Chromium is an element of a secondary 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 (Latin Chromium). A simple substance chromium is a bluish-white hard metal.

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 hot, 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.

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 metal surface explains its passivity in relation to concentrated solutions of acids - oxidizing agents.

Chromium compounds

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

Cr (OH) 2 + 2HCl → CrCl 2 + 2H 2 O

Chromium (II) compounds are strong reducing agents; are converted into chromium (III) compounds under the action 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. It can be obtained by calcining 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 (reaction "volcano")

Amphoteric oxide. When Cr 2 O 3 is fused with alkalis, soda and acidic salts, chromium compounds with the oxidation state (+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 fusion with a mixture of alkali and oxidizer, 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) C hydroxide r (OH) 3. Amphoteric hydroxide. Gray-green, decomposes when heated, losing water and forming green metahydroxide CrO (OH). Does not dissolve in water. It precipitates from solution in the form of blue-gray and bluish-green hydrates. 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 Сr (ОН) 3 + ЗН + \u003d Сr 3+ + 3H 2 O

Cr (OH) 3 + KOH → K, Cr (OH) 3 + ZOH - (conc.) \u003d [Cr (OH) 6] 3-

Cr (OH) 3 + KOH → KCrO 2 + 2H 2 O Cr (OH) 3 + MOH \u003d MCrO 2 (green) + 2H 2 O (300-400 ° C, M \u003d Li, Na)

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

2Сr (ОН) 3 + 4NаОН (conc.) + ЗН 2 O 2 (conc.) \u003d 2Na 2 СrO 4 + 8Н 2 0

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

Cr 3+ + 3 (NH 3 H 2 O) \u003d FROMr(OH) 3 ↓ + ЗNН 4+

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

Chromium (III) salts have a purple or dark green color. Chemical properties are similar to 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 are bright red crystals, soluble in water.

Prepared 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 - acidic oxide, with alkalis forms yellow chromates CrO 4 2-:

CrO 3 + 2KOH → K 2 CrO 4 + H 2 O

In an acidic medium, chromates are converted to 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:

К 2 Сr 2 O 7 + 4H 2 SO 4 + 3Na 2 SO 3 \u003d 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 \u003d Cr 2 (SO 4) 3 + 3NaNO 3 + K 2 SO 4 + 4H 2 O

K 2 Cr 2 O 7 + 7H 2 SO 4 + 6 KI \u003d 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 \u003d Cr 2 (SO 4) 3 + 3Fe 2 (SO 4) 3 + K 2 SO 4 + 7H 2 O

Potassium chromate K 2 Cr About 4 . Oxosol. Yellow, non-absorbent. Melts without decomposition, thermally stable. Let's well dissolve in water ( yellow the color of the solution corresponds to the CrO 4 2- ion), slightly hydrolyzed by the anion. In an acidic environment, it transforms into K 2 Cr 2 O 7. Oxidizing agent (weaker than K 2 Cr 2 O 7). It enters into ion exchange reactions.

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

Equations of the most important reactions:

2K 2 CrO 4 + H 2 SO 4 (30%) \u003d K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O

2K 2 CrO 4 (t) + 16HCl (end, hot) \u003d 2CrCl 3 + 3Cl 2 + 8H 2 O + 4KCl

2K 2 CrO 4 + 2H 2 O + 3H 2 S \u003d 2Cr (OH) 3 ↓ + 3S ↓ + 4KOH

2K 2 CrO 4 + 8H 2 O + 3K 2 S \u003d 2K [Cr (OH) 6] + 3S ↓ + 4KOH

2K 2 CrO 4 + 2AgNO 3 \u003d KNO 3 + Ag 2 CrO 4 (red) ↓

Qualitative response:

К 2 СгO 4 + ВаСl 2 \u003d 2КСl + ВаCrO 4 ↓

2ВаСrO 4 (t) + 2HCl (dil.) \u003d ВаСr 2 O 7 (p) + ВаС1 2 + Н 2 O

Receiving: sintering of chromite with potash in air:

4 (Сr 2 Fe ‖‖) O 4 + 8К 2 CO 3 + 7O 2 \u003d 8К 2 СrO 4 + 2Fе 2 O 3 + 8СO 2 (1000 ° С)

Potassium dichromate K 2 Cr 2 O 7 ... Oxosol. Technical name chrompeak... Orange red, non-absorbent. It melts without decomposition, decomposes upon further heating. Let's well dissolve in water ( orange the color of the solution corresponds to the ion Cr 2 O 7 2-). In an alkaline environment forms K 2 CrO 4. Typical oxidant in solution and fusion. It enters into ion exchange reactions.

