Carbon in the periodic table. Carbon dioxide

Physical properties: carbon forms many allotropic modifications: diamond- one of the hardest substances graphite, coal, soot.

A carbon atom has 6 electrons: 1s 2 2s 2 2p 2 . The last two electrons are located in separate p-orbitals and are unpaired. In principle, this pair could occupy the same orbital, but in this case the interelectron repulsion greatly increases. For this reason, one of them takes 2p x, and the other, either 2p y , or 2p z orbitals.

The difference in the energy of the s- and p-sublevels of the outer layer is small, so the atom quite easily goes into an excited state, in which one of the two electrons from the 2s orbital passes to a free one 2 rub. A valence state appears with the configuration 1s 2 2s 1 2p x 1 2p y 1 2p z 1 . It is this state of the carbon atom that is characteristic of the diamond lattice—tetrahedral spatial arrangement of hybrid orbitals, identical length and energy of bonds.

This phenomenon is known to be called sp 3 -hybridization, and the emerging functions are sp 3 -hybrid . The formation of four sp 3 bonds provides the carbon atom with a more stable state than three r-r- and one s-s-connection. In addition to sp 3 hybridization, sp 2 and sp hybridization is also observed at the carbon atom . In the first case, mutual overlap occurs s- and two p-orbitals. Three equivalent sp 2 hybrid orbitals are formed, located in the same plane at an angle of 120° to each other. The third orbital p is unchanged and directed perpendicular to the plane sp2.

During sp hybridization, the s and p orbitals overlap. An angle of 180° arises between the two equivalent hybrid orbitals that are formed, while the two p-orbitals of each atom remain unchanged.

Allotropy of carbon. Diamond and graphite

In a graphite crystal, carbon atoms are located in parallel planes, occupying the vertices of regular hexagons. Each carbon atom is connected to three neighboring sp 2 hybrid bonds. The connection between parallel planes is carried out due to van der Waals forces. The free p-orbitals of each atom are directed perpendicular to the planes of covalent bonds. Their overlap explains the additional π bond between the carbon atoms. Thus, from the valence state in which the carbon atoms in a substance are located determines the properties of this substance.

Chemical properties of carbon

The most characteristic oxidation states are: +4, +2.

At low temperatures carbon is inert, but when heated its activity increases.

Carbon as a reducing agent:

- with oxygen
C 0 + O 2 – t° = CO 2 carbon dioxide
with a lack of oxygen - incomplete combustion:
2C 0 + O 2 – t° = 2C +2 O carbon monoxide

- with fluorine
C + 2F 2 = CF 4

- with water vapor
C 0 + H 2 O – 1200° = C +2 O + H 2 water gas

- with metal oxides. This is how metal is smelted from ore.
C 0 + 2CuO – t° = 2Cu + C +4 O 2

- with acids - oxidizing agents:
C 0 + 2H 2 SO 4 (conc.) = C +4 O 2 + 2SO 2 + 2H 2 O
C 0 + 4HNO 3 (conc.) = C +4 O 2 + 4NO 2 + 2H 2 O

- forms carbon disulfide with sulfur:
C + 2S 2 = CS 2.

Carbon as an oxidizing agent:

- forms carbides with some metals

4Al + 3C 0 = Al 4 C 3

Ca + 2C 0 = CaC 2 -4

- with hydrogen - methane (as well as a huge number of organic compounds)

C0 + 2H2 = CH4

— with silicon, forms carborundum (at 2000 °C in an electric furnace):

Finding carbon in nature

Free carbon occurs in the form of diamond and graphite. In the form of compounds, carbon is found in minerals: chalk, marble, limestone - CaCO 3, dolomite - MgCO 3 *CaCO 3; hydrocarbonates - Mg(HCO 3) 2 and Ca(HCO 3) 2, CO 2 is part of the air; Carbon is the main component of natural organic compounds - gas, oil, coal, peat, and is part of organic substances, proteins, fats, carbohydrates, amino acids that make up living organisms.

Inorganic carbon compounds

Neither C 4+ nor C 4- ions are formed during any conventional chemical processes: carbon compounds contain covalent bonds of different polarities.

