Chromium(III) compounds. Chromium - a general characteristic of the element, the chemical properties of chromium and its compounds What acids does chromium form
1) Chromium (III) oxide.
Chromium oxide can be obtained:
Thermal decomposition of ammonium dichromate:
(NH 4) 2 C 2 O 7 Cr 2 O 3 + N 2 + 4H 2 O
Reduction of potassium dichromate with carbon (coke) or sulfur:
2K 2 Cr 2 O 7 + 3C 2Cr 2 O 3 + 2K 2 CO 3 + CO 2
K 2 Cr 2 O 7 + S Cr 2 O 3 + K 2 SO 4
Chromium(III) oxide has amphoteric properties.
With acids, chromium (III) oxide forms salts:
Cr 2 O 3 + 6HCl \u003d 2CrCl 3 + 3H 2 O
When chromium (III) oxide is fused with oxides, hydroxides and carbonates of alkali and alkaline earth metals, chromates (III), (chromites) are formed:
Cr 2 O 3 + Ba (OH) 2 Ba (CrO 2) 2 + H 2 O
Cr 2 O 3 + Na 2 CO 3 2NaCrO 2 + CO 2
With alkaline melts of oxidizing agents - chromates (VI) (chromates)
Cr 2 O 3 + 3KNO 3 + 4KOH = 2K 2 CrO 4 + 3KNO 2 + 2H 2 O
Cr 2 O 3 + 3Br 2 + 10NaOH = 2Na 2 CrO 4 + 6NaBr + 5H 2 O
Cr 2 O 3 + O 3 + 4KOH \u003d 2K 2 CrO 4 + 2H 2 O
Cr 2 O 3 + 3O 2 + 4Na 2 CO 3 \u003d 2Na 2 CrO 4 + 4CO 2
Cr 2 O 3 + 3NaNO 3 + 2Na 2 CO 3 2Na 2 CrO 4 + 2CO 2 + 3NaNO 2
Cr 2 O 3 + KClO 3 + 2Na 2 CO 3 = 2Na 2 CrO 4 + KCl + 2CO 2
2) Chromium(III) hydroxide
Chromium (III) hydroxide has amphoteric properties.
2Cr(OH) 3 \u003d Cr 2 O 3 + 3H 2 O
2Cr(OH) 3 + 3Br 2 + 10KOH = 2K 2 CrO 4 + 6KBr + 8H 2 O
3) Salts of chromium (III)
2CrCl 3 + 3Br 2 + 16KOH = 2K 2 CrO 4 + 6KBr + 6KCl + 8H 2 O
2CrCl 3 + 3H 2 O 2 + 10NaOH = 2Na 2 CrO 4 + 6NaCl + 8H 2 O
Cr 2 (SO 4) 3 + 3H 2 O 2 + 10NaOH \u003d 2Na 2 CrO 4 + 3Na 2 SO 4 + 8H 2 O
Cr 2 (SO 4) 3 + 3Br 2 + 16NaOH = 2Na 2 CrO 4 + 6NaBr + 3Na 2 SO 4 + 8H 2 O
Cr 2 (SO 4) 3 + 6KMnO 4 + 16KOH = 2K 2 CrO 4 + 6K 2 MnO 4 + 3K 2 SO 4 + 8H 2 O.
2Na 3 + 3Br 2 + 4NaOH \u003d 2Na 2 CrO 4 + 6NaBr + 8H 2 O
2K 3 + 3Br 2 + 4KOH = 2K 2 CrO 4 + 6KBr + 8H 2 O
2KCrO 2 + 3PbO 2 + 8KOH = 2K 2 CrO 4 + 3K 2 PbO 2 + 4H 2 O
Cr 2 S 3 + 30HNO 3 (conc.) \u003d 2Cr (NO 3) 3 + 3H 2 SO 4 + 24NO 2 + 12H 2 O
2CrCl 3 + Zn = 2CrCl 2 + ZnCl 2
Chromates (III) easily react with acids:
NaCrO 2 + HCl (lack) + H 2 O \u003d Cr (OH) 3 + NaCl
NaCrO 2 + 4HCl (excess) = CrCl 3 + NaCl + 2H 2 O
K 3 + 3CO 2 \u003d Cr (OH) 3 ↓ + 3NaHCO 3
Completely hydrolyzed in solution
NaCrO 2 + 2H 2 O \u003d Cr (OH) 3 ↓ + NaOH
Most chromium salts are highly soluble in water, but are easily hydrolyzed:
Cr 3+ + HOH ↔ CrOH 2+ + H +
CrCl 3 + HOH ↔ CrOHCl 2 + HCl
Salts formed by chromium (III) cations and an anion of a weak or volatile acid are completely hydrolyzed in aqueous solutions:
Cr 2 S 3 + 6H 2 O \u003d 2Cr (OH) 3 ↓ + 3H 2 S
Chromium (VI) compounds
1) Chromium oxide (VI).
