Publication date 01/07/2013 16:35

Sulfurous acid is an inorganic dibasic unstable acid medium strength. An unstable compound, known only in aqueous solutions at a concentration of no more than six percent. When attempting to isolate pure sulfurous acid, it breaks down into sulfur oxide (SO2) and water (H2O). For example, when concentrated sulfuric acid (H2SO4) reacts with sodium sulfite (Na2SO3), sulfur oxide (SO2) is released instead of sulfurous acid. This is what the reaction looks like:

Na2SO3 (sodium sulfite) + H2SO4 ( sulfuric acid) = Na2SO4 (sodium sulfate) + SO2 (sulfur dioxide) + H2O (water)

Sulfurous acid solution

When storing it, it is necessary to exclude access to air. Otherwise, sulfurous acid, slowly absorbing oxygen (O2), will turn into sulfuric acid.

2H2SO3 (sulfuric acid) + O2 (oxygen) = 2H2SO4 (sulfuric acid)

Solutions of sulfurous acid have a rather specific odor (reminiscent of the odor remaining after lighting a match), the presence of which can be explained by the presence of sulfur oxide (SO2), which is not chemically bound with water.

Chemical properties of sulfurous acid

1. Sulfurous acid (formula H2SO3) can be used as a reducing agent or an oxidizing agent.

H2SO3 is a good reducing agent. With its help, it is possible to obtain hydrogen halides from free halogens. For example:

H2SO3 (sulfuric acid) + Cl2 (chlorine, gas) + H2O (water) = H2SO4 (sulfuric acid) + 2HCl (hydrochloric acid)

But when interacting with strong reducing agents this acid will act as an oxidizing agent. An example is the reaction of sulfurous acid with hydrogen sulfide:

H2SO3 (sulfuric acid) + 2H2S (hydrogen sulfide) = 3S (sulfur) + 3H2O (water)

2. What we are considering chemical compound forms two types of salts - sulfites (medium) and hydrosulfites (acidic). These salts are reducing agents, just like (H2SO3) sulfurous acid. When they are oxidized, salts of sulfuric acid are formed. When sulfites of active metals are calcined, sulfates and sulfides are formed. This is a self-oxidation-self-healing reaction. For example:

4Na2SO3 (sodium sulfite) = Na2S (sodium sulfide) + 3Na2SO4 (sodium sulfate)

Sodium and potassium sulfites (Na2SO3 and K2SO3) are used in dyeing fabrics in the textile industry, in bleaching metals, and in photography. Calcium hydrosulfite (Ca(HSO3)2), which exists only in solution, is used to process wood material into a special sulfite pulp. It is then used to make paper.

Application of sulfurous acid

Sulfurous acid is used:

– for bleaching wool, silk, wood pulp, paper and others similar substances that cannot withstand bleaching using stronger oxidizing agents (for example, chlorine);

– as a preservative and antiseptic, for example, to prevent the fermentation of grain when producing starch or to prevent the fermentation process in wine barrels;

– for food preservation, for example, when canning vegetables and fruits;

– in the processing of wood chips into sulfite cellulose, from which paper is then produced. In this case, a solution of calcium hydrosulfite (Ca(HSO3)2) is used, which dissolves lignin, a special substance that binds cellulose fibers.

Sulfurous acid: preparation

This acid can be produced by dissolving sulfur dioxide (SO2) in water (H2O). You will need concentrated sulfuric acid (H2SO4), copper (Cu) and a test tube. Algorithm of actions:

1. Carefully pour concentrated sulfuric acid into a test tube and then place a piece of copper in it. Heat up. The following reaction occurs:

Cu (copper) + 2H2SO4 (sulfuric acid) = CuSO4 (sulfur sulfate) + SO2 (sulfur dioxide) + H2O (water)

2. The flow of sulfur dioxide must be directed into a test tube with water. When it dissolves, it partially occurs chemical reaction with water, resulting in the formation of sulfurous acid:

SO2 (sulfur dioxide) + H2O (water) = H2SO3

So, by passing sulfur dioxide through water, you can get sulfurous acid. It is worth considering that this gas has an irritating effect on the shells respiratory tract, can cause inflammation, as well as loss of appetite. Inhaling it for a long time may cause loss of consciousness. This gas must be handled with extreme caution and care.

