0 2 behaves as a typical reducing agent. For example, halogens are reduced to the corresponding hydrogen halides, and S(IV) usually transforms into S(VI):

In the presence of strong reducing agents, S0 2 behaves as an oxidizing agent:

It is also characterized by a disproportionation reaction:

SQ is an acidic oxide, easily soluble in water (1 volume of H 2 0 dissolves 40 volumes of S0 2). An aqueous solution of SOv is acidic and is called sulfurous acid. Typically, the bulk of S0 2 dissolved in water is in solution in the hydrated form of S0 2 azH 2 0, and only a small part of S0 2 interacts with water according to the scheme

Sulfurous acid, as a dibasic acid, forms two types of salts: medium - sulfites (Na 2 S0 3) and acidic - hydrosulfites (NaHS0 3). H 2 S0 3 exists in two tautomeric forms (Fig. 25.3).

Rice. 25.3.Structure of tautomeric forms of H 2 S0 3

Since sulfur in sulfurous acid has an oxidation state of +4, it exhibits, like S0 2, the properties of both an oxidizing agent and a reducing agent, as already mentioned, therefore sulfurous acid in oxidation-reduction reactions completely duplicates the properties of S0 9.

Salts H 2 S0 3 (sulfites) have the properties of both oxidizing and reducing agents. Thus, the SO 2 ion easily transforms into the SO 2 ion, exhibiting strong reducing properties, therefore, in solutions, sulfites are gradually oxidized by molecular oxygen, turning into sulfuric acid salts:

In the presence of strong reducing agents, sulfites behave as oxidizing agents. With strong heating, sulfites of the most active metals decompose at 600°C to form salts H 2 SO^ and H 2 S, i.e. disproportion occurs:

Of the salts of sulfurous acid, only salts of 5-elements of group I are dissolved, as well as hydrosulfites of the Me 2+ (HS0 3) 2 type.

Since H 2 S0 3 is a weak acid, when acids act on sulfites and hydrosulfites, S0 2 is released, which is usually used to obtain S0 2 in the laboratory:

Water-soluble sulfites easily undergo hydrolysis, as a result of which the concentration of OH ions in the solution increases:

When S0 2 is passed through aqueous solutions of hydrosulfites, pyrosulfites are formed:

If a solution of Na 2 S0 3 is boiled with sulfur powder, then sodium thiosulfate is formed. In thiosulfates, sulfur atoms are located in two different degrees oxidation - +6 and -2:

The resulting thiosulfate ion corresponds to the acid H 2 S 2 0 3, called thiosulfuric acid. Free acid under normal conditions it is unstable and easily decomposes:

The properties of thiosulfates are due to the presence of and in them, and

the presence of S determines the reducing properties of the S 2 0 3 _ ion:

Weaker oxidizing agents oxidize sodium thiosulfate to tetrathionic acid salts. An example is the interaction with iodine:

This reaction finds wide application V analytical chemistry, since it is the basis of one of the most important methods of volumetric analysis, called iodometry.

Alkali metal thiosulfates are produced industrially on a large scale. Among them highest value has sodium thiosulfate Na 2 S 2 0 3, which is used in medicine as an antidote for poisoning with halogens and cyanides. The action of this drug is based on its property of releasing sulfur, which, for example, with cyanide ions CN forms the less toxic thiocyanate ion SCN:

The drug can also be used for poisoning with compounds As, Pb, Hg, since non-toxic sulfides are formed. Na 2 S 2 0 3 is used when allergic diseases, arthritis, neuralgia. A characteristic reaction for Na 2 S 2 0 3 is its interaction with AgN0 3: a precipitate is formed white Ag. ; S.; 0 3, which over time under the influence of light and moisture turns black with the release of Ag 2 S:

These reactions are used for the qualitative detection of thiosulfate ion.

Thionyl chloride SOCl 2 is obtained by reacting S0 2 with PC1 5:

The SOCl 2 molecule has a pyramidal structure (Fig. 25.4). Bonds with sulfur are formed due to a set of orbitals, which can be very approximately considered as $/? 3-hybrid. One of them is occupied by a lone pair of electrons, so SOCl 2 can exhibit the properties of a weak Lewis base.

Rice. 25.4.

S()C1 2 - colorless, fuming liquid with a pungent odor, hydrolyzes in the presence of traces of moisture:

Volatile compounds formed during the reaction are easily removed. Therefore, SOCl 2 is often used to obtain anhydrous chlorides:

SOCl 2 is widely used as a chlorinating agent in organic syntheses.

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Educating:

Create conditions for the moral and aesthetic education of students towards the environment, the ability to work in pairs during self-analysis of control sections and tests.

Developmental:

develop the ability to work in an atmosphere of search, creativity, to give every student the opportunity to achieve success; the ability to self-assess activities in the classroom;

General education:

organize students’ activities to learn:

• knowledge
: chemical properties and methods for producing sulfur dioxide and sulfurous acid;
• skills
: write down equations chemical reactions, characterizing the chemical properties of sulfurous acid and its salts in ionic and redox form.

