Chromium compounds. Amphotericity of chromium(III) hydroxide

Chromium is 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). The 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, and 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

In a hot state, 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 - 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; pass 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 (volcano reaction)

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

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 an 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. Grey-green, decomposes on heating, losing water and forming green metahydroxide CrO(OH). Does not dissolve in water. It 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.) \u003d [Cr (OH) 6] 3-

Cr (OH) 3 + KOH → KCrO 2 + 2H 2 O Cr (OH) 3 + MON \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) Cr2O3

2Cr(OH) 3 + 4NaOH (conc.) + ZN 2 O 2 (conc.) \u003d 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 ↓+ ЗНН 4+

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

Salts of chromium (III) have a purple or dark green color. By chemical properties, they 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.

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, forms yellow chromates CrO 4 2- with alkalis:

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 \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 = 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 About 4 . Oksosol. Yellow, non-hygroscopic. Melts without decomposition, thermally stable. Highly 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 passes 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 ion CrO 4 2- - precipitation of a yellow precipitate of barium chromate, decomposing 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 (conc., horizon) \u003d 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[Сr(OH) 6]+3S↓+4KOH

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

Qualitative response:

K 2 CrO 4 + BaCl 2 \u003d 2KSl + BaCrO 4 ↓

2ВаСrO 4 (t) + 2НCl (razb.) = ВаСr 2 O 7(p) + ВаС1 2 + Н 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 ° С)

Potassium dichromate K 2 Cr 2 O 7 . Oksosol. technical name chrompeak. Orange-red, non-hygroscopic. Melts without decomposition, decomposes on further heating. Highly soluble in water orange the color of the solution corresponds to the ion Cr 2 O 7 2-). In an alkaline medium, it forms K 2 CrO 4 . A typical oxidizing agent in solution and when fused. Enters into ion exchange reactions.

Qualitative reactions- blue coloring of an ether solution in the presence of H 2 O 2, blue coloring 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, 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 (conc) \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 + (razb.) \u003d Ag 2 Cr 2 O 7 (so red) ↓

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

K 2 Cr 2 O 7 (t) + 6HCl + 8H 0 (Zn) \u003d 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

Description

Chromium(III) hydroxide is an amphoteric hydroxide. Gray-green in color, decomposes when heated, losing water and forming green metahydroxide CrO(OH). Does not dissolve in water. It precipitates from solution in the form of a gray-blue and bluish-green hydrate. When standing under the solution, it loses its reactivity (“gets old”). Reacts with acids and alkalis, does not interact with ammonia hydrate. It is used for the synthesis of chromium(III) compounds.

The molar electrical conductivity at infinite dilution at 25 ° C is 795.9 cm 2 / mol. It is obtained in the form of a gelatinous green precipitate during the treatment of chromium (III) salts with alkalis, during the hydrolysis of chromium (III) salts with alkali metal carbonates or ammonium sulfide.

Chemical properties

• It is formed by the action of alkalis or an aqueous solution of ammonia on solutions of chromium salts:

$\backslash mathsf(CrCl\_3\; +\; 3NH\_3\; +3H\_2O\; \backslash \; \backslash xrightarrow()\backslash \; Cr(OH)\_3\; +\; 3NH\_4Cl\; )$

• It is also formed by passing carbon dioxide through an alkaline solution of sodium hexahydroxochromate (III):

$\backslash mathsf(Na\_3\; +\; 3CO\_2\; \backslash \; \backslash xrightarrow()\backslash \; Cr(OH)\_3\; +\; 3NaHCO\_3)$

• When heated to ~100^oC in air, it decomposes to form green CrO(OH). At higher temperatures, trivalent chromium hydroxide decomposes with the formation of chromium (III) oxide and the release of water vapor:

$\backslash mathsf(Cr(OH)\_3\; \backslash \; \backslash xrightarrow(100^oC)\backslash \; CrO(OH)\; +\; H\_2O)$ $\backslash mathsf(2Cr(OH)\_3\; \backslash \; \backslash xrightarrow(430-1000^oC)\backslash \; Cr\_2O\_3\; +\; 3H\_2O)$