Chromium(VI) oxide. Highly poisonous!
Chromium (VI) oxide can be obtained by the action of concentrated sulfuric acid on dry chromates or dichromates:
Na 2 Cr 2 O 7 + 2H 2 SO 4 = 2CrO 3 + 2NaHSO 4 + H 2 O
Acid oxide that interacts with basic oxides, bases, water:
CrO 3 + Li 2 O → Li 2 CrO 4
CrO 3 + 2KOH → K 2 CrO 4 + H 2 O
CrO 3 + H 2 O \u003d H 2 CrO 4
2CrO 3 + H 2 O \u003d H 2 Cr 2 O 7
Chromium (VI) oxide is a strong oxidizing agent: it oxidizes carbon, sulfur, iodine, phosphorus, while turning into chromium (III) oxide
4CrO 3 → 2Cr 2 O 3 + 3O 2 .
4CrO 3 + 3S = 2Cr 2 O 3 + 3SO 2
Salt oxidation:
2CrO 3 + 3K 2 SO 3 + 3H 2 SO 4 \u003d 3K 2 SO 4 + Cr 2 (SO 4) 3 + 3H 2 O
Oxidation of organic compounds:
4CrO 3 + C 2 H 5 OH + 6H 2 SO 4 = 2Cr 2 (SO 4) 2 + 2CO 2 + 9H 2 O
Strong oxidizing agents are salts of chromic acids - chromates and dichromates. The reduction products of which are chromium (III) derivatives.
In a neutral medium, chromium (III) hydroxide is formed:
K 2 Cr 2 O 7 + 3Na 2 SO 3 + 4H 2 O \u003d 2Cr (OH) 3 ↓ + 3Na 2 SO 4 + 2KOH
2K 2 CrO 4 + 3(NH 4) 2 S + 2H 2 O = 2Cr(OH) 3 ↓ + 3S↓ + 6NH 3 + 4KOH
In alkaline - hydroxochromates (III):
2K 2 CrO 4 + 3NH 4 HS + 5H 2 O + 2KOH = 3S + 2K 3 + 3NH 3 H 2 O
2Na 2 CrO 4 + 3SO 2 + 2H 2 O + 8NaOH \u003d 2Na 3 + 3Na 2 SO 4
2Na 2 CrO 4 + 3Na 2 S + 8H 2 O \u003d 3S + 2Na 3 + 4NaOH
In acidic - chromium (III) salts:
3H 2 S + K 2 Cr 2 O 7 + 4H 2 SO 4 = K 2 SO 4 + Cr 2 (SO 4) 3 + 3S + 7H 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 + 3H 2 S + 4H 2 SO 4 = K 2 SO 4 + Cr 2 (SO 4) 3 + 3S + 7H 2 O
8K 2 Cr 2 O 7 + 3Ca 3 P 2 + 64HCl = 3Ca 3 (PO 4) 2 + 16CrCl 3 + 16KCl + 32H 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
K 2 Cr 2 O 7 + 4H 2 SO 4 + 3KNO 2 = Cr 2 (SO 4) 3 + 3KNO 3 + K 2 SO 4 + 4H 2 O
K 2 Cr 2 O 7 + 14HCl = 3Cl 2 + 2CrCl 3 + 7H 2 O + 2KCl
K 2 Cr 2 O 7 + 3SO 2 + 8HCl = 2KCl + 2CrCl 3 + 3H 2 SO 4 + H 2 O
2K 2 CrO 4 + 16HCl = 3Cl 2 + 2CrCl 3 + 8H 2 O + 4KCl
The recovery product in various environments can be represented schematically:
H 2 O Cr(OH) 3 gray-green precipitate
K 2 CrO 4 (CrO 4 2–)
OH - 3 - emerald green solution
K 2 Cr 2 O 7 (Cr 2 O 7 2–) H + Cr 3+ blue-violet solution
Salts of chromic acid - chromates - are yellow, and salts of dichromic acid - dichromates - are orange. By changing the reaction of the solution, it is possible to carry out the mutual transformation of chromates into dichromates:
2K 2 CrO 4 + 2HCl (diff.) = K 2 Cr 2 O 7 + 2KCl + H 2 O
2K 2 CrO 4 + H 2 O + CO 2 \u003d K 2 Cr 2 O 7 + KHCO 3
acidic environment
2СrO 4 2 – + 2H + Cr 2 O 7 2– + H 2 O
alkaline environment
Chromium. Chromium compounds.
