Chemical properties. Oxidation state of chromium Chromium and its chemical properties

Chromium is a chemical element with atomic number 24. It is a hard, shiny, steel-gray metal that polishes well and does not tarnish. Used in alloys such as stainless steel and as a coating. The human body requires small amounts of trivalent chromium to metabolize sugar, but Cr(VI) is highly toxic.

Various chromium compounds, such as chromium(III) oxide and lead chromate, are brightly colored and used in paints and pigments. The red color of ruby ​​is due to the presence of this chemical element. Some substances, especially sodium, are oxidizing agents used to oxidize organic compounds and (together with sulfuric acid) to clean laboratory glassware. In addition, chromium (VI) oxide is used in the production of magnetic tape.

Discovery and etymology

The history of the discovery of the chemical element chromium is as follows. In 1761, Johann Gottlob Lehmann found an orange-red mineral in the Ural Mountains and named it “Siberian red lead.” Although it was erroneously identified as a compound of lead with selenium and iron, the material was actually lead chromate with the chemical formula PbCrO 4 . Today it is known as the mineral croconte.

In 1770, Peter Simon Pallas visited the site where Lehmann found the red lead mineral, which had very useful properties as a pigment in paints. The use of Siberian red lead as paint developed rapidly. In addition, bright yellow crocont has become fashionable.

In 1797, Nicolas-Louis Vauquelin obtained samples of red. By mixing croconte with hydrochloric acid, he obtained CrO 3 oxide. Chromium was isolated as a chemical element in 1798. Vauquelin obtained it by heating the oxide with charcoal. He was also able to detect traces of chromium in gemstones such as ruby ​​and emerald.

In the 1800s, Cr was primarily used in dyes and tanning salts. Today, 85% of the metal is used in alloys. The remainder is used in the chemical, refractory and foundry industries.

The pronunciation of the chemical element chromium corresponds to the Greek χρῶμα, meaning "color", due to the variety of colored compounds that can be obtained from it.

Extraction and production

The element is produced from chromite (FeCr 2 O 4). About half of the world's ore is mined in South Africa. In addition, Kazakhstan, India and Türkiye are its major producers. There are enough explored deposits of chromite, but geographically they are concentrated in Kazakhstan and southern Africa.

Deposits of native chromium metal are rare, but they do exist. For example, it is mined at the Udachnaya mine in Russia. It is rich in diamonds, and the reducing environment helped produce pure chromium and diamonds.

For industrial metal production, chromite ores are treated with molten alkali (caustic soda, NaOH). In this case, sodium chromate (Na 2 CrO 4) is formed, which is reduced by carbon to the oxide Cr 2 O 3. The metal is produced by heating the oxide in the presence of aluminum or silicon.

In 2000, approximately 15 million tons of chromite ore were mined and processed into 4 million tons of ferrochrome, a 70% chromium-iron alloy, with an approximate market value of US$2.5 billion.

Main characteristics

The characteristics of the chemical element chromium are due to the fact that it is a transition metal of the fourth period of the periodic table and is located between vanadium and manganese. Included in group VI. Melts at a temperature of 1907 °C. In the presence of oxygen, chromium quickly forms a thin layer of oxide, which protects the metal from further interaction with oxygen.

As a transition element, it reacts with substances in different proportions. Thus, it forms compounds in which it has different oxidation states. Chromium is a chemical element with the basic states +2, +3 and +6, of which +3 is the most stable. In addition, in rare cases conditions +1, +4 and +5 are observed. Chromium compounds in the +6 oxidation state are strong oxidizing agents.

What color is chrome? The chemical element gives the ruby ​​hue. The Cr 2 O 3 used for is also used as a pigment called chrome green. Its salts color glass emerald green. Chromium is the chemical element whose presence makes rubies red. Therefore, it is used in the production of synthetic rubies.

Isotopes

Isotopes of chromium have atomic weights ranging from 43 to 67. Typically, this chemical element consists of three stable forms: 52 Cr, 53 Cr and 54 Cr. Of these, 52 Cr is the most common (83.8% of all natural chromium). In addition, 19 radioisotopes have been described, of which the most stable is 50 Cr with a half-life exceeding 1.8x10 17 years. 51 Cr has a half-life of 27.7 days, and for all other radioactive isotopes it does not exceed 24 hours, and for most of them it lasts less than one minute. The element also has two meta states.