• Chromium (III) hydroxide has amphoteric properties, due to which it easily dissolves in acids with the formation of chromium (III) salts:
  • With dilute acids:

$\backslash mathsf(Cr(OH)\_3\; +\; 3HCl\; \backslash \; \backslash xrightarrow()\backslash \; CrCl\_3\; +\; 3H2O)$ $\backslash mathsf(2Cr(OH)\_3\; +\; 3H\_2SO\_4\; \backslash \; \backslash xrightarrow()\backslash \; Cr\_2(SO\_4)\_3\; +\; 6H2O)$ $\backslash mathsf(Cr(OH)\_3\; +\; 3HNO\_3\; \backslash \; \backslash xrightarrow()\backslash \; Cr(NO\_3)\_3\; +\; 3H\_2O)$

• • With concentrated substances:

$\backslash mathsf(Cr(OH)\_3\; +\; 3HF\; \backslash \; \backslash xrightarrow()\backslash \; CrF\_3(\backslash downarrow)\; +\; 3H\_2O)$ $\backslash mathsf(Cr(OH)\_3\; +\; 3CH\_3COOH\; \backslash \; \backslash xrightarrow()\backslash \; Cr(CH\_3COO)\_3\; +\; 3H\_2O)$ $\backslash mathsf(Cr(OH)\_3\; +\; 3HCN\; +\; 3KCN\; \backslash \; \backslash xrightarrow()\backslash \; K\_3\; +\; 2H\_2O)$

• Also, due to the amphoteric properties, reactions easily occur with alkalis:

$\backslash mathsf(Cr(OH)\_3\; +\; 3NaOH\; \backslash \; \backslash xrightarrow()\backslash \; Na\_3)$ $\backslash mathsf2(Cr(OH)\_3\; +\; 4NaOH\; +\; 3H\_2O\_2\; \backslash \; \backslash xrightarrow()\backslash \; 2Na\_2CrO\_4\; +\; 8H\_2O)$

• When interacting with sodium or lithium hydroxides, it is possible to obtain chromites of these metals (M = Li, Na):

$\backslash mathsf(Cr(OH)\_3\; +\; MOH\; \backslash \; \backslash xrightarrow(300-400^oC)\backslash \; MCrO\_2\; +\; 2H\_2O)$

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Literature

• Inorganic chemistry in reactions / R.A. Lidin, V.A. Molochko, L.L. Andreeva. - Moscow: "Drofa", 2007. - T. 3. - 640 p. - ISBN 978-5-358-01303-2.

An excerpt characterizing Chromium(III) hydroxide

As soon as Natasha, who was sitting at the head of Prince Andrei, found out about the arrival of Princess Marya, she quietly left his room with those quick, as it seemed to Princess Marya, as if with cheerful steps, and ran to her.
On her excited face, when she ran into the room, there was only one expression - an expression of love, boundless love for him, for her, for everything that was close to a loved one, an expression of pity, suffering for others and a passionate desire to give herself all for in order to help them. It was evident that at that moment not a single thought about herself, about her relationship to him, was in Natasha's soul.
The sensitive Princess Marya, at the first glance at Natasha's face, understood all this and wept on her shoulder with sorrowful pleasure.
“Come on, let’s go to him, Marie,” Natasha said, taking her to another room.
Princess Mary raised her face, wiped her eyes, and turned to Natasha. She felt that she would understand and learn everything from her.
“What…” she began to question, but suddenly stopped. She felt that words could neither ask nor answer. Natasha's face and eyes should have said everything more clearly and deeply.
Natasha looked at her, but seemed to be in fear and doubt - to say or not to say everything that she knew; she seemed to feel that before those radiant eyes, penetrating into the very depths of her heart, it was impossible not to tell the whole, the whole truth as she saw it. Natasha's lip suddenly trembled, ugly wrinkles formed around her mouth, and she, sobbing, covered her face with her hands.
Princess Mary understood everything.
But she still hoped and asked in words in which she did not believe:
But how is his wound? In general, what position is he in?
“You, you ... will see,” Natasha could only say.
They sat for some time downstairs near his room in order to stop crying and come in to him with calm faces.
- How was the illness? Has he gotten worse? When did it happen? asked Princess Mary.
Natasha said that at first there was a danger from a feverish state and from suffering, but in the Trinity this passed, and the doctor was afraid of one thing - Antonov's fire. But that danger was over. When we arrived in Yaroslavl, the wound began to fester (Natasha knew everything about suppuration, etc.), and the doctor said that suppuration could go right. There was a fever. The doctor said that this fever was not so dangerous.
“But two days ago,” Natasha began, “it suddenly happened ...” She restrained her sobs. “I don't know why, but you'll see what he's become.
- Weakened? lost weight? .. - the princess asked.
No, not that, but worse. You will see. Ah, Marie, Marie, he's too good, he can't, can't live... because...