Carbon monoxide CO

Carbon monoxide; colorless, odorless, slightly soluble in water, soluble in organic solvents, toxic, boiling point = -192°C; t pl. = -205°C.

Receipt
1) In industry (in gas generators):
C + O 2 = CO 2

2) In the laboratory - thermal decomposition of formic or oxalic acid in the presence of H 2 SO 4 (conc.):
HCOOH = H2O + CO

H 2 C 2 O 4 = CO + CO 2 + H 2 O

Chemical properties

Under normal conditions, CO is inert; when heated - a reducing agent; non-salt-forming oxide.

1) with oxygen

2C +2 O + O 2 = 2C +4 O 2

2) with metal oxides

C +2 O + CuO = Cu + C +4 O 2

3) with chlorine (in the light)

CO + Cl 2 – hn = COCl 2 (phosgene)

4) reacts with alkali melts (under pressure)

CO + NaOH = HCOONa (sodium formate)

5) forms carbonyls with transition metals

Ni + 4CO – t° = Ni(CO) 4

Fe + 5CO – t° = Fe(CO) 5

Carbon monoxide (IV) CO2

Carbon dioxide, colorless, odorless, solubility in water - 0.9V CO 2 dissolves in 1V H 2 O (under normal conditions); heavier than air; t°pl. = -78.5°C (solid CO 2 is called “dry ice”); does not support combustion.

Receipt

1. Thermal decomposition of carbonic acid salts (carbonates). Limestone firing:

CaCO 3 – t° = CaO + CO 2

1. The action of strong acids on carbonates and bicarbonates:

CaCO 3 + 2HCl = CaCl 2 + H 2 O + CO 2

NaHCO 3 + HCl = NaCl + H 2 O + CO 2

ChemicalpropertiesCO2
Acid oxide: Reacts with basic oxides and bases to form carbonic acid salts

Na 2 O + CO 2 = Na 2 CO 3

2NaOH + CO 2 = Na 2 CO 3 + H 2 O

NaOH + CO 2 = NaHCO 3

At elevated temperatures may exhibit oxidizing properties

C +4 O 2 + 2Mg – t° = 2Mg +2 O + C 0

Qualitative reaction

Cloudiness of lime water:

Ca(OH) 2 + CO 2 = CaCO 3 ¯ (white precipitate) + H 2 O

It disappears when CO 2 is passed through lime water for a long time, because insoluble calcium carbonate turns into soluble bicarbonate:

CaCO 3 + H 2 O + CO 2 = Ca(HCO 3) 2

Carbonic acid and itssalt

H 2CO 3 - A weak acid, it exists only in aqueous solution:

CO 2 + H 2 O ↔ H 2 CO 3

Dibasic:
H 2 CO 3 ↔ H + + HCO 3 - Acid salts - bicarbonates, bicarbonates
HCO 3 - ↔ H + + CO 3 2- Medium salts - carbonates

All properties of acids are characteristic.

Carbonates and bicarbonates can transform into each other:

2NaHCO 3 – t° = Na 2 CO 3 + H 2 O + CO 2

Na 2 CO 3 + H 2 O + CO 2 = 2NaHCO 3

Metal carbonates (except alkali metals) decarboxylate when heated to form an oxide:

CuCO 3 – t° = CuO + CO 2

Qualitative reaction- “boiling” under the influence of a strong acid:

Na 2 CO 3 + 2HCl = 2NaCl + H 2 O + CO 2

CO 3 2- + 2H + = H 2 O + CO 2

Carbides

Calcium carbide:

CaO + 3 C = CaC 2 + CO

CaC 2 + 2 H 2 O = Ca(OH) 2 + C 2 H 2.

Acetylene is released when zinc, cadmium, lanthanum and cerium carbides react with water:

2 LaC 2 + 6 H 2 O = 2La(OH) 3 + 2 C 2 H 2 + H 2.

Be 2 C and Al 4 C 3 decompose with water to form methane:

Al 4 C 3 + 12 H 2 O = 4 Al(OH) 3 = 3 CH 4.

In technology, titanium carbides TiC, tungsten W 2 C (hard alloys), silicon SiC (carborundum - as an abrasive and material for heaters) are used.

