Chromium metal physical properties. Chromium in nature and its industrial extraction. Oxidation state of chromium in compounds

Chromium is a transition metal that is widely used in industry due to its strength and resistance to heat and corrosion. This article will give you an understanding of some of the important properties and uses of this transition metal.

Chromium belongs to the category of transition metals. It is a hard but brittle steel-gray metal with atomic number 24. This lustrous metal is placed in group 6 of the periodic table, and is designated by the symbol "Cr".

The name chromium is derived from the Greek word chromia, which means color.

True to its name, chromium forms several intensely colored compounds. Today, virtually all commercially used chromium is extracted from the ore iron chromite or chromium oxide (FeCr2O4).

Properties of chromium

• Chromium is the most common element on earth's crust, but it never happens in its pure form. Mainly extracted from mines such as chromite mines.

• Chromium melts at a temperature of 2180 K or 3465°F, and the boiling point is 2944 K or 4840°F. its atomic weight is 51.996 g/mol, and on the Mohs scale it is 5.5.

• Chromium occurs in many oxidation states, such as +1, +2, +3, +4, +5, and +6, of which +2, +3, and +6 are the most common, and +1, +4, A +5 is a rare oxidation. The +3 oxidation state is the most stable state of chromium. Chromium(III) can be prepared by dissolving elemental chromium in hydrochloric or sulfuric acid.

• This metal element is known for its unique magnetic properties. At room temperature, it exhibits antiferromagnetic ordering, which is shown in other metals at relatively low temperatures.

• Antiferromagnetism is where neighboring ions that behave like magnets attach to opposing or antiparallel mechanisms through a material. As a result, the magnetic field created by magnetic atoms or ions is oriented in one direction canceling magnetic atoms or ions aligned in the opposite direction, so that the material does not exhibit any gross external magnetic fields.

• At temperatures above 38°C, chromium becomes paramagnetic, i.e. it is attracted to an externally applied magnetic field. In other words, chromium is attracted to an external magnetic field at temperatures above 38°C.

• Chrome is not subject to hydrogen embrittlement, i.e. it does not become brittle when exposed to atomic hydrogen. But when exposed to nitrogen, it loses its plasticity and becomes brittle.

• Chrome is highly resistant to corrosion. A thin protective oxide film forms on the surface of a metal when it comes into contact with oxygen in the air. This layer prevents the diffusion of oxygen into the base material and thus protects it from further corrosion. This process is called passivation, passivation with chromium gives resistance to acids.

• There are three main isotopes of chromium, called 52Cr, 53Cr, and 54Cr, of which 52CR is the most common isotope. Chromium reacts with most acids, but does not react with water. At room temperature, it reacts with oxygen to form chromium oxide.

Application

Stainless steel production

Chrome has a wide range of uses due to its hardness and corrosion resistance. It is used mainly in three industries - metallurgical, chemical and refractory. It is widely used for making stainless steel as it prevents corrosion. Today it is a very important alloying material for steels. It is also used to make nichrome, which is used in resistance heating elements due to its ability to withstand high temperatures.

Surface coating

Acid chromate or dichromate is also used to coat surfaces. This is usually done using an electroplating method in which a thin layer of chromium is applied to a metal surface. Another method is chrome plating, through which chromates are used to apply a protective layer to certain metals such as aluminum (Al), cadmium (CD), zinc (Zn), silver, and also magnesium (MG).

Preservation of wood and tanning of leather

Chromium (VI) salts are toxic, so they are used to preserve wood from damage and destruction by fungus, insects and termites. Chromium(III), especially chromium alum or potassium sulfate, is used in the leather industry as it helps stabilize leather.

Dyes and pigments

Chromium is also used to make pigments or dyes. Chrome yellow and lead chromate were widely used as pigments in the past. Due to environmental concerns, its use declined significantly and was then finally replaced by lead and chrome pigments. Other pigments are based on chromium, red chromium, green chromium oxide, which is a mixture of yellow and Prussian blue. Chromium oxide is used to give the greenish color to glass.

