Chromium(lat. Cromium), Cr, a chemical element of Group VI of the Mendeleev periodic system, atomic number 24, atomic mass 51.996; steel-blue metal.

Natural stable isotopes: 50 Cr (4.31%), 52 Cr (87.76%), 53 Cr (9.55%) and 54 Cr (2.38%). Of the artificial radioactive isotopes, the most important is 51 Cr (half-life T ½ = 27.8 days), which is used as an isotope tracer.

History reference. Chromium was discovered in 1797 by LN Vauquelin in the mineral crocoite - natural lead chromate РbCrО 4 . Chrome got its name from the Greek word chroma - color, paint (because of the variety of colors of its compounds). Independently of Vauquelin, chromium was discovered in crocoite in 1798 by the German scientist M. G. Klaproth.

Distribution of Chromium in nature. The average content of Chromium in the earth's crust (clarke) is 8.3·10 -3%. This element is probably more characteristic of the Earth's mantle, since ultramafic rocks, which are believed to be closest in composition to the Earth's mantle, are enriched in Chromium (2·10 -4%). Chromium forms massive and disseminated ores in ultramafic rocks; the formation of the largest deposits of Chromium is associated with them. In basic rocks, the content of Chromium reaches only 2 10 -2%, in acidic rocks - 2.5 10 -3%, in sedimentary rocks (sandstones) - 3.5 10 -3%, shale - 9 10 -3 %. Chromium is a comparatively weak water migrant; Chromium content in sea water is 0.00005 mg/l.

In general, Chromium is a metal of the deep zones of the Earth; stony meteorites (analogues of the mantle) are also enriched in Chromium (2.7·10 -1%). Over 20 chromium minerals are known. Only chrome spinels (up to 54% Cr) are of industrial importance; in addition, chromium is contained in a number of other minerals that often accompany chromium ores, but are of no practical value in themselves (uvarovite, volkonskoite, kemerite, fuchsite).

Physical properties of Chromium. Chromium is a hard, heavy, refractory metal. Pure Chrome is plastic. Crystallizes in a body-centered lattice, a = 2.885Å (20 °C); at 1830°C, transformation into a modification with a face-centered lattice is possible, a = 3.69Å.

Atomic radius 1.27 Å; ionic radii Cr 2+ 0.83Å, Cr 3+ 0.64Å, Cr 6+ 0.52 Å. Density 7.19 g/cm 3 ; t pl 1890 °C; t kip 2480 °C. Specific heat capacity 0.461 kJ/(kg K) (25°C); thermal coefficient of linear expansion 8.24 10 -6 (at 20 °C); thermal conductivity coefficient 67 W/(m K) (20 °С); electrical resistivity 0.414 μm m (20 °C); the thermal coefficient of electrical resistance in the range of 20-600 °C is 3.01·10 -3 . Chromium is antiferromagnetic, specific magnetic susceptibility is 3.6·10 -6 . The hardness of high-purity Chromium according to Brinell is 7-9 MN / m 2 (70-90 kgf / cm 2).

Chemical properties of Chromium. The external electron configuration of the Chromium atom is 3d 5 4s 1 . In compounds, it usually exhibits oxidation states +2, +3, +6, among which Cr 3+ is the most stable; individual compounds are known in which Chromium has oxidation states +1, +4, +5. Chromium is chemically inactive. Under normal conditions, it is resistant to oxygen and moisture, but combines with fluorine, forming CrF 3 . Above 600 °C, it interacts with water vapor, giving Cr 2 O 3; nitrogen - Cr 2 N, CrN; carbon - Cr 23 C 6, Cr 7 C 3, Cr 3 C 2; gray - Cr 2 S 3. When fused with boron, it forms CrB boride; with silicon, it forms silicides Cr 3 Si, Cr 2 Si 3, CrSi 2. Chromium forms alloys with many metals. The interaction with oxygen proceeds at first quite actively, then it slows down sharply due to the formation of an oxide film on the metal surface. At 1200°C, the film breaks down and oxidation proceeds rapidly again. Chromium ignites in oxygen at 2000°C to form dark green chromium (III) oxide Cr 2 O 3 . In addition to the oxide (III), there are other compounds with oxygen, such as CrO, CrO 3 obtained indirectly. Chromium easily reacts with dilute solutions of hydrochloric and sulfuric acids to form chloride and chromium sulfate and release hydrogen; aqua regia and nitric acid passivate Chromium.

With an increase in the degree of oxidation, the acidic and oxidizing properties of Chromium increase. Cr 2+ derivatives are very strong reducing agents. The Cr 2+ ion is formed at the first stage of dissolution of Chromium in acids or during the reduction of Cr 3+ in an acidic solution with zinc. Nitrous hydrate Cr(OH) 2 during dehydration passes into Cr 2 O 3 . Cr 3+ compounds are stable in air. They can be both reducing and oxidizing agents. Cr 3+ can be reduced in an acidic solution with zinc to Cr 2+ or oxidized in an alkaline solution to CrO 4 2- with bromine and other oxidizing agents. Hydroxide Cr (OH) 3 (more precisely, Cr 2 O 3 nH 2 O) is an amphoteric compound that forms salts with the Cr 3+ cation or salts of chromic acid HCrO 2 - chromites (for example, KC-O 2, NaCrO 2). Cr 6+ compounds: CrO 3 chromic anhydride, chromic acids and their salts, among which the most important are chromates and dichromates - strong oxidizing agents. Chromium forms a large number of salts with oxygen-containing acids. Chromium complex compounds are known; complex compounds of Cr 3+ are especially numerous, in which Chromium has a coordination number of 6. There is a significant number of Chromium peroxide compounds

Get Chrome. Depending on the purpose of use, chromium is obtained in various degrees of purity. The raw material is usually chrome spinels, which are enriched and then fused with potash (or soda) in the presence of atmospheric oxygen. With regard to the main component of ores containing Cr 3 +, the reaction is as follows:

2FeCr 2 O 4 + 4K 2 CO 3 + 3.5O 2 \u003d 4K 2 CrO 4 + Fe 2 O 3 + 4CO 2.

The resulting potassium chromate K 2 CrO 4 is leached with hot water and the action of H 2 SO 4 converts it into dichromate K 2 Cr 2 O 7 . Further, by the action of a concentrated solution of H 2 SO 4 on K 2 Cr 2 O 7, chromic anhydride C 2 O 3 is obtained or by heating K 2 Cr 2 O 7 with sulfur - Chromium oxide (III) C 2 O 3.

The purest Chromium is obtained under industrial conditions either by electrolysis of concentrated aqueous solutions of CrO 3 or Cr 2 O 3 containing H 2 SO 4 , or by electrolysis of Chromium sulfate Cr 2 (SO 4) 3 . In this case, chromium is precipitated on an aluminum or stainless steel cathode. Complete purification from impurities is achieved by treating Chromium with highly pure hydrogen at high temperature (1500-1700 °C).

It is also possible to obtain pure Chromium by electrolysis of CrF 3 or CrCl 3 melts mixed with sodium, potassium, calcium fluorides at a temperature of about 900 °C in an argon atmosphere.

Chromium is obtained in small quantities by reduction of Cr 2 O 3 with aluminum or silicon. In the aluminothermic method, a preheated mixture of Cr 2 O 3 and Al powder or shavings with the addition of an oxidizing agent is loaded into a crucible, where the reaction is initiated by igniting a mixture of Na 2 O 2 and Al until the crucible is filled with Chromium and slag. Chromium is smelted silicothermally in arc furnaces. The purity of the resulting Chromium is determined by the content of impurities in Cr 2 O 3 and in Al or Si used for recovery.

In industry, chromium alloys are produced on a large scale - ferrochrome and silicochrome.

Chromium application. The use of Chromium is based on its heat resistance, hardness and corrosion resistance. Most of all Chromium is used for smelting chromium steels. Alumino- and silicothermic chromium is used for smelting nichrome, nimonic, other nickel alloys, and stellite.

A significant amount of Chromium is used for decorative corrosion-resistant coatings. Chromium powder has been widely used in the production of metal-ceramic products and materials for welding electrodes. Chromium in the form of the Cr 3+ ion is an impurity in ruby, which is used as a gemstone and laser material. Chromium compounds are used to etch fabrics during dyeing. Some Chromium salts are used as an ingredient in tanning solutions in the leather industry; PbCrO 4 , ZnCrO 4 , SrCrO 4 - as art paints. Chromite-magnesite refractory products are made from a mixture of chromite and magnesite.

Chromium compounds (especially Cr 6 + derivatives) are toxic.

