Hydroxides and salts of chromium (II) and (III). Complex compounds of chromium (III)
Chromium (II) hydroxide Cr (OH) 2 is obtained in the form of a yellow precipitate by treating solutions of chromium (II) salts with alkalis in the absence of oxygen:
CrCl 2 + 2NaOH \u003d Cr (OH) 2 ¯ + 2NaCl
Cr(OH) 2 has typical basic properties and is a strong reducing agent:
2Cr(OH) 2 +H 2 O+1/2O 2 =2Cr(OH) 3 ¯
Aqueous solutions of chromium (II) salts are obtained without access to air by dissolving chromium metal in dilute acids in a hydrogen atmosphere or by reducing trivalent chromium salts with zinc in an acidic medium. Anhydrous salts of chromium (II) are white, and aqueous solutions and crystalline hydrates are blue.
In terms of their chemical properties, chromium (II) salts are similar to ferrous iron salts, but differ from the latter in more pronounced reducing properties, i.e. easier than the corresponding ferrous compounds to oxidize. That is why it is very difficult to obtain and store divalent chromium compounds.
Chromium hydroxide (III) Cr (OH) 3 - a gelatinous gray-green precipitate, it is obtained by the action of alkalis on solutions of chromium (III) salts:
Cr 2 (SO 4) 3 + 6NaOH \u003d 2Cr (OH) 3 ¯ + 3Na 2 SO 4
Chromium (III) hydroxide has amphoteric properties, dissolving both in acids with the formation of chromium (III) salts:
2Cr (OH) 3 + 3H 2 SO 4 \u003d Cr 2 (SO 4) 3 + 6H 2 O and in alkalis with the formation of hydroxychromites: Cr (OH) 3 + NaOH \u003d Na 3
When Cr (OH) 3 is fused with alkalis, metachromites and orthochromites are formed:
Cr (OH) 3 + NaOH \u003d NaCrO 2 + 2H 2 O Cr (OH) 3 + 3NaOH \u003d Na 3 CrO 3 + 3H 2 O
When calcining chromium (III) hydroxide, chromium (III) oxide is formed:
2Cr (OH) 3 \u003d Cr 2 O 3 + 3H 2 O
Trivalent chromium salts are colored both in the solid state and in aqueous solutions. For example, anhydrous chromium (III) sulfate Cr 2 (SO 4) 3 is purple-red, aqueous solutions of chromium (III) sulfate, depending on the conditions, can change color from purple to green. This is explained by the fact that in aqueous solutions the Cr 3+ cation exists only in the form of a hydrated 3+ ion due to the propensity of trivalent chromium to form complex compounds. The violet color of aqueous solutions of chromium (III) salts is due precisely to the 3+ cation. When heated, complex salts of chromium (III) can
partially lose water, forming salts of various colors, up to green.
Trivalent chromium salts are similar to aluminum salts in composition, crystal lattice structure, and solubility; so, for chromium (III), as well as for aluminum, the formation of potassium chromium alum KCr (SO 4) 2 12H 2 O is typical, they are used for tanning leather and as a mordant in the textile business.
Salts of chromium (III)Cr 2 (SO 4) 3, CrCl 3, etc. when stored in air, they are stable, and in solutions they undergo hydrolysis:
Cr 3+ + 3Cl - + NON "Cr (OH) 2+ + 3Cl - + H +
Hydrolysis goes according to stage I, but there are salts that are completely hydrolyzed:
Cr 2 S 3 + H 2 O \u003d Cr (OH) 3 ¯ + H 2 S
In redox reactions in an alkaline medium, chromium (III) salts behave as reducing agents:
It should be noted that in the series of chromium hydroxides of various oxidation states Cr (OH) 2 - Cr (OH) 3 - H 2 CrO 4, the basic properties are naturally weakened and the acid ones are strengthened. Such a change in properties is due to an increase in the degree of oxidation and a decrease in the ionic radii of chromium. In the same series, the oxidizing properties are consistently enhanced. Cr (II) compounds are strong reducing agents, they are easily oxidized, turning into chromium (III) compounds. Chromium(VI) compounds are strong oxidizing agents, easily reduced to chromium(III) compounds. Compounds with an intermediate oxidation state, i.e. chromium (III) compounds, when interacting with strong reducing agents, can exhibit oxidizing properties, turning into chromium (II) compounds, and when interacting with strong oxidizing agents, they can exhibit reducing properties, turning into chromium (VI) compounds.
