Preparation of chromium oxide 2. Chromium oxides (II), (III) and (VI)

Chrome forms three oxide: CRO, CR 2 O 3, CRO 3.

Chromium oxide (II) CRO - pyrophoric black powder. It has basic properties.

In oxidative reaction, behaves like a reducing agent:

CRO is obtained by decomposition in the vacuum of chromium CR (CO) 6 at 300 ° C.

Chromium oxide (III) CR 2 O 3 - refractory green powder. The hardness is close to Corundum, therefore it is introduced into the composition of polishing agents. It is formed when CR and O 2 interact at high temperature. In the laboratory of chromium oxide (III), it is possible to obtain ammonium dichromate:

(N -3 H 4) 2 CR +6 2 O 7 \u003d Cr +3 2 O 3 + N 0 2 + 4N 2

Chromium (III) oxide has amphoteric properties. In the interaction with acids, chromium salts (III) are formed: CR 2 O 3 + 3H 2 SO 4 \u003d CR 2 (SO 4) 3 + 3N 2

When interacting with alkalis in the melt, chromium compounds (III) are formed - chromites (in the absence of oxygen): CR 2 O 3 + 2NAOH \u003d 2NACRO 2 + N 2

In water, chromium (III) oxide is insoluble.

In oxidative and recovery reactions, chromium oxide (III) behaves like a reducing agent:

Chromium oxide (VI) CRO 3 - chrome anhydride, is dark red needle crystals. When heated about 200 ° C decomposes:

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

It is easily dissolved in water, having an acidic character forms chromium acids. With excess water, chromic acid H 2 CRO 4 is formed:

CRO 3 + H 2 O \u003d H 2 CRO 4

At high concentration of CRO 3, dichromic acid H 2 CR 2 O 7 is formed:

2Cro 3 + H 2 O \u003d H 2 CR 2 O 7

which when diluted goes into chromium acid:

H 2 CR 2 O 7 + H 2 O \u003d 2N 2 CRO 4

Chromic acids exist only in aqueous solution, none of these acids in the free state is not highlighted. However, their salts are very sustainable.

Chromium oxide (VI) is a strong oxidizing agent:

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

Oxidizes iodine, sulfur, phosphorus, coal, turning into CR 2 O 3. CRO 3 is obtained by an excess of concentrated sulfuric acid to a saturated aqueous sodium dichromate solution: Na 2 Cr 2 O 7 + 2H 2 SO 4 \u003d 2Cro 3 + 2NAHSO 4 + H 2 O should be noted the strong toxicity of chromium oxide (VI).

Several chemical compounds consisting of two simple elements - CR and O, refer to the class of inorganic compounds - oxides. Their common name is chromium oxide, then in brackets, the Roman numerals are taken to indicate the valence of metal. Other names and chemical formulas:

• chrome (II) oxide - Zrom, CRO;
• chromium (III) oxide - chrome greens, chromium sesquioxide, CR2O3;
• chrome (IV) oxide - chromium oxide, CRO2;
• chrome (VI) oxide - chrome anhydride, trioxide chromium, CRO3.

A compound in which metal is hexavalenten is the highest chromium oxide. This solid is odorless, in appearance is (in the air they are broken due to severe hygroscopicity). Molar weight - 99.99 g / mol. The density at 20 ° C is 2.70 g / cm³. Melting point - 197 ° C, boiling - 251 ° C. At 0 ° C, 61.7 g / 100 dissolves in water, at 25 ° C - 63 g / 100 ml, at 100 ° C - 67.45 g / 100 ml. The oxide is also dissolved in sulfuric acid (this is a chromium mixture, which is used in laboratory practice for washing chemical dishes) and ethyl alcohol, ethyl ester, acetic acid, acetone. At 450 ° C decomposes to CR2O3.