Cyanide

obtained by heating soda in an atmosphere of ammonia and carbon monoxide:

Na 2 CO 3 + 2 NH 3 + 3 CO = 2 NaCN + 2 H 2 O + H 2 + 2 CO 2

Hydrocyanic acid HCN is an important product of the chemical industry and is widely used in organic synthesis. Its global production reaches 200 thousand tons per year. The electronic structure of the cyanide anion is similar to carbon monoxide (II); such particles are called isoelectronic:

C = O: [:C = N:] –

Cyanides (0.1-0.2% aqueous solution) are used in gold mining:

2 Au + 4 KCN + H 2 O + 0.5 O 2 = 2 K + 2 KOH.

When boiling solutions of cyanide with sulfur or melting solids, they form thiocyanates:
KCN + S = KSCN.

When cyanides of low-active metals are heated, cyanide is obtained: Hg(CN) 2 = Hg + (CN) 2. Cyanide solutions are oxidized to cyanates:

2 KCN + O 2 = 2 KOCN.

Cyanic acid exists in two forms:

H-N=C=O; H-O-C = N:

In 1828, Friedrich Wöhler (1800-1882) obtained urea from ammonium cyanate: NH 4 OCN = CO(NH 2) 2 by evaporating an aqueous solution.

This event is usually regarded as the victory of synthetic chemistry over "vitalistic theory".

There is an isomer of cyanic acid - explosive acid

H-O-N=C.
Its salts (mercuric fulminate Hg(ONC) 2) are used in impact igniters.

Synthesis urea(urea):

CO 2 + 2 NH 3 = CO(NH 2) 2 + H 2 O. At 130 0 C and 100 atm.

Urea is a carbonic acid amide; there is also its “nitrogen analogue” – guanidine.

Carbonates

The most important inorganic carbon compounds are salts of carbonic acid (carbonates). H 2 CO 3 is a weak acid (K 1 = 1.3 10 -4; K 2 = 5 10 -11). Carbonate buffer supports carbon dioxide balance in the atmosphere. The world's oceans have enormous buffer capacity because they are an open system. The main buffer reaction is the equilibrium during the dissociation of carbonic acid:

H 2 CO 3 ↔ H + + HCO 3 - .

When acidity decreases, additional absorption of carbon dioxide from the atmosphere occurs with the formation of acid:
CO 2 + H 2 O ↔ H 2 CO 3 .

As acidity increases, carbonate rocks (shells, chalk and limestone sediments in the ocean) dissolve; this compensates for the loss of hydrocarbonate ions:

H + + CO 3 2- ↔ HCO 3 —

CaCO 3 (solid) ↔ Ca 2+ + CO 3 2-

Solid carbonates turn into soluble bicarbonates. It is this process of chemical dissolution of excess carbon dioxide that counteracts the “greenhouse effect” - global warming due to the absorption of thermal radiation from the Earth by carbon dioxide. About a third of the world's production of soda (sodium carbonate Na 2 CO 3) is used in glass production.

Carbon dioxide, carbon monoxide, carbon dioxide - all these are names for one substance known to us as carbon dioxide. So what properties does this gas have, and what are its areas of application?

Carbon dioxide and its physical properties

Carbon dioxide consists of carbon and oxygen. The formula for carbon dioxide looks like this – CO₂. In nature, it is formed during the combustion or decay of organic substances. The gas content in the air and mineral springs is also quite high. In addition, humans and animals also emit carbon dioxide when they exhale.

Rice. 1. Carbon dioxide molecule.

Carbon dioxide is a completely colorless gas and cannot be seen. It also has no smell. However, with high concentrations, a person may develop hypercapnia, that is, suffocation. Lack of carbon dioxide can also cause health problems. As a result of a lack of this gas, the opposite condition to suffocation can develop - hypocapnia.

If you place carbon dioxide in low temperature conditions, then at -72 degrees it crystallizes and becomes like snow. Therefore, carbon dioxide in a solid state is called “dry snow.”

Rice. 2. Dry snow – carbon dioxide.

Carbon dioxide is 1.5 times denser than air. Its density is 1.98 kg/m³. The chemical bond in the carbon dioxide molecule is polar covalent. It is polar due to the fact that oxygen has a higher electronegativity value.