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) = FROMr(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
Target: deepen students' knowledge of the topic.
Tasks:
- characterize chromium as a simple substance;
- to introduce students to chromium compounds of different oxidation states;
- show the dependence of the properties of compounds on the degree of oxidation;
- show redox properties of chromium compounds;
- to continue the formation of students' skills to write down the equations of chemical reactions in molecular and ionic form, to draw up an electronic balance;
- continue the formation of skills to observe a chemical experiment.
Lesson form: lecture with elements of independent work of students and observation of a chemical experiment.
Lesson progress
I. Repetition of the material of 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 are the oxidation states in the compounds?
How do the atomic radius and ionization energy change from chromium to tungsten?
You can offer students to fill out a table using the tabular values of the radii of atoms, ionization energies and draw conclusions.
Sample table:
2. Listen to the student's message on the topic "Elements of the chromium subgroup in nature, obtaining and using."
II. Lecture.
Lecture plan:
- Chromium.
- Chromium compounds. (2)
- Chromium oxide; (2)
- Chromium hydroxide. (2)
- Chromium compounds. (3)
- Chromium oxide; (3)
- Chromium hydroxide. (3)
- Chromium compounds (6)
- Chromium oxide; (6)
- Chromic and dichromic acids.
- Dependence of the properties of chromium compounds on the degree of oxidation.
- Redox properties of chromium compounds.
1. Chrome.
Chromium is a white lustrous metal with a bluish tint, very hard (density 7.2 g/cm3), 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 temperature:
- 4Cr + 3O 2 \u003d 2Cr 2 O 3
- 2Cr + 3S = Cr 2 S 3
- 2Cr + 3Cl 2 = 2CrCl 3
Exercise: write 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 reducing agent.
The interaction of chromium with complex substances:
At very high temperatures, chromium reacts with water:
- 2Cr + 3 H 2 O \u003d Cr 2 O 3 + 3H 2
Exercise:
Chromium reacts with dilute sulfuric and hydrochloric acids:
- Cr + H 2 SO 4 = CrSO 4 + H 2
- Cr + 2HCl \u003d 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 - 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 the molecular and ionic form of the interaction of chromium oxide (2) with sulfuric acid.
Chromium oxide (2) is easily oxidized in air:
- 4CrO + O 2 \u003d 2Cr 2 O 3
Exercise: draw up an electronic balance, indicate the oxidizing agent and reducing agent.
Chromium oxide (2) is formed during the oxidation of chromium amalgam with atmospheric oxygen:
2Сr (amalgam) + О 2 = 2СrО
2. Chromium hydroxide (2)- Cr (OH) 2 - 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: compose reaction equations 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 Cr (OH) 2 + O 2 + 2H 2 O \u003d 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 to corundum in hardness (it corresponds to chromium hydroxide (3) - Cr (OH) 3). Chromium oxide (3) is amphoteric in nature, however, it 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 the molecular and ionic form of 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 \u003d 2K 3 [Cr (OH) 6]
- Cr 2 O 3 + 6HCl \u003d 2CrCl 3 + 3H 2 O
Exercise: compose reaction equations in the 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 decomposition of ammonium dichromate:
- (NH 4) 2Cr 2 O 7 \u003d N 2 + Cr 2 O 3 + 4H 2 O
2. Chromium hydroxide (3) Cr (OH) 3 is obtained by the action of alkalis on solutions of chromium salts (3):
- CrCl 3 + 3KOH \u003d Cr (OH) 3 ↓ + 3KSl
Exercise: write ionic equations
Chromium hydroxide (3) is a gray-green precipitate, upon receipt of which, alkali must be taken in short supply. Chromium (3) 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)
Exercise: compose 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 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
4. Chromium compounds. (6)
1. Chromium oxide (6)- 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
Exercise: make equations of chemical reactions of chromium oxide (6) with iodine, phosphorus, coal; to one of the equations, draw up an electronic balance, indicate the oxidizing agent and reducing agent
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
2. 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 - Cr 2O 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