Chromium in the body. Chromium is one of the biogenic elements that is constantly included in the tissues of plants and animals. The average content of Chromium in plants is 0.0005% (92-95% of Chromium accumulates in the roots), in animals - from ten thousandths to ten millionths of a percent. In planktonic organisms, the accumulation coefficient of Chromium is enormous - 10,000-26,000. Higher plants do not tolerate Chromium concentrations above 3-10 -4 mol/l. In leaves, it is present as a low molecular weight complex not associated with subcellular structures. In animals, chromium is involved in the metabolism of lipids, proteins (part of the trypsin enzyme), carbohydrates (a structural component of the glucose-resistant factor). The main source of Chromium in the body of animals and humans is food. A decrease in the content of Chromium in food and blood leads to a decrease in growth rate, an increase in blood cholesterol and a decrease in the sensitivity of peripheral tissues to insulin.

Chromium poisoning and its compounds occur during their production; in mechanical engineering (electroplated coatings); metallurgy (alloying additives, alloys, refractories); in the manufacture of leather, paints, etc. The toxicity of chromium compounds depends on their chemical structure: dichromates are more toxic than chromates, Cr (VI) compounds are more toxic than Cr (II), Cr (III) compounds. The initial forms of the disease are manifested by a feeling of dryness and pain in the nose, sore throat, difficulty breathing, coughing, etc.; they may disappear when contact with Chrome is discontinued. With prolonged contact with Chromium compounds, signs of chronic poisoning develop: headache, weakness, dyspepsia, weight loss, and others. Functions of a stomach, a liver and a pancreas are broken. Bronchitis, bronchial asthma, diffuse pneumosclerosis are possible. When exposed to Chromium, dermatitis and eczema may develop on the skin. According to some reports, Chromium compounds, mainly Cr(III), have a carcinogenic effect.

"National Research Tomsk Polytechnic University"

Institute of Natural Resources Geoecology and Geochemistry

Chromium

By discipline:

Chemistry

Completed:

student of group 2G41 Tkacheva Anastasia Vladimirovna 10/29/2014

Checked:

teacher Stas Nikolay Fedorovich

Position in the periodic system

Chromium- an element of a side subgroup of the 6th group of the 4th period of the periodic system of chemical elements of D. I. Mendeleev with atomic number 24. It is indicated by the symbol Cr(lat. Chromium). simple substance chromium- hard bluish-white metal. Chromium is sometimes referred to as a ferrous metal.

The structure of the atom

17 Cl) 2) 8) 7 - diagram of the structure of the atom

1s2s2p3s3p - electronic formula

The atom is located in period III, and has three energy levels

The atom is located in VII in the group, in the main subgroup - at the external energy level of 7 electrons

Element properties

Physical properties

Chromium is a white shiny metal with a cubic body-centered lattice, a \u003d 0.28845 nm, characterized by hardness and brittleness, with a density of 7.2 g / cm 3, one of the hardest pure metals (second only to beryllium, tungsten and uranium), with a melting point of 1903 degrees. And with a boiling point of about 2570 degrees. C. In air, the surface of chromium is covered with an oxide film, which protects it from further oxidation. The addition of carbon to chromium further increases its hardness.

Chemical properties

Chromium under normal conditions is an inert metal, when heated it becomes quite active.

Interaction with non-metals

When heated above 600°C, chromium burns in oxygen:

4Cr + 3O 2 \u003d 2Cr 2 O 3.

It reacts with fluorine at 350°C, with chlorine at 300°C, with bromine at a red heat temperature, forming chromium (III) halides:

2Cr + 3Cl 2 = 2CrCl 3 .

It reacts with nitrogen at temperatures above 1000°C to form nitrides:

2Cr + N 2 = 2CrN

or 4Cr + N 2 = 2Cr 2 N.

2Cr + 3S = Cr 2 S 3 .

Reacts with boron, carbon and silicon to form borides, carbides and silicides:

Cr + 2B = CrB 2 (the formation of Cr 2 B, CrB, Cr 3 B 4, CrB 4 is possible),

2Cr + 3C \u003d Cr 2 C 3 (the formation of Cr 23 C 6, Cr 7 B 3 is possible),

Cr + 2Si = CrSi 2 (possible formation of Cr 3 Si, Cr 5 Si 3, CrSi).

It does not interact directly with hydrogen.

Interaction with water

In a finely ground hot state, chromium reacts with water, forming chromium (III) oxide and hydrogen:

2Cr + 3H 2 O \u003d Cr 2 O 3 + 3H 2

Interaction with acids

In the electrochemical series of voltages of metals, chromium is before hydrogen, it displaces hydrogen from solutions of non-oxidizing acids:

Cr + 2HCl \u003d CrCl 2 + H 2;

Cr + H 2 SO 4 \u003d CrSO 4 + H 2.

In the presence of atmospheric oxygen, chromium (III) salts are formed:

4Cr + 12HCl + 3O 2 = 4CrCl 3 + 6H 2 O.

Concentrated nitric and sulfuric acids passivate chromium. Chromium can dissolve in them only with strong heating, chromium (III) salts and acid reduction products are formed:

2Cr + 6H 2 SO 4 = Cr 2 (SO 4) 3 + 3SO 2 + 6H 2 O;

Cr + 6HNO 3 \u003d Cr (NO 3) 3 + 3NO 2 + 3H 2 O.

Interaction with alkaline reagents

In aqueous solutions of alkalis, chromium does not dissolve; it slowly reacts with alkali melts to form chromites and release hydrogen:

2Cr + 6KOH \u003d 2KCrO 2 + 2K 2 O + 3H 2.

Reacts with alkaline melts of oxidizing agents, such as potassium chlorate, while chromium passes into potassium chromate:

Cr + KClO 3 + 2KOH = K 2 CrO 4 + KCl + H 2 O.

Recovery of metals from oxides and salts

Chromium is an active metal, capable of displacing metals from solutions of their salts: 2Cr + 3CuCl 2 = 2CrCl 3 + 3Cu.

Properties of a simple substance

Stable in air due to passivation. For the same reason, it does not react with sulfuric and nitric acids. At 2000 °C, it burns out with the formation of green chromium (III) oxide Cr 2 O 3, which has amphoteric properties.

Synthesized compounds of chromium with boron (borides Cr 2 B, CrB, Cr 3 B 4, CrB 2, CrB 4 and Cr 5 B 3), with carbon (carbides Cr 23 C 6, Cr 7 C 3 and Cr 3 C 2), with silicon (silicides Cr 3 Si, Cr 5 Si 3 and CrSi) and nitrogen (nitrides CrN and Cr 2 N).

Cr(+2) compounds

The oxidation state +2 corresponds to the basic oxide CrO (black). Cr 2+ salts (blue solutions) are obtained by reducing Cr 3+ salts or dichromates with zinc in an acidic environment (“hydrogen at the time of isolation”):

All these Cr 2+ salts are strong reducing agents, to the extent that they displace hydrogen from water upon standing. Oxygen in the air, especially in an acidic environment, oxidizes Cr 2+, as a result of which the blue solution quickly turns green.

Brown or yellow Cr(OH) 2 hydroxide precipitates when alkalis are added to solutions of chromium(II) salts.

Chromium dihalides CrF 2 , CrCl 2 , CrBr 2 and CrI 2 were synthesized

Cr(+3) compounds

The +3 oxidation state corresponds to the amphoteric oxide Cr 2 O 3 and the hydroxide Cr (OH) 3 (both green). This is the most stable oxidation state of chromium. Chromium compounds in this oxidation state have a color from dirty purple (ion 3+) to green (anions are present in the coordination sphere).

Cr 3+ is prone to the formation of double sulfates of the form M I Cr (SO 4) 2 12H 2 O (alum)

Chromium (III) hydroxide is obtained by acting with ammonia on solutions of chromium (III) salts:

Cr+3NH+3H2O→Cr(OH)↓+3NH

Alkali solutions can be used, but in their excess a soluble hydroxo complex is formed:

Cr+3OH→Cr(OH)↓

Cr(OH)+3OH→

By fusing Cr 2 O 3 with alkalis, chromites are obtained:

Cr2O3+2NaOH→2NaCrO2+H2O

Uncalcined chromium (III) oxide dissolves in alkaline solutions and in acids:

Cr2O3+6HCl→2CrCl3+3H2O

When chromium(III) compounds are oxidized in an alkaline medium, chromium(VI) compounds are formed:

2Na+3HO→2NaCrO+2NaOH+8HO

The same thing happens when chromium (III) oxide is fused with alkali and oxidizing agents, or with alkali in air (the melt becomes yellow in this case):

2Cr2O3+8NaOH+3O2→4Na2CrO4+4H2O

Chromium compounds (+4)[

With careful decomposition of chromium oxide (VI) CrO 3 under hydrothermal conditions, chromium oxide (IV) CrO 2 is obtained, which is ferromagnetic and has metallic conductivity.

Among chromium tetrahalides, CrF 4 is stable, chromium tetrachloride CrCl 4 exists only in vapor.

Chromium compounds (+6)

The +6 oxidation state corresponds to acidic chromium oxide (VI) CrO 3 and a number of acids between which there is an equilibrium. The simplest of them are chromic H 2 CrO 4 and two-chrome H 2 Cr 2 O 7 . They form two series of salts: yellow chromates and orange dichromates, respectively.