Isotopes of chromium in the earth's crust, as a rule, accompany isotopes of manganese, which is used in geology. 53 Cr is formed during the radioactive decay of 53 Mn. The Mn/Cr isotope ratio reinforces other clues about the early history of the Solar System. Changes in the 53 Cr/ 52 Cr and Mn/Cr ratios from different meteorites prove that new atomic nuclei were created just before the formation of the Solar System.

Chemical element chromium: properties, formula of compounds

Chromium(III) oxide Cr 2 O 3, also known as sesquioxide, is one of the four oxides of this chemical element. It is obtained from chromite. The green color compound is commonly called "chrome green" when used as a pigment for enamel and glass painting. The oxide can dissolve in acids, forming salts, and in molten alkali - chromites.

Potassium dichromate

K 2 Cr 2 O 7 is a powerful oxidizing agent and is preferred as a means for cleaning laboratory glassware from organic matter. For this purpose, its saturated solution is used. Sometimes, however, it is replaced with sodium bichromate, based on the higher solubility of the latter. In addition, it can regulate the oxidation process of organic compounds, converting primary alcohol into aldehyde and then into carbon dioxide.

Potassium bichromate can cause chrome dermatitis. Chromium is likely to cause sensitization leading to the development of dermatitis, especially of the hands and forearms, which is chronic and difficult to cure. Like other Cr(VI) compounds, potassium dichromate is carcinogenic. It must be handled with gloves and appropriate protective equipment.

Chromic acid

The compound has the hypothetical structure H 2 CrO 4 . Neither chromic nor dichromic acids occur in nature, but their anions are found in various substances. The “chromic acid” that can be found on sale is actually its acid anhydride - CrO 3 trioxide.

Lead(II) chromate

PbCrO 4 has a bright yellow color and is practically insoluble in water. For this reason, it has found use as a coloring pigment called crown yellow.

Cr and pentavalent bond

Chromium is distinguished by its ability to form pentavalent bonds. The compound is created by Cr(I) and a hydrocarbon radical. A pentavalent bond is formed between two chromium atoms. Its formula can be written as Ar-Cr-Cr-Ar, where Ar represents a specific aromatic group.

Application

Chromium is a chemical element whose properties have given it many different uses, some of which are listed below.

It gives metals corrosion resistance and a glossy surface. Therefore, chromium is included in alloys such as stainless steel, used, for example, in cutlery. It is also used for chrome plating.

Chromium is a catalyst for various reactions. It is used to make molds for firing bricks. Its salts are used to tan leather. Potassium bichromate is used for the oxidation of organic compounds such as alcohols and aldehydes, as well as for cleaning laboratory glassware. It serves as a fixing agent for fabric dyeing and is also used in photography and photo printing.

CrO 3 is used to make magnetic tapes (for example, for audio recording), which have better characteristics than films with iron oxide.

Role in biology

Trivalent chromium is a chemical element necessary for the metabolism of sugar in the human body. In contrast, hexavalent Cr is highly toxic.

Precautionary measures

Chromium metal and Cr(III) compounds are generally not considered a health hazard, but substances containing Cr(VI) can be toxic if ingested or inhaled. Most of these substances are irritating to the eyes, skin and mucous membranes. With chronic exposure, chromium(VI) compounds can cause eye damage if not treated properly. In addition, it is a recognized carcinogen. The lethal dose of this chemical element is about half a teaspoon. According to the recommendations of the World Health Organization, the maximum permissible concentration of Cr (VI) in drinking water is 0.05 mg per liter.

Because chromium compounds are used in dyes and to tan leather, they are often found in soil and groundwater from abandoned industrial sites requiring environmental cleanup and remediation. Primer containing Cr(VI) is still widely used in the aerospace and automotive industries.