1. Chromium (III) sulfide was treated with water, while gas was released and an insoluble substance remained. A solution of caustic soda was added to this substance and chlorine gas was passed through, while the solution acquired a yellow color. The solution was acidified with sulfuric acid, as a result, the color changed to orange; the gas released during the treatment of sulfide with water was passed through the resulting solution, and the color of the solution changed to green. Write the equations of the described reactions.
2. After briefly heating an unknown powdery substance, an orange substance of an orange color, a spontaneous reaction begins, which is accompanied by a change in color to green, the release of gas and sparks. The solid residue was mixed with caustic potash and heated, the resulting substance was introduced into a dilute solution of hydrochloric acid, and a green precipitate formed, which dissolves in excess acid. Write the equations of the described reactions.
3. Two salts color the flame purple. One of them is colorless, and when it is slightly heated with concentrated sulfuric acid, a liquid is distilled off, in which copper dissolves, the last transformation is accompanied by the evolution of brown gas. When the second salt of the sulfuric acid solution is added to the solution, the yellow color of the solution changes to orange, and when the resulting solution is neutralized with alkali, the original color is restored. Write the equations of the described reactions.
4. Trivalent chromium hydroxide treated with hydrochloric acid. Potash was added to the resulting solution, the precipitate was separated and added to a concentrated solution of potassium hydroxide, as a result, the precipitate dissolved. After adding excess hydrochloric acid, a green solution was obtained. Write the equations of the described reactions.
5. When adding dilute hydrochloric acid to a yellow salt solution, which turns the flame purple, the color changes to orange-red. After neutralization of the solution with concentrated alkali, the color of the solution returned to its original color. When barium chloride is added to the resulting mixture, a yellow precipitate is formed. The precipitate was filtered off and silver nitrate solution was added to the filtrate. Write the equations of the described reactions.
6. Soda ash was added to a solution of trivalent chromium sulfate. The precipitate formed was separated, transferred to a sodium hydroxide solution, bromine was added, and heated. After neutralization of the reaction products with sulfuric acid, the solution acquires an orange color, which disappears after passing sulfur dioxide through the solution. Write the equations of the described reactions.
7) Chromium(III) sulfide powder was treated with water. The gray-green precipitate that formed was treated with chlorine water in the presence of potassium hydroxide. A solution of potassium sulfite was added to the resulting yellow solution, and a gray-green precipitate fell out again, which was calcined until the mass was constant. Write the equations of the described reactions.
8) Chromium(III) sulfide powder was dissolved in sulfuric acid. In this case, gas was released and a solution was formed. An excess of ammonia solution was added to the resulting solution, and the gas was passed through a solution of lead nitrate. The resulting black precipitate turned white after treatment with hydrogen peroxide. Write the equations of the described reactions.
9) Ammonium dichromate decomposed on heating. The solid decomposition product was dissolved in sulfuric acid. Sodium hydroxide solution was added to the resulting solution until a precipitate formed. Upon further addition of sodium hydroxide to the precipitate, it dissolved. Write the equations of the described reactions.
10) Chromium(VI) oxide reacted with potassium hydroxide. The resulting substance was treated with sulfuric acid, an orange salt was isolated from the resulting solution. This salt was treated with hydrobromic acid. The resulting simple substance reacted with hydrogen sulfide. Write the equations of the described reactions.
11. Chrome burned in chlorine. The resulting salt reacted with a solution containing hydrogen peroxide and sodium hydroxide. An excess of sulfuric acid was added to the resulting yellow solution, the color of the solution changed to orange. When copper(I) oxide reacted with this solution, the color of the solution turned blue-green. Write the equations of the described reactions.
12. Sodium nitrate was fused with chromium (III) oxide in the presence of sodium carbonate. the resulting gas reacted with an excess of barium hydroxide solution to form a white precipitate. The precipitate was dissolved in an excess of hydrochloric acid solution, and silver nitrate was added to the resulting solution until the precipitation ceased. Write the equations of the described reactions.
13. Potassium was fused with sulfur. The resulting salt was treated with hydrochloric acid. the gas released in this process was passed through a solution of potassium bichromate in sulfuric acid. the precipitated yellow substance was filtered off and fused with aluminum. Write the equations of the described reactions.
14. Chrome burned in an atmosphere of chlorine. Potassium hydroxide was added dropwise to the resulting salt until the precipitation ceased. The resulting precipitate was oxidized with hydrogen peroxide in caustic potassium and evaporated. An excess of a hot solution of concentrated hydrochloric acid was added to the resulting solid residue. Write the equations of the described reactions.
Chromium. Chromium compounds.