Lesson progress

I. Organizational moment.

II. Learning new material:

1. Structure:

SO 2 (sulfur dioxide, sulfur oxide (IV)), molecular formula

Structural formula

2. Physical properties

1. A colorless gas with a pungent odor, poisonous.
2. Highly soluble in water (40 V SO 2 dissolves in 1 V H 2 O at standard conditions)
3. Heavier than air, poisonous.

3. Receipt

1. In industry: roasting of sulfides.

FeS 2 + O 2 → Fe 2 O 3 + SO 2

a) Compile an electronic balance sheet (EBR).

2. In laboratory conditions: interaction of sulfites with strong acids:

Na 2 SO 3 + 2HCl → 2NaCl + SO 2 + H 2 O

3. When oxidizing metals with concentrated sulfuric acid:

Cu + H 2 SO 4 (conc) → CuSO 4 + SO 2 + H 2 O

b) Compile an electronic balance (EB) .

4. Chemical properties of SO 2

1. Interaction with water

When dissolved in water, a weak and unstable sulfurous acid H 2 SO 3 is formed (exists only in aqueous solution).

SO 2 + H 2 O ↔ H 2 SO 3

2. Interaction with alkalis:

Ba(OH) 2 + SO 2 → BaSO 3 ↓(barium sulfite) + H 2 O

Ba(OH) 2 + 2SO 2 (excess) → Ba(HSO 3) 2 (barium hydrosulfite)

3. Interaction with basic oxides(salt is formed):

SO 2 + CaO = CaSO 3

4. Oxidation reactions, SO 2 – reducing agent:

SO 2 + O 2 → SO 3 (catalyst – V 2 O 5)

c) Compile an electronic balance (EB)

SO 2 + Br 2 + H 2 O → H 2 SO 4 + HBr

d) Compile an electronic balance (EB)

SO 2 + KMnO 4 + H 2 O → K 2 SO 4 + MnSO 4 + H 2 SO 4

e) Compile an electronic balance (EB)

5. Reduction reactions, SO 2 - oxidizing agent

SO 2 + C → S + CO 2 (when heated)

f) Compile an electronic balance (EB)

SO 2 + H 2 S → S + H 2 O

g) Compile an electronic balance (EB)

5. Chemical properties of H 2 SO 3

1. Sulfurous acid dissociates stepwise:

H 2 SO 3 ↔ H + + HSO 3 - (first step, hydrosulfite anion is formed)

HSO 3 - ↔ H+ + SO 3 2- (second stage, sulfite anion is formed)

H 2 SO 3 forms two series of salts:

Medium (sulfites)

Acidic (hydrosulfites)

2. A solution of sulfurous acid H 2 SO 3 has reducing properties:

H 2 SO 3 + I 2 + H 2 O = H 2 SO 4 + HI

h) Create an electronic balance (EB)

III. Self-control.

Carry out transformations according to the scheme:

S → H 2 S → SO 2 → Na 2 SO 3 → BaSO 3 → SO 2

Write the equations for ion exchange reactions in full and short ionic form.

Self-test answers are displayed on the screen.

IV. Reflection.

Answer the questions in the table “Questions for the student” (Appendix 1).

V. Homework(differentiated)

Complete the tasks highlighted in red:

Equations a, c, f, g – “3”

Equations a – e – “4”

Equations a – h – “5”

Appendix 1

Questions for the student

Date ___________________ Class ______________________

Try to remember exactly what you heard in class and answer the questions asked:

No.	Questions
1	What was the topic of the lesson?
2	What was your goal during the lesson?
3	What is the conclusion of the lesson?
4	How did your classmates work in class?
5	How did you work in class?
6	Do you think you can cope with the homework you received in class?

Sulfur dioxide is formed when sulfur is burned in air or oxygen. It is also obtained by calcining metal sulfides, such as iron pyrites, in air (“burning”):

By this reaction, sulfur dioxide is usually obtained industrially (for other industrial methods of production, see 9 § 131).

Sulfur dioxide is a colorless gas (“sulfur dioxide”) with a strong odor of hot sulfur. It condenses quite easily into colorless liquid, boiling at . When a liquid evaporates, a strong decrease in temperature occurs (to ).

Sulfur dioxide is highly soluble in water (about 40 volumes in 1 volume of water at ); in this case, a partial reaction with water occurs and sulfurous acid is formed:

Thus, sulfur dioxide is an anhydride of sulfurous acid. When heated, solubility decreases and the equilibrium shifts to the left; gradually all the sulfur dioxide is released from the solution again.

The molecule is constructed similarly to the ozone molecule. The nuclei of its constituent atoms form an isosceles triangle:

Here the sulfur atom, like the central oxygen atom in the ozone molecule, is in a state of -hybridization and the angle is close to . The -orbital of the sulfur atom, oriented perpendicular to the plane of the molecule, does not participate in hybridization. Due to this orbital and similarly oriented -orbitals of oxygen atoms, a three-center -bond is formed; the pair of electrons that carry it out belongs to all three atoms of the molecule.