Element properties

The main physical properties of chromium are as follows:

• Atomic number: 24.
• Atomic weight: 51.996.
• Melting point: 1890 °C.
• Boiling point: 2482 °C.
• Oxidation state: +2, +3, +6.
• Electron configuration: 3d 5 4s 1.

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

Exercise	Phosphorus has the same oxidation state in the following compounds: a) Ca 3 P 2 and H 3 PO 3; b) KH 2 PO 4 and KPO 3; c) P 4 O 6 and P 4 O 10; d) H 3 PO 4 and H 3 PO 3.
Solution	In order to give the correct answer to the question posed, we will alternately determine the degree of oxidation of phosphorus in each pair of proposed compounds. a) The oxidation state of calcium is (+2), oxygen and hydrogen - (-2) and (+1), respectively. Let us take the value of the oxidation state of phosphorus as “x” and “y” in the proposed compounds: 3 ×2 + x ×2 = 0; 3 + y + 3×(-2) = 0; The answer is incorrect. b) The oxidation state of potassium is (+1), oxygen and hydrogen are (-2) and (+1), respectively. Let us take the value of the oxidation state of chlorine as “x” and “y” in the proposed compounds: 1 + 2×1 +x + (-2)×4 = 0; 1 + y + (-2)×3 = 0; The answer is correct.
Answer	Option (b).

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 is formed.
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 oxides of aluminum and iron (III). 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 (Cr) is an element with atomic number 24 and atomic mass 51.996 of a secondary subgroup of the sixth group of the fourth period of the periodic system of chemical elements of D. I. Mendeleev. Chrome is a hard metal with a bluish-white color. 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 bright colors of various colors, which is why it got its name. After all, translated from Greek, “chrome” means “paint”.

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

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, a natural lead chromate. Around the same time (1798), independently of Vauquelin, chromium was discovered by German scientists M. G. Klaproth and Lowitz 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 metal chromium.

Metal chromium is used for chrome plating, and also as one of the most important components of alloy 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 environments at normal temperatures and in gases at elevated temperatures. Chromium steels are characterized by increased hardness. Chromium is used in thermochrome plating, a process in which the protective effect of Cr is due to the formation of a thin but durable oxide film on the surface of the steel, which prevents the interaction of the metal with the environment.

Chromium compounds are also widely used; 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 their leaves, where it is present in the form of a low-molecular complex not associated with subcellular structures. Until now, scientists have not been able to prove the necessity of this element for plants. However, in animals, Cr is involved in the metabolism of lipids, proteins (part of the enzyme trypsin), 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 nutrients, chromium enters the animal or human body through food. A decrease in this microelement in the body leads to slower growth, a sharp increase in blood cholesterol levels and a decrease in the sensitivity of peripheral tissues to insulin.

At the same time, chromium in its pure form is very toxic - Cr metal dust irritates lung tissue, 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 included in 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 coefficient of chromium accumulation is amazing - 10,000-26,000. In the adult human body, the Cr content ranges from 6 to 12 mg. Moreover, the physiological need for chromium for humans has not been established quite accurately. It largely depends on the diet - when eating food high in sugar, the body's need for chromium increases. It is generally accepted that a person needs approximately 20–300 mcg of this element per day. Like other biogenic elements, chromium can accumulate in body tissues, especially in hair. It is in them that the chromium content indicates the degree of provision of the body with this metal. Unfortunately, with age, the “reserves” of chromium in tissues are depleted, with the exception of the lungs.

Chromium is involved in the metabolism of lipids, proteins (present in the enzyme trypsin), carbohydrates (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 involving it. In addition, chromium takes part in the regulation of cholesterol metabolism and is an activator of certain enzymes.

