Chromium and its compounds. Chromium compounds

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.

Mining 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 dichromate 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 dichromate 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.

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, this meant 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 + 3H2O = Cr2O3 + 3H2

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 become 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 dichromate 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. As a result, red ruby ​​powder is formed.
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 is an element of the side subgroup of the 6th group of the 4th period of the periodic system of chemical elements of D.I. Mendeleev, with atomic number 24. It is designated by the symbol Cr (lat. Chromium). The simple substance chromium is a hard metal of a bluish-white color.

Chemical properties of chromium

Under normal conditions, chromium reacts only with fluorine. At high temperatures (above 600°C) it interacts with oxygen, halogens, nitrogen, silicon, boron, sulfur, phosphorus.

4Cr + 3O 2 – t° →2Cr 2 O 3

2Cr + 3Cl 2 – t° → 2CrCl 3

2Cr + N 2 – t° → 2CrN

2Cr + 3S – t° → Cr 2 S 3

When heated, it reacts with water vapor:

2Cr + 3H 2 O → Cr 2 O 3 + 3H 2

Chromium dissolves in dilute strong acids (HCl, H 2 SO 4)

In the absence of air, Cr 2+ salts are formed, and in air, Cr 3+ salts are formed.

Cr + 2HCl → CrCl 2 + H 2

2Cr + 6HCl + O 2 → 2CrCl 3 + 2H 2 O + H 2

The presence of a protective oxide film on the surface of the metal explains its passivity in relation to concentrated solutions of acids - oxidizers.

Chromium compounds

Chromium(II) oxide and chromium(II) hydroxide are basic in nature.

Cr(OH) 2 + 2HCl → CrCl 2 + 2H 2 O

Chromium (II) compounds are strong reducing agents; transform into chromium (III) compounds under the influence of atmospheric oxygen.

2CrCl 2 + 2HCl → 2CrCl 3 + H 2

4Cr(OH) 2 + O 2 + 2H 2 O → 4Cr(OH) 3

Chromium oxide (III) Cr 2 O 3 is a green, water-insoluble powder. Can be obtained by calcination of chromium(III) hydroxide or potassium and ammonium dichromates:

2Cr(OH) 3 – t° → Cr 2 O 3 + 3H 2 O

4K 2 Cr 2 O 7 – t° → 2Cr 2 O 3 + 4K 2 CrO 4 + 3O 2

(NH 4) 2 Cr 2 O 7 – t° → Cr 2 O 3 + N 2 + 4H 2 O (volcano reaction)

Amphoteric oxide. When Cr 2 O 3 is fused with alkalis, soda and acid salts, chromium compounds with an oxidation state of (+3) are obtained:

Cr 2 O 3 + 2NaOH → 2NaCrO 2 + H 2 O

Cr 2 O 3 + Na 2 CO 3 → 2NaCrO 2 + CO 2

When fused with a mixture of alkali and oxidizing agent, chromium compounds are obtained in the oxidation state (+6):

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

Chromium (III) hydroxide C r (OH) 3 . Amphoteric hydroxide. Gray-green, decomposes when heated, losing water and forming green metahydroxide CrO(OH). Does not dissolve in water. Precipitates from solution as a blue-gray and bluish-green hydrate. Reacts with acids and alkalis, does not interact with ammonia hydrate.

It has amphoteric properties - it dissolves in both acids and alkalis:

2Cr(OH) 3 + 3H 2 SO 4 → Cr 2 (SO 4) 3 + 6H 2 O Cr(OH) 3 + ZH + = Cr 3+ + 3H 2 O

Cr(OH) 3 + KOH → K, Cr(OH) 3 + ZON - (conc.) = [Cr(OH) 6 ] 3-

Cr(OH) 3 + KOH → KCrO 2 + 2H 2 O Cr(OH) 3 + MOH = MSrO 2 (green) + 2H 2 O (300-400 °C, M = Li, Na)

Cr(OH) 3 →(120 o C – H 2 O) CrO(OH) →(430-1000 0 C –H 2 O) Cr2O3

2Cr(OH) 3 + 4NaOH (conc.) + ZN 2 O 2 (conc.) = 2Na 2 CrO 4 + 8H 2 0

Receipt: precipitation with ammonia hydrate from a solution of chromium(III) salts:

Cr 3+ + 3(NH 3 H 2 O) = WITHr(OH) 3 ↓+ ЗNН 4+

Cr 2 (SO 4) 3 + 6NaOH → 2Cr(OH) 3 ↓+ 3Na 2 SO 4 (in excess alkali - the precipitate dissolves)

Chromium (III) salts have a purple or dark green color. Their chemical properties resemble colorless aluminum salts.