Synthesis of artificial rubies

Emeralds owe their green hue to chrome. Chromium oxide is also used to produce synthetic rubies. Natural rubies are corundum or aluminum oxide crystals that take on a red hue due to the presence of chromium. Synthetic or man-made rubies are made by doping chromium(III) onto synthetic corundum crystals.

Biological functions

Chromium(III) or trivalent chromium is essential in the human body, but in very small quantities. It is believed to play an important role in lipid and sugar metabolism. It is currently used in many dietary supplements that claim to have several health benefits, however, this is a controversial issue. The biological role of chromium has not been adequately tested, and many experts believe it is not important for mammals, while others view it as an essential micronutrient for humans.

Other uses

The high melting point and heat resistance make chrome an ideal fireproof material. It has found application in blast furnaces, cement kilns, and metal kilns. Many chromium compounds are used as catalysts for hydrocarbon processing. Chromium(IV) is used to produce magnetic tapes used in audio and video cassettes.

Hexavalent chromium or chromium(VI) is called a toxic and mutagenic substance, and chromium(IV) is known for its carcinogenic properties. Chromate salt also causes allergic reactions in some people. Thanks to health care and environmental problems, some restrictions have been placed on the use of chromium compounds in various parts of the world.

Target: deepen students' knowledge on the topic of the lesson.

Tasks:

• describe chromium as simple substance;
• introduce students to chromium compounds varying degrees oxidation;
• show the dependence of the properties of compounds on the degree of oxidation;
• show the redox properties of chromium compounds;
• continue to develop students’ skills in writing down equations of chemical reactions in molecular and ionic form and creating an electronic balance;
• continue to develop the skills to observe a chemical experiment.

Lesson form: lecture with elements independent work students and observing a chemical experiment.

Progress of the lesson

I. Repetition of material from the previous lesson.

1. Answer questions and complete tasks:

What elements belong to the chromium subgroup?

Write electronic formulas of atoms

What type of elements are they?

What oxidation states do the compounds exhibit?

How does the atomic radius and ionization energy change from chromium to tungsten?

You can ask students to complete the table using the tabulated values ​​of atomic radii, ionization energies and draw conclusions.

Sample table:

2. Listen to a student’s report on the topic “Elements of the chromium subgroup in nature, preparation and application.”

II. Lecture.

Lecture outline:

1. Chromium.
2. Chromium compounds. (2)

• Chromium oxide; (2)
• Chromium hydroxide. (2)

1. Chromium compounds. (3)

• Chromium oxide; (3)
• Chromium hydroxide. (3)

1. Chromium compounds (6)

• Chromium oxide; (6)
• Chromic and dichromic acids.

1. Dependence of the properties of chromium compounds on the degree of oxidation.
2. Redox properties of chromium compounds.

1. Chrome.

Chrome is a white, shiny metal with a bluish tint, very hard (density 7.2 g/cm3), melting point 1890˚C.

Chemical properties: chromium is an inactive metal under normal conditions. This is explained by the fact that its surface is covered with an oxide film (Cr 2 O 3). When heated, the oxide film is destroyed, and chromium reacts with simple substances at high temperatures:

• 4Сr + 3О 2 = 2Сr 2 О 3
• 2Сr + 3S = Сr 2 S 3
• 2Сr + 3Cl 2 = 2СrСl 3

Exercise: draw up equations for the reactions of chromium with nitrogen, phosphorus, carbon and silicon; Compose an electronic balance for one of the equations, indicate the oxidizing agent and the reducing agent.

Interaction of chromium with complex substances:

At very high temperatures, chromium reacts with water:

• 2Сr + 3Н2О = Сr2О3 + 3Н2

Exercise:

Chromium reacts with dilute sulfuric and hydrochloric acids:

• Cr + H 2 SO 4 = CrSO 4 + H 2
• Cr + 2HCl = CrCl 2 + H 2

Exercise: draw up an electronic balance, indicate the oxidizing agent and reducing agent.

Concentrated sulfuric hydrochloric and nitric acids passivate chromium.