When Natasha, with a habitual movement, opened his door, letting the princess pass in front of her, Princess Marya already felt ready sobs in her throat. No matter how much she prepared herself, or tried to calm down, she knew that she would not be able to see him without tears.
Princess Mary understood what Natasha meant in words: it happened to him two days ago. She understood that this meant that he suddenly softened, and that softening, tenderness, these were signs of death. As she approached the door, she already saw in her imagination that face of Andryusha, which she had known since childhood, tender, meek, tender, which he had so rarely seen and therefore always had such a strong effect on her. She knew that he would say to her quiet, tender words, like those that her father had said to her before his death, and that she could not bear it and burst into tears over him. But, sooner or later, it had to be, and she entered the room. Sobs came closer and closer to her throat, while with her short-sighted eyes she more and more clearly distinguished his form and searched for his features, and now she saw his face and met his gaze.
He was lying on the sofa, padded with pillows, in a squirrel-fur robe. He was thin and pale. One thin, transparently white hand held a handkerchief, with the other, with quiet movements of his fingers, he touched his thin overgrown mustache. His eyes were on those who entered.
Seeing his face and meeting his gaze, Princess Mary suddenly slowed down the speed of her step and felt that her tears had suddenly dried up and her sobs had stopped. Catching the expression on his face and eyes, she suddenly became shy and felt guilty.
“Yes, what am I guilty of?” she asked herself. “In the fact that you live and think about the living, and I! ..” answered his cold, stern look.
There was almost hostility in the deep, not out of himself, but looking into himself look, when he slowly looked around at his sister and Natasha.
He kissed his sister hand in hand, as was their habit.
Hello Marie, how did you get there? he said in a voice as even and alien as his eyes were. If he had squealed with a desperate cry, then this cry would have horrified Princess Marya less than the sound of this voice.
“And did you bring Nikolushka?” he said, also evenly and slowly, and with an obvious effort of recollection.
- How is your health now? - said Princess Marya, herself surprised at what she said.
“That, my friend, you need to ask the doctor,” he said, and, apparently making another effort to be affectionate, he said with one mouth (it was clear that he did not think at all what he was saying): “Merci, chere amie , d "etre venue. [Thank you, dear friend, for coming.]
Princess Mary shook his hand. He winced slightly as he shook her hand. He was silent and she didn't know what to say. She understood what had happened to him in two days. In his words, in his tone, and especially in that cold, almost hostile look, one could feel an estrangement from everything worldly, terrible for a living person. He apparently had difficulty understanding now all living things; but at the same time it was felt that he did not understand the living, not because he was deprived of the power of understanding, but because he understood something else, something that the living did not understand and could not understand and that swallowed him up.
- Yes, that's how strange fate brought us together! he said, breaking the silence and pointing to Natasha. - She keeps following me.
Princess Mary listened and did not understand what he was saying. He, sensitive, gentle Prince Andrei, how could he say this in front of the one he loved and who loved him! If he had thought to live, he would not have said it in such a coldly insulting tone. If he did not know that he was going to die, how could he not feel sorry for her, how could he say this in front of her! There could only be one explanation for this, that it was all the same to him, and all the same because something else, something more important, had been revealed to him.
The conversation was cold, incoherent, and interrupted incessantly.
“Marie passed through Ryazan,” said Natasha. Prince Andrei did not notice that she called his sister Marie. And Natasha, calling her that in his presence, noticed this for the first time.

