Where to find chrome metal. Chromium compounds. Oxides, hydroxides. Chromates. Dichromats. Oxidative properties of chromium (VI) compounds. Recognition of chromium compounds

DEFINITION

Chromium located in the fourth period of group VI of the secondary (B) subgroup of the Periodic table. Designation – Cr. In the form of a simple substance - a grayish-white shiny metal.

Chrome has a body-centered cubic lattice structure. Density - 7.2 g/cm3. The melting and boiling points are 1890 o C and 2680 o C, respectively.

Oxidation state of chromium in compounds

Chromium can exist in the form of a simple substance - a metal, and the oxidation state of metals in the elemental state is equal to zero, since the distribution of electron density in them is uniform.

Oxidation states (+2) And (+3) chromium appears in oxides (Cr +2 O, Cr +3 2 O 3), hydroxides (Cr +2 (OH) 2, Cr +3 (OH) 3), halides (Cr +2 Cl 2, Cr +3 Cl 3 ), sulfates (Cr +2 SO 4, Cr +3 2 (SO 4) 3) and other compounds.

Chromium is also characterized by its oxidation state (+6) : Cr +6 O 3, H 2 Cr +6 O 4, H 2 Cr +6 2 O 7, K 2 Cr +6 2 O 7, etc.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

Chromium

CHROMIUM-A; m.[from Greek chrōma - color, paint]

1. Chemical element (Cr), a hard metal of a steel-gray color (used in the manufacture of hard alloys and for coating metal products).

2. Soft thin leather tanned with salts of this metal. Boots made of chrome.

3. A type of yellow paint obtained from chromates.

◁ Chrome (see).

(lat. Chromium), chemical element of group VI of the periodic system. Named from the Greek. chrōma - color, paint (due to the bright color of the compounds). Bluish-silver metal; density 7.19 g/cm 3, t pl 1890°C. Does not oxidize in air. The main minerals are chrome spinels. Chromium is an essential component of stainless, acid-resistant, heat-resistant steels and a large number of other alloys (nichrome, chrome, stellite). Used for chrome plating. Chromium compounds are oxidizing agents, inorganic pigments, tanning agents.