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

However, even at concentrations of a fraction of a milligram per cubic meter in the atmosphere, all chromium compounds have a toxic effect on the body. Poisoning with chromium and its compounds is common during 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 include a feeling of dryness and pain in the nasal cavity, a sore throat, difficulty breathing, coughing and similar symptoms. If there is a slight excess of chromium vapors or dust, the signs of poisoning disappear soon after work in the workshop stops. With prolonged constant contact with chromium compounds, signs of chronic poisoning appear - weakness, constant headaches, weight loss, dyspepsia. Disturbances in the functioning of the gastrointestinal tract, pancreas, and liver begin. Bronchitis, bronchial asthma, and 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 includes periodic medical examinations of personnel working with chromium and its compounds; installation of ventilation, dust suppression and dust collection equipment; 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 is made up of two Greek words: chromo and soma. Literally this can be translated as “painted body” or “body that is painted.” The structural element of a chromosome, formed in the interphase of the cell nucleus as a result of chromosome duplication, is called “chromatid”. “Chromatin” is a substance of chromasomes 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 the concepts of “chromatic aberration” and “chromatic polarization”. “Chromatography” is a physical and chemical method for separating and analyzing mixtures. “Chromoscope” is a device for obtaining a color image by optically combining two or three color-separated photographic images, illuminated through specially selected differently colored filters.

The most toxic is chromium (VI) oxide CrO3; it belongs to hazard class I. Lethal dose for humans (orally) 0.6 g. Ethyl alcohol ignites on contact with freshly prepared CrO3!

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

Ferrochrome, used in the metallurgical industry in the production of chromium steels, was of very poor quality at the end of the 19th century. This is due to the low chromium content in it - only 7-8%. Then it was called “Tasmanian cast iron” due to the fact that the original iron-chrome ore was imported from Tasmania.

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

Many laboratories use a “chromic mixture” - a mixture of a saturated solution of potassium dichromate with concentrated sulfuric acid. It is used in degreasing the surfaces of glass and steel laboratory glassware. It oxidizes fat and removes its remains. Just handle this mixture with caution, 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, wood is impregnated with special compounds containing chromates and dichromates, 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 has occupied a special niche in printing. In 1839, it was discovered that paper impregnated with sodium bichromate suddenly turned brown when exposed to bright light. Then it turned out that bichromate coatings on paper, after exposure, do not dissolve in water, but, when wetted, acquire a bluish tint. Printers took advantage of this property. The desired pattern was photographed on a plate with a colloidal coating containing dichromate. The illuminated areas did not dissolve during washing, and the unexposed areas 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 ground 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 the unusual crystals with hydrochloric acid, resulting in a white precipitate in which lead was found. Based on the results obtained, Lehman named the mineral Siberian red lead. This is the story of the discovery of crocoite (from the Greek “krokos” - saffron) - a 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 mineral being studied were taken. This is how the traveler himself describes it: “This amazing red lead mineral is not found in any other deposit. When ground into powder it turns yellow and can be used in artistic miniatures.” German enterprise overcame all the difficulties of mining and delivering crocoite to Europe. Despite the fact that these operations took at least two years, soon the carriages of the noble gentlemen of Paris and London were traveling painted with finely ground crocoite. The collections of the mineralogical museums of many universities of the old world have been enriched with the best examples of this mineral from the Russian depths. However, European scientists could not figure out 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 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 treating 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). By 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 fused gray needle-shaped 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 explained by the admixture of chromium compounds in them. An example is emerald, which is a green-colored beryl in which aluminum is partially replaced by chromium.

It is clear that Vauquelin did not obtain pure metal, most likely its carbides, which is confirmed by the needle-shaped shape of light gray crystals. Pure chromium metal was later obtained by F. Tassert, probably in 1800.

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

Being in nature

In the bowels of the earth, chromium is a fairly common element, despite the fact that it is not found in free form. Its clarke (average content in the earth's crust) is 8.3.10-3% or 83 g/t. However, its distribution among breeds is uneven. This element is mainly characteristic of the Earth’s mantle; the fact is that ultramafic rocks (peridotites), which are presumably 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, and the formation of the largest deposits of this element is associated with them. The chromium content is also high in basic rocks (basalts, etc.) 2 10-2% or 200 g/t. Much less Cr is found in acidic rocks: 2.5 10-3%, sedimentary rocks (sandstones) - 3.5 10-3%, shales also contain chromium - 9 10-3%.

It can be concluded that chromium is a typical lithophile element and is almost entirely contained in deep minerals in the Earth’s interior.