1) Cr 2 S 3 + 6H 2 O \u003d 2Cr (OH) 3 ↓ + 3H 2 S
2Cr(OH) 3 + 3Cl 2 + 10NaOH = 2Na 2 CrO 4 + 6NaCl + 8H 2 O
Na 2 Cr 2 O 7 + 4H 2 SO 4 + 3H 2 S = Cr 2 (SO 4) 3 + Na 2 SO 4 + 3S↓ + 7H 2 O
2) (NH 4) 2 Cr 2 O 7 Cr 2 O 3 + N 2 + 4H 2 O
Cr 2 O 3 + 2KOH 2KCrO 2 + H 2 O
KCrO 2 + H 2 O + HCl \u003d KCl + Cr (OH) 3 ↓
Cr(OH) 3 + 3HCl = CrCl 3 + 3H 2 O
3) KNO 3 (solid) + H 2 SO 4 (conc.) HNO 3 + KHSO 4
4HNO 3 + Cu \u003d Cu (NO 3) 2 + 2NO 2 + 2H 2 O
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 \u003d 2K 2 CrO 4 + H 2 O
4) Cr(OH) 3 + 3HCl = CrCl 3 + 3H 2 O
2CrCl 3 + 3K 2 CO 3 + 3H 2 O \u003d 2Cr (OH) 3 ↓ + 3CO 2 + 6KCl
Cr(OH) 3 + 3KOH = K 3
K 3 + 6HCl \u003d CrCl 3 + 3KCl + 6H 2 O
5) 2K 2 CrO 4 + 2HCl = K 2 Cr 2 O 7 + 2KCl + H 2 O
K 2 Cr 2 O 7 + 2KOH \u003d 2K 2 CrO 4 + H 2 O
K 2 CrO 4 + BaCl 2 = BaCrO 4 ↓ + 2 KCl
KCl + AgNO 3 = AgCl↓ + KNO 3
6) Cr 2 (SO 4) 3 + 3Na 2 CO 3 + 6H 2 O \u003d 2Cr (OH) 3 ↓ + 3CO 2 + 3K 2 SO 4
2Cr(OH) 3 + 3Br 2 + 10NaOH = 2Na 2 CrO 4 + 6NaBr + 8H 2 O
2Na 2 CrO 4 + H 2 SO 4 = Na 2 Cr 2 O 7 + Na 2 SO 4 + H 2 O
Na 2 Cr 2 O 7 + H 2 SO 4 + 3SO 2 = Cr 2 (SO 4) 3 + Na 2 SO 4 + H 2 O
7) Cr 2 S 3 + 6H 2 O \u003d 2Cr (OH) 3 ↓ + 3H 2 S
2Cr(OH) 3 + 3Cl 2 + 10KOH = 2K 2 CrO 4 + 6KCl + 8H 2 O
2K 2 CrO 4 + 3K 2 SO 3 + 5H 2 O = 2Cr(OH) 2 + 3K 2 SO 4 + 4KOH
2Cr(OH)3Cr2O3 + 3H2O
8) Cr 2 S 3 + 3H 2 SO 4 = Cr 2 (SO 4) 3 + 3H 2 S
Cr 2 (SO 4) 3 + 6NH 3 + 6H 2 O \u003d 2Cr (OH) 3 ↓ + 3 (NH 4) 2 SO 4
H 2 S + Pb (NO 3) 2 \u003d PbS + 2HNO 3
PbS + 4H 2 O 2 \u003d PbSO 4 + 4H 2 O
9) (NH 4) 2 Cr 2 O 7 Cr 2 O 3 + N 2 + 4H 2 O
Cr 2 O 3 + 3H 2 SO 4 \u003d Cr 2 (SO 4) 3 + 3H 2 O
Cr 2 (SO 4) 3 + 6NaOH \u003d 2Cr (OH) 3 ↓ + 3Na 2 SO 4
Cr(OH) 3 + 3NaOH = Na 3
10) CrO 3 + 2KOH = K 2 CrO 4 + H 2 O
2K 2 CrO 4 + H 2 SO 4 (diff.) \u003d K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O
K 2 Cr 2 O 7 + 14HBr = 3Br 2 + 2CrBr 3 + 7H 2 O + 2KBr
Br 2 + H 2 S \u003d S + 2HBr
11) 2Cr + 3Cl 2 = 2CrCl 3
2CrCl 3 + 10NaOH + 3H 2 O 2 = 2Na 2 CrO 4 + 6NaCl + 8H 2 O
2Na 2 CrO 4 + H 2 SO 4 = Na 2 Cr 2 O 7 + Na 2 SO 4 + H 2 O
Na 2 Cr 2 O 7 + 3Cu 2 O + 10H 2 SO 4 = 6CuSO 4 + Cr 2 (SO 4) 3 + Na 2 SO 4 + 10H 2 O
12) 3NaNO 3 + Cr 2 O 3 + 2Na 2 CO 3 = 2Na 2 CrO 4 + 3NaNO 2 + 2CO 2
CO 2 + Ba(OH) 2 = BaCO 3 ↓ + H 2 O
BaCO 3 + 2HCl \u003d BaCl 2 + CO 2 + H 2 O
BaCl 2 + 2AgNO 3 \u003d 2AgCl ↓ + Ba (NO 3) 2
13) 2K + S = K 2 S
K 2 S + 2HCl \u003d 2KCl + H 2 S
3H 2 S + K 2 Cr 2 O 7 + 4H 2 SO 4 = 3S + Cr 2 (SO 4) 3 + K 2 SO 4 + 7H 2 O
3S + 2Al \u003d Al 2 S 3
14) 2Cr + 3Cl 2 = 2CrCl 3
CrCl 3 + 3KOH \u003d 3KCl + Cr (OH) 3 ↓
2Cr(OH) 3 + 3H 2 O 2 + 4KOH = 2K 2 CrO 4 + 8H 2 O
2K 2 CrO 4 + 16HCl = 2CrCl 3 + 4KCl + 3Cl 2 + 8H 2 O
Nonmetals.