Redox properties of chromium compounds with different degrees of oxidation.

Chromium. The structure of the atom. Possible oxidation states. Acid-base properties. Application.

Cr +24)2)8)13)1

Chromium has oxidation states +2, +3 and +6.

With an increase in the degree of oxidation, acidic and oxidizing properties increase. Chromium Cr2+ derivatives are very strong reducing agents. The Cr2+ ion is formed at the first stage of Chromium dissolution in acids or during the reduction of Cr3+ in an acidic solution with zinc. Nitrous oxide Cr(OH)2 passes into Cr2O3 upon dehydration. Cr3+ compounds are stable in air. They can be both reducing and oxidizing agents. Cr3+ can be reduced in an acidic solution with zinc to Cr2+ or oxidized in an alkaline solution to CrO42- with bromine and other oxidizing agents. Hydroxide Cr (OH) 3 (more precisely Cr2O3 nH2O) is an amphoteric compound that forms salts with the Cr3 + cation or salts of chromic acid HCrO2 - chromites (for example, KSrO2, NaCrO2). Cr6+ compounds: chromic anhydride CrO3, chromic acids and their salts, among which the most important are chromates and dichromates - strong oxidizing agents.salts.

It is used as wear-resistant and beautiful galvanic coatings (chrome plating). Chromium is used for the production of alloys: chromium-30 and chromium-90, indispensable for the production of high-power plasma torch nozzles and in the aerospace industry.

Chromium is chemically inactive. Under normal conditions, it reacts only with fluorine (from non-metals), forming a mixture of fluorides.

Chromates and dichromates

Chromates are formed by the interaction of CrO3, or solutions of chromic acids with alkalis:

CrO3 + 2NaOH = Na2CrO4 + H2O

Dichromates are obtained by the action of acids on chromates:

2 Na2Cr2O4 + H2SO4 = Na2Cr2O7 + Na2SO4 + H2O

Chromium compounds are characterized by redox reactions.

Chromium (II) compounds are strong reducing agents, they are easily oxidized

4(5rC12 + O2 + 4HCI = 4CrC13 + 2H2O

Chromium compounds (!!!) are characterized by reducing properties. Under the action of oxidizing agents, they pass:

to chromates - in an alkaline environment,

in dichromates - in an acidic environment.

Cr(OH)3. CrOH + HCl = CrCl + H2O, 3CrOH + 2NaOH = Cr3Na2O3 + 3H2O

Chromates(III) (old names for chromites).

Chromium compounds are characterized by reducing properties. Under the action of oxidizing agents, they pass:

to chromates - in an alkaline environment,

in dichromates - in an acidic environment.

2Na3 [Cr(OH)6] + 3Br2 + 4NaOH = 2Na2CrO4 + 6NaBr + 8H2O

5Cr2(SO4)3 + 6KMnO4 + 11H2O = 3K2Cr2O7 + 2H2Cr2O7 + 6MnSO4 + 9H2SO4

Salts of chromic acids in an acidic environment are strong oxidizing agents:

3Na2SO3 + K2Cr2O7 + 4H2SO4 = 3Na2SO4 + Cr2(SO4)3 + K2SO4 + 4H2O

The discovery of chromium belongs to the period of rapid development of chemical-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 the Siberian red lead ore (crocoite), described by Lomonosov. The mineral was investigated, but nothing but oxides of lead, iron and aluminum was found in it. However, in 1797, Vauquelin, by boiling a finely ground sample of the mineral with potash and precipitating lead carbonate, obtained an orange-red solution. From this solution, he crystallized a ruby-red salt, from which an oxide and a free metal, different from all known metals, were isolated. Vauquelin called him Chromium ( Chrome ) from the Greek word- coloring, color; True, here it was not the property of the metal that was meant, but its brightly colored salts.