2. Chromium compounds. (2)

1. Chromium oxide (2)- CrO is a solid, bright red substance, a typical basic oxide (it corresponds to chromium (2) hydroxide - Cr(OH) 2), does not dissolve in water, but dissolves in acids:

• CrO + 2HCl = CrCl 2 + H 2 O

Exercise: draw up a reaction equation in molecular and ionic form for the interaction of chromium oxide (2) with sulfuric acid.

Chromium oxide (2) is easily oxidized in air:

• 4CrO+ O 2 = 2Cr 2 O 3

Exercise: draw up an electronic balance, indicate the oxidizing agent and reducing agent.

Chromium oxide (2) is formed by the oxidation of chromium amalgam with atmospheric oxygen:

2Сr (amalgam) + O 2 = 2СrО

2. Chromium hydroxide (2)- Cr(OH) 2 is a yellow substance, poorly soluble in water, with a pronounced basic character, therefore it interacts with acids:

• Cr(OH) 2 + H 2 SO 4 = CrSO 4 + 2H 2 O

Exercise: draw up reaction equations in molecular and ionic form for the interaction of chromium oxide (2) with hydrochloric acid.

Like chromium(2) oxide, chromium(2) hydroxide is oxidized:

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

Exercise: draw up an electronic balance, indicate the oxidizing agent and reducing agent.

Chromium hydroxide (2) can be obtained by the action of alkalis on chromium salts (2):

• CrCl 2 + 2KOH = Cr(OH) 2 ↓ + 2KCl

Exercise: write ionic equations.

3. Chromium compounds. (3)

1. Chromium oxide (3)- Cr 2 O 3 – dark green powder, insoluble in water, refractory, close in hardness to corundum (chromium hydroxide (3) – Cr(OH) 3) corresponds to it. Chromium oxide (3) is amphoteric in nature, but is poorly soluble in acids and alkalis. Reactions with alkalis occur during fusion:

• Cr 2 O 3 + 2KOH = 2KSrO 2 (chromite K)+ H 2 O

Exercise: draw up a reaction equation in molecular and ionic form for the interaction of chromium oxide (3) with lithium hydroxide.

It is difficult to interact with concentrated solutions of acids and alkalis:

• Cr 2 O 3 + 6 KOH + 3H 2 O = 2K 3 [Cr(OH) 6 ]
• Cr 2 O 3 + 6HCl = 2CrCl 3 + 3H 2 O

Exercise: draw up reaction equations in molecular and ionic form for the interaction of chromium oxide (3) with concentrated sulfuric acid and a concentrated solution of sodium hydroxide.

Chromium oxide (3) can be obtained from the decomposition of ammonium dichromate:

• (NН 4)2Сr 2 О 7 = N 2 + Сr 2 О 3 +4Н 2 О

2. Chromium hydroxide (3) Cr(OH) 3 is obtained by the action of alkalis on solutions of chromium salts (3):

• CrCl 3 + 3KOH = Cr(OH) 3 ↓ + 3KCl

Exercise: write ionic equations

Chromium hydroxide (3) is a gray-green precipitate, upon receipt of which the alkali must be taken in deficiency. The chromium hydroxide (3) obtained in this way, in contrast to the corresponding oxide, easily interacts with acids and alkalis, i.e. exhibits amphoteric properties:

• Cr(OH) 3 + 3HNO 3 = Cr(NO 3) 3 + 3H 2 O
• Cr(OH) 3 + 3KOH = K 3 [Cr(OH)6] (hexahydroxochromite K)

Exercise: draw up reaction equations in molecular and ionic form for the interaction of chromium hydroxide (3) with hydrochloric acid and sodium hydroxide.