Chromium oxide (VI) CrO 3 is formed by the interaction of concentrated sulfuric acid with solutions of dichromates. A typical acid oxide, when interacting with water, it forms strong unstable chromic acids: chromic H 2 CrO 4, dichromic H 2 Cr 2 O 7 and other isopoly acids with the general formula H 2 Cr n O 3n+1. An increase in the degree of polymerization occurs with a decrease in pH, that is, an increase in acidity:

2CrO+2H→Cr2O+H2O

But if an alkali solution is added to an orange solution of K 2 Cr 2 O 7, how does the color turn yellow again, since chromate K 2 CrO 4 is formed again:

Cr2O+2OH→2CrO+HO

It does not reach a high degree of polymerization, as occurs in tungsten and molybdenum, since polychromic acid decomposes into chromium (VI) oxide and water:

H2CrnO3n+1→H2O+nCrO3

The solubility of chromates roughly corresponds to the solubility of sulfates. In particular, yellow barium chromate BaCrO 4 precipitates when barium salts are added to both chromate and dichromate solutions:

Ba+CrO→BaCrO↓

2Ba+CrO+H2O→2BaCrO↓+2H

The formation of a blood-red, poorly soluble silver chromate is used to detect silver in alloys using assay acid.

Chromium pentafluoride CrF 5 and unstable chromium hexafluoride CrF 6 are known. Volatile chromium oxyhalides CrO 2 F 2 and CrO 2 Cl 2 (chromyl chloride) have also been obtained.

Chromium(VI) compounds are strong oxidizing agents, for example:

K2Cr2O7+14HCl→2CrCl3+2KCl+3Cl2+7H2O

The addition of hydrogen peroxide, sulfuric acid, and an organic solvent (ether) to dichromates leads to the formation of blue chromium peroxide CrO 5 L (L is a solvent molecule), which is extracted into the organic layer; this reaction is used as an analytical one.

Chromium (Cr) is an element with atomic number 24 and atomic mass 51.996 of a side subgroup of the sixth group of the fourth period of the periodic system of chemical elements of D. I. Mendeleev. Chromium is a bluish-white hard metal. It has high chemical resistance. At room temperature, Cr is resistant to water and air. This element is one of the most important metals used in industrial alloying of steels. Chromium compounds have a bright color of various colors, for which, in fact, he got his name. After all, translated from Greek, “chrome” means “paint”.

There are 24 known isotopes of chromium from 42Cr to 66Cr. Stable natural isotopes 50Cr (4.31%), 52Cr (87.76%), 53Cr (9.55%) and 54Cr (2.38%). Of the six artificial radioactive isotopes, 51Cr is the most important, with a half-life of 27.8 days. It is used as an isotope tracer.

Unlike the metals of antiquity (gold, silver, copper, iron, tin and lead), chromium has its own “discoverer”. In 1766, a mineral was found in the vicinity of Yekaterinburg, which was called "Siberian red lead" - PbCrO4. In 1797, L. N. Vauquelin discovered element No. 24 in the mineral crocoite - natural lead chromate. At about the same time (1798), independently of Vauquelin, chromium was discovered by German scientists M. G. Klaproth and Lovitz in a sample of heavy black mineral (it was chromite FeCr2O4) found in the Urals. Later, in 1799, F. Tassert discovered a new metal in the same mineral found in southeastern France. It is believed that it was Tassert who first managed to obtain relatively pure metallic chromium.

Chromium metal is used for chromium plating, and also as one of the most important components of alloyed steels (in particular, stainless steels). In addition, chromium has found application in a number of other alloys (acid-resistant and heat-resistant steels). After all, the introduction of this metal into steel increases its resistance to corrosion both in aqueous media at ordinary temperatures and in gases at elevated temperatures. Chromium steels are characterized by increased hardness. Chromium is used in thermochromizing, a process in which the protective effect of Cr is due to the formation of a thin but strong oxide film on the steel surface, which prevents the metal from interacting with the environment.

Chromium compounds have also found wide application, so chromites are successfully used in the refractory industry: open-hearth furnaces and other metallurgical equipment are lined with magnesite-chromite bricks.

Chromium is one of the biogenic elements that are constantly included in the tissues of plants and animals. Plants contain chromium in the leaves, where it is present as a low molecular weight complex not associated with subcellular structures. Until now, scientists have not been able to prove the need for this element for plants. However, in animals, Cr is involved in the metabolism of lipids, proteins (part of the trypsin enzyme), and carbohydrates (a structural component of the glucose-resistant factor). It is known that only trivalent chromium is involved in biochemical processes. Like most other important biogenic elements, chromium enters the animal or human body through food. A decrease in this microelement in the body leads to growth retardation, a sharp increase in blood cholesterol levels and a decrease in the sensitivity of peripheral tissues to insulin.

At the same time, in its pure form, chromium is very toxic - Cr metal dust irritates lung tissues, chromium (III) compounds cause dermatitis. Chromium (VI) compounds lead to various human diseases, including cancer.

Biological properties

Chromium is an important biogenic element, which is certainly part of the tissues of plants, animals and humans. The average content of this element in plants is 0.0005%, and almost all of it accumulates in the roots (92-95%), the rest is contained in the leaves. Higher plants do not tolerate concentrations of this metal above 3∙10-4 mol/l. In animals, the chromium content ranges from ten thousandths to ten millionths of a percent. But in plankton, the chromium accumulation coefficient is amazing - 10,000-26,000. In an adult human body, the Cr content ranges from 6 to 12 mg. Moreover, the physiological need for chromium for humans has not been established accurately enough. It largely depends on the diet - when eating foods high in sugar, the body's need for chromium increases. It is generally accepted that a person needs about 20–300 mcg of this element per day. Like other biogenic elements, chromium is able to accumulate in body tissues, especially in hair. It is in them that the content of chromium indicates the degree of provision of the body with this metal. Unfortunately, with age, the "reserves" of chromium in the tissues are depleted, with the exception of the lungs.

Chromium is involved in the metabolism of lipids, proteins (it is present in the trypsin enzyme), carbohydrates (it is a structural component of the glucose-resistant factor). This factor ensures the interaction of cellular receptors with insulin, thereby reducing the body's need for it. Glucose tolerance factor (GTF) enhances the action of insulin in all metabolic processes with its participation. In addition, chromium is involved in the regulation of cholesterol metabolism and is an activator of certain enzymes.

The main source of chromium in the body of animals and humans is food. Scientists have found that the concentration of chromium in plant foods is much lower than in animal foods. The richest sources of chromium are brewer's yeast, meat, liver, legumes, and whole grains. A decrease in the content of this metal in food and blood leads to a decrease in growth rate, an increase in blood cholesterol, and a decrease in the sensitivity of peripheral tissues to insulin (a diabetic state). In addition, the risk of developing atherosclerosis and disorders of higher nervous activity increases.

However, already at concentrations of fractions of a milligram per cubic meter in the atmosphere, all chromium compounds have a toxic effect on the body. Chromium poisoning and its compounds are frequent in their production, in mechanical engineering, metallurgy, and in the textile industry. The degree of toxicity of chromium depends on the chemical structure of its compounds - dichromates are more toxic than chromates, Cr + 6 compounds are more toxic than Cr + 2 and Cr + 3 compounds. Signs of poisoning are manifested by a feeling of dryness and pain in the nasal cavity, acute sore throat, difficulty breathing, coughing and similar symptoms. With a slight excess of chromium vapor or dust, signs of poisoning disappear soon after the cessation of work in the workshop. With prolonged constant contact with chromium compounds, signs of chronic poisoning appear - weakness, constant headaches, weight loss, dyspepsia. Disturbances in the work of the gastrointestinal tract, pancreas, liver begin. Bronchitis, bronchial asthma, pneumosclerosis develop. Skin diseases appear - dermatitis, eczema. In addition, chromium compounds are dangerous carcinogens that can accumulate in body tissues, causing cancer.

Prevention of poisoning are periodic medical examinations of personnel working with chromium and its compounds; installation of ventilation, means of dust suppression and dust collection; use of personal protective equipment (respirators, gloves) by workers.

The root "chrome" in its concept of "color", "paint" is part of many words used in a wide variety of fields: science, technology and even music. So many names of photographic films contain this root: "orthochrome", "panchrome", "isopanchrome" and others. The word "chromosome" consists of two Greek words: "chromo" and "soma". Literally, this can be translated as "painted body" or "body that is painted." The structural element of the chromosome, which is formed in the interphase of the cell nucleus as a result of chromosome doubling, is called "chromatid". "Chromatin" - a substance of chromosomes, located in the nuclei of plant and animal cells, which is intensely stained with nuclear dyes. "Chromatophores" are pigment cells in animals and humans. In music, the concept of "chromatic scale" is used. "Khromka" is one of the types of Russian accordion. In optics, there are concepts of "chromatic aberration" and "chromatic polarization". "Chromatography" is a physicochemical method for separating and analyzing mixtures. "Chromoscope" - a device for obtaining a color image by optically combining two or three color-separated photographic images illuminated through specially selected differently colored light filters.