Chromium (VI) oxide is used in the electrolysis process (for obtaining pure chromium), to chromate galvanized products, in electrolytic chromium, as a strong oxidizing agent (for the production of indigo and yestin). Chrome is used to detect alcohol in exhaled air. The interaction occurs according to the scheme: 4CRO3 + 6H2SO4 + 3C2H5OH → 2CR2 (SO4) 3 + 3Ch3COOH + 9H2O. The presence of alcohol indicates the change in the color of the solution (acquires green).

Chromium (VI) oxide, as well as all the joints of the hexavalent CR, is a strong poison (lethal dose - 0.1 g). Because of its high activity, CRO3 causes fire (with explosions) when contacting them. Despite the small volatility, the highest chromium oxide is dangerous when inhalation, as it causes lung cancer. When contacting the skin (even with imminently removal) causes irritation, dermatitis, eczema, provokes the development of cancer.

He oxidized with four-dimensional chrome CRO2 in appearance is a solid in the form of ferrous tetrahedral ferromagnetic crystals. Chromium 4 oxide has a molar mass of 83.9949 g / mol, a density of 4.89 g / cm³. The substance melts, while simultaneously decomposing, at a temperature of 375 ° C. It does not dissolve in water. Used in magnetic recording media as a working substance. With the increase in the popularity of CDs and DVDs, the use of chromium (IV) oxide decreased. It was first synthesized in 1956 by a chemist from Ei DuPont Norman L. Koksya by decomposing chromium trioxide in the presence of water at a temperature of 640 ° C and a 200 MPa pressure. Dupont licenses are manufactured by Sony companies in Japan and BASF in Germany.

Chromium oxide 3 CR2O3 is a solid fine-crystalline substance from light to dark green. The molar mass is 151.99 g / mol. Density - 5.22 g / cm³. Melting point - 2435 ° C, boiling - 4000 ° C. The refractive index of pure substance is 2.551. This oxide does not dissolve in water, in alcohol, acetone, acid. Since its density is approaching the density of the corundum, it is injected into the composition of polishing agents (for example, GOI pastes). This is one of the chromium, which is used as a pigment. For the first time on the secret technology, it was obtained in 1838 in the form of a transparent hydrated form. In nature, it is found in the form of a chromium Zheleznyka FEO.cr2O3.

Oxidized bivalent chromium - a black or red solid with a melting point of 1550 ° C. Melts with decomposition. Molar weight - 67.996 g / mol. The chromium (II) oxide is not a pyrofoor, and the same substance of the black color is pyrophoric. The powder is self-propagated in air, so it can be stored only under the layer in the water, since it does not interact with it. Black rising chromium get in pure form is very difficult.

For chromium oxides with lower valence, the main properties are characteristic, and for oxide with higher valence - acidic.

Chromium - element of the side subgroup of the 6th group of the 4th period of the periodic system of chemical elements D. I. Mendeleev, with atomic number 24. It is indicated by the symbol of CR (Lat. Chromium). A simple substance chrome-solid metal bluish-white color.

Chemical properties chromium

Under normal conditions, the chromium reacts only with fluorine. At high temperatures (above 600 ° C) interacts with oxygen, halogens, nitrogen, silicon, boron, gray, 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 split state, reacts with water vapor:

2CR + 3H 2 O → CR 2 O 3 + 3H 2

Chrome dissolves in dilute strong acids (HCl, H 2 SO 4)

In the absence of air, CR 2+ salts are formed, and in air - CR 3+ salts.

CR + 2HCL → CRCL 2 + H 2

2CR + 6HCl + O 2 → 2CrCl 3 + 2H 2 O + H 2

The presence of a protective oxide film on the surface of the metal explains its passive in relation to concentrated solutions of acids - oxidizing agents.

Chromium compounds

Chromium oxide (II) and chromium hydroxide (II) are of the main character.

