Chalcogens. Presentation on the topic "Sulfur, selenium, tellurium." Selenium, tellurium, polonium and their compounds

Selenium is not widely distributed in nature. The content of selenium in the earth's crust is . Its compounds are found as impurities in natural sulfur compounds with metals and. Therefore, selenium is obtained from waste products generated in the production of sulfuric acid, in the electrolytic refining of copper, and in some other processes.

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. Their aqueous solutions are acids, the dissociation constants of which are somewhat 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.

ELEMENTS VI A subgroups

(O, S, Se, Te, Po)

general characteristics

Oxygen

Sulfur

Selenium and tellurium

General characteristics of the elements

The VI A subgroup of PS includes the elements: oxygen, sulfur, selenium, tellurium and polonium. For sulfur, selenium, tellurium and polonium, a common name is used - chalcogens. Oxygen, sulfur, selenium and tellurium are non-metals, while polonium is a metal. Polonium is a radioactive element, in nature it is formed in small quantities during the radioactive decay of radium, therefore its chemical properties are poorly understood.

Table 1

Main characteristics of chalcogens

Characteristics	ABOUT	S	Se	Those
Atomic radius, nm	0,066	0,104	0,117	0,136
Ionic radius E 2-, nm	0,140	0,184	0,198	0,221
Ionization potential, eV	13,62	10,36	9,75	9,01
Electron affinity, eV	1,47	2,08	2,02	1,96
Electronegativity (according to Pauling)	3,44	2,58	2,55	2,10
Bond enthalpy, kJ/mol E –E E = E	- 146 - 494	- 265 - 421	- 192 - 272	- 218 - 126
Melting point, °С
Boiling point, °C	- 183
Density, g / cm 3	1.43 (liquid)	2,07	4,80	6,33
Content in the earth's crust, % (wt.)	49,13	0,003	1.4 10 -5	1 10 -7
Mass numbers of natural isotopes	16, 17, 18	32, 33, 34, 35	74, 76, 77, 78, 80, 82	120, 122, 123, 124, 125, 126 128, 130
The state of aggregation at Art. conditions of the most stable allotropic form. color	colorless gas	Crystal. yellow substance	Crystal. gray matter	Crystal. silvery white substance
Crystal cell	Molecular in TV. form	molecular	molecular	molecular
Composition of molecules	About 2	S8	Se ∞	Te ∞

According to the structure of the outer electronic layer, the considered elements belong to the p-elements. Of the six electrons in the outer layer, two are unpaired, which determines their valence of two. For atoms of sulfur, selenium, tellurium and polonium in an excited state, the number of unpaired electrons can be 4 and 6. That is, these elements can be four - and hexavalent. All elements have high electronegativity values, and the EO of oxygen is second only to fluorine. Therefore, in compounds they exhibit art. oxidation -2, -1, 0. The ionization potentials of sulfur, selenium and tellurium atoms are small, and these elements in compounds with halogens have oxidation states of +4 and +6. Oxygen has a positive oxidation state in fluorine compounds and in ozone.

Atoms can form molecules with a double bond O 2, ... and join in chains E - E - ... - E -, which can exist both in simple and in complex substances. In terms of chemical activity and oxidizing ability, chalcogens are inferior to halogens. This is indicated by the fact that in nature oxygen and sulfur exist not only in a bound, but also in a free state. The lower activity of chalcogens is largely due to a stronger bond in the molecules. In general, chalcogens are among the highly reactive substances, the activity of which sharply increases with increasing temperature. Allotropic modifications are known for all substances of this subgroup. Sulfur and oxygen practically do not conduct electric current (dielectrics), selenium and tellurium are semiconductors.

When moving from oxygen to tellurium, the tendency of elements to form double bonds with small atoms (C, N, O) decreases. The inability of large atoms to form π-bonds with oxygen is especially evident in the case of tellurium. So, in tellurium there are no acid molecules H 2 TeO 3 and H 2 TeO 4 (meta-forms), as well as TeO 2 molecules. Tellurium dioxide exists only in the form of a polymer, where all oxygen atoms are bridging: Te - O - Te. Telluric acid, in contrast to sulfuric and selenic acid, occurs only in the ortho form - H 6 TeO 6, where, as in TeO 2, the Te atoms are connected to the O atoms only by σ-bonds.