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

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

Equations of the most important reactions:

4K 2 Cr 2 O 7 \u003d 4K 2 CrO 4 + 2Cr 2 O 3 + 3O 2 (500-600 o C)

K 2 Cr 2 O 7 (t) + 14HCl (end) \u003d 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) \u003d H 2 O + 2K 2 CrO 4

Cr 2 O 7 2- + 14H + + 6I - \u003d 2Cr 3+ + 3I 2 ↓ + 7H 2 O

Cr 2 O 7 2- + 2H + + 3SO 2 (g) \u003d 2Cr 3+ + 3SO 4 2- + H 2 O

Cr 2 O 7 2- + H 2 O + 3H 2 S (g) \u003d 3S ↓ + 2OH - + 2Cr 2 (OH) 3 ↓

Cr 2 O 7 2- (conc.) + 2Ag + (dil.) \u003d Ag 2 Cr 2 O 7 (t. Red) ↓

Cr 2 O 7 2- (dil.) + H 2 O + Pb 2+ \u003d 2H + + 2PbCrO 4 (red) ↓

K 2 Cr 2 O 7 (s) + 6HCl + 8H 0 (Zn) \u003d 2CrCl 2 (syn) + 7H 2 O + 2KCl

Receiving: treatment of К 2 СrO 4 with sulfuric acid:

2K 2 CrO 4 + H 2 SO 4 (30%) \u003d K 2Cr 2 O 7 + K 2 SO 4 + H 2 O

In 1766, Professor of Chemistry and Head of the Chemical Laboratory of the St. Petersburg Academy of Sciences I.G. Lehman described a new mineral found in the Urals at the Berezovsky mine, which was called "Siberian red lead", PbCrO 4. Modern name - crocoite. In 1797 the French chemist LN Vauquelin isolated a new refractory metal from it.
The element got its name from the Greek. χρῶμα - color, paint - because of the variety of colors of its compounds.

Being in nature and receiving:

The most common chromium mineral is chromium iron ore FeCr 2 O 4 (chromite), rich deposits of which are found in the Urals and Kazakhstan, the second most important mineral is crocoite PbCrO 4. Mass fraction of chromium in earth crust is 0.03%. Natural chromium consists of a mixture of five isotopes with mass numbers 50, 52, 53, 54, and 56; other radioactive isotopes have also been artificially obtained.
The main amounts of chromium are obtained and used in the form of an alloy with iron, ferrochrome, reducing chromite with coke: FeCr 2 O 4 + 4C \u003d Fe + 2Cr + 4CO
Pure chromium is obtained by reducing its oxide with aluminum: Cr 2 O 3 + 2Al \u003d 2Cr + Al 2 O 3
or by electrolysis of aqueous solutions of chromium compounds.

Physical properties:

Chrome is a grayish-white shiny metal, appearance similar to steel, one of the hardest metals, r \u003d 7.19 g / cm 3, Tm \u003d 2130K, Tboil \u003d 2945K. Chromium has all the properties characteristic of metals - it conducts heat well, electricity, has the luster inherent in most metals.

Chemical properties:

Chromium is stable in air due to passivation - the formation of a protective oxide film. For the same reason, it does not react with concentrated sulfuric and nitric acids. Burns at 2000 ° C with the formation of green chromium (III) oxide Cr 2 O 3.
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.

The most important connections:

Oxidation state +2 - basic oxide CrO (black), hydroxide Cr (OH) 2 (yellow). Chromium (II) salts (solutions blue) are obtained by the reduction of chromium (III) salts with zinc in an acidic medium. Very strong reducing agents, slowly oxidized with water to release hydrogen.