When Cr(OH) 3 is fused with alkalis, metachromites and orthochromites are obtained:

• Cr(OH) 3 + KOH = KCrO 2 (metachromite K)+ 2H 2 O
• Cr(OH) 3 + KOH = K 3 CrO 3 (orthochromite K)+ 3H 2 O

4. Chromium compounds. (6)

1. Chromium oxide (6)- СrО 3 – dark red crystalline substance, highly soluble in water - a typical acid oxide. This oxide corresponds to two acids:

• CrO 3 + H 2 O = H 2 CrO 4 (chromic acid - formed when there is excess water)
• CrO 3 + H 2 O =H 2 Cr 2 O 7 (dichromic acid - formed at a high concentration of chromium oxide (3)).

Chromium oxide (6) is a very strong oxidizing agent, therefore it energetically interacts with organic substances:

• C 2 H 5 OH + 4CrO 3 = 2CO 2 + 2Cr 2 O 3 + 3H 2 O

Also oxidizes iodine, sulfur, phosphorus, coal:

• 3S + 4CrO 3 = 3SO 2 + 2Cr 2 O 3

Exercise: draw up equations of chemical reactions of chromium oxide (6) with iodine, phosphorus, coal; create an electronic balance for one of the equations, indicate the oxidizing agent and reducing agent

When heated to 250 0 C, chromium oxide (6) decomposes:

• 4CrO3 = 2Cr2O3 + 3O2

Chromium oxide (6) can be obtained by the action of concentrated sulfuric acid on solid chromates and dichromates:

• K 2 Cr 2 O 7 + H 2 SO 4 = K 2 SO 4 + 2CrO 3 + H 2 O

2. Chromic and dichromic acids.

Chromic and dichromic acids exist only in aqueous solutions and form stable salts, chromates and dichromates, respectively. Chromates and their solutions are yellow in color, dichromates are orange.

Chromate - CrO 4 2- ions and dichromate - Cr 2O 7 2- ions easily transform into each other when the solution environment changes

In an acidic solution, chromates transform into dichromates:

• 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, dichromates turn into chromates:

• K 2 Cr 2 O 7 + 2 KOH = 2 K 2 CrO 4 + H 2 O

When diluted, dichromic acid turns into chromic acid:

• H 2 Cr 2 O 7 + H 2 O = 2H 2 CrO 4

5. Dependence of the properties of chromium compounds on the degree of oxidation.

Oxidation state	+2	+3	+6
Oxide	CrO	Cr 2 O 3	СrО 3
Character of the oxide	basic	amphoteric	acid
Hydroxide	Cr(OH) 2	Cr(OH) 3 – H 3 CrO 3	H 2 CrO 4
Nature of the hydroxide	basic	amphoteric	acid
→ weakening of basic properties and strengthening of acidic properties→

6. Redox properties of chromium compounds.

Reactions in an acidic environment.

In an acidic environment, Cr +6 compounds transform into Cr +3 compounds under the action of reducing agents: H 2 S, SO 2, FeSO 4

• K 2 Cr 2 O 7 + 3H 2 S + 4H 2 SO 4 = 3S + Cr 2 (SO 4) 3 + K 2 SO 4 + 7H 2 O
• S -2 – 2e → S 0
• 2Cr +6 + 6e → 2Cr +3

Exercise:

1. Equalize the reaction equation using the electronic balance method, indicate the oxidizing agent and reducing agent:

• Na 2 CrO 4 + K 2 S + H 2 SO 4 = S + Cr 2 (SO 4) 3 + K 2 SO 4 + Na 2 SO 4 + H 2 O

2. Add the reaction products, equalize the equation using the electronic balance method, indicate the oxidizing agent and reducing agent:

• K 2 Cr 2 O 7 + SO 2 + H 2 SO 4 =? +? +H 2 O

Reactions in an alkaline environment.

In an alkaline environment, chromium compounds Cr +3 transform into compounds Cr +6 under the action of oxidizing agents: J2, Br2, Cl2, Ag2O, KClO3, H2O2, KMnO4:

• 2KCrO 2 +3 Br 2 +8NaOH =2Na 2 CrO 4 + 2KBr +4NaBr + 4H 2 O
• Cr +3 - 3e → Cr +6
• Br2 0 +2e → 2Br -

Exercise:

Equalize the reaction equation using the electronic balance method, indicate the oxidizing agent and reducing agent:

• NaCrO 2 + J 2 + NaOH = Na 2 CrO 4 + NaJ + H 2 O

Add the reaction products, equalize the equation using the electronic balance method, indicate the oxidizing agent and reducing agent:

• Cr(OH) 3 + Ag 2 O + NaOH = Ag + ? + ?