IV A group (carbon, silicon).
Carbon. Carbon compounds.
I. Carbon.
Carbon can exhibit both reducing and oxidizing properties. Carbon exhibits reducing properties with simple substances formed by non-metals with a higher electronegativity value compared to it (halogens, oxygen, sulfur, nitrogen), as well as with metal oxides, water and other oxidizing agents.
When heated with excess air, graphite burns to form carbon monoxide (IV):
With a lack of oxygen, you can get CO
Amorphous carbon already at room temperature reacts with fluorine.
C + 2F 2 = CF 4
When heated with chlorine:
C + 2Cl 2 = CCl 4
With stronger heating, carbon reacts with sulfur, silicon:
Under the action of an electric discharge, carbon combines with nitrogen, forming diacin:
2C + N 2 → N ≡ C - C ≡ N
In the presence of a catalyst (nickel) and when heated, carbon reacts with hydrogen:
C + 2H 2 = CH 4
With water, hot coke forms a mixture of gases:
C + H 2 O \u003d CO + H 2
The reducing properties of carbon are used in pyrometallurgy:
C + CuO = Cu + CO
When heated with oxides of active metals, carbon forms carbides:
3C + CaO \u003d CaC 2 + CO
9С + 2Al 2 O 3 \u003d Al 4 C 3 + 6CO
2C + Na 2 SO 4 \u003d Na 2 S + CO 2
2C + Na 2 CO 3 \u003d 2Na + 3CO
Carbon is oxidized by such strong oxidizing agents as concentrated sulfuric and nitric acids, other oxidizing agents:
C + 4HNO 3 (conc.) = CO 2 + 4NO 2 + 2H 2 O
C + 2H 2 SO 4 (conc.) \u003d 2SO 2 + CO 2 + 2H 2 O
3C + 8H 2 SO 4 + 2K 2 Cr 2 O 7 \u003d 2Cr 2 (SO 4) 3 + 2K 2 SO 4 + 3CO 2 + 8H 2 O
In reactions with active metals, carbon exhibits the properties of an oxidizing agent. In this case, carbides are formed:
4C + 3Al \u003d Al 4 C 3
Carbides undergo hydrolysis, forming hydrocarbons:
Al 4 C 3 + 12H 2 O \u003d 4Al (OH) 3 + 3CH 4
CaC 2 + 2H 2 O \u003d Ca (OH) 2 + C 2 H 2
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
"National Research Tomsk Polytechnic University"
Institute of Natural Resources Geoecology and Geochemistry
Chromium
By discipline:
Chemistry
Completed:
student of group 2G41 Tkacheva Anastasia Vladimirovna 10/29/2014
Checked:
teacher Stas Nikolay Fedorovich
Position in the periodic system
Chromium- an element of a side subgroup of the 6th group of the 4th period of the periodic system of chemical elements of D. I. Mendeleev with atomic number 24. It is indicated by the symbol Cr(lat. Chromium). simple substance chromium- hard bluish-white metal. Chromium is sometimes referred to as a ferrous metal.
The structure of the atom
17 Cl) 2) 8) 7 - diagram of the structure of the atom
1s2s2p3s3p - electronic formula
The atom is located in period III, and has three energy levels
The atom is located in VII in the group, in the main subgroup - at the external energy level of 7 electrons
Element Properties
Physical Properties
Chromium is a white shiny metal with a cubic body-centered lattice, a = 0.28845 nm, characterized by hardness and brittleness, with a density of 7.2 g / cm 3, one of the hardest pure metals (second only to beryllium, tungsten and uranium), with a melting point of 1903 degrees. And with a boiling point of about 2570 degrees. C. In air, the surface of chromium is covered with an oxide film, which protects it from further oxidation. The addition of carbon to chromium further increases its hardness.