The most poisonous is chromium oxide (VI) CrO3, it belongs to the 1st hazard class. The lethal dose for humans (oral) is 0.6 g. Ethyl alcohol ignites when it comes into contact with freshly prepared CrO3!

The most common grade of stainless steel contains 18% Cr, 8% Ni, about 0.1% C. It resists corrosion and oxidation excellently and retains its strength at high temperatures. It is from this steel that the sheets used in the construction of the sculptural group of V.I. Mukhina "Worker and Collective Farm Girl".

Ferrochromium, used in the metallurgical industry in the production of chromium steels, was of very poor quality at the end of the 90th century. This is due to the low content of chromium in it - only 7-8%. Then it was called "Tasmanian pig iron" in view of the fact that the original iron-chromium ore was imported from Tasmania.

It was previously mentioned that chrome alum is used in the tanning of hides. Thanks to this, the concept of "chrome" boots appeared. Leather tanned with chromium compounds acquires shine, gloss and strength.

Many laboratories use a "chromium mixture" - a mixture of a saturated solution of potassium dichromate with concentrated sulfuric acid. It is used in the degreasing of surfaces of glass and steel laboratory glassware. It oxidizes fat and removes its residues. Just handle this mixture with care, because it is a mixture of a strong acid and a strong oxidizing agent!

Nowadays, wood is still used as a building material, because it is inexpensive and easy to process. But it also has many negative properties - susceptibility to fires, fungal diseases that destroy it. To avoid all these troubles, the tree is impregnated with special compounds containing chromates and bichromates plus zinc chloride, copper sulfate, sodium arsenate and some other substances. Thanks to such compositions, wood increases its resistance to fungi and bacteria, as well as to open fire.

Chrome occupied a special niche in the printing industry. In 1839, it was found that paper impregnated with sodium dichromate, after being illuminated with a bright light, suddenly turns brown. Then it turned out that bichromate coatings on paper, after exposure, do not dissolve in water, but, when wetted, acquire a bluish tint. This property was used by printers. The desired pattern was photographed on a plate with a colloidal coating containing bichromate. The illuminated areas did not dissolve during washing, but the non-exposed ones dissolved, and a pattern remained on the plate from which it was possible to print.

Story

The history of the discovery of element No. 24 began in 1761, when an unusual red mineral was found in the Berezovsky mine (the eastern foot of the Ural Mountains) near Yekaterinburg, which, when rubbed into dust, gave a yellow color. The find belonged to St. Petersburg University Professor Johann Gottlob Lehmann. Five years later, the scientist delivered the samples to the city of St. Petersburg, where he conducted a series of experiments on them. In particular, he treated unusual crystals with hydrochloric acid, obtaining a white precipitate in which lead was found. Based on the results obtained, Leman named the mineral Siberian red lead. This is the story of the discovery of crocoite (from the Greek "krokos" - saffron) - natural lead chromate PbCrO4.

Interested in this find, Peter Simon Pallas, a German naturalist and traveler, organized and led an expedition of the St. Petersburg Academy of Sciences to the heart of Russia. In 1770, the expedition reached the Urals and visited the Berezovsky mine, where samples of the studied mineral were taken. This is how the traveler himself describes it: “This amazing red lead mineral is not found in any other deposit. Turns yellow when ground into powder and can be used in miniature art. German enterprise overcame all the difficulties of extracting and delivering crocoite to Europe. Despite the fact that these operations took at least two years, soon the carriages of the noblemen of Paris and London were traveling painted with finely crushed crocoite. The collections of mineralogical museums of many universities of the Old World have been enriched with the best samples of this mineral from the Russian bowels. However, European scientists could not unravel the composition of the mysterious mineral.

This lasted for thirty years, until a sample of Siberian red lead fell into the hands of Nicolas Louis Vauquelin, professor of chemistry at the Paris Mineralogical School, in 1796. After analyzing the crocoite, the scientist found nothing in it except oxides of iron, lead and aluminum. Subsequently, Vauquelin treated the crocoite with a solution of potash (K2CO3) and, following the precipitation of a white precipitate of lead carbonate, isolated a yellow solution of an unknown salt. After conducting a series of experiments on the processing of the mineral with salts of various metals, the professor, using hydrochloric acid, isolated a solution of "red lead acid" - chromium oxide and water (chromic acid exists only in dilute solutions). After evaporating this solution, he obtained ruby-red crystals (chromic anhydride). Further heating of the crystals in a graphite crucible in the presence of coal gave a lot of intergrown gray needle-like crystals - a new, hitherto unknown metal. The next series of experiments showed the high refractoriness of the resulting element and its resistance to acids. The Paris Academy of Sciences immediately witnessed the discovery, the scientist, at the insistence of his friends, gave the name to the new element - chromium (from the Greek "color", "color") due to the variety of shades of the compounds it forms. In his further works, Vauquelin confidently stated that the emerald color of some precious stones, as well as natural beryllium and aluminum silicates, is due to the admixture of chromium compounds in them. An example is emerald, which is a green-colored beryl in which the aluminum is partly replaced by chromium.

It is clear that Vauquelin received not pure metal, most likely its carbides, which is confirmed by the acicular shape of light gray crystals. Pure metallic chromium was later obtained by F. Tassert, presumably in 1800.

Also, independently of Vauquelin, chromium was discovered by Klaproth and Lovitz in 1798.

Being in nature

In the bowels of the earth, chromium is a fairly common element, despite the fact that it does not occur in its free form. Its clarke (average content in the earth's crust) is 8.3.10-3% or 83 g/t. However, its distribution across breeds is uneven. This element is mainly characteristic of the Earth's mantle, the fact is that ultramafic rocks (peridotites), which are supposedly close in composition to the mantle of our planet, are the richest in chromium: 2 10-1% or 2 kg / t. In such rocks, Cr forms massive and disseminated ores, which are associated with the formation of the largest deposits of this element. The content of chromium is also high in basic rocks (basalts, etc.) 2 10-2% or 200 g/t. There is much less Cr in acidic rocks: 2.5 10-3%, sedimentary (sandstones) - 3.5 10-3%, shale also contains chromium - 9 10-3%.

It can be concluded that chromium is a typical lithophile element and almost all of it is contained in minerals of deep occurrence in the bowels of the Earth.

There are three main chromium minerals: magnochromite (Mn, Fe)Cr2O4, chrompicotite (Mg, Fe)(Cr, Al)2O4 and aluminochromite (Fe, Mg)(Cr, Al)2O4. These minerals have a single name - chromium spinel and the general formula (Mg, Fe)O (Cr, Al, Fe) 2O3. They are indistinguishable in appearance and are inaccurately referred to as "chromites". Their composition is changeable. The content of the most important components varies (weight%): Cr2O3 from 10.5 to 62.0; Al2O3 from 4 to 34.0; Fe2O3 from 1.0 to 18.0; FeO from 7.0 to 24.0; MgO from 10.5 to 33.0; SiO2 from 0.4 to 27.0; TiO2 impurities up to 2; V2O5 up to 0.2; ZnO up to 5; MnO up to 1. Some chromium ores contain 0.1-0.2 g/t of elements of the platinum group and up to 0.2 g/t of gold.

In addition to various chromites, chromium is part of a number of other minerals - chrome vesuvian, chromium chlorite, chrome tourmaline, chromium mica (fuxite), chromium garnet (uvarovite), etc., which often accompany ores, but do not have industrial significance. Chromium is a relatively weak water migrant. Under exogenous conditions, chromium, like iron, migrates in the form of suspensions and can be deposited in clays. Chromates are the most mobile form.

Of practical importance, perhaps, is only chromite FeCr2O4, which belongs to spinels - isomorphic minerals of the cubic system with the general formula MO Me2O3, where M is a divalent metal ion, and Me is a trivalent metal ion. In addition to spinels, chromium occurs in many less common minerals, such as melanochroite 3PbO 2Cr2O3, wokelenite 2(Pb,Cu)CrO4(Pb,Cu)3(PO4)2, tarapakaite K2CrO4, ditzeite CaIO3 CaCrO4 and others.

Chromites are usually found in the form of granular masses of black color, less often - in the form of octahedral crystals, have a metallic luster, occur in the form of continuous arrays.

At the end of the 20th century, chromium reserves (identified) in almost fifty countries of the world with deposits of this metal amounted to 1674 million tons. ). The second place in terms of chromium resources belongs to Kazakhstan, where very high quality ore is mined in the Aktobe region (Kempirsai massif). Other countries also have stocks of this element. Turkey (in Guleman), Philippines on the island of Luzon, Finland (Kemi), India (Sukinda), etc.