CHROME (Latin chromium, from the Greek chromium - color, color; chromium compounds are characterized by a wide color palette), Cr (read “chrome”), a chemical element with atomic number 24, atomic weight 51.9961. Located in group VIB in the 4th period of the periodic table of elements.
Natural chromium consists of a mixture of four stable nuclides: 50 Cr (mixture content 4.35%), 52 Cr (83.79%), 53 Cr (9.50%) and 54 Cr (2.36%). Configuration of two outer electronic layers 3s 2 R 6 d 5 4s 1 . Oxidation states range from 0 to +6, the most typical are +3 (the most stable) and +6 (valences III and VI).
Neutral atom radius 0.127 nm, ion radius (coordination number 6): Cr 2+ 0.073 nm, Cr 3+ 0.0615 nm, Cr 4+ 0.055 nm, Cr 5+ 0.049 nm and Cr 6+ 0.044 nm. The sequential ionization energies are 6.766, 16.49, 30.96, 49.1, 69.3 and 90.6 eV. Electron affinity 1.6 eV. Electronegativity according to Pauling (cm. PAULING Linus) 1,66.
History of discovery
In 1766, a mineral was discovered in the vicinity of Yekaterinburg, which was called “Siberian red lead,” PbCrO 4. The modern name is crocoite. In 1797, the French chemist L. N. Vauquelin (cm. VAUCLIN Louis Nicolas) isolated a new refractory metal from it (most likely Vauquelin obtained chromium carbide).
Being in nature
Content in the earth's crust is 0.035% by weight. The chromium content in sea water is 2·10 -5 mg/l. Chromium is practically never found in free form. It is part of more than 40 different minerals (chromite FeCr 2 O 4, volkonskoite, uvarovite, vokelenite, etc.). Some meteorites contain chromium sulfide compounds.
Receipt
The industrial raw material for the production of chromium and chromium-based alloys is chromite. By reducing chromite melting with coke (reducing agent), iron ore and other components, ferrochrome with a chromium content of up to 80% (by weight) is obtained.
To obtain pure metal chromium, chromite is fired with soda and limestone in furnaces:
2Cr 2 O 3 + 2Na 2 CO 3 + 3O 2 = 4Na 2 CrO 4 + 4CO 2
The resulting sodium chromate Na 2 CrO 4 is leached with water, the solution is filtered, evaporated and treated with acid. In this case, Na 2 CrO 4 chromate transforms into Na 2 Cr 2 O 7 dichromate:
2Na 2 CrO 4 + H 2 SO 4 = Na 2 Cr 2 O 7 + Na 2 SO 4 + H 2 O
The resulting dichromate is reduced with sulfur:
Na 2 Cr 2 O 7 + 3S = Na 2 S + Cr 2 O 3 + 2SO 2,
The resulting pure chromium(III) oxide Cr 2 O 3 is subjected to aluminothermy:
Cr 2 O 3 + 2Al = Al 2 O 3 + 2Cr.
Silicon is also used:
2Cr 2 O 3 + 3Si = 3SiO 2 + 4Cr
To obtain high-purity chromium, technical chromium is electrochemically purified from impurities.
Physical and chemical properties
In its free form, it is a bluish-white metal with a cubic body-centered lattice, A= 0.28845 nm. At a temperature of 39°C it changes from a paramagnetic state to an antiferromagnetic state (Néel point). Melting point 1890°C, boiling point 2680°C. Density 7.19 kg/dm3.
Stable in air. At 300°C it burns to form green chromium (III) oxide Cr 2 O 3, which has amphoteric properties. By fusing Cr 2 O 3 with alkalis, chromites are obtained:
Cr 2 O 3 + 2NaOH = 2NaCrO 2 + H 2 O
Uncalcined chromium(III) oxide easily dissolves in alkaline solutions and acids:
Cr 2 O 3 + 6HCl = 2CrCl 3 + 3H 2 O
The thermal decomposition of chromium carbonyl Cr(OH) 6 produces red basic chromium(II) oxide CrO. Brown or yellow hydroxide Cr(OH) 2 with weakly basic properties is precipitated when alkalis are added to solutions of chromium(II) salts.
Careful decomposition of chromium(VI) oxide CrO 3 under hydrothermal conditions produces chromium(IV) dioxide CrO 2, which is ferromagnetic and has metallic conductivity.
When concentrated sulfuric acid reacts with solutions of dichromates, red or violet-red crystals of chromium(VI) oxide CrO 3 are formed. A typically acidic oxide, when interacting with water it forms strong unstable chromic acids: chromic H 2 CrO 4 , dichromic H 2 Cr 2 O 7 and others.
Halides are known that correspond to different oxidation states of chromium. Chromium dihalides CrF 2, CrCl 2, CrBr 2 and CrI 2 and trihalides CrF 3, CrCl 3, CrBr 3 and CrI 3 were synthesized. However, unlike similar compounds of aluminum and iron, CrCl 3 trichloride and chromium tribromide CrBr 3 are non-volatile.
Among chromium tetrahalides, CrF 4 is stable, chromium tetrachloride CrCl 4 exists only in vapors. Chromium hexafluoride CrF 6 is known.
Chromium oxyhalides CrO 2 F 2 and CrO 2 Cl 2 were obtained and characterized.
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), were synthesized. with silicon (silicides Cr 3 Si, Cr 5 Si 3 and CrSi) and nitrogen (nitrides CrN and Cr 2 N).
Chromium(III) compounds are the most stable in solutions. In this oxidation state, chromium corresponds to both the cationic form and the anionic forms, for example, the 3- anion existing in an alkaline environment.
When chromium(III) compounds are oxidized in an alkaline environment, chromium(VI) compounds are formed:
2Na 3 + 3H 2 O 2 = 2Na 2 CrO 4 + 2NaOH + 8H 2 O
Cr (VI) corresponds to a number of acids that exist only in aqueous solutions: chromic H 2 CrO 4 , dichromic H 2 Cr 2 O 7 , trichromic H 3 Cr 3 O 10 and others that form salts - chromates, dichromates, trichromates, etc. .
Depending on the acidity of the environment, the anions of these acids easily convert into each other. For example, when a yellow solution of potassium chromate K 2 CrO 4 is acidified, orange potassium dichromate K 2 Cr 2 O 7 is formed:
2K 2 CrO 4 + 2HCl = K 2 Cr 2 O 7 + 2KCl + H 2 O
But if an alkali solution is added to the orange solution of K 2 Cr 2 O 7, the color turns yellow again because potassium chromate K 2 CrO 4 is formed again:
K 2 Cr 2 O 7 + 2KOH = 2K 2 CrO 4 + H 2 O
When a barium salt solution is added to a yellow solution containing chromate ions, a yellow precipitate of barium chromate BaCrO 4 precipitates:
Ba 2+ + CrO 4 2- = BaCrO 4
Chromium(III) compounds are strong oxidizing agents, for example:
K 2 Cr 2 O 7 + 14 HCl = 2CrCl 3 + 2KCl + 3Cl 2 + 7H 2 O
Application
The use of chromium is based on its heat resistance, hardness and corrosion resistance. They are used to produce alloys: stainless steel, nichrome, etc. A large amount of chromium is used for decorative corrosion-resistant coatings. Chromium compounds are fire-resistant materials. Chromium (III) oxide is a green paint pigment, also included in abrasive materials (GOI pastes). The color change upon reduction of chromium(VI) compounds is used to conduct a rapid analysis of the alcohol content in exhaled air.
The Cr 3+ cation is part of potassium chromium KCr(SO 4) 2 ·12H 2 O alum used in leather tanning.
Physiological action
Chromium is one of the biogenic elements and is constantly included in the tissues of plants and animals. In animals, chromium is involved in the metabolism of lipids, proteins (part of the enzyme trypsin), and carbohydrates. A decrease in chromium content in food and blood leads to a decrease in growth rate and an increase in cholesterol in the blood.
Chromium metal is virtually non-toxic, but chromium metal dust is irritating to lung tissue. Chromium(III) compounds cause dermatitis. Chromium(VI) compounds lead to various human diseases, including cancer. The maximum permissible concentration of chromium(VI) in atmospheric air is 0.0015 mg/m3.