CR (OH) 2 + 2HCl → CRCL 2 + 2H 2 O

Chromium (II) compounds - strong reducing agents; Transfer to the compounds of chromium (III) under the action of air 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 - green powder insoluble in water. It can be obtained by calcining chromium hydroxide (III) or dichromates of potassium and ammonium:

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 ("Vulcan" reaction)

Amphoteric oxide. When fusing CR 2 O 3 with alkalis, soda and acidic salts, chromium compounds with oxidation degree (+3) are obtained:

CR 2 O 3 + 2NAOH → 2NACRO 2 + H 2 O

CR 2 O 3 + Na 2 CO 3 → 2NACRO 2 + CO 2

When fusing with a chicken mixture and the oxidant, chromium compounds are obtained into the degree of oxidation (+6):

CR 2 O 3 + 4KOH + KCLO 3 → 2K 2 CRO 4 + KCL + 2H 2 O

Chromium hydroxide (III) with r. (He) 3. Amphoteric hydroxide. Gray-green, decomposes when heated, losing water and forming green metagidroxide Oh (he). Not dissolved in water. The solution is deposited in the form of a gray-blue and bluish-green hydrate. Reacts with acids and alkalis, does not interact with ammonia hydrate.

It has amphoteric properties - dissolves both in acids and alkalis:

2CR (OH) 3 + 3H 2 SO 4 → CR 2 (SO 4) 3 + 6H 2 O CR (OH) 3 + zn + \u003d CR 3+ + 3H 2 O

CR (OH) 3 + KOH → k, CR (OH) 3 + zones - (conc.) \u003d [CR (OH) 6] 3-

CR (OH) 3 + KOH → KCRO 2 + 2H 2 O CR (OH) 3 + MONC \u003d MSRO 2 (green) + 2n 2 O (300-400 ° C, m \u003d Li, Na)

CR (OH) 3 →(120 O. C. – H. 2 O.) Sro (he) → (430-1000 0 C -H. 2 O.) CR 2 O 3

2CR (OH) 3 + 4None (conc.) + Zn 2 O 2 (conc.) \u003d 2NA 2 sro 4 + 8n 2 0

Obtaining: precipitation of ammonia hydrate from chromium salts solution (W):

CR 3+ + 3 (NH 3 H 2 O) \u003d FROMr.(OH) 3 ↓ + ZNN 4+

CR 2 (SO 4) 3 + 6NAOH → 2CR (OH) 3 ↓ + 3NA 2 SO 4 (in excess alkali - precipitate dissolves)

Chromium (III) salts have a purple or dark green color. By chemical properties resemble colorless salts of aluminum.

CR (III) compounds can also be oxidative, and rehabilitation 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

Compounds of hexavalent chromium

Chromium oxide (VI) Cro 3 - bright red crystals soluble in water.

Obtain from chromate (or dichromate) potassium and H 2 SO 4 (conc.).

K 2 CRO 4 + H 2 SO 4 → CRO 3 + K 2 SO 4 + H 2 O

K 2 CR 2 O 7 + H 2 SO 4 → 2CRO 3 + K 2 SO 4 + H 2 O

CRO 3 - acidic oxide, with alkalis forms the yellow chromates CRO 4 2-:

CRO 3 + 2KOH → K 2 CRO 4 + H 2 O

In an acidic medium, chromat 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 medium, this reaction proceeds in the opposite direction:

K 2 Cr 2 O 7 + 2KOH → 2K 2 CRO 4 + H 2 O

Dichromat potassium - oxidizing agent:

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 \u003d CR 2 (SO 4) 3 + 3Nano 3 + K 2 SO 4 + 4H 2 O

K 2 Cr 2 O 7 + 7H 2 SO 4 + 6ki \u003d Cr 2 (SO 4) 3 + 3i 2 + 4k 2 SO 4 + 7H 2 O

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

Chromate potassium to 2 CR O 4. . Oxosol. Yellow, non-hygroscopic. Melts without decomposition, thermally stable. Well soluble in water ( yellow The color of the solution corresponds to the ion of SRO 4 2-), slightly hydrolyzed by anion. In the acidic medium proceeds to 2 Cr 2 O 7. Oxidizer (weaker than KR 2 O 7). Enters the reaction of ion exchange.