There are three main chromium minerals: magnochromite (Mn, Fe)Cr2O4, chromopicotite (Mg, Fe)(Cr, Al)2O4 and aluminochromite (Fe, Mg)(Cr, Al)2O4. These minerals have a single name - chrome spinel and the general formula (Mg, Fe)O (Cr, Al, Fe)2O3. They are indistinguishable in appearance and are inaccurately called “chromites.” Their composition is variable. 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 platinum group elements 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, chrome chlorite, chrome tourmaline, chrome mica (fuchsite), chrome garnet (uvarovite), etc., which often accompany ores, but are not of industrial importance. Chromium is a relatively weak aquatic migrant. Under exogenous conditions, chromium, like iron, migrates in the form of suspensions and can precipitate in clays. The most mobile form is chromates.

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 is found in many much less common minerals, for example, melanochroite 3PbO 2Cr2O3, vokelenite 2(Pb,Cu)CrO4(Pb,Cu)3(PO4)2, tarapacaite 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, and occur in the form of continuous masses.

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 leading position is occupied by the Republic of South Africa - 1050 million tons, where the main contribution is made by the Bushveld complex (about 1000 million tons ). The second place in chrome resources belongs to Kazakhstan, where very high quality ore is mined in the Aktobe region (Kempirsay massif). Other countries also have reserves of this element. Turkey (in Guleman), Philippines on the island of Luzon, Finland (Kemi), India (Sukinda), etc.

Our country has its own developed chromium deposits 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 chrome ores. It was only in 1827 that the American Isaac Tison discovered a large deposit of chrome ore on the border of Maryland and Pennsylvania, seizing the mining monopoly for many years. In 1848, deposits of high-quality chromite were found in Turkey, near Bursa, and soon (after the depletion of the Pennsylvania deposit) it was this country that took over the role of monopolist. This continued until 1906, when rich deposits of chromite were discovered in South Africa and India.

Application

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 total consumption.

Chromium is widely used to alloy steels and alloys, giving them corrosion and heat resistance. More than 40% of the resulting pure metal is consumed in the production 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 alloys - 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.

Metallic chromium is used for chrome 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 coating perfectly resists the effects of humid atmospheric air, salty sea air, water, nitric and most organic acids. Such coatings have two purposes: protective and decorative. The thickness of the 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 can withstand sudden, repeated temperature changes, which is why they are used in the structures 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 comparable to the hardness of corundum, which ensures 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 (chrompic) - K2Cr2O7 is also used for tanning leather, as a mordant for dyeing fabrics, and is a corrosion inhibitor for metals and alloys. 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.

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

Anhydrous chromium chloride CrCl3 is used to apply chromium coatings to the surface of steels by chemical vapor deposition and is a component of some catalysts. CrCl3 hydrates are a mordant for dyeing fabrics.

Various dyes are made from lead chromate PbCrO4.

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

Production

In nature, chromium is found mainly in the form of chromium iron ore FeO∙Cr2O3; when it is reduced with coal, an alloy of chromium with iron is obtained - ferrochrome, 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 obtain high-carbon chromium 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 prepared and then reduced by an aluminothermic method. In this case, a mixture of powdered or in the form of aluminum shavings (Al) and a charge of chromium oxide (Cr2O3) are first 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 adding 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%. This chromium is fragile and easily ground into powder.

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

When it is necessary to obtain high-purity chromium, industry uses electrolytic methods. Electrolysis is carried out using a mixture of chromic anhydride, chromoammonium alum or chromium sulfate with dilute sulfuric acid. Chromium deposited on aluminum or stainless steel cathodes during the electrolysis process 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 Cr metal by electrolysis of CrCl3 or CrF3 melts in a mixture with potassium, calcium, and sodium fluorides at a temperature of 900 ° C in an argon environment.

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 producing pure chromium. In this case, chromium is reduced from oxide by silicon:

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

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

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

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

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

Physical properties

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

The melting point of chromium directly depends on its purity. Therefore, determining 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 studying this issue for decades and received results that are far from each other: from 1513 to 1920 ° C. Previously, it was generally accepted that this metal melts at a temperature of 1890 ° C, but modern research indicates a temperature of 1907 ° C, chromium boils at temperatures above 2500° C - the data also varies: from 2199° C to 2671° C. The density of chromium is less than that of iron; it is 7.19 g/cm3 (at a temperature of 200° C).