Chemical properties
Chromium under normal conditions is an inert metal, when heated it becomes quite active.
Interaction with non-metals
When heated above 600°C, chromium burns in oxygen:
4Cr + 3O 2 \u003d 2Cr 2 O 3.
It reacts with fluorine at 350°C, with chlorine at 300°C, with bromine at a red heat temperature, forming chromium (III) halides:
2Cr + 3Cl 2 = 2CrCl 3 .
It reacts with nitrogen at temperatures above 1000°C to form nitrides:
2Cr + N 2 = 2CrN
or 4Cr + N 2 = 2Cr 2 N.
2Cr + 3S = Cr 2 S 3 .
Reacts with boron, carbon and silicon to form borides, carbides and silicides:
Cr + 2B = CrB 2 (the formation of Cr 2 B, CrB, Cr 3 B 4, CrB 4 is possible),
2Cr + 3C \u003d Cr 2 C 3 (the formation of Cr 23 C 6, Cr 7 B 3 is possible),
Cr + 2Si = CrSi 2 (possible formation of Cr 3 Si, Cr 5 Si 3, CrSi).
It does not interact directly with hydrogen.
Interaction with water
In a finely ground hot state, chromium reacts with water, forming chromium (III) oxide and hydrogen:
2Cr + 3H 2 O \u003d Cr 2 O 3 + 3H 2
Interaction with acids
In the electrochemical series of voltages of metals, chromium is before hydrogen, it displaces hydrogen from solutions of non-oxidizing acids:
Cr + 2HCl \u003d CrCl 2 + H 2;
Cr + H 2 SO 4 \u003d CrSO 4 + H 2.
In the presence of atmospheric oxygen, chromium (III) salts are formed:
4Cr + 12HCl + 3O 2 = 4CrCl 3 + 6H 2 O.
Concentrated nitric and sulfuric acids passivate chromium. Chromium can dissolve in them only with strong heating, chromium (III) salts and acid reduction products are formed:
2Cr + 6H 2 SO 4 = Cr 2 (SO 4) 3 + 3SO 2 + 6H 2 O;
Cr + 6HNO 3 \u003d Cr (NO 3) 3 + 3NO 2 + 3H 2 O.
Interaction with alkaline reagents
In aqueous solutions of alkalis, chromium does not dissolve; it slowly reacts with alkali melts to form chromites and release hydrogen:
2Cr + 6KOH \u003d 2KCrO 2 + 2K 2 O + 3H 2.
Reacts with alkaline melts of oxidizing agents, such as potassium chlorate, while chromium passes into potassium chromate:
Cr + KClO 3 + 2KOH = K 2 CrO 4 + KCl + H 2 O.
Recovery of metals from oxides and salts
Chromium is an active metal, capable of displacing metals from solutions of their salts: 2Cr + 3CuCl 2 = 2CrCl 3 + 3Cu.
Properties of a simple substance
Stable in air due to passivation. For the same reason, it does not react with sulfuric and nitric acids. At 2000 °C, it burns out with the formation of green chromium (III) oxide Cr 2 O 3, which has amphoteric properties.
Synthesized compounds of chromium with boron (borides Cr 2 B, CrB, Cr 3 B 4, CrB 2, CrB 4 and Cr 5 B 3), with carbon (carbides Cr 23 C 6, Cr 7 C 3 and Cr 3 C 2), with silicon (silicides Cr 3 Si, Cr 5 Si 3 and CrSi) and nitrogen (nitrides CrN and Cr 2 N).
Cr(+2) compounds
The oxidation state +2 corresponds to the basic oxide CrO (black). Cr 2+ salts (blue solutions) are obtained by reducing Cr 3+ salts or dichromates with zinc in an acidic environment (“hydrogen at the time of isolation”):
All these Cr 2+ salts are strong reducing agents, to the extent that they displace hydrogen from water upon standing. Oxygen in the air, especially in an acidic environment, oxidizes Cr 2+, as a result of which the blue solution quickly turns green.
Brown or yellow Cr(OH) 2 hydroxide precipitates when alkalis are added to solutions of chromium(II) salts.
Chromium dihalides CrF 2 , CrCl 2 , CrBr 2 and CrI 2 were synthesized
Cr(+3) compounds
The +3 oxidation state corresponds to the amphoteric oxide Cr 2 O 3 and the hydroxide Cr (OH) 3 (both green). This is the most stable oxidation state of chromium. Chromium compounds in this oxidation state have a color from dirty purple (ion 3+) to green (anions are present in the coordination sphere).