An important concept in the study of substances is molecular and molar mass. The molar mass of carbon dioxide is 44. This number is formed from the sum of the relative atomic masses of the atoms that make up the molecule. The values ​​of relative atomic masses are taken from the table of D.I. Mendeleev and are rounded to whole numbers. Accordingly, the molar mass of CO₂ = 12+2*16.

To calculate the mass fractions of elements in carbon dioxide, it is necessary to follow the formula for calculating the mass fractions of each chemical element in a substance.

n– number of atoms or molecules.
A r– relative atomic mass of a chemical element.
Mr– relative molecular mass of the substance.
Let's calculate the relative molecular mass of carbon dioxide.

Mr(CO₂) = 14 + 16 * 2 = 44 w(C) = 1 * 12 / 44 = 0.27 or 27% Since the formula of carbon dioxide includes two oxygen atoms, then n = 2 w(O) = 2 * 16 / 44 = 0.73 or 73%

Answer: w(C) = 0.27 or 27%; w(O) = 0.73 or 73%

Chemical and biological properties of carbon dioxide

Carbon dioxide has acidic properties because it is an acidic oxide, and when dissolved in water it forms carbonic acid:

CO₂+H₂O=H₂CO₃

Reacts with alkalis, resulting in the formation of carbonates and bicarbonates. This gas does not burn. Only certain active metals, such as magnesium, burn in it.

When heated, carbon dioxide breaks down into carbon monoxide and oxygen:

2CO₃=2CO+O₃.

Like other acidic oxides, this gas easily reacts with other oxides:

СaO+Co₃=CaCO₃.

Carbon dioxide is part of all organic substances. The circulation of this gas in nature is carried out with the help of producers, consumers and decomposers. In the process of life, a person produces approximately 1 kg of carbon dioxide per day. When we inhale, we receive oxygen, but at this moment carbon dioxide is formed in the alveoli. At this moment, an exchange occurs: oxygen enters the blood, and carbon dioxide comes out.

Carbon dioxide is produced during the production of alcohol. This gas is also a by-product in the production of nitrogen, oxygen and argon. The use of carbon dioxide is necessary in the food industry, where carbon dioxide acts as a preservative, and carbon dioxide in liquid form is found in fire extinguishers.

Rice. 3. Fire extinguisher.

What have we learned?

Carbon dioxide is a substance that under normal conditions is colorless and odorless. In addition to its common name, carbon dioxide, it is also called carbon monoxide or carbon dioxide.

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Carbon (English Carbon, French Carbone, German Kohlenstoff) in the form of coal, soot and soot has been known to mankind since time immemorial; about 100 thousand years ago, when our ancestors mastered fire, they dealt with coal and soot every day. Probably, very early people became acquainted with allotropic modifications of carbon - diamond and graphite, as well as fossil coal. It is not surprising that the combustion of carbon-containing substances was one of the first chemical processes to interest man. Since the burning substance disappeared when consumed by fire, combustion was considered a process of decomposition of the substance, and therefore coal (or carbon) was not considered an element. The element was fire - a phenomenon accompanying combustion; In ancient teachings about the elements, fire usually appears as one of the elements. At the turn of the XVII - XVIII centuries. The phlogiston theory arose, put forward by Becher and Stahl. This theory recognized the presence in each combustible body of a special elementary substance - a weightless fluid - phlogiston, which evaporates during the combustion process. Since when a large amount of coal is burned, only a little ash remains, phlogistics believed that coal was almost pure phlogiston. This is what explained, in particular, the “phlogisticating” effect of coal - its ability to restore metals from “limes” and ores. Later phlogistics, Reaumur, Bergman and others, already began to understand that coal is an elementary substance. However, “clean coal” was first recognized as such by Lavoisier, who studied the process of combustion of coal and other substances in air and oxygen. In the book "Method of Chemical Nomenclature" (1787) by Guiton de Morveau, Lavoisier, Berthollet and Fourcroix, the name "carbon" (carbone) appeared instead of the French "pure coal" (charbone pur). Under the same name, carbon appears in the “Table of Simple Bodies” in Lavoisier’s “Elementary Textbook of Chemistry.” In 1791, the English chemist Tennant was the first to obtain free carbon; he passed phosphorus vapor over calcined chalk, resulting in the formation of calcium phosphate and carbon. It has been known for a long time that diamond burns without leaving a residue when heated strongly. Back in 1751, the French king Francis I agreed to give diamond and ruby ​​for combustion experiments, after which these experiments even became fashionable. It turned out that only diamond burns, and ruby ​​(aluminum oxide with an admixture of chromium) can withstand prolonged heating at the focus of the ignition lens without damage. Lavoisier carried out a new experiment on burning diamonds using a large incendiary machine and came to the conclusion that diamond is crystalline carbon. The second allotrope of carbon - graphite in the alchemical period was considered a modified lead luster and was called plumbago; It was only in 1740 that Pott discovered the absence of any lead impurity in graphite. Scheele studied graphite (1779) and, being a phlogistician, considered it a special kind of sulfur body, a special mineral coal containing bound “aerial acid” (CO 2) and a large amount of phlogiston.

Twenty years later, Guiton de Morveau turned diamond into graphite and then into carbonic acid by careful heating.

The international name Carboneum comes from the Latin. carbo (coal). This word is of very ancient origin. It is compared with cremare - to burn; root sag, cal, Russian gar, gal, gol, Sanskrit sta means to boil, cook. The word "carbo" is associated with the names of carbon in other European languages ​​(carbon, charbone, etc.). German Kohlenstoff comes from Kohle - coal (Old German kolo, Swedish kylla - to heat). Old Russian ugorati, or ugarati (to burn, scorch) has the root gar, or mountains, with a possible transition to gol; coal in Old Russian yugal, or coal, of the same origin. The word diamond (Diamante) comes from the ancient Greek - indestructible, unyielding, hard, and graphite from the Greek - I write.

Carbon(Latin carboneum), C, chemical element of group IV of the periodic system of Mendeleev, atomic number 6, atomic mass 12.011. Two stable isotopes are known: 12 c (98.892%) and 13 c (1.108%). Of the radioactive isotopes, the most important is 14 s with a half-life (T = 5.6 × 10 3 years). Small amounts of 14 c (about 2 × 10 -10% by mass) are constantly formed in the upper layers of the atmosphere under the action of cosmic radiation neutrons on the nitrogen isotope 14 n. Based on the specific activity of the 14 c isotope in residues of biogenic origin, their age is determined. 14 c is widely used as .

Historical reference . U. has been known since ancient times. Charcoal served to restore metals from ores, diamond - as a precious stone. Much later, graphite began to be used to make crucibles and pencils.

In 1778 K. Scheele, heating graphite with saltpeter, I discovered that in this case, as when heating coal with saltpeter, carbon dioxide is released. The chemical composition of diamond was established as a result of experiments by A. Lavoisier(1772) on the study of diamond combustion in air and the research of S. Tennant(1797), who proved that equal amounts of diamond and coal produce equal amounts of carbon dioxide during oxidation. U. was recognized as a chemical element in 1789 by Lavoisier. U. received the Latin name carboneum from carbo - coal.

Distribution in nature. The average uranium content in the earth's crust is 2.3? 10 -2% by weight (1 ? 10 -2 in ultrabasic, 1 ? 10 -2 - in basic, 2 ? 10 -2 - in medium, 3 ? 10 -2 - V acidic rocks). U. accumulates in the upper part of the earth's crust (biosphere): in living matter 18% U., wood 50%, coal 80%, oil 85%, anthracite 96%. A significant part of the U. lithosphere is concentrated in limestones and dolomites.

The number of U.'s own minerals is 112; The number of organic compounds of hydrocarbons and their derivatives is exceptionally large.

The accumulation of carbon in the earth's crust is associated with the accumulation of many other elements that are sorbed by organic matter and precipitated in the form of insoluble carbonates, etc. Co 2 and carbonic acid play a major geochemical role in the earth's crust. A huge amount of co2 is released during volcanism - in the history of the Earth this was the main source of carbon dioxide for the biosphere.

Compared to the average content in the earth's crust, humanity extracts uranium from the subsoil (coal, oil, natural gas) in exceptionally large quantities, since these minerals are the main source of energy.

The uranium cycle is of great geochemical importance.

U. is also widespread in space; on the Sun it ranks 4th after hydrogen, helium and oxygen.

Physical and chemical properties. Four crystalline modifications of carbon are known: graphite, diamond, carbine, and lonsdaleite. Graphite is a gray-black, opaque, greasy to the touch, scaly, very soft mass with a metallic sheen. Constructed from crystals of hexagonal structure: a=2.462 a, c=6.701 a. At room temperature and normal pressure (0.1 Mn/m 2, or 1 kgf/cm 2) graphite is thermodynamically stable. Diamond is a very hard, crystalline substance. The crystals have a face-centered cubic lattice: a = 3,560 a. At room temperature and normal pressure, diamond is metastable (for details on the structure and properties of diamond and graphite, see the relevant articles). A noticeable transformation of diamond into graphite is observed at temperatures above 1400 °C in a vacuum or in an inert atmosphere. At atmospheric pressure and a temperature of about 3700 °C, graphite sublimes. Liquid U. can be obtained at pressures above 10.5 Mn/m 2(105 kgf/cm 2) and temperatures above 3700 °C. For hard U. ( coke, soot, charcoal) a state with a disordered structure is also characteristic - the so-called “amorphous” U., which does not represent an independent modification; Its structure is based on the structure of fine-crystalline graphite. Heating some varieties of “amorphous” carbon above 1500-1600 °C without access to air causes their transformation into graphite. The physical properties of “amorphous” carbon are very dependent on the dispersion of particles and the presence of impurities. The density, heat capacity, thermal conductivity, and electrical conductivity of “amorphous” carbon are always higher than that of graphite. Carbyne is obtained artificially. It is a fine-crystalline black powder (density 1.9-2 g/cm 3) . Constructed from long chains of C atoms arranged parallel to each other. Lonsdaleite is found in meteorites and obtained artificially; its structure and properties have not been definitively established.

Configuration of the outer electron shell of the U atom. 2s 2 2p 2 . Carbon is characterized by the formation of four covalent bonds, due to the excitation of the outer electron shell to state 2 sp3. Therefore, carbon is equally capable of both attracting and donating electrons. Chemical bonding can occur due to sp 3 -, sp 2 - And sp-hybrid orbitals, which correspond to coordination numbers of 4, 3, and 2. The number of valence electrons of the electron and the number of valence orbitals are the same; This is one of the reasons for the stability of the bond between U atoms.

The unique ability of uranium atoms to connect with each other to form strong and long chains and cycles has led to the emergence of a huge number of different uranium compounds being studied. organic chemistry.

In compounds, uranium exhibits an oxidation state of -4; +2; +4. Atomic radius 0.77 a, covalent radii 0.77 a, 0.67 a, 0.60 a, respectively, in single, double and triple bonds; ionic radius c 4- 2.60 a , c 4+ 0.20 a . Under normal conditions, uranium is chemically inert; at high temperatures it combines with many elements, exhibiting strong reducing properties. Chemical activity decreases in the following order: “amorphous” carbon, graphite, diamond; interaction with air oxygen (combustion) occurs respectively at temperatures above 300-500 °C, 600-700 °C and 850-1000 °C with the formation of carbon dioxide co 2 and carbon monoxide co.