Cr(III) compounds can exhibit both oxidizing and reducing properties:

Zn + 2Cr +3 Cl 3 → 2Cr +2 Cl 2 + ZnCl 2

2Cr +3 Cl 3 + 16NaOH + 3Br 2 → 6NaBr + 6NaCl + 8H 2 O + 2Na 2 Cr +6 O 4

Hexavalent chromium compounds

Chromium(VI) oxide CrO 3 - bright red crystals, soluble in water.

Obtained from potassium chromate (or dichromate) and H 2 SO 4 (conc.).

K 2 CrO 4 + H 2 SO 4 → CrO 3 + K 2 SO 4 + H 2 O

K 2 Cr 2 O 7 + H 2 SO 4 → 2CrO 3 + K 2 SO 4 + H 2 O

CrO 3 is an acidic oxide, with alkalis it forms yellow chromates CrO 4 2-:

CrO 3 + 2KOH → K 2 CrO 4 + H 2 O

In an acidic environment, chromates turn into orange dichromates Cr 2 O 7 2-:

2K 2 CrO 4 + H 2 SO 4 → K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O

In an alkaline environment, this reaction proceeds in the opposite direction:

K 2 Cr 2 O 7 + 2KOH → 2K 2 CrO 4 + H 2 O

Potassium dichromate is an oxidizing agent in an acidic environment:

K 2 Cr 2 O 7 + 4H 2 SO 4 + 3Na 2 SO 3 = Cr 2 (SO 4) 3 + 3Na 2 SO 4 + K 2 SO 4 + 4H 2 O

K 2 Cr 2 O 7 + 4H 2 SO 4 + 3NaNO 2 = Cr 2 (SO 4) 3 + 3NaNO 3 + K 2 SO 4 + 4H 2 O

K 2 Cr 2 O 7 + 7H 2 SO 4 + 6KI = Cr 2 (SO 4) 3 + 3I 2 + 4K 2 SO 4 + 7H 2 O

K 2 Cr 2 O 7 + 7H 2 SO 4 + 6FeSO 4 = Cr 2 (SO 4) 3 + 3Fe 2 (SO 4) 3 + K 2 SO 4 + 7H 2 O

Potassium chromate K 2 Cr O 4 . Oxosol. Yellow, non-hygroscopic. Melts without decomposition, thermally stable. Very soluble in water ( yellow the color of the solution corresponds to the CrO 4 2- ion), slightly hydrolyzes the anion. In an acidic environment it turns into K 2 Cr 2 O 7 . Oxidizing agent (weaker than K 2 Cr 2 O 7). Enters into ion exchange reactions.

Qualitative reaction on the CrO 4 2- ion - the precipitation of a yellow precipitate of barium chromate, which decomposes in a strongly acidic environment. It is used as a mordant for dyeing fabrics, a leather tanning agent, a selective oxidizing agent, and a reagent in analytical chemistry.

Equations of the most important reactions:

2K 2 CrO 4 +H 2 SO 4(30%)= K 2 Cr 2 O 7 +K 2 SO 4 +H 2 O

2K 2 CrO 4 (t) +16HCl (concentration, horizon) = 2CrCl 3 +3Cl 2 +8H 2 O+4KCl

2K 2 CrO 4 +2H 2 O+3H 2 S=2Cr(OH) 3 ↓+3S↓+4KOH

2K 2 CrO 4 +8H 2 O+3K 2 S=2K[Cr(OH) 6 ]+3S↓+4KOH

2K 2 CrO 4 +2AgNO 3 =KNO 3 +Ag 2 CrO 4(red) ↓

Qualitative reaction:

K 2 CrO 4 + BaCl 2 = 2KCl + BaCrO 4 ↓

2BaCrO 4 (t) + 2HCl (dil.) = BaCr 2 O 7 (p) + BaC1 2 + H 2 O

Receipt: sintering of chromite with potash in air:

4(Cr 2 Fe ‖‖)O 4 + 8K 2 CO 3 + 7O 2 = 8K 2 CrO 4 + 2Fe 2 O 3 + 8СO 2 (1000 °C)

Potassium dichromate K 2 Cr 2 O 7 . Oxosol. Technical name chrome peak. Orange-red, non-hygroscopic. Melts without decomposition, and decomposes upon further heating. Very soluble in water ( orange The color of the solution corresponds to the Cr 2 O 7 2- ion. In an alkaline environment it forms K 2 CrO 4 . A typical oxidizing agent in solution and during fusion. Enters into ion exchange reactions.

Qualitative reactions- blue color of an ethereal solution in the presence of H 2 O 2, blue color of an aqueous solution under the action of atomic hydrogen.

It is used as a leather tanning agent, a mordant for dyeing fabrics, a component of pyrotechnic compositions, a reagent in analytical chemistry, a metal corrosion inhibitor, in a mixture with H 2 SO 4 (conc.) - for washing chemical dishes.