Oxidation state +3 - the most stable oxidation state of chromium, it corresponds to: amphoteric oxide Cr 2 O 3 and hydroxide Cr (OH) 3 (both are gray-green), chromium (III) salts - gray-green or purple, chromites MCrO2, which are obtained when fusing chromium oxide with alkalis, tetra- and hexahydroxochromates (III) obtained by dissolving chromium (III) hydroxide in alkali solutions (green), numerous complex chromium compounds.

Oxidation state +6 - the second characteristic oxidation state of chromium, which corresponds to acid chromium oxide (VI) CrO 3 (red crystals, dissolves in water, forming chromic acids), chromic H 2 CrO 4, dichromic H 2 Cr 2 O 7 and polychromic acids, corresponding salts : yellow chromates and orange dichromates. Chromium (VI) compounds are strong oxidants, especially in acidic conditions, are reduced to chromium (III) compounds
In an aqueous solution, chromates turn into dichromates when the acidity of the medium changes:
2CrO 4 2- + 2H + Cr 2 O 7 2- + H 2 O, which is accompanied by a color change.

Application

Chromium, in the form of ferrochromium, is used in the production of alloyed types of steel (in particular, stainless steel), and other alloys. Chromium alloys: chromium-30 and chromium-90, indispensable for the production of powerful plasma torch nozzles and in the aerospace industry, an alloy with nickel (nichrome) - for the production of heating elements. Large amounts of chromium are used as wear-resistant and beautiful electroplated coatings (chrome plating).

Biological role and physiological action

Chromium is one of the biogenic elements that is constantly included in the tissues of plants and animals. In animals, chromium is involved in the metabolism of lipids, proteins (included in the enzyme trypsin), carbohydrates. A decrease in the chromium content in food and blood leads to a decrease in the growth rate, an increase in blood cholesterol.

In its pure form, chromium is quite toxic; chromium metal dust irritates lung tissue. Chromium (III) compounds cause dermatitis. Chromium (VI) compounds lead to various human diseases, including cancer. MAC of chromium (VI) in atmospheric air 0.0015 mg / m 3

Kononova A.S., Nakov D.D., Tyumen State University, 501 (2) group, 2013

Sources:
Chrome (element) // Wikipedia. URL: http://ru.wikipedia.org/wiki/Chrome (date of access: 01/06/2014).
Popular library of chemical elements: Chromium. // URL:

Purpose: to deepen the knowledge of students on the topic of the lesson.

Tasks:

• characterize chromium as simple substance;
• to introduce students to chromium compounds of different oxidation states;
• show the dependence of the properties of compounds on the oxidation state;
• show the redox properties of chromium compounds;
• continue the formation of students' skills to write down the equations of chemical reactions in molecular and ionic form, make an electronic balance;
• continue the formation of skills to observe a chemical experiment.

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

Course of the lesson

I. Repetition of the material of the previous lesson.

1. Answer questions and complete tasks:

What elements are in the chromium subgroup?

Write electronic formulas of atoms

What type of elements are they?

What are the oxidation states in the compounds?

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

You can invite students to fill out the table using the tabular values \u200b\u200bof the radii of atoms, ionization energy and draw conclusions.

Sample table:

2. Listen to a student's message on the topic "Elements of the chromium subgroup in nature, production and use."

II. Lecture.

Lecture plan:

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 oxidation state.
2. Redox properties of chromium compounds.

1. Chrome.

Chromium is a shiny metal, white with a bluish sheen, very hard (density 7, 2 g / cm 3), melting point 1890˚С.

Chemical properties: chromium is an inactive metal under normal conditions. This is due to 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 \u003d 2Сr 2 О 3
• 2Сr + 3S \u003d Сr 2 S 3
• 2Сr + 3Cl 2 \u003d 2СrСl 3

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

Interaction of chromium with complex substances:

At very high temperatures, chromium reacts with water:

• 2Сr + 3 Н 2 О \u003d Сr 2 О 3 + 3Н 2

The task:

Chromium reacts with dilute sulfuric and hydrochloric acids:

• Сr + Н 2 SO 4 \u003d СrSО 4 + Н 2
• Cr + 2HCl \u003d CrCl 2 + H 2

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

Concentrated sulfuric hydrochloric and nitric acids passivate chromium.

2. Compounds of chromium. (2)

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

• CrO + 2HCl \u003d CrCl 2 + H 2 O

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

Chromium oxide (2) is easily oxidized in air:

• 4СrО + О 2 \u003d 2Сr 2 О 3

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

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

2Сr (amalgam) + О 2 \u003d 2СrО

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

• Cr (OH) 2 + H 2 SO 4 \u003d CrSO 4 + 2H 2 O

The task: to draw up the equations of reactions in the molecular and ionic form of the interaction of chromium oxide (2) with hydrochloric acid.

Like chromium (2) oxide, chromium (2) hydroxide oxidizes:

• 4 Сr (ОH) 2 + О 2 + 2Н 2 О \u003d 4Сr (ОН) 3

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

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

• CrCl 2 + 2KOH \u003d Cr (OH) 2 ↓ + 2KCl

The task: draw up ionic equations.

3. Compounds of chromium. (3)

1. Chromium Oxide (3) - Cr 2 O 3 - a dark green powder, insoluble in water, refractory, close to corundum in hardness (it corresponds to chromium (3) hydroxide - Cr (OH) 3). Chromium oxide (3) has an amphoteric character; however, it dissolves poorly in acids and alkalis. Reactions with alkalis occur during fusion:

• Cr 2 O 3 + 2KOH \u003d 2KSrO 2 (chromite K) + H 2 O

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

Interacts with concentrated solutions of acids and alkalis with difficulty:

• Cr 2 O 3 + 6 KOH + 3H 2 O \u003d 2K 3 [Cr (OH) 6]
• Cr 2 O 3 + 6HCl \u003d 2CrCl 3 + 3H 2 O

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

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

• (NH 4) 2Сr 2 О 7 \u003d 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):

• СrСl 3 + 3KOH \u003d Сr (OH) 3 ↓ + 3КСl

The task: write ionic equations

Chromium hydroxide (3) is a gray-green precipitate, when obtained, alkali must be taken in a deficit. The chromium (3) hydroxide thus obtained, in contrast to the corresponding oxide, easily interacts with acids and alkalis, i.e. exhibits amphoteric properties:

• Cr (OH) 3 + 3HNO 3 \u003d Cr (NO 3) 3 + 3H 2 O
• Cr (OH) 3 + 3KON \u003d K 3 [Cr (OH) 6] (hexahydroxochromite K)

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

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

• Cr (OH) 3 + KOH \u003d KCrO 2 (metachromitis K) + 2H 2 O
• Cr (OH) 3 + KOH \u003d K 3 CrO 3 (orthochromite K)+ 3H 2 O

4. Compounds of chromium. (6)

1. Chromium Oxide (6) - CrO 3 - dark red crystalline substance, well soluble in water - typical acidic oxide. Two acids correspond to this oxide:

• СrО 3 + Н 2 О \u003d Н 2 СrО 4 (chromic acid - formed with excess water)
• СrО 3 + Н 2 О \u003d Н 2 Сr 2 О 7 (dichromic acid - formed at a high concentration of chromium oxide (3)).

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

• С 2 Н 5 ОН + 4СrО 3 \u003d 2СО 2 + 2Сr 2 О 3 + 3Н 2 О

It also oxidizes iodine, sulfur, phosphorus, coal:

• 3S + 4CrO 3 \u003d 3SO 2 + 2Cr 2 O 3

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

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

• 4CrO 3 \u003d 2Cr 2 O 3 + 3O 2

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

• К 2 Сr 2 О 7 + Н 2 SO 4 \u003d К 2 SO 4 + 2СrО 3 + Н 2 О

2. Chromic and dichromic acids.

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

Chromate - ions СrО 4 2- and dichromate - ions Сr 2О 7 2- easily pass into each other when the medium of solutions changes

In the acidic environment of the solution, chromates turn into dichromates:

• 2К 2 СrО 4 + Н 2 SO 4 \u003d К 2 Сr 2 О 7 + К 2 SO 4 + Н 2 О

In an alkaline environment, dichromates turn into chromates:

• К 2 Сr 2 О 7 + 2КОН \u003d 2К 2 СrО 4 + Н 2 О

When diluted, dichromic acid transforms into chromic acid:

• H 2 Cr 2 O 7 + H 2 O \u003d 2H 2 CrO 4

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

Oxidation state	+2	+3	+6
Oxide	CrO	Cr 2 O 3	CrO 3
The nature of the oxide	main	amphoteric	acid
Hydroxide	Cr (OH) 2	Cr (OH) 3 - H 3 CrO 3	H 2 CrO 4
The nature of the hydroxide	main	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 are converted to Cr +3 compounds under the action of reducing agents: H 2 S, SO 2, FeSO 4

• К 2 Сr 2 О 7 + 3Н 2 S + 4Н 2 SO 4 \u003d 3S + Сr 2 (SO 4) 3 + K 2 SO 4 + 7Н 2 О
• S -2 - 2e → S 0
• 2Cr +6 + 6e → 2Cr +3

The task:

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

• Na 2 CrO 4 + K 2 S + H 2 SO 4 \u003d 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 by the electronic balance method, indicate the oxidizing agent and the reducing agent:

• K 2 Cr 2 O 7 + SO 2 + H 2 SO 4 \u003d? +? + H 2 O

Reactions in an alkaline environment.

In an alkaline medium, chromium compounds Cr +3 are converted to Cr +6 compounds under the action of oxidants: J2, Br2, Cl2, Ag2O, KClO3, H2O2, KMnO4:

• 2KCrO 2 +3 Br 2 + 8NaOH \u003d 2Na 2 CrO 4 + 2KBr + 4NaBr + 4H 2 O
• Cr +3 - 3e → Cr +6
• Br2 0 + 2e → 2Br -

The task:

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

• NaCrO 2 + J 2 + NaOH \u003d Na 2 CrO 4 + NaJ + H 2 O

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

• Cr (OH) 3 + Ag 2 O + NaOH \u003d Ag +? +?

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

To the lecture technique:

1. To activate cognitive activities students and maintaining interest, it is advisable to conduct a demonstration experiment during the lecture. Depending on the possibilities learning laboratory You can demonstrate to students the following experiences:

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

1. Self-study assignments can be differentiated taking into account the real learning opportunities of students.
2. You can complete the lecture by completing the following tasks: write the equations of chemical reactions with which you can carry out the following transformations:

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

1. Vasilyeva Z.G. Laboratory works 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. Drafting chemical equations... - M., "High 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 .: "Education", 2000. - 223 p.

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 system

Chromium - an element of a 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). Simple substance chromium - hard metal, bluish-white. Chromium is sometimes referred to as ferrous metals.

Atom 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 VII in the group, in the main subgroup - at the external energy level of 7 electrons

Element properties

Physical properties

Chromium is a white, shiny metal with a cubic body-centered lattice, a \u003d 0.28845 nm, characterized by hardness and fragility, 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. The addition of carbon to chromium further increases its hardness.

Chemical properties

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

Interaction with non-metals

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

4Cr + 3O 2 \u003d 2Cr 2 O 3.

It reacts with fluorine at 350 ° С, with chlorine - at 300 ° С, with bromine - at the temperature of red heat, forming chromium (III) halides:

2Cr + 3Cl 2 \u003d 2CrCl 3.

Reacts with nitrogen at temperatures above 1000 ° C with the formation of nitrides:

2Cr + N 2 \u003d 2CrN

or 4Cr + N 2 \u003d 2Cr 2 N.

2Cr + 3S \u003d Cr 2 S 3.

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

Cr + 2B \u003d CrB 2 (formation of Cr 2 B, CrB, Cr 3 B 4, CrB 4 is possible),

2Cr + 3C \u003d Cr 2 C 3 (formation of Cr 23 C 6, Cr 7 B 3 is possible),

Cr + 2Si \u003d CrSi 2 (formation of Cr 3 Si, Cr 5 Si 3, CrSi is possible).

Does not directly interact with hydrogen.

Interaction with water

In a finely divided incandescent state, chromium reacts with water, forming chromium (III) oxide and hydrogen:

2Cr + 3H 2 O \u003d Cr 2 O 3 + 3H 2

5interactions with acids

In the electrochemical series of voltages of metals, chromium is up to hydrogen, it displaces hydrogen from solutions of non-oxidizing acids:

Cr + 2HCl \u003d CrCl 2 + H 2;

Cr + H 2 SO 4 \u003d CrSO 4 + H 2.