co 2 dissolves in water to form carbonic acid. In 1906 O. Diels received suboxide U. c 3 o 2. All forms of U. are resistant to alkalis and acids and are slowly oxidized only by very strong oxidizing agents (chromic mixture, a mixture of concentrated hno 3 and kclo 3, etc.). “Amorphous” U. reacts with fluorine at room temperature, graphite and diamond - when heated. The direct connection of carbon dioxide with chlorine occurs in an electric arc; U. does not react with bromine and iodine, therefore numerous carbon halides synthesized indirectly. Of the oxyhalides of the general formula cox 2 (where X is halogen), the best known is oxychloride cocl 2 ( phosgene) . Hydrogen does not interact with diamond; reacts with graphite and “amorphous” carbon at high temperatures in the presence of catalysts (ni, pt): at 600-1000 °C, mainly methane ch 4 is formed, at 1500-2000 ° C - acetylene c 2 h 2 , Other hydrocarbons may also be present in the products, for example ethane c 2 h 6 , benzene c 6 h 6 . The interaction of sulfur with “amorphous” carbon and graphite begins at 700-800 °C, with diamond at 900-1000 °C; in all cases, carbon disulfide cs 2 is formed. Dr. U. compounds containing sulfur (cs thioxide, c 3 s 2 thioxide, cos sulfide and thiophosgene cscl 2) are obtained indirectly. When cs 2 interacts with metal sulfides, thiocarbonates are formed - salts of weak thiocarbonic acid. The interaction of carbon dioxide with nitrogen to produce cyanogen (cn) 2 occurs when an electric discharge is passed between carbon electrodes in a nitrogen atmosphere. Among the nitrogen-containing compounds of uranium, hydrogen cyanide hcn and its numerous derivatives: cyanides, halo-halogenates, nitriles, etc. are of great practical importance. At temperatures above 1000 °C, uranium interacts with many metals, giving carbides. All forms of carbon, when heated, reduce metal oxides with the formation of free metals (zn, cd, cu, pb, etc.) or carbides (cac 2, mo 2 c, wo, tac, etc.). U. reacts at temperatures above 600-800 ° C with water vapor and carbon dioxide . A distinctive feature of graphite is the ability, when moderately heated to 300-400 °C, to interact with alkali metals and halides to form switching connections type c 8 me, c 24 me, c 8 x (where X is halogen, me is metal). Known compounds include graphite with hno 3, h 2 so 4, fecl 3, etc. (for example, graphite bisulfate c 24 so 4 h 2). All forms of uranium are insoluble in ordinary inorganic and organic solvents, but dissolve in some molten metals (for example, fe, ni, co).

The national economic importance of energy is determined by the fact that over 90% of all primary sources of energy consumed in the world come from organic sources. fuel, whose dominant role will continue for the coming decades, despite the intensive development of nuclear energy. Only about 10% of extracted fuel is used as raw material for basic organic synthesis And petrochemical synthesis, for getting plastics and etc.

B. A. Popovkin.

U. in the body . U. is the most important biogenic element that forms the basis of life on Earth, a structural unit of a huge number of organic compounds involved in the construction of organisms and ensuring their vital functions ( biopolymers, as well as numerous low-molecular biologically active substances - vitamins, hormones, mediators, etc.). A significant part of the energy necessary for organisms is formed in cells due to the oxidation of carbon. The emergence of life on Earth is considered in modern science as a complex process of the evolution of carbon compounds .

The unique role of carbon in living nature is due to its properties, which in the aggregate are not possessed by any other element of the periodic system. Strong chemical bonds are formed between carbon atoms, as well as between carbon and other elements, which, however, can be broken under relatively mild physiological conditions (these bonds can be single, double, or triple). The ability of carbon to form four equivalent valence bonds with other carbon atoms makes it possible to construct carbon skeletons of various types—linear, branched, and cyclic. It is significant that only three elements - C, O and H - make up 98% of the total mass of living organisms. This achieves a certain efficiency in living nature: with an almost limitless structural diversity of carbon compounds, a small number of types of chemical bonds makes it possible to significantly reduce the number of enzymes necessary for the breakdown and synthesis of organic substances. The structural features of the carbon atom underlie the various types isomerism organic compounds (the ability for optical isomerism turned out to be decisive in the biochemical evolution of amino acids, carbohydrates and some alkaloids).

According to the generally accepted hypothesis of A.I. Oparina, The first organic compounds on Earth were of abiogenic origin. The sources of hydrogen were methane (ch 4) and hydrogen cyanide (hcn), contained in the primary atmosphere of the Earth. With the emergence of life, the only source of inorganic carbon, due to which all organic matter of the biosphere is formed, is carbon dioxide(co 2), located in the atmosphere, and also dissolved in natural waters in the form of hco - 3. The most powerful mechanism for assimilation (assimilation) of U. (in the form of co 2) - photosynthesis - carried out everywhere by green plants (about 100 billion are assimilated annually). T co 2). On Earth, there is an evolutionarily more ancient method of assimilating co 2 by chemosynthesis; in this case, chemosynthetic microorganisms use not the radiant energy of the Sun, but the energy of oxidation of inorganic compounds. Most animals consume uranium with food in the form of ready-made organic compounds. Depending on the method of assimilation of organic compounds, it is customary to distinguish autotrophic organisms And heterotrophic organisms. Use of microorganisms for the biosynthesis of protein and other nutrients using U as the only source. hydrocarbons oil is one of the important modern scientific and technical problems.