Equations of the most important reactions:

4K 2 Cr 2 O 7 =4K 2 CrO 4 +2Cr 2 O 3 +3O 2 (500-600 o C)

K 2 Cr 2 O 7 (t) +14HCl (conc) = 2CrCl 3 +3Cl 2 +7H 2 O+2KCl (boiling)

K 2 Cr 2 O 7 (t) +2H 2 SO 4(96%) ⇌2KHSO 4 +2CrO 3 +H 2 O (“chromium mixture”)

K 2 Cr 2 O 7 +KOH (conc) =H 2 O+2K 2 CrO 4

Cr 2 O 7 2- +14H + +6I — =2Cr 3+ +3I 2 ↓+7H 2 O

Cr 2 O 7 2- +2H + +3SO 2 (g) =2Cr 3+ +3SO 4 2- +H 2 O

Cr 2 O 7 2- +H 2 O +3H 2 S (g) =3S↓+2OH - +2Cr 2 (OH) 3 ↓

Cr 2 O 7 2- (conc.) +2Ag + (dil.) =Ag 2 Cr 2 O 7 (red) ↓

Cr 2 O 7 2- (dil.) +H 2 O +Pb 2+ =2H + + 2PbCrO 4 (red) ↓

K 2 Cr 2 O 7(t) +6HCl+8H 0 (Zn)=2CrCl 2(syn) +7H 2 O+2KCl

Receipt: treatment of K 2 CrO 4 with sulfuric acid:

2K 2 CrO 4 + H 2 SO 4 (30%) = K 2Cr 2 O 7 + K 2 SO 4 + H 2 O

"National Research Tomsk Polytechnic University"

Institute of Natural Resources Geoecology and Geochemistry

Chromium

By discipline:

Chemistry

Completed:

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

Checked:

teacher Stas Nikolay Fedorovich

Position in the periodic table

Chromium- element of the side subgroup of the 6th group of the 4th period of the periodic system of chemical elements of D. I. Mendeleev with atomic number 24. Denoted by the symbol Cr(lat. Chromium). Simple substance chromium- hard metal of bluish-white color. Chrome is sometimes classified as a ferrous metal.

Atomic structure

17 Cl)2)8)7 - atomic structure diagram

1s2s2p3s3p - electronic formula

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

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

Element properties

Physical properties

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

Chemical properties

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

Interaction with non-metals

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

4Cr + 3O 2 = 2Cr 2 O 3.

Reacts with fluorine at 350°C, with chlorine at 300°C, with bromine at red heat, forming chromium (III) halides:

2Cr + 3Cl2 = 2CrCl3.

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

2Cr + N 2 = 2CrN

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

2Cr + 3S = Cr 2 S 3.

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

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

2Cr + 3C = Cr 2 C 3 (possible formation of Cr 23 C 6, Cr 7 B 3),

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

Does not interact directly with hydrogen.

Interaction with water

When finely ground and hot, chromium reacts with water to form chromium(III) oxide and hydrogen:

2Cr + 3H2O = Cr2O3 + 3H2

Interaction with acids

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

Cr + 2HCl = CrCl 2 + H 2;

Cr + H 2 SO 4 = CrSO 4 + H 2.

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

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

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

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

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

Interaction with alkaline reagents

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

2Cr + 6KOH = 2KCrO 2 + 2K 2 O + 3H 2.

Reacts with alkaline melts of oxidizing agents, for example potassium chlorate, and chromium is converted into potassium chromate:

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

Recovery of metals from oxides and salts

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

Properties of a simple substance

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

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).

Cr(+2) compounds

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

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

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

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

Cr(+3) compounds

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

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

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

Cr+3NH+3H2O→Cr(OH)↓+3NH

You can use alkali solutions, but in their excess a soluble hydroxo complex is formed:

Cr+3OH→Cr(OH)↓

Cr(OH)+3OH→

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

Cr2O3+2NaOH→2NaCrO2+H2O

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

Cr2O3+6HCl→2CrCl3+3H2O

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

2Na+3HO→2NaCrO+2NaOH+8HO

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

2Cr2O3+8NaOH+3O2→4Na2CrO4+4H2O

Chromium compounds (+4)[

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

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

Chromium compounds (+6)

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

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

2CrO+2H→Cr2O+H2O

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

Cr2O+2OH→2CrO+HO

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

H2CrnO3n+1→H2O+nCrO3

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

Ba+CrO→BaCrO↓

2Ba+CrO+H2O→2BaCrO↓+2H

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

Chromium pentafluoride CrF 5 and low-stable chromium hexafluoride CrF 6 are known. Volatile chromium oxyhalides CrO 2 F 2 and CrO 2 Cl 2 (chromyl chloride) were also obtained.

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

K2Cr2O7+14HCl→2CrCl3+2KCl+3Cl2+7H2O

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