Thus, oxidizing properties successively increase with a change in oxidation states in the series: Cr +2 → Cr +3 → Cr +6. Chromium compounds (2) are strong reducing agents and are easily oxidized, turning into chromium compounds (3). Chromium compounds (6) are strong oxidizing agents and are easily reduced to chromium compounds (3). Chromium compounds (3) when interacting with strong reducing agents exhibit oxidizing properties, turning into chromium compounds (2), and when interacting with strong oxidizing agents they exhibit reducing properties, turning into chromium compounds (6)

To the methodology of the lecture:

1. To activate cognitive activity students and maintaining interest, it is advisable to conduct a demonstration experiment during the lecture. Depending on the capabilities of the educational laboratory, the following experiments can be demonstrated to students:

• obtaining chromium oxide (2) and chromium hydroxide (2), proof of their basic properties;
• obtaining chromium oxide (3) and chromium hydroxide (3), proving their amphoteric properties;
• obtaining chromium oxide (6) and dissolving it in water (preparation of chromic and dichromic acids);
• transition of chromates to dichromates, dichromates to chromates.

1. Independent work tasks can be differentiated taking into account the real learning capabilities of students.
2. You can complete the lecture by completing the following tasks: write equations of chemical reactions that can be used to carry out the following transformations:

.III. Homework: improve the lecture (add the equations of chemical reactions)

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2. Egorov A.S. Chemistry tutor. – Rostov-on-Don: “Phoenix”, 2006.-765 p.
3. Kudryavtsev A.A. Compilation chemical equations. - M., “Higher School”, 1979. - 295 p.
4. Petrov M.M. Inorganic chemistry. – Leningrad: “Chemistry”, 1989. – 543 p.
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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 ore of chromium, which has practical significance, is chromite, the approximate composition of which corresponds to the formula FeCrO ​​4.

It is found in Asia Minor, the Urals, North America, in southern Africa. The above-mentioned mineral crocoite – PbCrO 4 – is also of technical importance. Chromium oxide (3) and some of its other compounds are also found in nature. In the earth's crust, the chromium content in terms of metal is 0.03%. Chromium 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 strongly heated 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. Part of it is soluble in water (it acquires 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 becomes deep Blue colour due to the formation of chromium peroxide. When shaken with a certain amount of ether, the peroxide will transform into an organic solvent and color it blue. This reaction is specific for chromium and is very sensitive. It can be used to detect chromium in metals and alloys. First of all, you need to dissolve the metal. During prolonged boiling with 30% sulfuric acid (you can also add hydrochloric acid), chromium and many steels are partially dissolved. The resulting solution contains chromium (III) sulfate. To be able to carry out a detection reaction, we first neutralize it with caustic soda. Gray-green chromium(III) hydroxide precipitates, which dissolves in excess NaOH to form green sodium chromite. Filter the solution and add 30% hydrogen peroxide. When heated, the solution will turn yellow as chromite oxidizes to chromate. Acidification will cause the solution to appear blue. The colored compound can be extracted by shaking with ether.

Analytical reactions for chromium ions.

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

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

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

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

Perchromic acid is much more stable in organic solvents.

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

Entertaining experiments with chromium compounds

1. A mixture of CuSO 4 and K 2 Cr 2 O 7 turns green when alkali is added, and turns yellow in the presence of acid. By heating 2 mg of glycerol with a small amount of (NH 4) 2 Cr 2 O 7 and then adding alcohol, after filtration a bright green solution is obtained, which turns yellow when acid is added, and turns green in a neutral or alkaline environment.
2. Place a “ruby mixture” in the center of a tin can with thermite - carefully ground and placed in aluminum foil Al 2 O 3 (4.75 g) with the addition of Cr 2 O 3 (0.25 g). To prevent the jar from cooling down longer, it is necessary to bury it under the top edge in sand, and after the thermite is set on fire and the reaction begins, cover it with an iron sheet and cover it with sand. Dig out the jar in a day. The result is a red ruby ​​powder.
3. 10 g of potassium dichromate is ground with 5 g of sodium or potassium nitrate and 10 g of sugar. The mixture is moistened and mixed with collodion. If the powder is compressed in a glass tube, and then the stick is pushed out and set on fire at the end, a “snake” will begin to crawl out, first black, and after cooling - green. A stick with a diameter of 4 mm burns at a speed of about 2 mm per second and extends 10 times.
4. If you mix solutions of copper sulfate and potassium dichromate and add a little ammonia solution, an amorphous brown precipitate of the composition 4СuCrO 4 * 3NH 3 * 5H 2 O will form, which dissolves in hydrochloric acid to form a yellow solution, and in excess of ammonia a green solution is obtained. If you further add alcohol to this solution, a green precipitate will form, which after filtration becomes blue, and after drying, blue-violet with red sparkles, clearly visible in strong light.
5. The chromium oxide remaining after the “volcano” or “pharaoh’s snakes” experiments can be regenerated. To do this, you need to fuse 8 g of Cr 2 O 3 and 2 g of Na 2 CO 3 and 2.5 g of KNO 3 and treat the cooled alloy with boiling water. The result is a soluble chromate, which can be converted into other Cr(II) and Cr(VI) compounds, including the original ammonium dichromate.

Examples of redox transitions involving chromium and its compounds

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

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

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

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

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

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

Chromium element as an artist

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

Green chrome is the most durable and light-resistant paint that is not susceptible to atmospheric gases. Chromium green ground in oil has great covering power and is capable of drying quickly, which is why it has been used since the 19th century. it is widely used in painting. It is of great importance in porcelain painting. The fact is that porcelain products can be decorated with both underglaze and overglaze painting. In the first case, paints are applied to the surface of only a lightly fired product, which is then covered with a layer of glaze. This is followed by the main, high-temperature firing: to sinter the porcelain mass and melt the glaze, the products are heated to 1350 - 1450 0 C. Such a high temperature without chemical changes Very few paints can withstand, 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. The result is bright blue cobalt silicates. 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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And fats.

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

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

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

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

Properties of chromium

Chrome – chemical element related to metals. The color of the substance is silver-blue.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Application of chromium

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

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

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

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

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

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

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

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

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

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

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

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

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

Chromium mining

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

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

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

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

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

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

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

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

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

Chrome price

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

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

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

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

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

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

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

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

Hard metal of bluish-white color. Chrome is sometimes classified as a ferrous metal. This metal is capable of painting compounds in different colors, which is why it was named “chrome”, which means “paint”. Chromium is a trace element necessary for normal development and functioning human body. His most important biological role consists of regulating carbohydrate metabolism and blood glucose levels.

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STRUCTURE

Depending on the types chemical bond— like all metals, chromium has a metallic type of crystal lattice, that is, the lattice nodes contain metal atoms.
Depending on the spatial symmetry - cubic, body-centered a = 0.28839 nm. A feature of chrome is sudden change its physical properties at a temperature of about 37°C. Crystal cell a metal consists of its ions and mobile electrons. Similarly, the chromium atom in its ground state has an electronic configuration. At 1830 °C it is possible to transform into a modification with a face-centered lattice, a = 3.69 Å.

PROPERTIES

Chromium has a Mohs hardness of 9, one of the hardest pure metals (second only to iridium, beryllium, tungsten and uranium). Very pure chrome can be machined quite well. 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. When heated, it reacts with many non-metals, often forming compounds of non-stoichiometric composition: carbides, borides, silicides, nitrides, etc. Chromium forms numerous compounds in various oxidation states, mainly +2, +3, +6. Chrome has all the properties characteristic of metals - it conducts heat well, electricity, has the shine inherent in most metals. It is antiferromagnetic and paramagnetic, that is, at a temperature of 39 °C it changes from a paramagnetic state to an antiferromagnetic state (Néel point).