Chrome has all the basic characteristics of metals - it conducts heat well, its resistance to electric current is very low, like most metals, chrome has a characteristic shine. In addition, this element has one very interesting feature: the fact is that at a temperature of 37 ° C its behavior cannot be explained - a sharp change in many physical properties occurs, this change has an abrupt nature. Chrome, like a sick person at a temperature of 37° C, begins to act up: the internal friction of chromium reaches a maximum, the elastic modulus drops to minimum values. The value of electrical conductivity jumps, the thermoelectromotive force and the coefficient of linear expansion constantly change. Scientists cannot yet 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 resistivity of 0.414 μΩ 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 the steel hard and wear-resistant, because chromium is the hardest of all metals - it cuts glass like diamond! The Brinell hardness of high-purity chromium is 7-9 Mn/m2 (70-90 kgf/cm2). Spring, spring, tool, stamp and ball bearing steels are alloyed with chromium. In them (except for ball bearing steels) chromium is present along with manganese, molybdenum, nickel, and vanadium. The addition of chromium to conventional steels (up to 5% Cr) improves their physical properties and makes the metal more susceptible to heat treatment.

Chromium is antiferromagnetic, specific magnetic susceptibility 3.6 10-6. Electrical resistivity 12.710-8 Ohm. The temperature coefficient of linear expansion of chromium is 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 quite inert, this is explained by the presence of a durable thin oxide film on its surface. Cr does not oxidize in air, even in the presence of moisture. When heated, oxidation occurs exclusively on the metal surface. At 1200°C the film is destroyed and oxidation occurs much faster. At 2000° C, chromium burns to form green chromium (III) oxide Cr2O3, which has amphoteric properties. By fusing Cr2O3 with alkalis, chromites are obtained:

Cr2O3 + 2NaOH = 2NaCrO2 + H2O

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

Cr2O3 + 6HCl = 2CrCl3 + 3H2O

In compounds, chromium mainly exhibits 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 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 by nitric acid does not dissolve in dilute sulfuric and hydrochloric acids even after prolonged boiling in their solutions, but at some point dissolution does occur, accompanied by violent foaming from the liberated hydrogen. This process is explained by the fact that chromium goes 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 during the dissolution process, 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 (III) oxide Cr2O3:

4Cr + 3O2 = 2Cr2O3

Cr2O3 are 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 and difficult to dissolve in aqueous acids and alkalis. Chromium(III) oxide is quite toxic. When it comes into contact with the skin, it 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 from 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.

Due to the fact that it has excellent anti-corrosion properties. Chrome plating protects any other alloy from rust. In addition, alloying steels with chromium gives them the same resistance to corrosion that is characteristic of the metal itself.

So, let's discuss today what are the technical and oxidation characteristics of the chromium material, the main amphoteric, reducing properties and metal production will also be affected. We will also find out what the effect of chromium is on the properties of steel.

Chromium is a metal of period 4 of group 6 of the secondary subgroup. Atomic number 24, atomic mass 51.996. It is a hard metal with a silvery-bluish color. In its pure form it is malleable and tough, but the slightest admixtures of nitrogen or carbon give it brittleness and hardness.

Chromium is often classified as a ferrous metal due to the color of its main mineral, chromium iron ore. But it got its name from the Greek “color”, “paint”, thanks to its compounds: metal salts and oxides with varying degrees of oxidation are painted in all the colors of the rainbow.

• Under normal conditions, chromium is inert and does not react with oxygen, nitrogen or water.
• In air, it is immediately passivated - covered with a thin oxide film, which completely blocks oxygen from accessing the metal. For the same reason, the substance does not interact with sulfuric and nitric acid.
• When heated, the metal becomes active and reacts with water, oxygen, acids and alkalis.

It is characterized by a body-centered cubic lattice. There are no phase transitions. At a temperature of 1830 C, a transition to a face-centered lattice is possible.