The chemical properties of oxygen differ from those of sulfur, selenium and tellurium. On the contrary, there is much in common in the properties of sulfur, selenium and tellurium. When moving through the group from top to bottom, one should note an increase in acidic and reducing properties in a series of compounds with hydrogen H 2 E; an increase in oxidizing properties in a series of similar compounds (H 2 EO 4, EO 2); decrease in thermal stability of hydrogen chalcogens and salts of oxygen acids.

Chemistry of the Elements Non-metals of VIA-subgroup

Elements of the VIA subgroup are non-metals, except for Po.

Oxygen is very different from other subgroup elements and plays a special role in chemistry. Therefore, the chemistry of oxygen is highlighted in a separate lecture.

Sulfur is the most important among the other elements. The chemistry of sulfur is very extensive, since sulfur forms a huge variety of compounds. Its compounds are widely used in chemical practice and in various industries. When discussing nonmetals of the VIA subgroup, the greatest attention will be paid to the chemistry of sulfur.

Key Issues Addressed in the Lecture

General characteristics of non-metals of the VIA-subgroup. Natural compounds Sulfur

Simple substance Sulfur compounds

Hydrogen sulfide, sulfides, polysulfides

Sulphur dioxide. Sulfites

Sulfur trioxide

Sulphuric acid. oxidative properties. sulfates

Other sulfur compounds

selenium, tellurium

Simple substances Compounds of selenium and tellurium

Selenides and tellurides

Se and Te compounds in oxidation state (+4)

Selenic and telluric acids. oxidative properties.

				Elements of the VIA subgroup
					general characteristics
						The p-elements belong to the VIA subgroup: acid-
					genus O, sulfur S, selenium Se, tellurium Te, polonium Po.
									The general formula for valence electrons
									thrones - ns 2 np 4 .
			oxygen
						Oxygen, sulfur, selenium and tellurium are non-metals.
					They are often grouped under the common name "chalcogens",
					which means "forming ores". Indeed many
					metals are found in nature in the form of oxides and sulfides;
					in sulfide ores				in small quantities with
					there are selenides and tellurides.
						Polonium is a very rare radioactive element that
					which is a metal.
			molybdenum
						To create a stable eight-electron
					chalcogen atoms lack only two electro-
					new The minimum oxidation state (–2) is
			tungsten		resistant to all elements. It is this degree of oxidation
					elements show in natural compounds - ok-
					sides, sulfides, selenides and tellurides.
						All elements of the VIA-subgroup, except for O, exhibit
			seaborgium		positive oxidation states +6 and +4. Most-
					the highest oxidation state of oxygen is +2, it exhibits
	only in conjunction with F.

The most characteristic oxidation states for S, Se, Te are

xia: (–2), 0, +4, +6, for oxygen: (–2), (–1), 0.

In the transition from S to Te, the stability of the highest oxidation state is +6

decreases, and the stability of the +4 oxidation state increases.

For Se, Te, Po, - the most stable oxidation state is +4.

Some characteristics of atoms of elements ViB - subgroups

		Relative	First energy
		elektrootri-	ionization,
		value	kJ/mol
		(according to Polling)
				an increase in the number of
				throne layers;
				an increase in the size of an atom;
				decrease in energy io-
				decrease in electrical
				values

As can be seen from the above data , oxygen is very different from other elements of the subgroup high value of ionization energy, ma-

large orbital radius of the atom and high electronegativity, only F has a higher electronegativity.

Oxygen, which plays a very special role in chemistry, was considered from

sensibly. Among the other elements of the VIA group, sulfur is the most important.

			Sulfur forms a very large number of various
			different connections. Its compounds are known from almost all
			mi elements, except for Au, Pt, I and noble gases. Cro-
			me of widespread compounds S in powers
	3s2 3p4
			oxidation (–2), +4, +6, are known, as a rule,
			stable compounds in oxidation states: +1 (S2 O), +2

(SF2 , SCl2 ), +3 (S2 O3 , H2 S2 O4 ). The variety of sulfur compounds is also confirmed by the fact that only about 20 oxygen-containing acids S are known.

The strength of the bond between S atoms turns out to be commensurate with the

bonds S with other non-metals: O, H, Cl, therefore, S is characterized by

including the very common mineral pyrite, FeS2, and polythionic acids (eg H2 S4 O6 ). Thus, the chemistry of sulfur is quite extensive.

The most important sulfur compounds used in industry

The most widely used sulfur compound in industry and the laboratory is sulfuric acid. The world volume of production of ser-

acid is 136 million tons. (no other acid is produced in such large quantities). Common compounds include

whether sulfuric acid - sulfates, as well as salts of sulfurous acid - sulfites.

natural sulfides are used to obtain the most important non-ferrous metals

thalls: Cu, Zn, Pb, Ni, Co, etc. Other common sulfur compounds include: hydrosulfide acid H2 S, di- and trioxides of sulfur: SO2

and SO3, thiosulfate Na2 S2 O3 ; acids: disulfuric (pyrosulfuric) H2 S2 O7, perox-

codisulfate H2 S2 O8 and peroxodisulfates (persulphates): Na2 S2 O8 and

(NH4 )2 S2 O8 .

Sulfur in nature

tea in the form of a simple substance, forming large underground deposits,

and in the form of sulfide and sulfate minerals , as well as in the form of compounds,

which are impurities in coal and oil. Coal and oil are obtained as a result of

those decompositions of organic substances, and sulfur is a part of animals and plants

body proteins. Therefore, when coal and oil are burned, sulfur oxides are formed,

polluting the environment.

Natural sulfur compounds

Rice. Pyrite FeS2 is the main mineral used to produce sulfuric acid.

native sulfur;

sulfide minerals:

FeS2 - pyrite or iron pyrite

FeCuS2 - chalcopyrite (copper quanti-

FeAsS - arsenopyrite

PbS - galena or lead luster

ZnS - sphalerite or zinc blende

HgS - cinnabar

Cu2 S- chalcocite or copper luster

Ag2 S - argentite or silver sheen

MoS2 - molybdenite

Sb2 S3 - stibnite or antimony shine

As4 S4 - realgar;

sulfates:

Na2 SO4 . 10 H2 O - mirabilite

CaSO4 . 2H2 O - gypsum

CaSO4 - anhydrite

BaSObarite or heavy spar

SrSO4 is celestine.

Rice. Gypsum CaSO4. 2H2O

simple substance

In a simple substance, sulfur atoms are bonded with two neighboring ones.

The most stable is the structure consisting of eight sulfur atoms,

united in a corrugated ring resembling a crown. There are several modifications of sulfur: rhombic sulfur, monoclinic and plastic sulfur. At ordinary temperature, sulfur is in the form of yellow brittle crystals.

rhombic shape (-S), formed by

ionic molecules S8 . Another modification - monoclinic sulfur (-S) also consists of eight-membered rings, but differs in location

arrangement of S8 molecules in the crystal. When dis-

melting sulfur rings are torn. At the same time, mo-

tangled threads can form, which

Rice. Sulfur

make the melt viscous, with further

As the temperature rises, the polymer chains can break down and the viscosity will decrease. Plastic sulfur is formed during the sharp cooling of the molten

sulfur and consists of entangled chains. Over time (within a few days), it will be converted into rhombic sulfur.

Sulfur boils at 445o C. Equilibria take place in sulfur vapor:

450 o C	650 o C	900 o C	1500 o C
S 8  S 6	 S 4	 S 2	 S

S2 molecules have a structure similar to O2.

Sulfur can be oxidized (usually to SO2) and can be reduced

upgraded to S(-2). At ordinary temperatures, almost all reactions involving solid sulfur are inhibited; only reactions with fluorine, chlorine, and mercury proceed.

This reaction is used to bind the smallest droplets of spilled mercury.

Liquid and vaporous sulfur are highly reactive . Sulfur vapor burns Zn, Fe, Cu. When passing H 2 over molten sulfur is formed

H 2 S. In reactions with hydrogen and metals, sulfur acts as an oxidizing

Sulfur can be easily oxidized under the action of halogens.

and oxygen. When heated in air, sulfur burns with a blue flame, oxidizing

up to SO2.

S + O2 = SO2

Sulfur is oxidized with concentrated sulfuric and nitric acids:

S + 2H2 SO4 (conc.) = 3SO2 + 2H2 O,

S + 6HNO3 (conc.) = H2 SO4 + 6 NO2 + 2H2 O

In hot alkali solutions, sulfur disproportionates.

3S + 6 NaOH = 2 Na2 S + Na2 SO3 + 3 H2 O.

When sulfur reacts with a solution of ammonium sulfide, yellow-red polysulfide ions(–S–S–)n or Sn 2– .

When sulfur is heated with a solution of sulfite, thiosulfate is obtained, and

when heated with a solution of cyanide - thiocyanate:

S + Na 2 SO3 = Na2 S2 O3, S + KCN = KSCN

Potassium thiocyanate or thiocyanate is used for the analytical detection of Fe3+ ions:

3+ + SCN – = 2+ + H2O

The resulting complex compound has a blood-red color,

even at a low concentration of hydrated Fe3+ ions in the

About 33 million tons of native sulfur are mined annually in the world. The main amount of extracted sulfur is processed into sulfuric acid and used

used in the rubber industry for the vulcanization of rubber. Add sulfur

binds to double bonds of rubber macromolecules, forming disulfide bridges

ki -S- S-, thereby, as if "stitching" them, which gives the rubber strength and elasticity. When a large amount of sulfur is introduced into rubber, ebo-

nit, which is a good insulating material used in electrical engineering. Sulfur is also used in pharmaceuticals to make skin ointments and in agriculture to control plant pests.

Sulfur compounds

Hydrogen sulfide, sulfides, polysulfides

Hydrogen sulfide H 2 S occurs naturally in sulfuric mineral waters,

present in volcanic and natural gas, formed during the decay of white

kov bodies.

Hydrogen sulfide is a colorless gas with a rotten egg odor and is highly toxic.

It is slightly soluble in water; at room temperature, three volumes of gaseous H2 S dissolve in one volume of water. The concentration of H 2 S in saturated

nom solution is ~ 0.1 mol/l . When dissolved in water, it forms

hydrosulfide acid, which is one of the weakest acids:

		H2 S  H+ + HS – , K1 = 6. 10 –8 ,
		HS -  H+ + S 2–,								K2 = 1.10 –14
Executor:											Many natural sulfides are known (see the list of sulfide minerals). Sulfides of many heavy non-ferrous metals (Cu, Zn, Pb, Ni, Co, Cd, Mo) are are industrially important ores. They are converted into oxides by firing in air, for example, 2 ZnS + 3 O2 = 2 ZnO + 2 SO2 then the oxides are most often reduced with coal: ZnO + C = Zn + CO Sometimes oxides are brought into solution by the action of an acid, and then the solution is subjected to electrolysis in order to reduce the metal. Sulfides of alkali and alkaline earth metals are practically chemically ionic compounds. Sulfides of other metals - the advantage vein-covalent compounds, as a rule, of non-stoichiometric composition. Many nonmetals also form covalent sulfides: B, C, Si, Ge, P, As, Sb. Natural sulfides As and Sb are known. Sulfides of alkali and alkaline earth metals, as well as sulfides ammonium feed is highly soluble in water, the rest of the sulfides are insoluble rhymes. They are isolated from solutions in the form of characteristically colored precipitates, for example, Pb(NO3 )2 + Na2 S = PbS (t.) + 2 NaNO3 This reaction is used to detect H2 S and S2– in solution. Some of the water-insoluble sulfides can be brought into solution by acids, due to the formation of a very weak and volatile hydrosulphuric acid. native acid, for example, NiS + H2SO4 = H2S + NiSO4 Sulfides can be dissolved in acids: FeS, NiS, CoS, MnS, ZnS. Metal sulfides and PR values Sulfides Sediment color PR value 5 . 10–18 1 . 10–24 2 . 10–25 2 . 10–27 6 . 10–36 4 . 10–53 Brown 2 . 10–27 2 . 10–28 2 . 10–10 2 . 10–24 Sulfides, characterized by a very low value of the solubility product, cannot dissolve in acids with the formation of H2 S. In ki- sulfides do not dissolve in slots: CuS, PbS, Ag2 S, HgS, SnS, Bi2 S3, Sb2 S3, Sb2 S5, CdS, As2 S3, As2 S5, SnS2. If the reaction of dissolution of sulfide due to the formation of H2 S is impossible, then it can be transferred into a solution by the action of concentrated nitric acid slots or aqua regia. CuS + 8HNO3 = CuSO4 + 8NO2 + 4H2O The sulfide anion S 2– is a strong proton acceptor (os- innovation according to Brønsted). That's why highly soluble sulfides

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

Properties of elements of the oxygen subgroup

Properties				Those	Ro
1. Order number
2. Valence electrons	2 s 2 2p 4	Z s 2 3r 4	4 s 2 4r 4	5s 2 5p 4	6s 2 6p 4
3. Energy Ionization of atom, eV	13,62	10,36	9,75	9,01	8,43
4. Relative electronegativity	3,50		2,48	2,01	1,76
5. The oxidation state in connections	1, -2,	2, +2, +4, +6	4, +6	4, +6	2, +2
6. Atomic radius, nm	0,066	0,104	0,117 0,137		0,164

Chalcogen atoms have the same structure of the external energy level - ns 2 nr 4 . This explains the similarity of their 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, i.e. 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.

The properties of simple substances naturally change: with an increase incharge 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.