Our country has its own chromium deposits being developed - in the Urals (Donskoye, Saranovskoye, Khalilovskoye, Alapaevskoye and many others). Moreover, at the beginning of the 19th century, it was the Ural deposits that were the main sources of chromium ores. Only in 1827, the American Isaac Tison discovered a large deposit of chromium ore on the border of Maryland and Pennsylvania, seizing the monopoly of mining for many years. In 1848, deposits of high quality chromite were found in Turkey, not far from Bursa, and soon (after the depletion of the Pennsylvania deposit) it was this country that seized the role of a monopolist. This continued until 1906, when rich deposits of chromites were discovered in South Africa and India.

Application

The total consumption of pure chromium metal today is approximately 15 million tons. The production of electrolytic chromium - the purest - accounts for 5 million tons, which is a third of the total consumption.

Chromium is widely used for alloying steels and alloys, giving them corrosion resistance and heat resistance. More than 40% of the resulting pure metal is spent on the manufacture of such "superalloys". The most well-known resistance alloys are nichrome with a Cr content of 15-20%, heat-resistant alloys - 13-60% Cr, stainless - 18% Cr and ball-bearing steels 1% Cr. The addition of chromium to conventional steels improves their physical properties and makes the metal more susceptible to heat treatment.

Chromium metal is used for chromium plating - applying a thin layer of chromium to the surface of steel alloys in order to increase the corrosion resistance of these alloys. The chrome-plated coating perfectly resists the effects of humid atmospheric air, salty sea air, water, nitric and most organic acids. Such coatings are of two purposes: protective and decorative. The thickness of protective coatings is about 0.1 mm, they are applied directly to the product and give it increased wear resistance. Decorative coatings have an aesthetic value, they are applied to a layer of another metal (copper or nickel), which actually performs a protective function. The thickness of such a coating is only 0.0002–0.0005 mm.

Chromium compounds are also actively used in various fields.

The main chromium ore - chromite FeCr2O4 is used in the production of refractories. Magnesite-chromite bricks are chemically passive and heat-resistant, they withstand sharp multiple temperature changes, so they are used in the construction of the arches of open-hearth furnaces and the working space of other metallurgical devices and structures.

The hardness of chromium (III) oxide crystals - Cr2O3 is commensurate with the hardness of corundum, which ensured its use in the compositions of grinding and lapping pastes used in mechanical engineering, jewelry, optical and watch industries. It is also used as a catalyst for the hydrogenation and dehydrogenation of certain organic compounds. Cr2O3 is used in painting as a green pigment and for coloring glass.

Potassium chromate - K2CrO4 is used in leather tanning, as a mordant in the textile industry, in the production of dyes, and in wax bleaching.

Potassium dichromate (chromic) - K2Cr2O7 is also used in the tanning of leather, mordant when dyeing fabrics, is a corrosion inhibitor of metals and alloys. It is used in the manufacture of matches and for laboratory purposes.

Chromium (II) chloride CrCl2 is a very strong reducing agent, easily oxidized even by atmospheric oxygen, which is used in gas analysis for the quantitative absorption of O2. In addition, it is used to a limited extent in the production of chromium by electrolysis of molten salts and chromatometry.

Potassium chromium alum K2SO4.Cr2(SO4)3 24H2O is mainly used in the textile industry - in leather tanning.

Anhydrous chromium chloride CrCl3 is used to apply chromium coatings on the surface of steels by chemical vapor deposition, and is an integral part of some catalysts. Hydrates CrCl3 - mordant when dyeing fabrics.

Various dyes are made from lead chromate PbCrO4.

A solution of sodium dichromate is used to clean and pickle the surface of steel wire before galvanizing, and also brighten brass. Chromic acid is obtained from sodium bichromate, which is used as an electrolyte in chromium plating of metal parts.

Production

In nature, chromium occurs mainly in the form of chromium iron ore FeO ∙ Cr2O3, when it is reduced with coal, an alloy of chromium with iron is obtained - ferrochromium, which is directly used in the metallurgical industry in the production of chromium steels. The chromium content in this composition reaches 80% (by weight).

The reduction of chromium (III) oxide with coal is intended to produce high-carbon chromium, which is necessary for the production of special alloys. The process is carried out in an electric arc furnace.

To obtain pure chromium, chromium (III) oxide is first obtained, and then it is reduced by the aluminothermic method. At the same time, a mixture of powdered or in the form of aluminum shavings (Al) and a charge of chromium oxide (Cr2O3) is heated to a temperature of 500-600 ° C. Then, reduction is initiated with a mixture of barium peroxide with aluminum powder, or by igniting part of the charge, followed by the addition of the remaining part . In this process, it is important that the resulting thermal energy is sufficient to melt the chromium and separate it from the slag.

Cr2O3 + 2Al = 2Cr + 2Al2O3

The chromium obtained in this way contains a certain amount of impurities: iron 0.25-0.40%, sulfur 0.02%, carbon 0.015-0.02%. The content of pure substance is 99.1–99.4%. Such chromium is brittle and easily ground into powder.

The reality of this method was proven and demonstrated as early as 1859 by Friedrich Wöhler. On an industrial scale, the aluminothermic reduction of chromium became possible only after the method of obtaining cheap aluminum became available. Goldschmidt was the first to develop a safe way to control a highly exothermic (hence explosive) reduction process.

If it is necessary to obtain high-purity chromium in industry, electrolytic methods are used. Electrolysis is subjected to a mixture of chromic anhydride, ammonium chromium alum or chromium sulfate with dilute sulfuric acid. Chromium deposited during electrolysis on aluminum or stainless cathodes contains dissolved gases as impurities. Purity of 99.90–99.995% can be achieved using high-temperature (1500–1700°C) purification in a hydrogen flow and vacuum degassing. Advanced electrolytic chromium refining techniques remove sulfur, nitrogen, oxygen and hydrogen from the "raw" product.

In addition, it is possible to obtain metallic Cr by electrolysis of CrCl3 or CrF3 melts mixed with potassium, calcium, and sodium fluorides at a temperature of 900°C in argon.

The possibility of an electrolytic method for obtaining pure chromium was proved by Bunsen in 1854, by subjecting an aqueous solution of chromium chloride to electrolysis.

The industry also uses a silicothermic method for obtaining pure chromium. In this case, chromium oxide is reduced by silicon:

2Cr2O3 + 3Si + 3CaO = 4Cr + 3CaSiO3

Chromium is smelted silicothermally in arc furnaces. The addition of quicklime makes it possible to convert refractory silicon dioxide into a low-melting calcium silicate slag. The purity of silicothermal chromium is approximately the same as that of aluminothermic chromium, however, naturally, the content of silicon in it is somewhat higher, and that of aluminum is somewhat lower.

Cr can also be obtained by reduction of Cr2O3 with hydrogen at 1500°C, reduction of anhydrous CrCl3 with hydrogen, alkali or alkaline earth metals, magnesium and zinc.

To obtain chromium, they tried to use other reducing agents - carbon, hydrogen, magnesium. However, these methods are not widely used.

In the Van Arkel-Kuchman-De Boer process, decomposition of chromium (III) iodide is used on a wire heated to 1100 ° C with the deposition of pure metal on it.

Physical properties

Chromium is a hard, very heavy, refractory, malleable steel-gray metal. Pure chromium is quite plastic, crystallizes in a body-centered lattice, a = 2.885Å (at a temperature of 20°C). At a temperature of about 1830 ° C, the probability of transformation into a modification with a face-centered lattice is high, a = 3.69 Å. Atomic radius 1.27 Å; ionic radii Cr2+ 0.83Å, Cr3+ 0.64Å, Cr6+ 0.52 Å.

The melting point of chromium is directly related to its purity. Therefore, the determination of this indicator for pure chromium is a very difficult task - after all, even a small content of nitrogen or oxygen impurities can significantly change the value of the melting point. Many researchers have been working on this issue for decades and have obtained results that are far from each other: from 1513 to 1920 ° C. It was previously believed that this metal melts at a temperature of 1890 ° C, but modern studies indicate a temperature of 1907 ° C, chromium boils at temperatures above 2500 ° C - the data also vary: from 2199 ° C to 2671 ° C. The density of chromium is less than that of iron; it is 7.19 g/cm3 (at 200°C).

Chromium is characterized by all the main characteristics of metals - it conducts heat well, its resistance to electric current is very low, like most metals, chromium has a characteristic luster. In addition, this element has one very interesting feature: the fact is that at a temperature of 37 ° C its behavior cannot be explained - there is a sharp change in many physical properties, this change has an abrupt character. Chromium, like a sick person at a temperature of 37 ° C, begins to act up: the internal friction of chromium reaches a maximum, the modulus of elasticity drops to a minimum. The value of the electrical conductivity jumps, the thermoelectromotive force and the coefficient of linear expansion constantly change. Scientists have not yet been able to explain this phenomenon.