5. Dependence of the properties of chromium compounds on the degree of oxidation.
| Oxidation state | +2 | +3 | +6 |
| Oxide | CrO | Cr 2 O 3 | CrO 3 |
| The nature of the oxide | basic | amphoteric | acid |
| Hydroxide | Cr(OH) 2 | Cr(OH) 3 - H 3 CrO 3 | H 2 CrO 4 |
| The nature of the hydroxide | basic | amphoteric | acid |
|
→ weakening of basic properties and strengthening of acidic→ |
|||
6. 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
Exercise:
1. Equalize the reaction equation using the electron 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, equate the equation using the electron balance method, indicate the oxidizing agent and reducing agent:
- K 2 Cr 2 O 7 + SO 2 + H 2 SO 4 \u003d? +? +H 2 O
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 Br 2 +8NaOH \u003d 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 electron 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, equate the equation using the electron balance method, indicate the oxidizing agent and reducing agent:
- Cr(OH) 3 + Ag 2 O + NaOH = Ag + ? +?
Thus, the oxidizing properties are consistently enhanced with a change in the oxidation states in the series: Cr +2 → Cr +3 → Cr +6. Chromium compounds (2) are strong reducing agents, they are easily oxidized, turning into chromium compounds (3). Chromium compounds (6) are strong oxidizers, easily reduced to chromium compounds (3). Chromium (3) compounds, when interacting with strong reducing agents, exhibit oxidizing properties, turning into chromium (2) compounds, and when interacting with strong oxidizing agents, they exhibit reducing properties, turning into chromium compounds (6)
To the lecture method:
- To enhance the cognitive activity of students and maintain interest, it is advisable to conduct a demonstration experiment during the lecture. Depending on the capabilities of the educational laboratory, students can demonstrate the following experiments:
- 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);
- the transition of chromates to dichromates, dichromates to chromates.
- Tasks of independent work can be differentiated taking into account the real learning opportunities of students.
- 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: finalize the lecture (add the equations of chemical reactions)
- Vasilyeva Z.G. Laboratory work on general and inorganic chemistry. -M.: "Chemistry", 1979 - 450 p.
- Egorov A.S. Chemistry tutor. - Rostov-on-Don: "Phoenix", 2006.-765 p.
- Kudryavtsev A.A. Compilation of chemical equations. - M., "Higher School", 1979. - 295 p.
- Petrov M.M. Inorganic chemistry. - Leningrad: "Chemistry", 1989. - 543 p.
- Ushkalova V.N. Chemistry: competitive tasks and answers. - M.: "Enlightenment", 2000. - 223 p.
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, 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.
- 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 .
- 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.
- 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.
- 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.
- 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.
- 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?
- 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 ?
- 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?
- 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.
- 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.
- When CrO 4 2- and Ba 2+ ions interact, a yellow precipitate of barium chromate BaCrO 4 precipitates.
- Silver nitrate forms brick red precipitate of silver chromate with CrO 4 2 ions.
- 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
- 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 acid is added, and turns green in a neutral or alkaline medium.
- 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.
- 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.
- 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.
- 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). determines its diverse coloration - from the color of a darkened winter fir to the bright green color of a swamp 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's 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 adsorbed, was also popular with artists. This pigment gives the paint an emerald hue.
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Chemistry Tutor
Continuation. See in No. 22/2005; 1, 2, 3, 5, 6, 8, 9, 11, 13, 15, 16, 18, 22/2006;
3, 4, 7, 10, 11, 21/2007;
2, 7, 11, 18/2008
ACTIVITY 25
10th grade(first year of study)
Chromium and its compounds
1. Position in the table of D.I. Mendeleev, the structure of the atom.
2. Origin of the name.
3. Physical properties.
4. Chemical properties.
5. Being in nature.
6. Basic methods of obtaining.
7. The most important chromium compounds:
a) chromium(II) oxide and hydroxide;
b) chromium(III) oxide and hydroxide, their amphoteric properties;
c) chromium(VI) oxide, chromic and dichromic acid, chromates and dichromates.