However, chromium has one interesting anomaly. At a temperature of 37 C, some physical properties of the metal change sharply: electrical resistance and linear expansion coefficient change, the elastic modulus drops to a minimum and internal friction increases.

This is due to the passage of the Néel point: at this temperature, the substance changes its antiferromagnetic properties to paramagnetic ones, which represents a first-level transition and means a sharp increase in volume.

The chemical properties of chromium and its compounds are described in this video:

Chemical and physical properties of chromium

Melting and boiling points

• The physical characteristics of a metal are affected by impurities to such an extent that even the melting point has proven difficult to determine.
• According to modern measurements, the melting point is considered to be 1907 C. The metal is a refractory substance.

The boiling point is 2671 C.

Below we will give a general description of the physical and magnetic properties of chromium metal.

General properties and characteristics of chromium

Physical Features

• Chromium is one of the most stable of all refractory metals.
• Density under normal conditions is 7200 kg/cubic meter. m, this is less than .
• Hardness on the Mohs scale is 5, on the Brinell scale 7–9 Mn/m2. Chromium is the hardest metal known, second only to uranium, iridium, tungsten and beryllium.

Due to its structure - a body-centered lattice, chromium has such a characteristic as the temperature of the brittle-ductile period. But when it comes to this metal, this value turns out to be highly dependent on the degree of purity and ranges from -50 to +350 C. In practice, crystallized chromium does not have any ductility, but after soft annealing and molding it becomes malleable.

The strength of the metal also increases with cold working. Alloying additives also significantly enhance this quality.

Thermophysical characteristics

As a rule, refractory metals have a high level of thermal conductivity and, accordingly, a low coefficient of thermal expansion. However, chromium differs noticeably in its qualities.

At the Néel point, the coefficient of thermal expansion makes a sharp jump, and then continues to increase noticeably with increasing temperature. At 29 C (before the jump), the value of the coefficient is 6.2 · 10-6 m/(m K).

Thermal conductivity obeys the same pattern: at the Néel point it falls, although not so sharply and decreases with increasing temperature.

• Under normal conditions, the thermal conductivity of the substance is 93.7 W/(m K).
• The specific heat capacity under the same conditions is 0.45 J/(g K).

Electrical properties

Despite the atypical “behavior” of thermal conductivity, chromium is one of the best conductors of current, second only to silver and gold in this parameter.

• At normal temperature, the electrical conductivity of the metal will be 7.9 · 106 1/(Ohm m).
• Electrical resistivity – 0.127 (Ohm mm2)/m.

Up to the Néel point - 38 C, the substance is antiferromagnet, that is, under the influence of a magnetic field and in its absence, no magnetic properties appear. Above 38 C, chromium becomes paramagnetic: it exhibits magnetic properties under the influence of an external magnetic field.

Toxicity

In nature, chromium is found only in bound form, so the entry of pure chromium into the human body is excluded. However, it is known that metal dust irritates lung tissue and is not absorbed through the skin. The metal itself is not toxic, but the same cannot be said about its compounds.

• Trivalent chromium appears in the environment during its processing. However, it can also enter the human body as part of a dietary supplement - chromium picolinate, used in weight loss programs. As a trace element, the trivalent metal is involved in the synthesis of glucose and is essential. Excess of it, judging by research, does not pose a certain danger, since it is not absorbed by the intestinal walls. However, it can accumulate in the body.
• Hexavalent chromium compounds toxic by more than 100–1000 times. It can enter the body during the production of chromates, during chrome plating of objects, and during some welding operations. Compounds of the hexavalent element are strong oxidizing agents. Once in the gastrointestinal tract, they cause bleeding of the stomach and intestines, possibly with perforation of the intestine. The substances are almost not absorbed through the skin, but have a strong corrosive effect - burns, inflammation, and ulcers are possible.

Chromium is a mandatory alloying element when producing stainless and heat-resistant materials. Its ability to resist corrosion and transfer this quality to alloys remains the most sought-after quality of the metal.

The chemical properties of chromium compounds and its redox properties are discussed in this video: