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

Tasks:

• characterize chromium as a simple substance;
• introduce students to chromium compounds of different oxidation states;
• 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 of independent work of students and observation of 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)- CrO 3 – dark red crystalline substance, highly soluble in water – a typical acidic 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:

• 4CrO 3 = 2Cr 2 O 3 + 3O 2

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.

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, the oxidizing properties consistently 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 lecture methodology:

1. To enhance students’ cognitive activity and maintain 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)

1. Vasilyeva Z.G. Laboratory work in general and inorganic chemistry. -M.: “Chemistry”, 1979 – 450 p.
2. Egorov A.S. Chemistry tutor. – Rostov-on-Don: “Phoenix”, 2006.-765 p.
3. Kudryavtsev A.A. Writing chemical equations. - M., “Higher School”, 1979. - 295 p.
4. Petrov M.M. Inorganic chemistry. – Leningrad: “Chemistry”, 1989. – 543 p.
5. Ushkalova V.N. Chemistry: competition tasks and answers. - M.: “Enlightenment”, 2000. – 223 p.

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 gray-blue 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 + 3H 2 O = Cr 2 O 3 + 3H 2

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

Ba+CrO→BaCrO↓

2Ba+CrO+H2O→2BaCrO↓+2H

The formation of 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.

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

Cr(OH)+2HCl→CrCl+2HO

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

2CrCl+ 2HCl → 2CrCl+ H

4Cr(OH)+O+ 2HO→4Cr(OH)

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

2Cr(OH)-→CrO+ 3HO

4KCrO-→ 2CrO + 4KCrO + 3O

(NH)CrO-→ CrO+ N+ HO

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

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

CrCl 3 + 3KOH = Cr(OH) 3 ↓ + 3KCl

Chromium (III) hydroxide is a gray-green precipitate, upon receipt of which the alkali must be taken in deficiency. The chromium (III) hydroxide 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)

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

Chromium compounds(VI).

Chromium oxide (VI) - CrO 3 – dark red crystalline substance, highly soluble in water – a typical acidic 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

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

4CrO 3 = 2Cr 2 O 3 + 3O 2

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

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 - Cr2O 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

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

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

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 Br2 +8NaOH =2Na 2 CrO 4 + 2KBr +4NaBr + 4H 2 O

Cr +3 - 3e → Cr +6