9. Redox properties of chromium compounds.
Chromium is located in the secondary subgroup of group VI of the table of D.I. Mendeleev. When compiling the electronic formula of chromium, it must be remembered that due to the greater stability of configuration 3 d 5 at the chromium atom, an electron slip is observed and the electronic formula has the form: 1 s 2 2s 2 p 6 3s 2 p 6 4s 1 3d 5 . In compounds, chromium can exhibit oxidation states +2, +3, and +6 (the +3 oxidation state is the most stable):
Chrome gets its name from the Greek word chroma(color, paint) due to the bright varied color of its compounds.
Chrome is a white shiny metal, very hard, brittle, refractory. Resistant to corrosion. In air, it becomes covered with an oxide film, due to which the surface becomes dull.
Chemical properties
Under normal conditions, chromium is an inactive metal and only reacts with fluorine. But when heated, the oxide film of chromium is destroyed, and chromium reacts with many simple and complex substances (similar to Al).
4Cr + 3O 2 2Cr 2 O 3 .
Metals (-).
Nonmetals (+):
2Cr + 3Cl 2 2CrCl 3 ,
2Cr + 3F 2 \u003d 2CrF 3,
2Cr + 3SCr 2 S 3 ,
H 2 O (+/-): *
2Cr + 3H 2 O (steam) Cr 2 O 3 + 3H 2.
Basic oxides (-).
Acid oxides (-).
Bases (+/-):
2Cr + 6NaOH + 6H 2 O \u003d 2Na 3 + 3H 2.
Non-oxidizing acids (+).
Cr + 2HCl \u003d CrCl 2 + H 2.
Oxidizing acids (-). Passivation.
Salts (+/-):
2Cr + 3CuSO 4 \u003d Cr 2 (SO 4) 3 + 3Cu,
Cr + CaCl 2 no reaction.
In nature, the element chromium is represented by four isotopes with mass numbers 50, 52, 53 and 54. In nature, chromium occurs only in the form of compounds, the most important of which are chromium iron ore, or chromite (FeOzhCr 2 O 3) and lead red ore (PbCrO 4).
Metal chromium is obtained: 1) from its oxide using aluminothermy:
Cr 2 O 3 + 2Al 2Cr + Al 2 O 3,
2) electrolysis of aqueous solutions or melts of its salts:
From chromium iron ore in industry, an alloy of iron with chromium is obtained - ferrochromium, which is widely used in metallurgy:
FeO Cr 2 O 3 + 4CFe + 2Cr + 4CO.
The most important chromium compounds
Chromium forms three oxides and their corresponding hydroxides, the nature of which naturally changes with an increase in the oxidation state of chromium:
Chromium oxide(II) (CrO) is a solid, bright red or brown-red substance, insoluble under normal conditions in water, a typical basic oxide. Chromium(II) oxide is easily oxidized in air when heated, and is reduced to pure chromium.
CrO + 2HCl \u003d CrCl 2 + H 2 O,
4CrO + O 2 2Cr 2 O 3,
CrO + H 2 Cr + H 2 O.
Chromium(II) oxide is obtained by direct oxidation of chromium:
2Cr + O 2 2CrO.
Chromium hydroxide(II) (Cr(OH) 2) - water-insoluble yellow substance, weak electrolyte, exhibits basic properties, readily soluble in concentrated acids; easily oxidized in the presence of moisture by atmospheric oxygen; when calcined in air, it decomposes to form chromium(III) oxide:
Cr(OH) 2 + 2HCl = CrCl 2 + 2H 2 O,
4Cr(OH) 2 + O 2 2Cr 2 O 3 + 4H 2 O.
Chromium(II) hydroxide is obtained by the exchange reaction between a chromium(II) salt and an alkali solution in the absence of oxygen:
CrCl 2 + 2NaOH \u003d Cr (OH) 2 + 2NaCl.
Chromium oxide(III) (Cr 2 O 3) exhibits amphoteric properties. It is a refractory (hardness comparable to corundum) green powder, insoluble in water. Carcinogen! It is obtained by decomposition of ammonium dichromate, chromium (III) hydroxide, reduction of potassium dichromate or direct oxidation of chromium:
(NH 4) 2 Cr 2 O 7 N 2 + Cr 2 O 3 + 4H 2 O,
2Cr (OH) 3 Cr 2 O 3 + 3H 2 O,
2K 2 Cr 2 O 7 + 3С2Cr 2 O 3 + 2K 2 CO 3 + CO 2,
4Cr + 3O 2 2Cr 2 O 3 .
Under normal conditions, chromium(III) oxide is poorly soluble in acids and alkalis; it exhibits amphoteric properties when fused with alkalis or with alkali metal carbonates (forming chromites); at high temperatures, chromium(III) oxide can be reduced to a pure metal:
Cr 2 O 3 + 2KOH 2KCrO 2 + H 2 O,
Cr 2 O 3 + Na 2 CO 3 2NaCrO 2 + CO 2,
Cr 2 O 3 + 6HCl \u003d 2CrCl 3 + 3H 2 O,
2Cr 2 O 3 + 3C4Cr + 3CO 2.
Chromium hydroxide(III) (Cr (OH) 3) is precipitated by the action of alkalis on salts of trivalent chromium (gray-green precipitate):
CrCl 3 + 3NaOH (lack) = Cr(OH) 3 + 3NaCl.
It exhibits amphoteric properties, dissolving both in acids and in an excess of alkalis; thermally unstable:
Cr(OH) 3 + 3HCl = CrCl 3 + 3H 2 O,
Cr(OH) 3 + 3KOH \u003d K 3,
Cr(OH) 3 + KOH KCrO 2 + 2H 2 O,
2Cr(OH)3Cr2O3 + 3H2O.
Chromium oxide(VI) (CrO 3) - a dark red crystalline substance, poisonous, exhibits acidic properties. Let's well dissolve in water, at dissolution of this oxide in water chromic acids are formed; how the acid oxide CrO 3 interacts with basic oxides and with alkalis; thermally unstable; is the strongest oxidizing agent
CrO 3 + H 2 O =
2CrO 3 + H 2 O =
CrO 3 + K 2 OK 2 CrO 4 ,
CrO 3 + 2NaOH \u003d Na 2 CrO 4 + H 2 O,
4CrO 3 2Cr 2 O 3 + 3O 2,
This oxide is obtained by the interaction of dry chromates and dichromates with concentrated sulfuric acid:
K 2 Cr 2 O 7 + H 2 SO 4 (conc.) 2CrO 3 + K 2 SO 4 + H 2 O,
K 2 CrO 4 + H 2 SO 4 (conc.) CrO 3 + K 2 SO 4 + H 2 O.
Chrome and dichromic acid exist only in aqueous solutions, but form stable salts - chromates and dichromates. Chromates and their solutions are yellow, and dichromates are orange. Chromate ions and dichromate ions easily pass into each other when the solution medium changes. AT acidic environment chromates turn into dichromates, the solution acquires an orange color; in an alkaline environment dichromates turn into chromates, the solution turns yellow:
2K 2 CrO 4 + H 2 SO 4) K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O,
K 2 Cr 2 O 7 + 2KOH)2K 2 CrO 4 + H 2 O.
The ion is stable in an alkaline medium, but in an acidic one.
Oxidation–reduction properties
chromium compound
Of all chromium compounds, the most stable are compounds with a chromium oxidation state of +3. Chromium compounds with an oxidation state of +2 are strong reducing agents and are easily oxidized to +3:
4Cr(OH) 2 + O 2 + 2H 2 O = 4Cr(OH) 3,
4CrCl 2 + 4HCl + O 2 = 4CrCl 3 + 2H 2 O.
Compounds containing chromium in the +6 oxidation state are strong oxidizers, while chromium is reduced from +6 to +3:
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.
To detect alcohol in exhaled air, a reaction based on the oxidizing ability of chromium(VI) oxide is used:
4CrO 3 + 3С 2 H 5 OH 2Cr 2 O 3 + 3CH 3 COOH + 3H 2 O.
A solution of potassium dichromate in concentrated sulfuric acid is called chrome mixture and is used to clean chemical glassware.
Test on the topic "Chromium and its compounds"
1. Some element forms all three types of oxides (basic, amphoteric, and acidic). The oxidation state of an element in an amphoteric oxide will be:
a) minimum;
b) maximum;
c) intermediate between the minimum and maximum;
d) can be anyone.
2. When a freshly prepared precipitate of chromium (III) hydroxide reacts with an excess of an alkali solution, the following is formed:
a) medium salt; b) basic salt;
c) double salt; d) complex salt.
3. The total number of electrons in the preexternal level of the chromium atom is:
a) 12; b) 13; in 1; d) 2.
4. Which of the metal oxides is acidic?
a) Copper(II) oxide; b) chromium(VI) oxide;
c) chromium(III) oxide; d) iron(III) oxide.
5. What mass of potassium dichromate (in g) is required to oxidize 11.2 g of iron in sulfuric acid solution?
a) 58.8; b) 14.7; c) 294; d) 29.4.
6. What mass of water (in g) must be evaporated from 150 g of a 10% solution of chromium(III) chloride to obtain a 30% solution of this salt?
a) 100; b) 20; c) 50; d) 40.
7. The molar concentration of sulfuric acid in the solution is 11.7 mol/l, and the density of the solution is 1.62 g/ml. The mass fraction of sulfuric acid in this solution is (in%):
a) 35.4; b) 98; c) 70.8; d) 11.7.
8. The number of oxygen atoms in 19.4 g of potassium chromate is:
a) 0.602 10 23; b) 2.408 10 23;
c) 2.78 10 23; d) 6.02 10 23 .
9. Litmus will show a red color in an aqueous solution (multiple correct answers are possible):
a) chromium(III) chloride; b) chromium(II) chloride;
c) potassium chloride; d) hydrochloric acid.
10. The transition of chromate to dichromate occurs in ... environment and is accompanied by the process:
a) acidic, recovery process;
b) acidic, there is no change in oxidation states;
c) alkaline, recovery process;
d) alkaline, there is no change in oxidation states.
Key to the test
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| in | G | b | b | G | a | in | b | a, b, d | b |
Qualitative tasks for the identification of substances 1. An aqueous solution of some salt is divided into two parts. One of them was treated with an excess of alkali and heated, the released gas changed the color of red litmus to blue. The other part was treated with hydrochloric acid, the released gas caused the lime water to become cloudy. What salt was analyzed? Support your answer with reaction equations.
Answer. ammonium carbonate.
2. When ammonia, sodium sulfide and silver nitrate are added (separately) to an aqueous solution of substance A, white precipitates are formed, two of which are of the same composition. What is Substance A? Write reaction equations.
Solution
Substance A - AlCl 3.
AlCl 3 + 3NH 4 OH \u003d Al (OH) 3 + 3NH 4 Cl,
2AlCl 3 + 3Na 2 S + 6H 2 O 2Al(OH) 3 + 3H 2 S + 6NaCl,
AlCl 3 + 3AgNO 3 \u003d 3AgCl + Al (NO 3) 3.
Answer. aluminum chloride.
3. When a colorless gas A with a pungent characteristic odor is burned in the presence of oxygen, another gas B is formed, without color and odor, which reacts with lithium at room temperature to form a solid substance C. Identify the substances, write the reaction equations.
Solution
Substance A - NH 3,
substance B - N 2,
substance C - Li 3 N.
4NH 3 + 3O 2 2N 2 + 6H 2 O,
N 2 + 6Li = 2Li 3 N.
Answer. NH 3 , N 2 , Li 3 N.
4. Colorless gas A, with a characteristic pungent odor, reacts with another colorless gas, B, which has the smell of rotten eggs. As a result of the reaction, a simple C and a complex substance are formed. Substance C reacts with copper to form a black salt. Identify substances, give reaction equations.
Answer. SO 2 , H 2 S, S.
5. Colorless gas A with a pungent characteristic odor, lighter than air, reacts with strong acid B to form salt C, an aqueous solution of which does not form precipitates with either barium chloride or silver nitrate. Identify substances, give reaction equations (one of the possible options).
Answer. NH 3 , HNO 3 , NH 4 NO 3 .
6. A simple substance A, formed by atoms of the second most common element of the earth's crust, reacts when heated with iron (II) oxide, resulting in the formation of compound B, which is insoluble in aqueous solutions of alkalis and acids (except hydrofluoric). Substance B, when fused with quicklime, forms an insoluble salt C. Identify the substances, give the reaction equations (one of the possible options).