The U content in living organisms calculated on a dry matter basis is: 34.5-40% in aquatic plants and animals, 45.4-46.5% in terrestrial plants and animals, and 54% in bacteria. During the life of organisms, mainly due to tissue respiration, oxidative decomposition of organic compounds occurs with the release of co 2 into the external environment. U. is also released as part of more complex metabolic end products. After the death of animals and plants, part of the carbon is again converted into co2 as a result of decay processes carried out by microorganisms. This is how the cycle of carbon occurs in nature . A significant part of the uranium is mineralized and forms deposits of fossil uranium: coal, oil, limestone, etc. In addition to the main functions - the source of uranium - co 2, dissolved in natural waters and in biological fluids, participates in maintaining the optimal acidity of the environment for life processes . As part of caco 3, U. forms the exoskeleton of many invertebrates (for example, mollusk shells), and is also found in corals, bird eggshells, etc. U. compounds such as hcn, co, ccl 4, which prevailed in the primary atmosphere of the Earth in prebiological period, later, in the process of biological evolution, turned into strong antimetabolites metabolism.

In addition to the stable isotopes of carbon, radioactive 14c is widespread in nature (the human body contains about 0.1 mccurie) . The use of uranium isotopes in biological and medical research is associated with many major achievements in the study of metabolism and the uranium cycle in nature. . Thus, with the help of a radiocarbon tag, the possibility of fixation of h 14 co - 3 by plants and animal tissues was proven, the sequence of photosynthesis reactions was established, the metabolism of amino acids was studied, the paths of biosynthesis of many biologically active compounds were traced, etc. The use of 14 c contributed to the success of molecular biology in the study of the mechanisms of protein biosynthesis and the transmission of hereditary information. Determining the specific activity of 14 c in carbon-containing organic residues makes it possible to judge their age, which is used in paleontology and archeology.

N. N. Chernov.

Lit.: Shafranovsky I.I., Almazy, M. - L., 1964; Ubbelohde A.R., Lewis F.A., Graphite and its crystalline compounds, trans. from English, M., 1965; Remi G., Course of inorganic chemistry, trans. from German, vol. 1, M., 1972; Perelman A.I., Geochemistry of elements in the hypergenesis zone, M., 1972; Nekrasov B.V., Fundamentals of General Chemistry, 3rd ed., M., 1973; Akhmetov N.S., Inorganic chemistry, 2nd ed., M., 1975; Vernadsky V.I., Essays on Geochemistry, 6th ed., M., 1954; Roginsky S.Z., Shnol S.E., Isotopes in biochemistry, M., 1963; Horizons of biochemistry, trans. from English, M., 1964; Problems of evolutionary and technical biochemistry, M., 1964; Calvin M., Chemical evolution, trans. from English, M., 1971; Löwy A., Sikiewitz F., Cell structure and function, trans. from English, 1971, ch. 7; Biosphere, trans. from English, M., 1972.

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Carbon C is number 6 in Mendeleev's periodic table. Even primitive people noticed that after burning wood, coal is formed, which can be used to draw on the walls of a cave. All organic compounds contain carbon. The two most studied allotropic modifications of carbon are graphite and diamond.

Carbon in organic chemistry

Carbon occupies a special place in the periodic table. Due to its structure, it forms long chains of bonds of a linear or cyclic structure. More than 10 million organic compounds are known. Despite their diversity, in air and under the influence of temperature they will always turn into carbon dioxide and.

The role of carbon in our daily lives is enormous. Without carbon dioxide, photosynthesis, one of the main biological processes, will not occur.

Application of carbon

Carbon is widely used in medicine to create various organic medicines. Carbon isotopes allow radiocarbon dating. Without carbon, the metallurgical industry cannot operate. Coal burned in solid fuel pyrolysis boilers serves as a source of energy. In the oil refining industry, gasoline and diesel fuel are produced from organic carbon compounds. Much of the carbon is needed to produce sugar. It is also used in the synthesis of organic compounds important for all areas of everyday life.