Sulfur dioxide is used to produce sulfuric acid, and also (in much smaller quantities) for bleaching straw, wool, silk, etc. disinfectant(for the destruction of mold fungi in basements, cellars, wine barrels, fermentation vats).

Sulfurous acid is a very fragile compound. It is known only in aqueous solutions. When trying to separate sulfurous acid, it breaks down into water. For example, when concentrated sulfuric acid acts on sodium sulfite, sulfur dioxide is released instead of sulfurous acid:

The sulfurous acid solution must be protected from access to air, otherwise it, absorbing oxygen from the air, slowly oxidizes into sulfuric acid:

Sulfurous acid is a good reducing agent. For example, free halogens are reduced by it into hydrogen halides:

However, when interacting with strong reducing agents, sulfurous acid can play the role of an oxidizing agent. So, its reaction with hydrogen sulfide mainly proceeds according to the equation:

Being dibasic, sulfurous acid forms two series of salts. Its average salts are called sulfites, acidic ones - hydrosulfites.

Like acid, sulfites and hydrosulfites are reducing agents. When they are oxidized, salts of sulfuric acid are obtained.

When calcined, sulfites of the most active metals decompose to form sulfides and sulfates (self-oxidation - self-healing reaction):

Potassium and sodium sulfites are used for bleaching certain materials, in the textile industry for dyeing fabrics, and in photography. The solution (this salt exists only in solution) is used to process wood into so-called sulfite pulp, from which paper is then obtained.

Undiluted sulfuric acid is a covalent compound.

In the molecule, sulfuric acid is tetrahedrally surrounded by four oxygen atoms, two of which are part of the hydroxyl groups. The S–O bonds are double, and the S–OH bonds are single.

The colorless, ice-like crystals have a layered structure: each H 2 SO 4 molecule is connected to four neighboring strong hydrogen bonds, forming a single spatial framework.

The structure of liquid sulfuric acid is similar to the structure of solid one, only the integrity of the spatial framework is broken.

Physical properties of sulfuric acid

Under normal conditions, sulfuric acid is a heavy, oily liquid without color or odor. In technology, sulfuric acid is a mixture of both water and sulfuric anhydride. If the molar ratio of SO 3: H 2 O is less than 1, then it is an aqueous solution of sulfuric acid; if it is greater than 1, it is a solution of SO 3 in sulfuric acid.

100% H 2 SO 4 crystallizes at 10.45 ° C; T kip = 296.2 °C; density 1.98 g/cm3. H 2 SO 4 mixes with H 2 O and SO 3 in any ratio to form hydrates; the heat of hydration is so high that the mixture can boil, splash and cause burns. Therefore, it is necessary to add acid to water, and not vice versa, since when water is added to acid, lighter water will end up on the surface of the acid, where all the heat generated will be concentrated.

When aqueous solutions of sulfuric acid containing up to 70% H 2 SO 4 are heated and boiled, only water vapor is released into the vapor phase. Sulfuric acid vapor also appears above more concentrated solutions.

In terms of structural features and anomalies, liquid sulfuric acid is similar to water. There is the same system of hydrogen bonds, almost the same spatial framework.

Chemical properties of sulfuric acid

Sulfuric acid– one of the strongest mineral acids; due to its high polarity, the H–O bond is easily broken.

Sulfuric acid dissociates in aqueous solution , forming a hydrogen ion and an acidic residue:

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

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

Summary equation:

H 2 SO 4 = 2H + + SO 4 2- .

Shows properties of acids , reacts with metals, metal oxides, bases and salts.

Dilute sulfuric acid does not exhibit oxidizing properties; when it interacts with metals, hydrogen and a salt containing the metal are released. lowest degree oxidation. In the cold, the acid is inert towards metals such as iron, aluminum and even barium.

Concentrated acid has oxidizing properties. Possible interaction products simple substances with concentrated sulfuric acid are given in the table. The dependence of the reduction product on the acid concentration and the degree of activity of the metal is shown: the more active the metal, the more deeply it reduces the sulfate ion of sulfuric acid.

Interaction with oxides:

CaO + H 2 SO 4 = CaSO 4 = H 2 O.

Interaction with bases:

2NaOH + H 2 SO 4 = Na 2 SO 4 + 2H 2 O.

Interaction with salts:

Na 2 CO 3 + H 2 SO 4 = Na 2 SO 4 + CO 2 + H 2 O.

Oxidative properties

Sulfuric acid oxidizes HI and HBr to free halogens:

H 2 SO 4 + 2HI = I 2 + 2H 2 O + SO 2.

Sulfuric acid takes away chemically bound water from organic compounds containing hydroxyl groups. Dehydration of ethyl alcohol in the presence of concentrated sulfuric acid leads to the production of ethylene:

C 2 H 5 OH = C 2 H 4 + H 2 O.

The charring of sugar, cellulose, starch and other carbohydrates upon contact with sulfuric acid is also explained by their dehydration:

C 6 H 12 O 6 + 12H 2 SO 4 = 18H 2 O + 12SO 2 + 6CO 2.