Quality reaction On Ion CRO 4 2- - the fallout of the yellow precipitate of the barium chromate, decomposing in a strong-acid medium. It is used as a rubbish with the collapse of tissues, a tubeer of leather, a selective oxidizing agent, a reagent in analytical chemistry.

Equations of the most important reactions:

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

2K 2 CRO 4 (T) + 16HCl (Con., Mountains.) \u003d 2CrCl 3 + 3Cl 2 + 8H 2 O + 4KCL

2K 2 CRO 4 + 2H 2 O + 3H 2 S \u003d 2CR (OH) 3 ↓ + 3S ↓ + 4KOH

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

2K 2 CRO 4 + 2AGNO 3 \u003d KNO 3 + AG 2 CRO 4 (Krasn.) ↓

Quality reaction:

K 2 СГO 4 + youl 2 \u003d 2xl + vascro 4 ↓

2VASRO 4 (T) + 2NSL (RSC) \u003d VOR 2 O 7 (P) + You1 2 + H 2 O

Obtaining: sintering chromite with potash in air:

4 (CR 2 Fe ‖‖) O 4 + 8K 2 CO 3 + 7O 2 \u003d 8K 2 sro 4 + 2F 2 O 3 + 8CO 2 (1000 ° C)

Dichromat potassium K. 2 CR 2 O. 7 . Oxosol. Technical name chrompik. Orange-red, non-hygroscopic. Melts without decomposition, with further heating decomposes. Well soluble in water ( orange The color of the solution corresponds to the ion of CR 2 O 7 2-). The alkaline medium forms to 2 CRO 4. A typical oxidizing agent in the solution and when fusing. Enters the reaction of ion exchange.

Qualitative reactions - Blue staining of the essential solution in the presence of H 2 O 2, blue staining of aqueous solution under the action of atomic hydrogen.

It is used as a tubeer of leather, rubbing with tissue daring, component of pyrotechnic compositions, reagent in analytical chemistry, inhibitor of corrosion of metals, in a mixture with H 2 SO 4 (conc.) - for washing chemical dishes.

Equations of the most important reactions:

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

K 2 Cr 2 O 7 (T) + 14HCl (KOB C) \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 ("Chrome mixture")

K 2 CR 2 O 7 + KOH (concluding) \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 (D) \u003d 3S ↓ + 2OH - + 2CR 2 (OH) 3 ↓

CR 2 O 7 2- (COND) + 2AG + (RSC) \u003d AG 2 CR 2 O 7 (T. red) ↓

CR 2 O 7 2- (RSC) + H 2 O + PB 2+ \u003d 2H + + 2PBCRO 4 (red) ↓

K 2 Cr 2 O 7 (T) + 6HCl + 8H 0 (Zn) \u003d 2CrCl 2 (SIN) + 7H 2 O + 2KCL

Getting: Treatment to 2 srox 4 sulfuric acid:

2k 2 sro 4 + H 2 SO 4 (30%) \u003d To 2.CR 2 O. 7 + K 2 SO 4 + H 2 O

Chromium discovery refers to the period of rapid development of chemical-analytical studies of salts and minerals. In Russia, chemists showed 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 League Ore (Curont) described by Lomonosov. The mineral was studied, but nothing but the oxides of lead, iron and aluminum in it was not found. However, in 1797, the boilers, a thin-crushed sample of a mineral with potash, and precipitated a lead carbonate, got a solution painted in orange - red. From this solution, he crystallized the ruby-red salt, from which the oxide was isolated and free metal other than all known metals. The ones called it Chromium (Chrome. ) from the Greek word- coloring, color; True, it was meant the property not metal, but his brightly colored salts.

Finding in nature.

The most important ore chromium, which is of practical importance, is chromisy, the approximate composition of which meets the FECRO 4 formula.

He is found in Malaya Asia, in the Urals, in North America, in southern Africa. The aforementioned mineral of the Crocken is also technical significance - PBCro 4. In nature there are also chromium oxide (3) and some other connections. In the earth's crust, the chromium content in terms of metal is 0.03%. Chrome discovered in the sun, stars, meteorites.