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See what "chrome" is in other dictionaries:

chromium- chrome, and... Russian spelling dictionary

chromium- chrome/… Morphemic-spelling dictionary

- (from the Greek chroma color, paint). A grayish metal mined from chrome ore. Dictionary of foreign words included in the Russian language. Chudinov A.N., 1910. CHROME metal of grayish color; in its pure form x. not used; connections with... Dictionary of foreign words of the Russian language

CHROMIUM- see CHROME (Cr). Chromium compounds are found in wastewater from many industrial enterprises producing chromium salts, acetylene, tannins, aniline, linoleum, paper, paints, pesticides, plastics, etc. Trivalent compounds are found in water... ... Fish Diseases: A Guide

CHROME, ah, husband. 1. Chemical element, hard light gray shiny metal. 2. Kind of yellow paint (special). | adj. chromium, aya, oh (to 1 value) and chrome, aya, oh. Chromium steel. Chrome ore. II. CHROME, ah, husband. A type of soft, thin leather. | adj... Ozhegov's Explanatory Dictionary

chromium- a, m. chrome m. novolat. chromium lat. chroma gr. dye. 1. Chemical element is a hard silvery metal used in the manufacture of hard alloys and for coating metal products. BAS 1. Metal discovered by Vauquelin... ... Historical Dictionary of Gallicisms of the Russian Language

CHROMIUM- CHROME, Chromium (from the Greek chroma paint), I symbol. SG, chem. element with at. weighing 52.01 (isotopes 50, 52, 53, 54); serial number 24, for! occupies a place in the even subgroup VI of group j of the periodic table. Compounds X. are often found in nature... Great Medical Encyclopedia

- (lat. Chromium) Cr, chemical element of group VI of Mendeleev’s periodic table, atomic number 24, atomic mass 51.9961. Name from Greek. chroma color, paint (due to the bright color of the Compound). Bluish silver metal; density 7.19… … Big Encyclopedic Dictionary