Finding in nature.

The most important chromium ore of practical importance is chromite, the approximate composition of which corresponds to the formula FeCrO ​​4.

It is found in Asia Minor, in the Urals, in 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 is found on the Sun, stars, meteorites.

Physical properties.

Chromium is a white, hard and brittle metal, exceptionally chemically resistant to acids and alkalis. It oxidizes in air and has a thin transparent oxide film on the surface. Chromium has a density of 7.1 g / cm 3, its melting point is +1875 0 C.

Receipt.

With strong heating of chromium iron ore with coal, chromium and iron are reduced:

FeO * Cr 2 O 3 + 4C = 2Cr + Fe + 4CO

As a result of this reaction, an alloy of chromium with iron 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 \u003d Al 2 O 3 + 2Cr

Two oxides are usually used in this process - Cr 2 O 3 and CrO 3

Chemical properties.

Thanks to a thin protective oxide film covering the surface of chromium, 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 \u003d Cr 2 (SO 4) 3 + 3H 2
2Cr + 6HCl = 2CrCl 3 + 3H 2

At high temperatures, chromium burns in oxygen to form oxide(III).

Hot chromium reacts with water vapor:

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

Chromium also reacts with halogens at high temperatures, halogens with hydrogens, sulfur, nitrogen, phosphorus, coal, silicon, boron, for example:

Cr + 2HF = CrF 2 + H 2
2Cr + N2 = 2CrN
2Cr + 3S = Cr2S3
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 obtain high-strength, corrosion-resistant coatings in mechanical engineering. Alloys in the form of ferrochrome are used as metal cutting tools. Chrome-plated alloys have found application in medical technology, in the manufacture of chemical process equipment.

The position of chromium in the periodic table of chemical elements:

Chromium heads the side subgroup of group VI of the periodic system 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 at the chromium atom, the regularity is violated, according to which the 4S orbital should have been filled first to the state 4S 2 . However, due to the fact that the 3d orbital occupies a more favorable energy position in the chromium atom, it is filled up to the value 4d 5 . Such a phenomenon is observed in the atoms of some other elements of the secondary subgroups. Chromium can exhibit oxidation states from +1 to +6. The most stable are chromium compounds with oxidation states +2, +3, +6.

Divalent chromium compounds.

Chromium oxide (II) CrO - pyrophoric black powder (pyrophoric - the ability to ignite in air in a finely divided 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 by dissolving chromium metal in acids. These reactions take place in an atmosphere of an inactive gas (for example, H 2), because in the presence of air, Cr(II) is easily oxidized to Cr(III).

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, is a reducing agent. The hydrated Cr2+ ion is colored pale blue. An aqueous solution of CrCl 2 has a blue color. In air in aqueous solutions, Cr(II) compounds transform into Cr(III) compounds. This is especially pronounced for Cr(II) hydroxide:

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

Trivalent chromium compounds.

Chromium oxide (III) Cr 2 O 3 is a refractory green powder. It is close to corundum in hardness. In the laboratory, it can be obtained by heating ammonium dichromate:

(NH 4) 2 Cr 2 O 7 \u003d Cr 2 O 3 + N 2 + 4H 2

Cr 2 O 3 - amphoteric oxide, when fused with alkalis, it forms chromites: Cr 2 O 3 + 2NaOH \u003d 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 sulfate (III) Cr 2 (SO 4) 3 pink, 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 are also known, containing a smaller amount of water. Chrome alum KCr(SO 4) 2 *12H 2 O crystallizes from solutions containing violet chromium sulfate and potassium sulfate. A solution of chromic 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 conclude with a high degree of probability that chromium is absent.