The specific heat capacity of chromium is 0.461 kJ / (kg.K) or 0.11 cal / (g ° C) (at a temperature of 25 ° C); thermal conductivity coefficient 67 W / (m K) or 0.16 cal / (cm sec ° C) (at a temperature of 20 ° C). Thermal coefficient of linear expansion 8.24 10-6 (at 20 °C). Chromium at a temperature of 20 ° C has a specific electrical resistance of 0.414 μm m, and its thermal coefficient of electrical resistance in the range of 20-600 ° C is 3.01 10-3.

It is known that chromium is very sensitive to impurities - the smallest fractions of other elements (oxygen, nitrogen, carbon) can make chromium very brittle. It is extremely difficult to obtain chromium without these impurities. For this reason, this metal is not used for structural purposes. But in metallurgy, it is actively used as an alloying material, since its addition to the alloy makes steel hard and wear-resistant, because chromium is the hardest of all metals - it cuts glass like a diamond! The hardness of high-purity chromium according to Brinell is 7-9 MN/m2 (70-90 kgf/cm2). Chromium is alloyed with spring, spring, tool, die and ball bearing steels. In them (except for ball-bearing steels), chromium is present together with manganese, molybdenum, nickel, vanadium. The addition of chromium to ordinary steels (up to 5% Cr) improves their physical properties and makes the metal more susceptible to heat treatment.

Chromium is antiferromagnetic, specific magnetic susceptibility is 3.6 10-6. Specific electrical resistance 12.710-8 Ohm. Temperature coefficient of linear expansion of chromium 6.210-6. The heat of vaporization of this metal is 344.4 kJ/mol.

Chrome is resistant to corrosion in air and water.

Chemical properties

Chemically, chromium is rather inert, this is due to the presence of a strong thin oxide film on its surface. Cr does not oxidize in air, even in the presence of moisture. When heated, oxidation proceeds exclusively on the surface of the metal. At 1200°C the film breaks down and the oxidation proceeds much faster. At 2000°C, chromium burns to form green chromium (III) oxide Cr2O3, which has amphoteric properties. Fusing Cr2O3 with alkalis, chromites are obtained:

Cr2O3 + 2NaOH = 2NaCrO2 + H2O

Uncalcined chromium (III) oxide is easily soluble in alkaline solutions and acids:

Cr2O3 + 6HCl = 2CrCl3 + 3H2O

In compounds, chromium mainly exhibits the oxidation states Cr+2, Cr+3, Cr+6. The most stable are Cr+3 and Cr+6. There are also some compounds where chromium has the oxidation states Cr+1, Cr+4, Cr+5. Chromium compounds are very diverse in color: white, blue, green, red, purple, black and many others.

Chromium easily reacts with dilute solutions of hydrochloric and sulfuric acids to form chromium chloride and sulfate and release hydrogen:

Cr + 2HCl = CrCl2 + H2

Aqua regia and nitric acid passivate chromium. Moreover, chromium passivated with nitric acid does not dissolve in dilute sulfuric and hydrochloric acids, even with prolonged boiling in their solutions, but at some point the dissolution still occurs, accompanied by rapid foaming from the released hydrogen. This process is explained by the fact that chromium passes from a passive state to an active one, in which the metal is not protected by a protective film. Moreover, if nitric acid is added again in the process of dissolution, the reaction will stop, since chromium is again passivated.

Under normal conditions, chromium reacts with fluorine to form CrF3. At temperatures above 600 ° C, interaction with water vapor occurs, the result of this interaction is chromium oxide (III) Cr2O3:

4Cr + 3O2 = 2Cr2O3

Cr2O3 is green microcrystals with a density of 5220 kg/m3 and a high melting point (2437°C). Chromium (III) oxide exhibits amphoteric properties, but is very inert, it is difficult to dissolve it in aqueous acids and alkalis. Chromium(III) oxide is quite toxic. Contact with the skin can cause eczema and other skin diseases. Therefore, when working with chromium (III) oxide, it is imperative to use personal protective equipment.

In addition to the oxide, other compounds with oxygen are known: CrO, CrO3, obtained indirectly. The greatest danger is the inhaled oxide aerosol, which causes severe diseases of the upper respiratory tract and lungs.

Chromium forms a large number of salts with oxygen-containing components.

• Designation - Cr (Chromium);
• Period - IV;
• Group - 6 (VIb);
• Atomic mass - 51.9961;
• Atomic number - 24;
• Radius of an atom = 130 pm;
• Covalent radius = 118 pm;
• Electron distribution - 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1 ;
• melting point = 1857°C;
• boiling point = 2672°C;
• Electronegativity (according to Pauling / according to Alpred and Rochov) = 1.66 / 1.56;
• Oxidation state: +6, +3, +2, 0;
• Density (n.a.) \u003d 7.19 g / cm 3;
• Molar volume = 7.23 cm 3 / mol.

Chrome (color, paint) was first found at the Berezovsky gold deposit (Middle Urals), the first mentions date back to 1763, in his work "The First Foundations of Metallurgy" M.V. Lomonosov calls it "red lead ore".

Rice. The structure of the chromium atom.

The electronic configuration of the chromium atom is 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1 (see Electronic structure of atoms). In the formation of chemical bonds with other elements, 1 electron located at the outer 4s level + 5 electrons of the 3d sublevel (6 electrons in total) can participate, therefore, in compounds, chromium can take oxidation states from +6 to +1 (the most common are +6 , +3, +2). Chromium is a chemically inactive metal, it reacts with simple substances only at high temperatures.

Physical properties of chromium:

• bluish-white metal;
• very hard metal (in the presence of impurities);
• fragile at n. y.;
• plastic (in its pure form).

Chemical properties of chromium

• at t=300°C it reacts with oxygen:
  4Cr + 3O 2 \u003d 2Cr 2 O 3;
• at t>300°C it reacts with halogens, forming mixtures of halides;
• at t>400°C it reacts with sulfur to form sulfides:
  Cr + S = CrS;
• at t=1000°C, finely ground chromium reacts with nitrogen to form chromium nitride (a semiconductor with high chemical resistance):
  2Cr + N 2 = 2CrN;
• reacts with dilute hydrochloric and sulfuric acids to release hydrogen:
  Cr + 2HCl \u003d CrCl 2 + H 2;
  Cr + H 2 SO 4 \u003d CrSO 4 + H 2;
• warm concentrated nitric and sulfuric acids dissolve chromium.

With concentrated sulfuric and nitric acid at n.o. chromium does not interact, chromium also does not dissolve in aqua regia, it is noteworthy that pure chromium does not react even with dilute sulfuric acid, the reason for this phenomenon has not yet been established. During long-term storage in concentrated nitric acid, chromium is covered with a very dense oxide film (passivated), and ceases to react with dilute acids.

Chromium compounds

It has already been said above that the "favorite" oxidation states of chromium are +2 (CrO, Cr (OH) 2), +3 (Cr 2 O 3, Cr (OH) 3), +6 (CrO 3, H 2 CrO 4 ).

Chrome is chromophore, i.e., an element that gives color to the substance in which it is contained. For example, in the +3 oxidation state, chromium gives a lilac-red or green color (ruby, spinel, emerald, garnet); in oxidation state +6 - yellow-orange color (crocoite).

Chromophores, in addition to chromium, are also iron, nickel, titanium, vanadium, manganese, cobalt, copper - all these are d-elements.

The color of common compounds that include chromium:

• chromium in oxidation state +2:
  • chromium oxide CrO - red;
  • chromium fluoride CrF 2 - blue-green;
  • chromium chloride CrCl 2 - has no color;
  • chromium bromide CrBr 2 - has no color;
  • chromium iodide CrI 2 - red-brown.
• chromium in oxidation state +3:
  • Cr 2 O 3 - green;
  • CrF 3 - light green;
  • CrCl 3 - violet-red;
  • CrBr 3 - dark green;
  • CrI 3 - black.
• chromium in oxidation state +6:
  • CrO 3 - red;
  • potassium chromate K 2 CrO 4 - lemon yellow;
  • ammonium chromate (NH 4) 2 CrO 4 - golden yellow;
  • calcium chromate CaCrO 4 - yellow;
  • lead chromate PbCrO 4 - light brown-yellow.

Chromium oxides:

• Cr +2 O - basic oxide;
• Cr 2 +3 O 3 - amphoteric oxide;
• Cr +6 O 3 - acid oxide.

Chromium hydroxides:

• ".

  Application of chromium

  • as a alloying additive in the smelting of heat-resistant and corrosion-resistant alloys;
  • for chrome plating of metal products in order to give them high corrosion resistance, abrasion resistance and a beautiful appearance;
  • chromium-30 and chromium-90 alloys are used in plasma torch nozzles and in the aviation industry.

The discovery of chromium belongs to the period of rapid development of chemical-analytical studies of salts and minerals. In Russia, chemists took a special interest in the analysis of minerals found in Siberia and almost unknown in Western Europe. One of these minerals was the Siberian red lead ore (crocoite), described by Lomonosov. The mineral was investigated, but nothing but oxides of lead, iron and aluminum was found in it. However, in 1797, Vauquelin, by boiling a finely ground sample of the mineral with potash and precipitating lead carbonate, obtained an orange-red solution. From this solution, he crystallized a ruby-red salt, from which an oxide and a free metal, different from all known metals, were isolated. Vauquelin called him Chromium ( Chrome ) from the Greek word- coloring, color; True, here it was not the property of the metal that was meant, but its brightly colored salts.

