slide 12

Chemical properties

In air, sulfur burns, forming sulfur dioxide - a colorless gas with a pungent odor: S + O2 → SO2.

slide 13

The sulfur melt reacts with chlorine, and the formation of two lower chlorides 2S + Cl2 → S2Cl2 S + Cl2 → SCl2 is possible. When heated, sulfur also reacts with phosphorus, forming a mixture of phosphorus sulfides, among which is the higher sulfide P2S5: 5S + 2P → P2S2 In addition , when heated, sulfur reacts with hydrogen, carbon, silicon: S + H2 → H2S (hydrogen sulfide) C + 2S → CS2 (carbon disulfide)

Slide 14

Of the complex substances, first of all, the reaction of sulfur with molten alkali should be noted, in which sulfur disproportionates similarly to chlorine: 3S + 6KOH → K2SO3 + 2K2S + 3H2O Sulfur reacts with concentrated oxidizing acids only during prolonged heating: S + 6HNO3 (conc) → H2SO4 + 6NO2 + 2H2O S+ 2 H2SO4 (conc) → 3SO2 + 2H2O

slide 15

At 100–160°C, it is oxidized by water: Te+2H2O= TeO2+2H2 When boiled in alkaline solutions, tellurium disproportionates to form telluride and tellurite: 8Te+6KOH=2K2Te+ K2TeO3+3H2O.

slide 16

Dilute HNO3 oxidizes Te to tellurous acid H2TeO3: 3Te+4HNO3+H2O=3H2TeO3+4NO. Strong oxidizers (HClO3, KMnO4) oxidize Te to weak telluric acid H6TeO6: Te+HClO3+3H2O=HCl+H6TeO6. Tellurium compounds (+2) are unstable and prone to disproportionation: 2TeCl2=TeCl4+Te.

Slide 17

When heated in air, it burns to form colorless crystalline SeO2: Se + O2 = SeO2. It interacts with water when heated: 3Se + 3H2O = 2H2Se + H2SeO3. Selenium reacts when heated with nitric acid to form selenious acid H2SeO3: 3Se + 4HNO3 + H2O = 3H2SeO3 + 4NO.

Slide 18

When boiling in alkaline solutions, selenium disproportionates: 3Se + 6KOH = K2SeO3 + 2K2Se + 3H2O. If selenium is boiled in alkaline solution, through which air or oxygen is passed, then red-brown solutions containing polyselenides are formed: K2Se + 3Se = K2Se4

The oxygen subgroup includes five elements: oxygen, sulfur, selenium, tellurium and polonium (a radioactive metal). These are p-elements of group VI periodic system D.I. Mendeleev. They have a group name - chalcogens, which means "forming ores."

Properties of elements of the oxygen subgroup

Chalcogen atoms have the same structure of the external energy level - ns 2 nr 4 . This explains their similarity chemical properties. All chalcogens in compounds with hydrogen and metals exhibit an oxidation state of -2, and in compounds with oxygen and other active non-metals, usually +4 and +6. For oxygen, as well as for fluorine, an oxidation state equal to the group number is not typical. It exhibits an oxidation state of usually -2 and in combination with fluorine +2. Such values ​​of oxidation states follow from the electronic structure of chalcogens

The oxygen atom has two unpaired electrons in the 2p sublevel. Its electrons cannot be separated, since there is no d-sublevel at the outer (second) level, that is, there are no free orbitals. Therefore, the valency of oxygen is always equal to two, and the oxidation state is -2 and +2 (for example, in H 2 O and OF 2). These are the same valencies and oxidation states of the atom of sulfur in the unexcited state. Upon transition to an excited state (which takes place during the supply of energy, for example, during heating), at the sulfur atom, the 3 R— and then 3s electrons (shown by arrows). The number of unpaired electrons, and, consequently, the valency in the first case is four (for example, in SO 2), and in the second - six (for example, in SO 3). Obviously, even valencies 2, 4, 6 are characteristic of sulfur analogues - selenium, tellurium and polonium, and their oxidation states can be equal to -2, +2, +4 and +6.

Hydrogen compounds of elements of the oxygen subgroup are responsible formula H 2 R (R - element symbol): H 2 O, H 2 S , H 2 S e, H 2 Te. They callare hydrogen chalcides. When dissolved in water, they formacids. The strength of these acids increases with increasing atomic number of the element, which is explained by a decrease in energy bonds in the series of compounds H 2 R . Water dissociating into H + and O ions H - , is amphoteric electrolyte.

Sulfur, selenium and tellurium form the same forms of compounds with oxygen of the type R O 2 and R About 3- . They correspond to acids of the type H 2 R O 3 and H 2 R About 4- . With an increase in the ordinal number of the element, the strength of these acids decreases.vaet. All of them exhibit oxidizing properties, and acids of the type H 2 R About 3 are also restorative.

Properties change regularly simple substances: with increasecharge of the nucleus, non-metallic ones weaken and metallic ones increase. properties. So, oxygen and tellurium are non-metals, but the latter hasmetallic luster and conducts electricity.

Tellurium is one of the rare elements: its content in the earth's crust is only .

In the free state, selenium, like sulfur, forms several allotropic modifications, of which the most famous are amorphous selenium, which is a red-brown powder, and gray selenium, which forms brittle crystals with a metallic sheen.

Tellurium is also known in the form of an amorphous modification and in the form of light gray crystals with a metallic luster.

Selenium is a typical semiconductor (see § 190). An important property of it as a semiconductor is a sharp increase in electrical conductivity when illuminated. At the boundary of selenium with a metal conductor, a barrier layer is formed - a section of the circuit that can pass electric current in only one direction. In connection with these properties, selenium is used in semiconductor technology for the manufacture of rectifiers and photocells with a barrier layer. Tellurium is also a semiconductor, but its use is more limited. Selenides and tellurides of some metals also have semiconductor properties and are used in electronics. In small amounts, tellurium serves as an alloying addition to lead, improving its mechanical properties.

Hydrogen selenide and hydrogen telluride are colorless gases with a disgusting odor. Aqueous solutions they are acids, the dissociation constants of which are slightly larger than the dissociation constant of hydrogen sulfide.

Chemically, hydrogen selenide and hydrogen telluride are extremely similar to hydrogen sulfide. Like hydrogen sulfide, they are highly reducing properties. When heated, they both decompose. At the same time, it is less stable than: just as it happens in the series of hydrogen halides, the strength of the molecules decreases during the transition. Salts of hydrogen selenide and hydrogen telluride - selenides and tellurides - are similar to sulfides in terms of solubility in water and acids. By acting on selenides and tellurides with strong acids, hydrogen selenide and hydrogen telluride can be obtained.

When selenium and tellurium are burned in air or in oxygen, dioxides and are obtained, which under normal conditions are in a solid state and are anhydrides of selenous and tellurous acids.

Unlike sulfur dioxide, and exhibit predominantly oxidizing properties, easily recovering to free selenium and tellurium, for example:

By the action of strong oxidizing agents, selenium and tellurium dioxides can be converted into selenic and telluric acids, respectively.

Selenium and tellurium are in group VI of the periodic system and are analogues of sulfur. At the external electronic level, selenium and tellurium have 6 electrons each: Se 4s 2 4p 4; Te 5s 2 5p 4 , so they exhibit oxidation states IV, VI and -II. As in any group of the periodic system, as it grows atomic mass element, the acidic properties of the element weaken, and the basic ones increase, therefore, tellurium exhibits a number of basic (metallic properties) and it is not surprising that the discoverers mistook it for a metal.

Selenium is characterized by polymorphism, there are 3 crystalline and 2 amorphous modifications.

vitreous selenium obtained by rapidly chilled molten selenium, consists of Se 8 ring molecules and rings up to 1000 atoms.

Red amorphous selenium it is formed if Se vapor is quickly cooled, it mainly consists of improperly oriented Se 8 molecules, it dissolves in CS 2 during crystallization, two crystalline modifications are obtained:

t pl 170 0 C t pl 180 0 C

slow fast

built from Se 8 molecules.

Most stable gray hexagonal selenium , consisting of endless chains of selenium atoms. When heated, all modifications are transferred to the last one. This is the only semiconductor modification. It has: t pl 221 0 С and t kip 685 0 С. Along with Se 8, molecules with a smaller number of atoms up to Se 2 are also present in vapors.

Tellurium is more simple - hexagonal tellurium is the most stable, with t pl 452 0 С and t kip 993 0 С. Amorphous tellurium is finely dispersed hexagonal tellurium.

Selenium and tellurium are stable in air; when heated, they burn, forming dioxides SeO 2 and TeO 2 . At room temperature they do not react with water.

When amorphous selenium is heated to t 60 0 C, it begins to react with water:

3Se + 3H 2 O = 2H 2 Se + H 2 SeO 3 (17)

Tellurium is less active and reacts with water above 100 0 C. They react with alkalis under milder conditions, forming:

3Se + 6NaOH = 2Na 2 Se + Na 2 SeO 3 + 3H 2 O (18)

3Te + 6NaOH = 2Na 2 Te + Na 2 TeO 3 + 3H 2 O (19)

They do not react with acids (HCl and dilute H 2 SO 4), dilute HNO 3 oxidizes them to H 2 SeO 3; H 2 TeO 3 , if the acid is concentrated, then it oxidizes tellurium to basic nitrate Te 2 O 3 (OH)NO 3 .

Concentrated H 2 SO 4 dissolves selenium and tellurium, forming

Se 8 (HSO 4) 2 - green H 2 SeO 3

Te 4 (HSO 4) 2 - red Te 2 O 3 SO 4

½ solutions

unstable

Se and Te are released

For Se, as well as for S, addition reactions are characteristic:

Na 2 S + 4Se = Na 2 SSe 4 (most stable) (20)

Na 2 S + 2Te \u003d Na 2 STe 2 (most stable) (21)

in the general case, Na 2 SE n, where E \u003d Se, Te.

Na 2 SO 3 + Se Na 2 SeSO 3 (22)

selenosulfate

For tellurium, such a reaction occurs only in autoclaves.

Se + KCN = KSeCN (unknown for tellurium) (23)

Selenium interacts with hydrogen at a temperature of 200 0 C:

Se + H 2 \u003d H 2 Se (24)

For tellurium, the reaction proceeds with difficulty and the yield of hydrogen telluride is low.

Selenium and tellurium interact with most metals. In compounds, selenium and tellurium are characterized by oxidation states of -2, +4, and +6 are also known.

Compounds with oxygen. Dioxides. SeO 2 - white, t sub. - 337 0 С, dissolves in water, forming H 2 SeO 3 - unstable, at a temperature of 72 0 С it decomposes by peritectic reaction.

TeO 2 - more refractory, t pl. – 733 0 С, t b.p. – 1260 0 C, non-volatile, slightly soluble in water, easily soluble in alkalis, solubility minimum falls at pH ~ 4, H 2 TeO 3 precipitate is released from the solution, unstable and decomposes upon drying.

trioxides. Higher oxides are obtained by the action of strong oxidizing agents.

SeO 3 (reminiscent of SO 3) reacts with water, forming H 2 SeO 4, t pl. ~ 60 0 C, strong oxidizing agent, dissolves Au:

2Au + 6H 2 SeO 4 = Au 2 (SeO 4) 3 + 3H 2 SeO 3 + 3H 2 O (25)

dissolves Pt in a mixture with HCl.

TeO 3 is an inactive substance, exists in amorphous and crystalline modifications. Amorphous trioxide hydrates under prolonged exposure to hot water, turning into ortho-telluric acid H 6 TeO 6 . It dissolves in concentrated alkali solutions when heated, forming tellurates.

H 2 TeO 4 has three varieties: ortho-telluric acid H 6 TeO 6 is highly soluble in H 2 O, its solutions do not give an acidic reaction, a very weak acid, upon dehydration polymetatelluric acid (H 2 TeO 4) n is obtained insoluble in water. Allotelluric acid is obtained by heating ortho-telluric acid in a sealed ampoule, it is miscible with water in any ratio and has an acidic character. It is intermediate, there are 6-10 molecules in the chain, it is unstable, at room temperature it turns into ortho-telluric acid, and when heated in air it quickly turns into H 2 TeO 4 .

Salt. For selenates, salts of heavy metals are highly soluble in water, selenates of alkaline earth metals, lead and, unlike sulfates, Ag and Tl, are slightly soluble. When heated, they form selenites (different from sulfates). Selenites are more stable than sulfites and can be melted unlike sulfites.

Tellurates Na 2 H 4 TeO 6 - orthotellurate exists in two modifications, obtained at low temperatures, soluble in water, at high temperatures - insoluble. Upon dehydration, Na 2 TeO 4 is obtained, which is insoluble in water. Tellurates of heavy and alkaline earth metals differ in low solubility. Unlike tellurate, sodium tellurite is soluble in water.

Hydrides. H 2 Se and H 2 Te gases, dissolve in water and give more strong acids than H 2 S. When neutralized with alkalis, they form salts similar to Na 2 S. For tellurides and selenides, as well as for Na 2 S, addition reactions are characteristic:

Na 2 Se + Se = Na 2 Se 2 (26)

Na 2 Se + nS = Na 2 SeS n (27)

In the general case, Na 2 ES 3 and Na 2 ES 4 are formed, where E is selenium and tellurium.

Chlorides. If for sulfur S 2 Cl 2 is the most stable, then for selenium a similar compound is known, however, SeCl 4 is the most stable, for tellurium TeCl 4. When dissolved in water, SeCl 4 hydrolyzes:

SeCl 4 + 3H 2 O = 4НCl + H 2 SeO 3 (28)

TeCl 4 dissolves without noticeable hydrolysis.

For TeCl 4 complexes are known: K 2 TeCl 6 and KTeCl 5, with aluminum chloride forms cationic complexes + -. In some cases, it also forms complexes with selenium, but only hexachloroselenates are known for it: M 2 SeCl 6 .

When heated, they sublimate and dissociate:

SeCl 4 \u003d SeCl 2 + Cl 2 (29)

during condensation they disproportionate:

2TeCl 2 \u003d Te + TeCl 4 (30)

Fluorides, bromides, and iodides are known to form only in tellurium.

Sulfides. When fused with sulfur compounds are not formed. Under the action of H 2 S on salts of selenium and tellurium, TeS 2 and a mixture of SeS 2 and SeS can be precipitated (it is believed that this is a mixture of S and Se).

Synthesis, by replacing sulfur with selenium in the S 8 molecule, Se 4 S 4, Se 3 S 5, Se 2 S 6, SeS 7 are obtained, the substitution occurs through one sulfur atom.