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

4Cr + 12HCl + 3O 2 \u003d 4CrCl 3 + 6H 2 O.

Concentrated nitrogen and sulfuric acid 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 \u003d Cr 2 (SO 4) 3 + 3SO 2 + 6H 2 O;

Cr + 6HNO 3 \u003d Cr (NO 3) 3 + 3NO 2 + 3H 2 O.

Interaction with alkaline reagents

In aqueous solutions of alkalis, chromium does not dissolve, slowly reacts with alkali melts with the formation of chromites and the release of hydrogen:

2Cr + 6KOH \u003d 2KCrO 2 + 2K 2 O + 3H 2.

Reacts with alkaline melts of oxidizing agents, for example potassium chlorate, while chromium passes into potassium chromate:

Cr + KClO 3 + 2KOH \u003d 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 \u003d 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. Burns at 2000 ° C 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), with silicon (silicides Cr 3 Si, Cr 5 Si 3 and CrSi) and nitrogen (nitrides CrN and Cr 2 N).

Cr compounds (+2)

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 (“with hydrogen at the time of isolation”):

All these Cr 2+ salts are strong reducing agents to the extent that they displace hydrogen from water upon standing. 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 amphoteric oxide Cr 2 O 3 and hydroxide Cr (OH) 3 (both are green). This is the most stable oxidation state of chromium. Chromium compounds in this oxidation state have a color from dirty lilac (ion 3+) to green (anions are present in the coordination sphere).

Cr 3+ tends to form double sulfates like M I Cr (SO 4) 2 12H 2 O (alum)

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

Cr + 3NH + 3H2O → Cr (OH) ↓ + 3NH

You can use solutions of alkalis, 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 happens when chromium (III) oxide is fused with alkali and oxidizing agents, or with alkali in air (the melt becomes yellow in this case):

2Cr2O3 + 8NaOH + 3O2 → 4Na2CrO4 + 4H2O

Chromium compounds (+4)[

With the 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 chromic H 2 CrO 4 and two-chromic H 2 Cr 2 O 7. They form two series of salts: yellow chromates and orange dichromates, respectively.

Chromium oxide (VI) CrO 3 is formed by the interaction of concentrated sulfuric acid with dichromate solutions. 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 isopolyacids 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, since the chromate K 2 CrO 4 is again formed:

Cr2O + 2OH → 2CrO + HO

It does not reach a high degree of polymerization, as it happens in 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, the yellow chromate of barium BaCrO 4 precipitates when barium salts are added, both to chromate solutions and to dichromate solutions:

Ba + CrO → BaCrO ↓

2Ba + CrO + H2O → 2BaCrO ↓ + 2H

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

Known chromium pentafluoride CrF 5 and unstable chromium hexafluoride CrF 6. 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 the 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.

DEFINITION

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

Chromium has a body-centered cubic lattice structure. Density - 7.2 g / cm 3. The melting and boiling points are 1890 ° C and 2680 ° C, respectively.

The oxidation state of chromium in compounds

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

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

Chromium also has an oxidation state (+6) : Cr +6 O 3, H 2 Cr +6 O 4, H 2 Cr +6 2 O 7, K 2 Cr +6 2 O 7, etc.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

The task	Phosphorus has the same oxidation state in the compounds: a) Ca 3 P 2 and H 3 PO 3; b) KH 2 PO 4 and KPO 3; c) P 4 O 6 and P 4 O 10; d) H 3 PO 4 and H 3 PO 3.
Decision	In order to give the correct answer to the question posed, we will alternately determine the oxidation state of phosphorus in each pair of the proposed compounds. a) The oxidation state of calcium is (+2), oxygen and hydrogen are (-2) and (+1), respectively. Let's take the value of the oxidation state of phosphorus for "x" and "y" in the proposed compounds: 3 × 2 + x × 2 \u003d 0; 3 + y + 3 × (-2) \u003d 0; The answer is wrong. b) The oxidation state of potassium is (+1), oxygen and hydrogen are (-2) and (+1), respectively. Let's take the value of the oxidation state of chlorine for "x" and "y" in the proposed compounds: 1 + 2 × 1 + x + (-2) × 4 \u003d 0; 1 + y + (-2) × 3 \u003d 0; The answer is correct.
Answer	Option (b).