Physical properties.

Chrome - white, solid and fragile metal, exceptionally chemically resistant to the effects of acids and alkalis. It is oxidized in the air, it 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.

Getting.

With a strong heating of chromium iron with coal, chromium and iron is restored:

Feo * Cr 2 O 3 + 4C \u003d 2CR + FE + 4CO

As a result of this reaction, a fusion of chromium with iron, characterized by high strength, is formed. To obtain pure chromium, it is reduced from chromium oxide (3) aluminum:

CR 2 O 3 + 2AL \u003d Al 2 O 3 + 2CR

In this process, two oxide are usually used - CR 2 O 3 and CRO 3

Chemical properties.

Due to the thin protective film of the oxide covering the surface of chromium, it is very resistant to aggressive acids and alkalis. Chrome does not react with concentrated nitric and sulfuric acids, as well as with phosphoric acid. With alkalishes, chromium joins at T \u003d 600-700 O C. However, the chrome interacts with diluted sulfur and hydrochloric acids, displacing hydrogen:

2CR + 3H 2 SO 4 \u003d CR 2 (SO 4) 3 + 3H 2
2CR + 6HCl \u003d 2CrCl 3 + 3H 2

At high temperatures, chromium is lit in oxygen, forming oxide (III).

Red chrome reacts with water vapor:

2CR + 3H 2 O \u003d CR 2 O 3 + 3H 2

The chrome at high temperatures also reacts with halogens, halogen - hydrogen, gray, nitrogen, phosphorus, coal, silicon, boron, for example:

CR + 2HF \u003d CRF 2 + H 2
2Cr + N2 \u003d 2CRN
2CR + 3S \u003d Cr 2 S 3
CR + SI \u003d CRSI

The above physical and chemical properties of chromium found their use 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. Ferrochrome alloys are used as metal-cutting tools. Chrome alloys found applied in medical equipment, in the manufacture of chemical technological equipment.

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

Chrome is headed by a side subgroup VI of a periodic system of elements. His electronic formula is as follows:

24 CR IS 2 2S 2 2P 6 3S 2 3P 6 3D 5 4S 1

In the filling of orbitals, electrons at the chromium atom disrupts the pattern, according to which 4S orbital should be filled first to the 4S 2 state. However, due to the fact that the 3D orbital occupies a more favorable energy position in the chromium atom, it is filling up to 4D 5. Such a phenomenon is observed at atoms of some other elements of side subgroups. Chrome can exhibit the degree of oxidation from +1 to +6. The most stable are chromium compounds with oxidation degrees +2, +3, +6.

Compounds of bivalent chromium.

Chromium oxide (II) CRO - Pyrophoric Black Powder (Piroforming - the ability in a finely dredged state will flamm in air). CRO is dissolved in dilute hydrochloric acid:

CRO + 2HCl \u003d CRCL 2 + H 2 O

In air, when heated, over 100 0 С Cro is converted to CR 2 O 3.

Bivalent chromium salts are formed when dissolved metal chromium in acids. These reactions pass in an atmosphere of low-active gas (for example H 2), because In the presence of air, the CR (II) is easily occurring to CR (III).

Chromium hydroxide is obtained as a yellow sediment under the action of alkali solutions on chromium chromium (II):

CRCL 2 + 2NAOH \u003d CR (OH) 2 + 2NACL

CR (OH) 2 has basic properties, is a reducing agent. Hydrated Ion CR2 + painted in pale - blue color. CRCL 2 aqueous solution has a blue color. In air in aqueous solutions of CR (II) compounds, CR (III) compounds are transmitted. This is especially pronounced in CR (II) hydroxide:

4CR (OH) 2 + 2H 2 O + O 2 \u003d 4CR (OH) 3

Combination of trivalent chromium.