Answer. Si, SiO 2 , CaSiO 3 .
7. Water-insoluble brown compound A decomposes upon heating to form two oxides, one of which is water. The other oxide, B, is reduced by carbon to form metal C, the second most abundant metal in nature. Identify substances, write reaction equations.
Answer. Fe (OH) 3, Fe 2 O 3, Fe.
8. Substance A, which is part of one of the most common minerals, forms gas B when treated with hydrochloric acid. When substance B reacts when heated with simple substance C, only one compound is formed - a combustible gas without color and odor. Identify substances, give reaction equations.
Answer. CaCO 3 , CO 2 , C.
9. Light metal A, which reacts with dilute sulfuric acid, but does not react in cold with concentrated sulfuric acid, interacts with a solution of sodium hydroxide, and gas and salt B are formed. When hydrochloric acid is added to substance B, salt C is formed. Identify substances, give equations reactions.
Answer. Al, NaAlO 2 , NaCl.
10. Substance A is a soft, well-cut silver-white metal, lighter than water. When substance A interacts with a simple substance B, compound C is formed, which is soluble in water to form an alkaline solution. When substance C is treated with hydrochloric acid, a gas with an unpleasant odor is released and salt is formed, which turns the flame of the burner purple. Identify substances, give reaction equations.
Answer. K, S, K2S.
11. Colorless gas A with a pungent characteristic odor is oxidized by oxygen in the presence of a catalyst to compound B, which is a volatile liquid. Substance B, reacting with quicklime, forms salt C. Identify the substances, give the reaction equations.
Answer. SO 2 , SO 3 , CaSO 4 .
12. A simple substance A, liquid at room temperature, reacts with a silvery-white light metal B, forming salt C, which, when treated with an alkali solution, gives a white precipitate that dissolves in an excess of alkali. Identify substances, give reaction equations.
Answer. Br 2 , Al, AlBr 3 .
13. A yellow simple solid substance A reacts with a silvery-white light metal B, resulting in the formation of salt C, which is completely hydrolyzed in an aqueous solution with the formation of a white precipitate and a poisonous gas with an unpleasant odor. Identify substances, give reaction equations.
Answer. S, Al, Al 2 S 3 .
14. A simple unstable gaseous substance A turns into another simple substance B, in the atmosphere of which metal C burns; The product of this reaction is an oxide in which the metal is in two oxidation states. Identify substances, give reaction equations.
Answer. O 3 , O 2 , Fe.
15. A dark purple crystalline substance A, when heated, decomposes to form a simple gaseous substance B, in the atmosphere of which a simple substance C burns, forming a colorless, odorless gas, which is part of the air in small quantities. Identify substances, give reaction equations.
Answer. KMnO 4 , O 2 , C.
16. A simple substance A, which is a semiconductor, reacts with a simple gaseous substance B to form compound C, which is insoluble in water. When fused with alkalis, substance C forms compounds called soluble glasses. Identify substances, give reaction equations (one of the possible options).
Answer. Si, O 2 , SiO 2 .
17. Toxic, colorless gas A with an unpleasant odor decomposes when heated into simple substances, one of which B is a yellow solid. When substance B is burned, a colorless gas C with an unpleasant odor is formed, which decolorizes many organic paints. Identify substances, give reaction equations.
Answer. H2S, S, SO2.
18. Volatile hydrogen compound A burns in air to form substance B, which is soluble in hydrofluoric acid. When substance B is fused with sodium oxide, a water-soluble salt C is formed. Identify the substances, give the reaction equations.
Answer. SiH 4 , SiO 2 , Na 2 SiO 3 .
19. Compound A, sparingly soluble in water, of white color, as a result of calcination at high temperature with coal and sand in the absence of oxygen, forms a simple substance B, which exists in several allotropic modifications. When this substance is burned in air, compound C is formed, which dissolves in water to form an acid capable of forming three series of salts. Identify substances, write reaction equations.
Answer. Ca 3 (PO 4) 2, P, P 2 O 5.
* The +/– sign means that this reaction does not proceed with all reagents or under specific conditions.
To be continued