Cr 3+ is prone to the formation of double sulfates of the form M I Cr (SO 4) 2 12H 2 O (alum)
Chromium (III) hydroxide is obtained by acting with ammonia on solutions of chromium (III) salts:
Cr+3NH+3H2O→Cr(OH)↓+3NH
Alkali solutions can be used, but in their excess a soluble hydroxo complex is formed:
Cr+3OH→Cr(OH)↓
Cr(OH)+3OH→
By fusing Cr 2 O 3 with alkalis, chromites are obtained:
Cr2O3+2NaOH→2NaCrO2+H2O
Uncalcined chromium (III) oxide dissolves in alkaline solutions and in acids:
Cr2O3+6HCl→2CrCl3+3H2O
When chromium(III) compounds are oxidized in an alkaline medium, chromium(VI) compounds are formed:
2Na+3HO→2NaCrO+2NaOH+8HO
The same thing happens when chromium (III) oxide is fused with alkali and oxidizing agents, or with alkali in air (the melt becomes yellow in this case):
2Cr2O3+8NaOH+3O2→4Na2CrO4+4H2O
Chromium compounds (+4)[
With careful decomposition of chromium oxide (VI) CrO 3 under hydrothermal conditions, chromium oxide (IV) CrO 2 is obtained, which is ferromagnetic and has metallic conductivity.
Among chromium tetrahalides, CrF 4 is stable, chromium tetrachloride CrCl 4 exists only in vapor.
Chromium compounds (+6)
The +6 oxidation state corresponds to acidic chromium oxide (VI) CrO 3 and a number of acids between which there is an equilibrium. The simplest of them are chromic H 2 CrO 4 and two-chrome H 2 Cr 2 O 7. They form two series of salts: yellow chromates and orange dichromates, respectively.
Chromium oxide (VI) CrO 3 is formed by the interaction of concentrated sulfuric acid with solutions of dichromates. A typical acid oxide, when interacting with water, it forms strong unstable chromic acids: chromic H 2 CrO 4, dichromic H 2 Cr 2 O 7 and other isopoly acids with the general formula H 2 Cr n O 3n+1. An increase in the degree of polymerization occurs with a decrease in pH, that is, an increase in acidity:
2CrO+2H→Cr2O+H2O
But if an alkali solution is added to an orange solution of K 2 Cr 2 O 7, how does the color turn yellow again, since chromate K 2 CrO 4 is formed again:
Cr2O+2OH→2CrO+HO
It does not reach a high degree of polymerization, as occurs in tungsten and molybdenum, since polychromic acid decomposes into chromium (VI) oxide and water:
H2CrnO3n+1→H2O+nCrO3
The solubility of chromates roughly corresponds to the solubility of sulfates. In particular, yellow barium chromate BaCrO 4 precipitates when barium salts are added to both chromate and dichromate solutions:
Ba+CrO→BaCrO↓
2Ba+CrO+H2O→2BaCrO↓+2H
The formation of a blood-red, poorly soluble silver chromate is used to detect silver in alloys using assay acid.
Chromium pentafluoride CrF 5 and unstable chromium hexafluoride CrF 6 are known. Volatile chromium oxyhalides CrO 2 F 2 and CrO 2 Cl 2 (chromyl chloride) have also been obtained.
Chromium(VI) compounds are strong oxidizing agents, for example:
K2Cr2O7+14HCl→2CrCl3+2KCl+3Cl2+7H2O
The addition of hydrogen peroxide, sulfuric acid, and an organic solvent (ether) to dichromates leads to the formation of blue chromium peroxide CrO 5 L (L is a solvent molecule), which is extracted into the organic layer; this reaction is used as an analytical one.
Chromium (II) oxide CrO- pyrophoric black powder (pyrophoric - the ability to ignite in air in a finely divided state). It is obtained by oxidation of chromium amalgam with atmospheric oxygen. Soluble in dilute hydrochloric acid:
In air, when heated above 100 ° C, chromium (II) oxide turns into chromium (III) oxide.
Salts of chromium (II). In terms of their chemical properties, Cr 2+ salts are similar to Fe 2+ salts. By treating their solutions with alkalis in the absence of oxygen, a yellow precipitate can be obtained. chromium (II) hydroxide:
which has typical basic properties. Is a restorer. When Cr(OH) 2 is calcined in the absence of oxygen, chromium oxide (II) CrO is formed. When calcined in air, it turns into Cr 2 O 3.
All chromium (II) compounds are rather unstable and are easily oxidized by atmospheric oxygen to chromium (III) compounds:
Salts of chromium (III). Trivalent chromium salts are similar to aluminum salts in composition, crystal lattice structure and solubility. In aqueous solutions, the Cr 3+ cation occurs only as a hydrated ion [Cr(H 2 O) 6 ] 3+ , which imparts a violet color to the solution (for simplicity, write Cr 3+).