Chromium oxide (III) CR 2 O 3 - refractory green powder. Hardness is close to Corundum. 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 fusing with alkalis, it forms chromites: CR 2 O 3 + 2NAOH \u003d 2NACRO 2 + H 2 O

Chromium hydroxide is also an amphoter compound:

CR (OH) 3 + HCl \u003d CRCl 3 + 3H 2 O
CR (OH) 3 + NaOH \u003d Nacro 2 + 2H 2 O

Anhydrous CRCL 3 has the form of dark purple leaves, completely insoluble in cold water, with boiling it dissolves very slowly. Anhydrous chromium sulfate (III) CR 2 (SO 4) 3 pink color is also poorly soluble in water. In the presence of reducing agents, purple chromium sulfate CR 2 (SO 4) 3 * 18H 2 O is formed. Green hydrates of chromium sulfate containing fewer water are also known. Kcr (SO 4) 2 * 12H 2 O chromium alum is crystallized from solutions containing purple chromium sulfate and potassium sulfate. The solution of chromium alum when heated becomes green due to the formation of sulfates.

Reactions with chrome and its connections

Almost all chromium compounds and their solutions are intensely painted. Having a colorless solution or white precipitate, we can with a lot of probability to conclude about the absence of chromium.

1. Strongly minimize in the flame burner on a porcelain cup such a quantity of potassium bichromate, which will fit on the tip of the knife. The salt will not allocate crystallization water, and melts at a temperature of about 400 0 ° C with the formation of dark fluid. I melous it for a few more minutes on a strong flame. After cooling on the shard, a green precipitate is formed. Part it is soluble in water (it acquires yellow), and leave another part on the shard. Salt when heated was decomposed, as a result, the soluble yellow chromat potassium K 2 CRO 4 and the green Cr 2 O 3 was formed.
2. Solubam 3G powdered potassium bichromate in 50ml of water. To one part will 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 bichromate. If you now add a 50% solution of sulfuric acid by portions, then the red - yellow painting of the bichromate will appear again.
3. Nallem in the test tube 5ml. The solution of potassium bichromate, boil with 3 ml of concentrated hydrochloric acid under the burden. From the solution, yellow-green poisonous chlorine gaseous chlorine is distinguished, because chromat is oxidized by HCl to Cl 2 and H 2 O. Chromate itself will turn into green chloride trivalent chromium. It can be distinguished by evaporation of the solution, and then, sprumeivation with soda and saltier, translate to chromat.
4. When adding a solution of lead nitrate drops yellow chromate; When interacting with a solution of silver nitrate, a red-brown sediment of silver chromat is formed.
5. We will add hydrogen peroxide to a solution of potassium bichromate and acidify a solution with sulfuric acid. The solution acquires a deep blue color due to the formation of chromium peroxide. Peroxide when sculpting with some ether will go into an organic solvent and paint it into a blue color. This reaction is specific for chrome and very sensitive. With it, it is possible to detect chrome in metals and alloys. First of all, it is necessary to dissolve the metal. With long-term boiling with 30% sulfuric acid (you can add hydrochloric acid) chromium and many steel partially dissolve. The resulting solution contains chromium sulfate (III). To make a detection reaction, first neutralize it with caustic. The sediment drops the gray-green chromium hydroxide (III), which dissolves in an excess of NaOH and forms green sodium chromite. Cool solution and add 30% hydrogen peroxide. When heated, the solution is painted in yellow, as the chromite will oxidize to the chromat. The acidification will lead to the appearance of blue solutions. The painted compound can be extracted, shaking with ether.

Analytical reactions to chromium ions.