Sulfurous acid is capable of reacting with oxygen. This produces sulfuric acid. This reaction takes a very long time and is only possible if storage rules are violated. Sulfurous acid has both oxidizing and reducing properties. It can be used to produce halogen acids. An aqueous solution reacts with chlorine to form hydrochloric and sulfuric acid.

When reacting with strong reducing agents, sulfurous acid plays the role of an oxidizing agent. One such substance is hydrogen sulfide, a gas with very unpleasant smell. Interacting with an aqueous solution of sulfuric acid, it forms sulfur and water. Salts of sulfurous acid also have reducing properties. They are divided into sulfites and hydrosulfites. The oxidation reactions of these salts produce sulfuric acid.

Preparation of sulfurous acid

Sulfurous acid is formed only by the interaction of sulfur dioxide and water. You need to get sulfur dioxide. This can be done using copper and sulfuric acid. Carefully pour concentrated sulfuric acid into a test tube and drop a piece of copper into it. Heat the test tube using an alcohol lamp.

As a result of heating, copper sulfate (copper sulfate), water and sulfur dioxide are formed, which must be brought to the flask using a special tube. clean water. In this way, sulfurous acid can be obtained.

Remember that sulfur dioxide is harmful to humans. It causes damage to the respiratory tract, loss of appetite and headache. Prolonged inhalation may cause fainting. Caution is required when working with it.

Application of sulfurous acid

Sulfurous acid has antiseptic properties. It is used for surface disinfection and grain fermentation. It can be used to decompose certain substances that, when interacting with strong oxidizing agents (for example, chlorine), decompose. Such substances include wool, silk, paper and some others. Her antibacterial properties used to prevent wine from fermenting. Thus, the noble drink can be stored for a very long time, acquiring a noble taste and unique aroma.

Sulfurous acid is used in paper production. The addition of this acid is part of the technology for producing sulfite cellulose. It is then treated with a solution of calcium hydrosulfite to bind the fibers together.

H2SO3, weak dibasic acid. It is not isolated in free form; it exists in aqueous solutions. Salts of sulfurous acid sulfites... Big Encyclopedic Dictionary

SULFURIC ACID- (H2SO3) weak dibasic acid. Exists only in aqueous solutions. Salts S. to. sulfites. Used in pulp and paper and food industry. See also Acids and anhydrides... Russian encyclopedia of labor protection

sulfurous acid- - [A.S. Goldberg. English-Russian energy dictionary. 2006] Energy topics in general EN sulfurous acid ... Technical Translator's Guide

H2SO3, weak dibasic acid. It is not isolated in free form; it exists in aqueous solutions. Sulfuric acid salts sulfites. * * * SULFURIC ACID SULFURIC ACID, H2SO3, a weak dibasic acid. Not highlighted in free form,... ... Encyclopedic Dictionary

sulfurous acid- sulfito rūgštis statusas T sritis chemija formulė H₂SO₃ atitikmenys: angl. sulfurous acid rus. sulfurous acid ryšiai: sinonimas – vandenilio trioksosulfatas (2–) … Chemijos terminų aiškinamasis žodynas

H2SO3, a weak dibasic acid corresponding to the oxidation state of sulfur +4. Known only in dilute aqueous solutions. Dissociation constants: K1 = 1.6 10 2, K2 = 1.0 10 7 (18°C). Gives two series of salts: normal sulfites and acidic... ... Great Soviet Encyclopedia

H2SO3, weak dibasic acid. It is not isolated in free form; it exists in waters. r rah. Salts S. k. sulfites ... Natural science. Encyclopedic Dictionary

See Sera... Encyclopedic Dictionary F.A. Brockhaus and I.A. Efron

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.

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 is 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.