Finding in nature.

The most important chromium ore of practical importance is chromite, the approximate composition of which corresponds to the formula FeCrO ​​4.

It is found in Asia Minor, in the Urals, in North America, in southern Africa. The above-mentioned mineral crocoite - PbCrO 4 - is also of technical importance. Chromium oxide (3) and some of its other compounds are also found in nature. In the earth's crust, the chromium content in terms of metal is 0.03%. Chromium is found on the Sun, stars, meteorites.

Physical properties.

Chromium is a white, hard and brittle metal, exceptionally chemically resistant to acids and alkalis. It oxidizes in air and has a thin transparent oxide film on the surface. Chromium has a density of 7.1 g / cm 3, its melting point is +1875 0 C.

Receipt.

With strong heating of chromium iron ore with coal, chromium and iron are reduced:

FeO * Cr 2 O 3 + 4C = 2Cr + Fe + 4CO

As a result of this reaction, an alloy of chromium with iron is formed, which is characterized by high strength. To obtain pure chromium, it is reduced from chromium(3) oxide with aluminum:

Cr 2 O 3 + 2Al \u003d Al 2 O 3 + 2Cr

Two oxides are usually used in this process - Cr 2 O 3 and CrO 3

Chemical properties.

Thanks to a thin protective oxide film covering the surface of chromium, it is highly resistant to aggressive acids and alkalis. Chromium does not react with concentrated nitric and sulfuric acids, as well as with phosphoric acid. Chromium interacts with alkalis at t = 600-700 o C. However, chromium interacts with dilute sulfuric and hydrochloric acids, displacing hydrogen:

2Cr + 3H 2 SO 4 \u003d Cr 2 (SO 4) 3 + 3H 2
2Cr + 6HCl = 2CrCl 3 + 3H 2

At high temperatures, chromium burns in oxygen to form oxide(III).

Hot chromium reacts with water vapor:

2Cr + 3H 2 O \u003d Cr 2 O 3 + 3H 2

Chromium also reacts with halogens at high temperatures, halogens with hydrogens, sulfur, nitrogen, phosphorus, coal, silicon, boron, for example:

Cr + 2HF = CrF 2 + H 2
2Cr + N2 = 2CrN
2Cr + 3S = Cr2S3
Cr + Si = CrSi

The above physical and chemical properties of chromium have found their application in various fields of science and technology. For example, chromium and its alloys are used to obtain high-strength, corrosion-resistant coatings in mechanical engineering. Alloys in the form of ferrochrome are used as metal cutting tools. Chrome-plated alloys have found application in medical technology, in the manufacture of chemical process equipment.

The position of chromium in the periodic table of chemical elements:

Chromium heads the side subgroup of group VI of the periodic system of elements. Its electronic formula is as follows:

24 Cr IS 2 2S 2 2P 6 3S 2 3P 6 3d 5 4S 1

In filling the orbitals with electrons at the chromium atom, the regularity is violated, according to which the 4S orbital should have been filled first to the state 4S 2 . However, due to the fact that the 3d orbital occupies a more favorable energy position in the chromium atom, it is filled up to the value 4d 5 . Such a phenomenon is observed in the atoms of some other elements of the secondary subgroups. Chromium can exhibit oxidation states from +1 to +6. The most stable are chromium compounds with oxidation states +2, +3, +6.

Divalent chromium compounds.

Chromium oxide (II) CrO - pyrophoric black powder (pyrophoric - the ability to ignite in air in a finely divided state). CrO dissolves in dilute hydrochloric acid:

CrO + 2HCl = CrCl 2 + H 2 O

In air, when heated above 100 0 C, CrO turns into Cr 2 O 3.

Divalent chromium salts are formed by dissolving chromium metal in acids. These reactions take place in an atmosphere of an inactive gas (for example, H 2), because in the presence of air, Cr(II) is easily oxidized to Cr(III).

Chromium hydroxide is obtained in the form of a yellow precipitate by the action of an alkali solution on chromium (II) chloride:

CrCl 2 + 2NaOH = Cr(OH) 2 + 2NaCl

Cr(OH) 2 has basic properties, is a reducing agent. The hydrated Cr2+ ion is colored pale blue. An aqueous solution of CrCl 2 has a blue color. In air in aqueous solutions, Cr(II) compounds transform into Cr(III) compounds. This is especially pronounced for Cr(II) hydroxide:

4Cr(OH) 2 + 2H 2 O + O 2 = 4Cr(OH) 3

Trivalent chromium compounds.

Chromium oxide (III) Cr 2 O 3 is a refractory green powder. It is close to corundum in hardness. In the laboratory, it can be obtained by heating ammonium dichromate:

(NH 4) 2 Cr 2 O 7 \u003d Cr 2 O 3 + N 2 + 4H 2

Cr 2 O 3 - amphoteric oxide, when fused with alkalis, forms chromites: Cr 2 O 3 + 2NaOH \u003d 2NaCrO 2 + H 2 O

Chromium hydroxide is also an amphoteric compound:

Cr(OH) 3 + HCl = CrCl 3 + 3H 2 O
Cr(OH) 3 + NaOH = NaCrO 2 + 2H 2 O

Anhydrous CrCl 3 has the appearance of dark purple leaves, is completely insoluble in cold water, and dissolves very slowly when boiled. Anhydrous chromium sulfate (III) Cr 2 (SO 4) 3 pink, also poorly soluble in water. In the presence of reducing agents, it forms purple chromium sulfate Cr 2 (SO 4) 3 *18H 2 O. Green chromium sulfate hydrates are also known, containing a smaller amount of water. Chrome alum KCr(SO 4) 2 *12H 2 O crystallizes from solutions containing violet chromium sulfate and potassium sulfate. A solution of chromic alum turns green when heated due to the formation of sulfates.

Reactions with chromium and its compounds

Almost all chromium compounds and their solutions are intensely colored. Having a colorless solution or a white precipitate, we can conclude with a high degree of probability that chromium is absent.

1. We strongly heat in the flame of a burner on a porcelain cup such an amount of potassium dichromate that will fit on the tip of a knife. Salt will not release water of crystallization, but will melt at a temperature of about 400 0 C with the formation of a dark liquid. Let's heat it for a few more minutes on a strong flame. After cooling, a green precipitate forms on the shard. Part of it is soluble in water (it turns yellow), and the other part is left on the shard. The salt decomposed when heated, resulting in the formation of soluble yellow potassium chromate K 2 CrO 4 and green Cr 2 O 3 .
2. Dissolve 3g of powdered potassium dichromate in 50ml of water. To one part add some potassium carbonate. It will dissolve with the release of CO 2 , and the color of the solution will become light yellow. Chromate is formed from potassium dichromate. If we now add a 50% solution of sulfuric acid in portions, then the red-yellow color of the bichromate will appear again.
3. Pour into a test tube 5 ml. potassium dichromate solution, boil with 3 ml of concentrated hydrochloric acid under draft. Yellow-green poisonous gaseous chlorine is released from the solution, because chromate will oxidize HCl to Cl 2 and H 2 O. The chromate itself will turn into green trivalent chromium chloride. It can be isolated by evaporating the solution, and then, fusing with soda and nitrate, converted to chromate.
4. When a solution of lead nitrate is added, yellow lead chromate precipitates; when interacting with a solution of silver nitrate, a red-brown precipitate of silver chromate is formed.
5. Add hydrogen peroxide to a solution of potassium bichromate and acidify the solution with sulfuric acid. The solution acquires a deep blue color due to the formation of chromium peroxide. Peroxide, when shaken with some ether, will turn into an organic solvent and turn it blue. This reaction is specific for chromium and is very sensitive. It can be used to detect chromium in metals and alloys. First of all, it is necessary to dissolve the metal. With prolonged boiling with 30% sulfuric acid (hydrochloric acid can also be added), chromium and many steels partially dissolve. The resulting solution contains chromium (III) sulfate. To be able to conduct a detection reaction, we first neutralize it with caustic soda. Gray-green chromium (III) hydroxide precipitates, which dissolves in excess NaOH and forms green sodium chromite. Filter the solution and add 30% hydrogen peroxide. When heated, the solution will turn yellow, as chromite is oxidized to chromate. Acidification will result in a blue color of the solution. The colored compound can be extracted by shaking with ether.

Analytical reactions for chromium ions.