CHROME 1, a, m. Ozhegov’s Explanatory Dictionary. S.I. Ozhegov, N.Yu. Shvedova. 1949 1992 … Ozhegov's Explanatory Dictionary

CHROME 2, a, m. A type of soft, thin leather. Ozhegov's explanatory dictionary. S.I. Ozhegov, N.Yu. Shvedova. 1949 1992 … Ozhegov's Explanatory Dictionary

The discovery of chromium dates back to a period of rapid development of chemical and 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 Siberian red lead ore (crocoite), described by Lomonosov. The mineral was examined, but nothing but oxides of lead, iron and aluminum were found in it. However, in 1797, Vaukelin, boiling a finely ground sample of the mineral with potash and precipitating lead carbonate, obtained a solution colored orange-red. From this solution he crystallized a ruby-red salt, from which the oxide and free metal, different from all known metals, were isolated. Vauquelin called him Chromium ( Chrome ) from the Greek word- coloring, color; True, what was meant here was not the property of the metal, but its brightly colored salts.

Being 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, the Urals, North America, and 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 has been found in the Sun, stars, and meteorites.

Physical properties.

Chrome is a white, hard and brittle metal, extremely chemically resistant to acids and alkalis. In air it oxidizes and has a thin transparent film of oxide on the surface. Chromium has a density of 7.1 g/cm3, its melting point is +1875 0 C.

Receipt.

When chromium iron ore is heated strongly with coal, chromium and iron are reduced:

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

As a result of this reaction, a chromium-iron alloy 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 = Al 2 O 3 + 2Cr

In this process, two oxides are usually used - Cr 2 O 3 and CrO 3

Chemical properties.

Thanks to the thin protective film of oxide covering the surface of chrome, 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 = Cr 2 (SO 4) 3 + 3H 2
2Cr + 6HCl = 2CrCl3 + 3H2

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

Hot chromium reacts with water vapor:

2Cr + 3H 2 O = Cr 2 O 3 + 3H 2

At high temperatures, chromium also reacts with halogens, halogen with hydrogen, sulfur, nitrogen, phosphorus, carbon, silicon, boron, for example:

Cr + 2HF = CrF 2 + H 2
2Cr + N2 = 2CrN
2Cr + 3S = Cr 2 S 3
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 produce high-strength, corrosion-resistant coatings in mechanical engineering. Alloys in the form of ferrochrome are used as metal-cutting tools. Chrome alloys have found application in medical technology and in the manufacture of chemical technological equipment.

Position of chromium in the periodic table of chemical elements:

Chromium heads the secondary subgroup of group VI of the periodic table 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 in the chromium atom, the pattern according to which the 4S orbital should first be filled to the 4S 2 state is violated. However, due to the fact that the 3d orbital occupies a more favorable energy position in the chromium atom, it is filled to the value 4d 5 . This phenomenon is observed in atoms of some other elements of secondary subgroups. Chromium can exhibit oxidation states from +1 to +6. The most stable are chromium compounds with oxidation states +2, +3, +6.

Compounds of divalent chromium.

Chromium (II) oxide CrO is a pyrophoric black powder (pyrophoricity - the ability to ignite in air in a finely crushed 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 when chromium metal is dissolved in acids. These reactions take place in an atmosphere of low-active gas (for example H 2), because in the presence of air, oxidation of Cr(II) to Cr(III) easily occurs.

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

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

Trivalent chromium compounds.

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

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

Cr 2 O 3 is an amphoteric oxide, when fused with alkalis it forms chromites: Cr 2 O 3 + 2NaOH = 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 (III) sulfate Cr 2 (SO 4) 3 is pink in color and is 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 containing less water are also known. Chromium alum KCr(SO 4) 2 *12H 2 O crystallizes from solutions containing violet chromium sulfate and potassium sulfate. A solution of chrome 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 with a high degree of probability conclude that chromium is absent.