RESERVES AND PRODUCTION

The largest chromium deposits are located in South Africa (1st place in the world), Kazakhstan, Russia, Zimbabwe, and Madagascar. There are also deposits in Turkey, India, Armenia, Brazil, and the Philippines.nThe main deposits of chromium ores in the Russian Federation are known in the Urals (Don and Saranovskoye). Explored reserves in Kazakhstan amount to over 350 million tons (2nd place in the world). Chromium is found in nature mainly in the form of chromium iron ore Fe(CrO 2) 2 (iron chromite). Ferrochrome is obtained from it by reduction in electric furnaces with coke (carbon). To obtain pure chromium, the reaction is carried out as follows:
1) iron chromite is fused with sodium carbonate (soda ash) in air;
2) dissolve sodium chromate and separate it from iron oxide;
3) convert the chromate to dichromate, acidifying the solution and crystallizing the dichromate;
4) pure chromium oxide is obtained by reducing sodium dichromate with coal;
5) metallic chromium is obtained using aluminothermy;
6) using electrolysis, electrolytic chromium is obtained from a solution of chromic anhydride in water containing the addition of sulfuric acid.

ORIGIN

The average content of Chromium in the earth's crust (clarke) is 8.3·10 -3%. This element is probably more characteristic of the Earth's mantle, since ultramafic rocks, which are believed to be closest in composition to the Earth's mantle, are enriched in Chromium (2·10 -4%). Chromium forms massive and disseminated ores in ultramafic rocks; The formation of the largest chromium deposits is associated with them. In basic rocks, the Chromium content reaches only 2·10 -2%, in acidic rocks - 2.5·10 -3%, in sedimentary rocks (sandstones) - 3.5·10 -3%, in clay shales - 9·10 -3 %. Chromium is a relatively weak aquatic migrant; Chromium content in sea water is 0.00005 mg/l.
In general, Chromium is a metal in the deep zones of the Earth; stony meteorites (analogues of the mantle) are also enriched in Chromium (2.7·10 -1%). Over 20 chromium minerals are known. Only chrome spinels (up to 54% Cr) are of industrial importance; in addition, Chromium is contained in a number of other minerals, which often accompany chromium ores, but are not of practical value themselves (uvarovite, volkonskoite, kemerite, fuchsite).
There are three main chromium minerals: magnochromite (Mg, Fe)Cr 2 O 4 , chrompicotite (Mg, Fe)(Cr, Al) 2 O 4 and aluminochromite (Fe, Mg)(Cr, Al) 2 O 4 . By appearance they are indistinguishable and are inaccurately called "chromites".

APPLICATION

Chromium is an important component in many alloy steels (in particular stainless steels), as well as in a number of other alloys. The addition of chromium significantly increases the hardness and corrosion resistance of alloys. The use of Chrome is based on its heat resistance, hardness and corrosion resistance. Most of all, Chromium is used for smelting chromium steels. Aluminum- and silicothermic chromium is used for smelting nichrome, nimonic, other nickel alloys and stellite.
A significant amount of Chromium is used for decorative corrosion-resistant coatings. Powdered Chromium is widely used in the production of metal-ceramic products and materials for welding electrodes. Chromium, in the form of Cr 3+ ion, is an impurity in ruby, which is used as a gemstone and laser material. Chromium compounds are used to etch fabrics during dyeing. Some Chromium salts are used as a component of tanning solutions in the leather industry; PbCrO 4 , ZnCrO 4 , SrCrO 4 - like art paints. Chromium-magnesite refractory products are made from a mixture of chromite and magnesite.
Used as wear-resistant and beautiful galvanic coatings (chrome plating).
Chromium is used for the production of alloys: chromium-30 and chromium-90, which are indispensable for the production of nozzles for powerful plasma torches and in the aerospace industry.

Chrome (eng. Chromium) - Cr