1. To 3-4 drops of a solution of chromium chloride CrCl 3 add a 2M solution of NaOH until the initial precipitate dissolves. Note the color of the sodium chromite formed. Heat the resulting solution in a water bath. What is happening?
2. To 2-3 drops of CrCl 3 solution add an equal volume of 8M NaOH solution and 3-4 drops of 3% H 2 O 2 solution. Heat the reaction mixture in a water bath. What is happening? What precipitate is formed if the resulting colored solution is neutralized, CH 3 COOH is added to it, and then Pb (NO 3) 2 ?
3. Pour 4-5 drops of solutions of chromium sulfate Cr 2 (SO 4) 3, IMH 2 SO 4 and KMnO 4 into a test tube. Heat the reaction site for several minutes on a water bath. Note the change in color of the solution. What caused it?
4. To 3-4 drops of K 2 Cr 2 O 7 solution acidified with nitric acid, add 2-3 drops of H 2 O 2 solution and mix. The blue color of the solution that appears is due to the appearance of perchromic acid H 2 CrO 6:

Cr 2 O 7 2- + 4H 2 O 2 + 2H + = 2H 2 CrO 6 + 3H 2 O

Pay attention to the rapid decomposition of H 2 CrO 6:

2H 2 CrO 6 + 8H+ = 2Cr 3+ + 3O 2 + 6H 2 O
blue color green color

Perchromic acid is much more stable in organic solvents.

1. To 3-4 drops of K 2 Cr 2 O 7 solution acidified with nitric acid, add 5 drops of isoamyl alcohol, 2-3 drops of H 2 O 2 solution and shake the reaction mixture. The layer of organic solvent that floats to the top is colored bright blue. The color fades very slowly. Compare the stability of H 2 CrO 6 in organic and aqueous phases.
2. When CrO 4 2- and Ba 2+ ions interact, a yellow precipitate of barium chromate BaCrO 4 precipitates.
3. Silver nitrate forms brick red precipitate of silver chromate with CrO 4 2 ions.
4. Take three test tubes. Place 5-6 drops of K 2 Cr 2 O 7 solution in one of them, the same volume of K 2 CrO 4 solution in the second, and three drops of both solutions in the third. Then add three drops of potassium iodide solution to each tube. Explain the result. Acidify the solution in the second tube. What is happening? Why?

Entertaining experiments with chromium compounds

1. A mixture of CuSO 4 and K 2 Cr 2 O 7 turns green when alkali is added, and turns yellow in the presence of acid. By heating 2 mg of glycerol with a small amount of (NH 4) 2 Cr 2 O 7 and then adding alcohol, a bright green solution is obtained after filtration, which turns yellow when an acid is added, and turns green in a neutral or alkaline medium.
2. Place in the center of the can with thermite "ruby mixture" - thoroughly ground and placed in aluminum foil Al 2 O 3 (4.75 g) with the addition of Cr 2 O 3 (0.25 g). So that the jar does not cool down longer, it is necessary to bury it under the upper edge in the sand, and after the thermite is ignited and the reaction begins, cover it with an iron sheet and fill it with sand. Bank to dig out in a day. The result is a red-ruby powder.
3. 10 g of potassium bichromate is triturated with 5 g of sodium or potassium nitrate and 10 g of sugar. The mixture is moistened and mixed with collodion. If the powder is compressed in a glass tube, and then the stick is pushed out and set on fire from the end, then a “snake” will begin to crawl out, first black, and after cooling - green. A stick with a diameter of 4 mm burns at a speed of about 2 mm per second and lengthens 10 times.
4. If you mix solutions of copper sulfate and potassium dichromate and add a little ammonia solution, then an amorphous brown precipitate of the composition 4СuCrO 4 * 3NH 3 * 5H 2 O will fall out, which dissolves in hydrochloric acid to form a yellow solution, and in excess of ammonia a green solution is obtained. If further alcohol is added to this solution, a green precipitate will form, which, after filtration, becomes blue, and after drying, blue-violet with red sparkles, clearly visible in strong light.
5. The chromium oxide left after the “volcano” or “pharaoh snake” experiments can be regenerated. To do this, it is necessary to fuse 8 g of Cr 2 O 3 and 2 g of Na 2 CO 3 and 2.5 g of KNO 3 and treat the cooled alloy with boiling water. Soluble chromate is obtained, which can also be converted into other Cr(II) and Cr(VI) compounds, including the original ammonium dichromate.

Examples of redox transitions involving chromium and its compounds

1. Cr 2 O 7 2- -- Cr 2 O 3 -- CrO 2 - -- CrO 4 2- -- Cr 2 O 7 2-

a) (NH 4) 2 Cr 2 O 7 = Cr 2 O 3 + N 2 + 4H 2 O b) Cr 2 O 3 + 2NaOH \u003d 2NaCrO 2 + H 2 O
c) 2NaCrO 2 + 3Br 2 + 8NaOH = 6NaBr + 2Na 2 CrO 4 + 4H 2 O
d) 2Na 2 CrO 4 + 2HCl = Na 2 Cr 2 O 7 + 2NaCl + H 2 O

2. Cr(OH) 2 -- Cr(OH) 3 -- CrCl 3 -- Cr 2 O 7 2- -- CrO 4 2-

a) 2Cr(OH) 2 + 1/2O 2 + H 2 O = 2Cr(OH) 3
b) Cr(OH) 3 + 3HCl = CrCl 3 + 3H 2 O
c) 2CrCl 3 + 2KMnO 4 + 3H 2 O = K 2 Cr 2 O 7 + 2Mn(OH) 2 + 6HCl
d) K 2 Cr 2 O 7 + 2KOH = 2K 2 CrO 4 + H 2 O

3. CrO - Cr (OH) 2 - Cr (OH) 3 - Cr (NO 3) 3 - Cr 2 O 3 - CrO - 2
Cr2+

a) CrO + 2HCl = CrCl 2 + H 2 O
b) CrO + H 2 O \u003d Cr (OH) 2
c) Cr(OH) 2 + 1/2O 2 + H 2 O = 2Cr(OH) 3
d) Cr(OH) 3 + 3HNO 3 = Cr(NO 3) 3 + 3H 2 O
e) 4Cr (NO 3) 3 \u003d 2Cr 2 O 3 + 12NO 2 + O 2
f) Cr 2 O 3 + 2 NaOH = 2NaCrO 2 + H 2 O

Chrome element as an artist

Chemists quite often turned to the problem of creating artificial pigments for painting. In the 18th-19th centuries, a technology for obtaining many pictorial materials was developed. Louis Nicolas Vauquelin in 1797, who discovered the previously unknown element chromium in Siberian red ore, prepared a new, remarkably stable paint - chrome green. Its chromophore is aqueous chromium (III) oxide. Under the name "emerald green" it began to be produced in 1837. Later, L. Vauquelen proposed several new paints: barite, zinc and chrome yellow. Over time, they were replaced by more persistent yellow, orange pigments based on cadmium.

Chrome green is the most durable and lightfast paint that is not affected by atmospheric gases. Rubbed in oil, chrome green has great hiding power and is capable of drying quickly, therefore, since the 19th century. it is widely used in painting. It is of great importance in porcelain painting. The fact is that porcelain products can be decorated with both underglaze and overglaze painting. In the first case, paints are applied to the surface of only a slightly fired product, which is then covered with a layer of glaze. This is followed by the main, high-temperature firing: for sintering the porcelain mass and melting the glaze, the products are heated to 1350 - 1450 0 C. Very few paints can withstand such a high temperature without chemical changes, and in the old days there were only two of them - cobalt and chromium. Black oxide of cobalt, applied to the surface of a porcelain item, fuses with the glaze during firing, chemically interacting with it. As a result, bright blue cobalt silicates are formed. This cobalt blue chinaware is well known to everyone. Chromium oxide (III) does not interact chemically with the components of the glaze and simply lies between the porcelain shards and the transparent glaze with a "deaf" layer.

In addition to chrome green, artists use paints derived from Volkonskoite. This mineral from the group of montmorillonites (a clay mineral of the subclass of complex silicates Na (Mo, Al), Si 4 O 10 (OH) 2) was discovered in 1830 by the Russian mineralogist Kemmerer and named after M.N. Volkonskaya, the daughter of the hero of the Battle of Borodino, General N N. Raevsky, wife of the Decembrist S. G. Volkonsky. Volkonskoite is a clay containing up to 24% chromium oxide, as well as oxides of aluminum and iron (III). The variability of the composition of the mineral found in the Urals, in the Perm and Kirov regions determines its varied coloration - from the color of a darkened winter fir to the bright green color of a swamp frog.

Pablo Picasso turned to the geologists of our country with a request to study the reserves of Volkonskoite, which gives the paint a uniquely fresh tone. At present, a method has been developed for obtaining artificial wolkonskoite. It is interesting to note that, according to modern research, Russian icon painters used paints from this material as early as the Middle Ages, long before its “official” discovery. Guinier's green (created in 1837), whose chromoform is a hydrate of chromium oxide Cr 2 O 3 * (2-3) H 2 O, where part of the water is chemically bound and part adsorbed, was also popular among artists. This pigment gives the paint an emerald hue.

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