1. Let us 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. The salt will not release water of crystallization, but will melt at a temperature of about 400 0 C to form a dark liquid. Let's heat it for a few more minutes over high heat. After cooling, a green precipitate forms on the shard. Let's dissolve part of it in water (it turns yellow), and leave the other part 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 bichromate in 50ml of water. Add a little potassium carbonate to one part. It will dissolve with the release of CO 2, and the color of the solution will turn light yellow. Chromate is formed from potassium dichromate. If you now add a 50% sulfuric acid solution in portions, the red-yellow color of the dichromate will appear again.
3. Pour 5 ml into a test tube. potassium bichromate solution, boil with 3 ml of concentrated hydrochloric acid under pressure. Yellow-green toxic chlorine gas is released from the solution because the 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, fused with soda and saltpeter, converted into 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 the potassium bichromate solution and acidify the solution with sulfuric acid. The solution acquires a deep blue color due to the formation of chromium peroxide. When shaken with a certain amount of ether, the peroxide will transform into an organic solvent and color 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, you need to dissolve the metal. During prolonged boiling with 30% sulfuric acid (you can also add hydrochloric acid), chromium and many steels are partially dissolved. The resulting solution contains chromium (III) sulfate. To be able to carry out a detection reaction, we first neutralize it with caustic soda. Gray-green chromium(III) hydroxide precipitates, which dissolves in excess NaOH to form green sodium chromite. Filter the solution and add 30% hydrogen peroxide. When heated, the solution will turn yellow as chromite oxidizes to chromate. Acidification will cause the solution to appear blue. The colored compound can be extracted by shaking with ether.

Analytical reactions for chromium ions.

1. Add a 2M NaOH solution to 3-4 drops of chromium chloride solution CrCl 3 until the initial precipitate dissolves. Note the color of the sodium chromite formed. Heat the resulting solution in a water bath. What happens?
2. To 2-3 drops of CrCl 3 solution, add an equal volume of 8 M NaOH solution and 3-4 drops of 3% H 2 O 2 solution. Heat the reaction mixture in a water bath. What happens? 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 the test tube. Heat the reaction mixture for several minutes in 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 emerging blue color of the solution 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 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- interacts with Ba 2+ ions, a yellow precipitate of barium chromate BaCrO 4 precipitates.
3. Silver nitrate forms a brick-red silver chromate precipitate with CrO 4 2 ions.
4. Take three test tubes. Place 5-6 drops of K 2 Cr 2 O 7 solution into one of them, the same volume of K 2 CrO 4 solution into the second, and three drops of both solutions into the third. Then add three drops of potassium iodide solution to each test tube. Explain your result. Acidify the solution in the second test tube. What happens? 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, after filtration a bright green solution is obtained, which turns yellow when acid is added, and turns green in a neutral or alkaline environment.
2. Place a “ruby mixture” in the center of a tin can with thermite - carefully ground and placed in aluminum foil Al 2 O 3 (4.75 g) with the addition of Cr 2 O 3 (0.25 g). To prevent the jar from cooling down longer, it is necessary to bury it under the top edge in sand, and after the thermite is set on fire and the reaction begins, cover it with an iron sheet and cover it with sand. Dig out the jar in a day. The result is a red ruby ​​powder.
3. 10 g of potassium dichromate is ground 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 at the end, 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 extends 10 times.
4. If you mix solutions of copper sulfate and potassium dichromate and add a little ammonia solution, an amorphous brown precipitate of the composition 4СuCrO 4 * 3NH 3 * 5H 2 O will form, which dissolves in hydrochloric acid to form a yellow solution, and in excess of ammonia a green solution is obtained. If you further add alcohol 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 remaining after the “volcano” or “pharaoh’s snakes” experiments can be regenerated. To do this, you need 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. The result is a soluble chromate, which can 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 = 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
Cr 2+

a) CrO + 2HCl = CrCl 2 + H 2 O
b) CrO + H 2 O = 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) 4Сr(NO 3) 3 = 2Cr 2 O 3 + 12NO 2 + O 2
e) Cr 2 O 3 + 2 NaOH = 2NaCrO 2 + H 2 O

Chromium element as an artist

Chemists quite often turned to the problem of creating artificial pigments for painting. In the 18th-19th centuries, the technology for producing many painting 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 hydrous chromium(III) oxide. It began to be produced under the name “emerald green” in 1837. Later, L. Vauquelin proposed several new paints: barite, zinc and chrome yellow. Over time, they were replaced by more persistent yellow and orange cadmium-based pigments.