1. To 3-4 drops of chloride solution CRCl 3, add a 2M solution of NaOH to dissolve the initial precipitate. Pay attention to the color of the sodium chromite formed. Heat the resulting solution on a water bath. What happens?
2. To 2-3 drops of R-RC CRCl 3, add an equal volume of the 8m solution of NaOH and 3-4 drops of 3% p-ra H 2 O 2. Heat the reaction mixture on a water bath. What happens? Which precipitate is formed if the resulting painted solution is neutralized, add CH 3 COOH to it, and then PB (NO 3) 2?
3. Pour into a test tube of 4-5 drops of chromium sulfate solutions CR 2 (SO 4) 3, IMH 2 SO 4 and KMNO 4. Heat the reactionary mention within a few minutes in the water bath. Pay attention to the change in the color of the solution. What is it caused?
4. To 3-4 drops of acidified nitric acid solution K 2 CR 2 O 7, add 2-3 drops of the H 2 O 2 solution and mix. The appearing blue decomposition of the solution is due to the occurrence of suprachic acid H 2 CRO 6:

CR 2 O 7 2- + 4H 2 O 2 + 2H + \u003d 2H 2 CRO 6 + 3H 2 O

Pay attention to the rapid decomposition H 2 CRO 6:

2H 2 CRO 6 + 8H + \u003d 2CR 3+ + 3O 2 + 6H 2 O
blue color green

Suchrome acid is much more stable in organic solvents.

1. K 2-4 drops of acidified nitric acid solution K 2 CR 2 O 7 add 5 drops of isoamyl alcohol, 2-3 drops of solution H 2 O 2 and shake the reaction mixture. A pop-up layer of an organic solvent is painted in bright blue color. The coloring disappears very slowly. Compare the stability of H 2 CRO 6 in the organic and aqueous phases.
2. With the interaction of CRO 4 2- and Ba 2+ ions, the Bacro 4 chromium sediment drops out.
3. Silver nitrate forms with cro 4 ions of brick-red silver chromat sediment.
4. Take three test tubes. In one of them, place 5-6 drops of the solution K 2 Cr 2 O 7, in the second - the same volume of the solution K 2 CRO 4, and in the third - three drops of both solutions. Then add three drops of potassium iodide solution to each test tube. Explain the result. Sweese the solution in the second test tube. What happens? Why?

Entertaining experiments with chromium compounds

1. A mixture of Cuso 4 and k 2 Cr 2 O 7 when adding alkali becomes green, and in the presence of an acid becomes yellow. Heated 2mg glycerin with a small amount (NH 4) 2 CR 2 O 7 followed by adding alcohol, after filtration, a bright green solution is obtained, which when acid addition becomes yellow, and in the neutral or alkaline medium it becomes green.
2. Place in the center of the canning can with termit "Ruby mixture" - carefully loss and placed in aluminum foil Al 2 O 3 (4.75 g) with the addition of CR 2 O 3 (0.25 g). In order for the bank longer than cooled, it is necessary to bury under the upper edge in the sand, and after the ignition of the termit and the beginning of the reaction, cover it with an iron sheet and flood with sand. Drop into the jar in a day. As a result, red - ruby \u200b\u200bpowder is formed.
3. 10G Potassium Bichromate is triturated with 5g sodium or potassium nitrate and 10g sugar. The mixture is moisturized and mixed with a college. If the powder is pressed in a glass tube, and then push the wand and set fire to it from the end, it will begin to crawl "snake", first black, and after cooling - green. A wand with a diameter of 4 mm is lit at a speed of about 2mm per second and is lengthened 10 times.
4. If mixed solutions of copper sulfate and potassium dichromate and add a little ammonia solution, then the amorphous brown sediment of the composition 4cucro 4 * 3NH 3 * 5H 2 O will fall out, which is dissolved in hydrochloric acid to form a yellow solution, and a green solution is obtained in an excess of ammonia. If you add alcohol further to this solution, then a green precipitate will fall, which becomes blue after filtering, and after drying - blue-purple with red sparkles, well visible with strong lighting.
5. The remaining chrome oxide can be regenerated after the experiments "volcano" or "pharaohon of the snake". To do this, we need to rush 8g CR 2 O 3 and 2G Na 2 CO 3 and 2.5G KNO 3 and process the cooled boiling water alloy. A soluble chromate is obtained, which can be transformed into other CR (II) and CR (VI) compounds, including the original ammonium dichromate.