1. We 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. Salt will not release water of crystallization, but will melt at a temperature of about 400 0 C with the formation of a dark liquid. Let's heat it for a few more minutes on a strong flame. After cooling, a green precipitate forms on the shard. Part of it is soluble in water (it turns yellow), and the other part is left 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 dichromate in 50ml of water. To one part add some potassium carbonate. It will dissolve with the release of CO 2 , and the color of the solution will become light yellow. Chromate is formed from potassium dichromate. If we now add a 50% solution of sulfuric acid in portions, then the red-yellow color of the bichromate will appear again.
3. Pour into a test tube 5 ml. potassium dichromate solution, boil with 3 ml of concentrated hydrochloric acid under draft. Yellow-green poisonous gaseous chlorine is released from the solution, because 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, fusing with soda and nitrate, converted to 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 a solution of potassium bichromate and acidify the solution with sulfuric acid. The solution acquires a deep blue color due to the formation of chromium peroxide. Peroxide, when shaken with some ether, will turn into an organic solvent and turn 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, it is necessary to dissolve the metal. With prolonged boiling with 30% sulfuric acid (hydrochloric acid can also be added), chromium and many steels partially dissolve. The resulting solution contains chromium (III) sulfate. To be able to conduct a detection reaction, we first neutralize it with caustic soda. Gray-green chromium (III) hydroxide precipitates, which dissolves in excess NaOH and forms green sodium chromite. Filter the solution and add 30% hydrogen peroxide. When heated, the solution will turn yellow, as chromite is oxidized to chromate. Acidification will result in a blue color of the solution. The colored compound can be extracted by shaking with ether.

Analytical reactions for chromium ions.

1. To 3-4 drops of a solution of chromium chloride CrCl 3 add a 2M solution of NaOH until the initial precipitate dissolves. Note the color of the sodium chromite formed. Heat the resulting solution in a water bath. What is happening?
2. To 2-3 drops of CrCl 3 solution add an equal volume of 8M NaOH solution and 3-4 drops of 3% H 2 O 2 solution. Heat the reaction mixture in a water bath. What is happening? 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 a test tube. Heat the reaction site for several minutes on 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 blue color of the solution that appears 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 color 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- and Ba 2+ ions interact, a yellow precipitate of barium chromate BaCrO 4 precipitates.
3. Silver nitrate forms brick red precipitate of silver chromate with CrO 4 2 ions.
4. Take three test tubes. Place 5-6 drops of K 2 Cr 2 O 7 solution in one of them, the same volume of K 2 CrO 4 solution in the second, and three drops of both solutions in the third. Then add three drops of potassium iodide solution to each tube. Explain the result. Acidify the solution in the second tube. What is happening? 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, a bright green solution is obtained after filtration, which turns yellow when an acid is added, and turns green in a neutral or alkaline medium.
2. Place in the center of the can with thermite "ruby mixture" - thoroughly ground and placed in aluminum foil Al 2 O 3 (4.75 g) with the addition of Cr 2 O 3 (0.25 g). So that the jar does not cool down longer, it is necessary to bury it under the upper edge in the sand, and after the thermite is ignited and the reaction begins, cover it with an iron sheet and fill it with sand. Bank to dig out in a day. The result is a red-ruby powder.
3. 10 g of potassium bichromate is triturated 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 from the end, then 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 lengthens 10 times.
4. If you mix solutions of copper sulfate and potassium dichromate and add a little ammonia solution, then an amorphous brown precipitate of the composition 4СuCrO 4 * 3NH 3 * 5H 2 O will fall out, which dissolves in hydrochloric acid to form a yellow solution, and in excess of ammonia a green solution is obtained. If further alcohol is added 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 left after the “volcano” or “pharaoh snake” experiments can be regenerated. To do this, it is necessary 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. Soluble chromate is obtained, which can also 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 \u003d 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
Cr2+

a) CrO + 2HCl = CrCl 2 + H 2 O
b) CrO + H 2 O \u003d 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) 4Cr (NO 3) 3 \u003d 2Cr 2 O 3 + 12NO 2 + O 2
f) Cr 2 O 3 + 2 NaOH = 2NaCrO 2 + H 2 O

Chrome 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 obtaining many pictorial 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 aqueous chromium (III) oxide. Under the name "emerald green" it began to be produced in 1837. Later, L. Vauquelen proposed several new paints: barite, zinc and chrome yellow. Over time, they were replaced by more persistent yellow, orange pigments based on cadmium.