Green chrome is the most durable and light-resistant paint that is not susceptible to atmospheric gases. Chromium green ground in oil has great covering power and is capable of drying quickly, which is why it has been used 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 lightly fired product, which is then covered with a layer of glaze. This is followed by the main, high-temperature firing: to sinter the porcelain mass and melt 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 chrome. Black cobalt oxide applied to the surface of a porcelain product fuses with the glaze during firing, chemically interacting with it. As a result, bright blue cobalt silicates are formed. Everyone knows this cobalt-decorated blue porcelain tableware well. Chromium (III) oxide does not react chemically with the components of the glaze and simply lies between the porcelain shards and the transparent glaze as a “blind” layer.

In addition to chrome green, artists use paints obtained 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 in honor of 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 aluminum and iron (III) oxides. The composition of the mineral, found in the Urals, Perm and Kirov regions, is inconsistent. determines its varied color - from the color of winter darkened fir to the bright green color of a marsh frog.

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

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Chromium(lat. Cromium), Cr, chemical element of group VI of the periodic system of Mendeleev, atomic number 24, atomic mass 51.996; bluish-steel colored 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 indicator.

Historical reference. Chromium was discovered in 1797 by L. N. Vauquelin in the mineral crocoite - natural lead chromate PbCrO 4 . Chrome got its name from the Greek word chroma - color, paint (due to 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 chromium deposits is associated with them. In basic rocks, the Chromium content reaches only 2·10 -2%, in acidic rocks - 2.5·10 -3%, in sedimentary rocks (sandstones) - 3.5·10 -3%, in clay shales - 9·10 -3 %. Chromium is a relatively weak aquatic migrant; Chromium content in sea water is 0.00005 mg/l.

In general, Chromium is a metal in 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, which often accompany chromium ores, but are not of practical value themselves (uvarovite, volkonskoite, kemerite, fuchsite).

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

Atomic radius 1.27 Å; ionic radii of Cr 2+ 0.83 Å, Cr 3+ 0.64 Å, Cr 6+ 0.52 Å. Density 7.19 g/cm3; t pl 1890 °C; boiling point 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 °C); electrical resistivity 0.414 μΩ 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 3.6·10 -6. The Brinell hardness of high-purity Chromium is 7-9 Mn/m2 (70-90 kgf/cm2).

Chemical properties of Chromium. The external electronic configuration of the Chromium atom is 3d 5 4s 1. In compounds it usually exhibits oxidation states +2, +3, +6, among them 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 to form 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; sulfur - Cr 2 S 3. When fused with boron, it forms boride CrB, and with silicon it forms silicides Cr 3 Si, Cr 2 Si 3, CrSi 2. Chromium forms alloys with many metals. The interaction with oxygen is quite active at first, then slows down sharply due to the formation of an oxide film on the metal surface. At 1200 °C the film is destroyed and oxidation proceeds quickly again. Chromium ignites in oxygen at 2000 °C to form the dark green oxide of Chromium (III) Cr 2 O 3. In addition to oxide (III), other compounds with oxygen are known, for example CrO, CrO 3, obtained indirectly. Chromium easily reacts with dilute solutions of hydrochloric and sulfuric acids to form chromium chloride and sulfate and release hydrogen; Regia vodka and nitric acid passivate chromium.

As the degree of oxidation increases, the acidic and oxidizing properties of Chromium increase. Derivatives of Cr 2+ are very strong reducing agents. The Cr 2+ ion is formed at the first stage of the dissolution of Chromium in acids or during the reduction of Cr 3+ in an acidic solution with zinc. Oxide hydrate Cr(OH) 2 upon dehydration turns 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 (or rather Cr 2 O 3 nH 2 O) is an amphoteric compound that forms salts with the Cr 3+ cation or salts of chromous acid HC-O 2 - chromites (for example, KS-O 2, NaCrO 2). Compounds Cr 6+: chromic anhydride CrO 3, 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; Cr 3+ complex compounds, in which Chromium has a coordination number of 6, are especially numerous. There is a significant number of Chromium peroxide compounds

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

2FeCr 2 O 4 + 4K 2 CO 3 + 3.5 O 2 = 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 turns it into dichromate K 2 Cr 2 O 7 . Next, 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 (III) oxide C 2 O 3.