Examples of redox - restoration 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 \u003d 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 \u003d 6NABr + 2NA 2 CRO 4 + 4H 2 O
d) 2NA 2 CRO 4 + 2HCl \u003d 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 \u003d 2CR (OH) 3
b) CR (OH) 3 + 3HCl \u003d CRCl 3 + 3H 2 O
c) 2CrCl 3 + 2kmno 4 + 3H 2 O \u003d k 2 Cr 2 O 7 + 2mn (OH) 2 + 6HCl
d) k 2 Cr 2 O 7 + 2KOH \u003d 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
CR 2+

a) CRO + 2HCl \u003d CRCL 2 + H 2 O
b) Cro + H 2 O \u003d CR (OH) 2
c) CR (OH) 2 + 1 / 2O 2 + H 2 O \u003d 2CR (OH) 3
d) cr (oh) 3 + 3hno 3 \u003d Cr (no 3) 3 + 3H 2 O
e) 4CR (NO 3) 3 \u003d 2Cr 2 O 3 + 12NO 2 + O 2
e) CR 2 O 3 + 2 NaOH \u003d 2NACRO 2 + H 2 O

Chrome element as artist

Chemists quite often appealed to the problem of creating artificial pigments for painting. In the XVIII-XIXVV, the technology of obtaining many picturesque materials was developed. Louis Nichan Nichana Voklen in 1797, who found an unknown chrome element in the Siberian red ore, prepared a new, remarkably steady paint - chrome greens. The chromophore is the water oxide (III). Under the name "Emerald Green" began to be released in 1837. Later, L. Voclen offered several new colors: barite, zinc and chrome yellow. Over time, they were ousted with more resistant yellow, orange pigments based on cadmium.

Green chromic - the most durable and light-resistant paint, not amenable to atmospheric gases. A chrome green greenery has a large crumbling force and is capable of rapid drying, so from the XIX century. It is widely used in painting. It has a great importance in porcelain painting. The fact is that porcelain products can be decorated with both under-ferry and supervised painting. In the first case, paints are applied to the surface only a slightly burned product, which is then covered with a layer of glaze. Next, the main, high-temperature firing should be followed: for sintering the porcelain mass and refueling glaze products are heated to 1350 - 1450 0 C. Such a high temperature without chemical changes are kept very few paints, and in the old days there were only two - cobalt and chromium. Black cobalt oxide, applied to the surface of the porcelain product, is melted with icing, chemically interacting with it. As a result, bright blue cobalt silicates are formed. Such a decared cobalt blue porcelain dishes are well known. Chromium (III) oxide does not interact with the chemically with the components of the glaze and simply runs between porcelain shards and a transparent glaze "deaf" layer.

In addition to chrome greenery, artists apply paints obtained from volkonskoyt. This mineral from the Montmorillonite group (clay mineral subclass of complex silicates Na (Mo, Al), Si 4 O 10 (OH) 2 was found in 1830. Russian mineralogue Cermers and named after M.N. Volkonskaya - Daughter of the Hero of the Battle of Borodino, General N . N. Raevsky, Wife of the Decembrist S.G.Volkomsky. Volkonsky is a clay containing up to 24% chromium oxide, as well as aluminum and iron oxides (III). The impermanence of the composition of the mineral that is engaged in the Urals, in the Perm and Kirov regions, It causes its diverse color - from the color of winter darkened fir to the bright green color of the swamp frog.

Pablo Picasso addressed the geologists of our country with a request to study the stocks of a volklockite, giving paint uniquely fresh tone. Currently, a method for producing artificial volkonskoyt has been developed. It is interesting to note that according to modern research, the Russian icon paints used paints from this material in the Middle Ages, long before its "official" discovery. Artists and Green Guinea used in a well-known popularity (created in 1837), the chromoform of which is a hydraulic chromium hydroxide CR 2 O 3 * (2-3) H 2 O, where part of the water is chemically connected, and the part is adsorbed. This pigment gives the paint an emerald shade.

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