Under the action of alkalis on chromium (III) salts, a gelatinous precipitate chromium hydroxide (III) - Cr (OH) 3 Green colour:
Chromium(III) hydroxide has amphoteric properties, dissolving as in acids with the formation of chromium (III) salts:
and in alkalis with the formation of tetrahydroxychromites, i.e. salts in which Cr 3+ is part of the anion:
As a result of calcination of Cr (OH) 3, one can obtain chromium oxide (III) Cr 2 O 3 :
Chromium oxide (III) Cr 2 O 3- refractory green powder. It is close to corundum in hardness, therefore it is introduced into the composition of polishing agents. It is obtained by combining elements at high temperature.
Cr 2 O 3 is a green crystal, practically insoluble in water. Cr 2 O 3 can also be obtained by calcining potassium and ammonium dichromates:
When Cr 2 O 3 is fused with alkalis, soda and acid salts, Cr 3+ compounds are obtained that are soluble in water:
Chromium(VI) oxide- acid oxide, anhydride chromic H 2 CrO 4 and dichromic H 2 Cr 2 O 7 acids.
It is obtained by reacting concentrated sulfuric acid with a saturated solution of sodium or potassium dichromate:
CrO 3 is acidic in nature: it readily dissolves in water, forming chromic acids. With excess water, it forms chromic acid H 2 CrO 4:
At a high concentration of CrO 3, dichromic acid H 2 Cr 2 O 7 is formed:
which, when diluted, becomes chromic acid:
Chromic acids exist only in aqueous solution. However, their salts are quite stable.
CrO 3 is a bright red crystal, easily soluble in water. Strong oxidizing agent: oxidizes iodine, sulfur, phosphorus, coal, while turning into Cr 2 O 3 . For example:
When heated to 250 ° C, it decomposes:
It reacts with alkalis to form yellow chromates CrO 4 2-:
In an acidic environment, the CrO 4 2- ion turns into the Cr 2 O 7 2- ion.
In an alkaline environment, this reaction proceeds in the opposite direction:
IN acidic environment dichromate ion is reduced to Cr 3+:
If we compare chromium hydroxides with different degrees of oxidation
Cr 2+ (OH) 2, Cr 3+ (OH) 3 and H 2 Cr 6+ O 4, it is easy to conclude that with an increase in the degree of oxidation, the basic properties of hydroxides weaken, and the acid ones increase.
Cr(OH) 2 exhibits basic properties, Cr(OH) 3 is amphoteric, and H 2 CrO 4 is acidic.
Chromates and dichromates (VI). The most important chromium compounds in the highest oxidation state 6+ are potassium chromate (VI) K 2 CrO 4 and potassium dichromate (VI) K 2 Cr 2 O 7 .
Chromic acids form two series of salts: chromates, the so-called salts of chromic acid, and dichromates, the so-called salts of dichromic acid. Chromates are colored yellow (the color of the chromate ion CrO 4 2-), dichromates are colored orange (the color of the dichromate ion Cr 2 O 7 2-).
Dichromates Na 2 Cr 2 O 7 × 2H 2 O and K 2 Cr 2 O 7 are called chromic peaks. They are used as oxidizing agents in the leather (tanning), paint and varnish, match and textile industries. Chromium mixture - the so-called 3% solution of potassium dichromate in concentrated sulfuric acid - is used in chemical laboratories for washing glassware.
Salts of chromic acids in an acidic environment are strong oxidizing agents:
Compounds of chromium (III) in an alkaline environment play the role of reducing agents. Under the action of various oxidizing agents - Cl 2, Br 2, H 2 O 2, KmnO 4, etc. - they turn into chromium (IV) compounds - chromates:
Here, the Cr (III) compound is shown in the form of Na, since it exists in the form of Na + and - ions in an excess of alkali solution.
Strong oxidizing agents, such as KMnO 4, (NH 4) 2 S 2 O 8 in an acidic environment, convert Cr (III) compounds into dichromates:
Thus, the oxidizing properties are consistently enhanced with a change in the oxidation states in the series: Cr 2+ ® Cr 3+ ® Cr 6+ . Cr (II) compounds are strong reducing agents, they are easily oxidized, turning into chromium compounds. (III). Chromium (VI) compounds are strong oxidizers, easily reduced to chromium (III) compounds. Compounds with an intermediate oxidation state, i.e., chromium (III) compounds, can, when interacting with strong reducing agents, exhibit oxidizing properties, turning into chromium (II) compounds, and when interacting with strong oxidizing agents (for example, bromine, KMnO 4) exhibit reducing properties, turning into chromium (VI) compounds.
Salts of chromium (III) are very diverse in color: purple, blue, green, brown, orange, red and black. All chromic acids and their salts, as well as chromium (VI) oxide, are poisonous: they affect the skin, respiratory tract, cause inflammation of the eyes, therefore, when working with them, all precautions must be observed.