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

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

Chromium is obtained in small quantities by reducing 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 oxidizing agent additives is loaded into a crucible, where the reaction is excited by igniting the mixture of Na 2 O 2 and Al until the crucible is filled with Chromium and slag. Silicothermic chromium is smelted 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 reduction.

Chromium alloys - ferrochrome and silicon chromium - are produced on a large scale in industry.

Application of Chromium. The use of Chrome is based on its heat resistance, hardness and corrosion resistance. Most of all, Chromium is used for smelting chromium steels. Aluminum- 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. Powdered Chromium is widely used in the production of metal-ceramic products and materials for welding electrodes. Chromium in the form of 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 a component of tanning solutions in the leather industry; PbCrO 4 , ZnCrO 4 , SrCrO 4 - like art paints. Chromium-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 and 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 higher than 3-10 -4 mol/l. In leaves it is present in the form of a low-molecular complex not associated with subcellular structures. In animals, Chromium is involved in the metabolism of lipids, proteins (part of the enzyme trypsin), and carbohydrates (a structural component of the glucose-resistant factor). The main source of Chromium in animals and humans is food. A decrease in chromium content 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.

Poisoning with Chromium and its compounds occurs during their production; in mechanical engineering (galvanic 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, cough, etc.; they can go away when contact with Chromium is stopped. With prolonged contact with chromium compounds, signs of chronic poisoning develop: headache, weakness, dyspepsia, weight loss and others. The functions of the stomach, liver and pancreas are impaired. Possible bronchitis, bronchial asthma, diffuse pneumosclerosis. When exposed to Chromium on the skin, dermatitis and eczema can develop. According to some data, chromium compounds, mainly Cr(III), have a carcinogenic effect.

And fats.

Scientists say cholesterol levels are affected by chromium. Element It is considered biogenic, that is, it is necessary for the body, not only the human one, but also all mammals.

With a lack of chromium, their growth slows down and cholesterol “jumps.” The norm is 6 milligrams of chromium from the total weight of a person.

Ions of the substance are found in all tissues of the body. You should get 9 micrograms per day.

You can take them from seafood, pearl barley, beets, liver and duck meat. While you are purchasing products, we will tell you about other purposes and properties of chromium.

Properties of chromium

Chromium is a chemical element related to metals. The color of the substance is silver-blue.

The element has the 24th atomic number, or, as they also say, atomic number.

The number indicates the number of protons in the nucleus. As for the electrons rotating near it, they have a special property - to fall through.

This means that one or two particles can move from one sublevel to another.

As a result, the 24th element is able to half fill the 3rd sublevel. A stable electronic configuration is obtained.

Electron failure is a rare phenomenon. Apart from chromium, the only ones that come to mind are, perhaps, , , and .

Like the 24th substance, they are chemically inactive. It is not then that the atom reaches a stable state in order to react with everyone.

Under normal conditions chromium is an element of the periodic table, which can only be “stirred up”.

The latter is the antipode of the 24th substance and is maximally active. The reaction produces fluoride chromium.

Element, properties which are discussed, does not oxidize, is not afraid of moisture and refractory materials.

The latter characteristic “delays” reactions that are possible during heating. Thus, interaction with water vapor starts only at 600 degrees Celsius.

The result is chromium oxide. The reaction with also starts, giving the nitride of the 24th element.

At 600 degrees, several compounds with and the formation of sulfide are also possible.

If the temperature is increased to 2000, the chromium will ignite upon contact with oxygen. The result of combustion will be a dark green oxide.

This precipitate easily reacts with solutions and acids. The result of the interaction is chromium chloride and sulfide. All compounds of the 24th substance are, as a rule, brightly colored.