Chrome green is the most durable and lightfast paint that is not affected by atmospheric gases. Rubbed in oil, chrome green has great hiding power and is capable of drying quickly, therefore, 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 slightly fired product, which is then covered with a layer of glaze. This is followed by the main, high-temperature firing: for sintering the porcelain mass and melting the glaze, the products are heated to 1350 - 1450 0 C. Very few paints can withstand such a high temperature without chemical changes, and in the old days there were only two of them - cobalt and chromium. Black oxide of cobalt, applied to the surface of a porcelain item, fuses with the glaze during firing, chemically interacting with it. As a result, bright blue cobalt silicates are formed. This cobalt blue chinaware is well known to everyone. Chromium oxide (III) does not interact chemically with the components of the glaze and simply lies between the porcelain shards and the transparent glaze with a "deaf" layer.

In addition to chrome green, artists use paints derived 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 after 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). Variability in the composition of the mineral found in the Urals, in the Perm and Kirov regions determines its diverse coloration - from the color of a darkened winter 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 gives the paint a uniquely fresh tone. At present, a method has been developed for obtaining artificial wolkonskoite. It is interesting to note that, according to modern research, Russian icon painters used paints from this material as early as the Middle Ages, long before its “official” discovery. Guinier green (created in 1837), whose chromoform is a hydrate of chromium oxide Cr 2 O 3 * (2-3) H 2 O, where part of the water is chemically bound, and part is adsorbed, was also known among artists. This pigment gives the paint an emerald hue.

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

Cr(OH)+2HCl→CrCl+2HO

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

2CrCl+ 2HCl → 2CrCl+ H

4Cr(OH)+O+ 2HO→4Cr(OH)

Chromium oxide(III) CrO is a green, water-insoluble powder. It can be obtained by calcining 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 \u003d 2K 3 [Cr (OH) 6]

Cr 2 O 3 + 6HCl \u003d 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 \u003d Cr (OH) 3 ↓ + 3KSl

Chromium hydroxide (III) is a gray-green precipitate, upon receipt of which, alkali must be taken in short supply. Chromium (III) hydroxide obtained in this way, unlike 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 + 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)+ 2H2O

Cr(OH) 3 + KOH = K 3 CrO 3 (orthochromite K)+ 3H2O

Chromium compounds(VI).

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

CrO 3 + H 2 O \u003d H 2 CrO 4 (chromic acid - formed with excess water)

CrO 3 + H 2 O \u003d H 2 Cr 2 O 7 (dichromic acid - is 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:

C 2 H 5 OH + 4CrO 3 \u003d 2CO 2 + 2Cr 2 O 3 + 3H 2 O

It also oxidizes iodine, sulfur, phosphorus, coal:

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

When heated to 250 0 C, 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:

K 2 Cr 2 O 7 + H 2 SO 4 \u003d K 2 SO 4 + 2CrO 3 + H 2 O

Chromic and dichromic acids.

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

Chromate - CrO 4 2- ions and dichromate - Cr2O 7 2- ions easily pass into each other when the solution environment changes

In the acidic environment of the solution, chromates turn 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 + 2KOH \u003d 2K 2 CrO 4 + H 2 O

When diluted, dichromic acid becomes chromic acid:

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

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

Oxidation state
The nature of the oxide	basic	amphoteric	acid
Hydroxide		Cr(OH) 3 - H 3 CrO 3
The nature of the hydroxide	basic	amphoteric	acid
→ weakening of basic properties and strengthening of acidic →

Redox properties of chromium compounds.

Reactions in an acid medium.

In an acidic environment, Cr +6 compounds turn 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 \u003d 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 medium.

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

2KCrO 2 +3 Br2 +8NaOH \u003d 2Na 2 CrO 4 + 2KBr + 4NaBr + 4H 2 O

Cr +3 - 3e → Cr +6



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