In its pure form, basic chromium element characteristics– toxicity. Metal dust irritates lung tissue.

Dermatitis, that is, allergic diseases, may appear. Accordingly, it is better not to exceed the norm of chromium for the body.

There is also a standard for the content of element 24 in the air. There should be 0.0015 milligrams per cubic meter of atmosphere. Exceeding the standard is considered pollution.

Chromium metal has a high density - more than 7 grams per cubic centimeter. This means the substance is quite heavy.

The metal is also quite high. It depends on the electrolyte temperature and current density. Fungi and mold seem to respect this.

If you impregnate wood with a chrome composition, microorganisms will not begin to destroy it. Builders use this.

They are also happy with the fact that treated wood burns worse, because chromium is a refractory metal. We will tell you further how and where else it can be applied.

Application of chromium

Chromium is an alloying element during smelting. Remember that under normal conditions the 24th metal does not oxidize or rust?

The basis of steels is . It cannot boast of such properties. That's why chromium is added, which increases corrosion resistance.

In addition, the addition of the 24th substance reduces the critical cooling rate point.

Siliconothermic chromium is used for smelting. This is a duet of the 24th element with nickel.

The additives used are silicon, . Nickel is responsible for its ductility, and chromium is responsible for its oxidation resistance and hardness.

Combine chrome and s. The result is super-hard stellite. Additives to it are molybdenum and.

The composition is expensive, but is necessary for surfacing machine parts in order to increase their wear resistance. Stellite is also sprayed onto working machines.

As a rule, decorative corrosion-resistant coatings use chromium compounds.

The bright range of their colors comes in handy. In metal-ceramics, color is not needed, therefore, powdered chrome is used. It is added, for example, for strength to the bottom layer of crowns for.

Chromium formula- component . This is a mineral from the group, but it does not have the usual color.

Uvarovite is a stone, and it is chromium that makes it so. It's no secret that they are used.

The green variety of the stone is no exception, and is valued higher than the red one because it is rare. Also, it will boil down a little to the standard ones.

This is also a plus, because mineral inserts are more difficult to scratch. The stone is cut facetted, that is, by forming angles, which increases the play of light.

Chromium mining

It is not profitable to extract chromium from minerals. Most with the 24th element are used entirely.

In addition, the chromium content in, as a rule, is low. The substance is extracted, basically, from ores.

Associated with one of them opening chrome. He was found in Siberia. In the 18th century, crocoite was found there. This is a red lead ore.

Its base is , the second element is chrome. A German chemist named Lehmann managed to discover it.

At the time of the discovery of crocoite, he was visiting St. Petersburg, where he conducted experiments. Now, the 24th element is obtained by electrolysis of concentrated aqueous solutions of chromium oxide.

Electrolysis of sulfate is also possible. These are 2 ways to get the purest chromium. Molecule oxide or sulfate is destroyed in a crucible, where the original compounds are set on fire.

The 24th element is separated, the rest goes to slag. All that remains is to smelt the chromium in an arc. This is how the purest metal is extracted.

There are other ways to get chromium element, for example, the reduction of its oxide with silicon.

But this method produces metal with a large number of impurities and, moreover, is more expensive than electrolysis.

Chrome price

In 2016, the cost of chromium is still decreasing. January started at $7,450 per ton.

By mid-summer they are asking for only 7,100 conventional units per 1,000 kilograms of metal. Data provided by Infogeo.ru.

That is, Russian prices were considered. The global cost of chromium reached almost $9,000 per ton.

The lowest summer mark differs from the Russian one by only 25 dollars upward.

If we are not considering the industrial sector, for example, metallurgy, but benefits of chromium for the body, you can study the offers of pharmacies.

So, “Picolinate” of the 24th substance costs about 200 rubles. For “Cartnitin Chrome Forte” they ask for 320 rubles. This is the price tag for a package of 30 tablets.

Turamine Chrome can also compensate for the deficiency of the 24th element. Its cost is 136 rubles.

Chromium, by the way, is part of tests for detecting drugs, in particular marijuana. One test costs 40-45 rubles.