The content of the article

CHROMIUM– (Chromium) Cr, chemical element 6(VIb) of group of the Periodic table. Atomic number 24, atomic mass 51.996. There are 24 known isotopes of chromium from 42 Cr to 66 Cr. The isotopes 52 Cr, 53 Cr, 54 Cr are stable. Isotopic composition of natural chromium: 50 Cr (half-life 1.8 10 17 years) – 4.345%, 52 Cr – 83.489%, 53 Cr – 9.501%, 54 Cr – 2.365%. The main oxidation states are +3 and +6.

In 1761, chemistry professor at St. Petersburg University Johann Gottlob Lehmann, at the eastern foot of the Ural Mountains at the Berezovsky mine, discovered a wonderful red mineral, which, when crushed into powder, gave a bright yellow color. In 1766 Lehman brought samples of the mineral to St. Petersburg. Having treated the crystals with hydrochloric acid, he obtained a white precipitate, in which he discovered lead. Lehmann called the mineral Siberian red lead (plomb rouge de Sibérie); it is now known that it was crocoite (from the Greek “krokos” - saffron) - a natural lead chromate PbCrO 4.

The German traveler and naturalist Peter Simon Pallas (1741–1811) led an expedition of the St. Petersburg Academy of Sciences to the central regions of Russia and in 1770 visited the Southern and Middle Urals, including the Berezovsky mine and, like Lehmann, became interested in crocoite. Pallas wrote: “This amazing red lead mineral is not found in any other deposit. When ground into powder it turns yellow and can be used in artistic miniatures.” Despite the rarity and difficulty of delivering crocoite from the Berezovsky mine to Europe (it took almost two years), the use of the mineral as a coloring agent was appreciated. In London and Paris at the end of the 17th century. all noble persons rode in carriages painted with finely ground crocoite; in addition, the best examples of Siberian red lead replenished the collections of many mineralogical cabinets in Europe.

In 1796, a sample of crocoite came to the professor of chemistry at the Paris Mineralogical School, Nicolas-Louis Vauquelin (1763–1829), who analyzed the mineral, but found nothing in it except oxides of lead, iron and aluminum. Continuing his research on Siberian red lead, Vaukelin boiled the mineral with a solution of potash and, after separating the white precipitate of lead carbonate, obtained a yellow solution of an unknown salt. When treated with lead salt, a yellow precipitate was formed, with mercury salt, a red one, and when tin chloride was added, the solution became green. By decomposing crocoite with mineral acids, he obtained a solution of “red lead acid,” the evaporation of which gave ruby-red crystals (it is now clear that it was chromic anhydride). Having calcined them with coal in a graphite crucible, after the reaction I discovered many fused gray needle-shaped crystals of a metal unknown to that time. Vaukelin noted the high refractoriness of the metal and its resistance to acids.

Vaukelin named the new element chromium (from the Greek crwma - color, color) due to the many multi-colored compounds it forms. Based on his research, Vauquelin was the first to state that the emerald color of some precious stones is explained by the admixture of chromium compounds in them. For example, natural emerald is a deep green colored beryl in which aluminum is partially replaced by chromium.

Most likely, Vauquelin obtained not pure metal, but its carbides, as evidenced by the needle-shaped shape of the resulting crystals, but the Paris Academy of Sciences nevertheless registered the discovery of a new element, and now Vauquelin is rightly considered the discoverer of element No. 24.

Yuri Krutyakov

"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 table

Chromium- element of the 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. Denoted by the symbol Cr(lat. Chromium). Simple substance chromium- hard metal of bluish-white color. Chrome is sometimes classified as a ferrous metal.

Atomic structure

17 Cl)2)8)7 - atomic structure diagram

1s2s2p3s3p - electronic formula

The atom is located in the III period, and has three energy levels

The atom is located in group VII, in the main subgroup - at the outer energy level 7 electrons

Element properties

Physical properties

Chrome 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. Adding carbon to chromium further increases its hardness.

Chemical properties

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

Interaction with non-metals

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

4Cr + 3O 2 = 2Cr 2 O 3.

Reacts with fluorine at 350°C, with chlorine at 300°C, with bromine at red heat, forming chromium (III) halides:

2Cr + 3Cl2 = 2CrCl3.

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 (possible formation of Cr 2 B, CrB, Cr 3 B 4, CrB 4),

2Cr + 3C = Cr 2 C 3 (possible formation of Cr 23 C 6, Cr 7 B 3),

Cr + 2Si = CrSi 2 (possible formation of Cr 3 Si, Cr 5 Si 3, CrSi).

Does not interact directly with hydrogen.

Interaction with water

When finely ground and hot, chromium reacts with water to form chromium(III) oxide and hydrogen:

2Cr + 3H 2 O = Cr 2 O 3 + 3H 2

Interaction with acids

In the electrochemical voltage series of metals, chromium is located before hydrogen; it displaces hydrogen from solutions of non-oxidizing acids:

Cr + 2HCl = CrCl 2 + H 2;

Cr + H 2 SO 4 = 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 = Cr(NO 3) 3 + 3NO 2 + 3H 2 O.

Interaction with alkaline reagents

Chromium does not dissolve in aqueous solutions of alkalis; it slowly reacts with alkali melts to form chromites and release hydrogen:

2Cr + 6KOH = 2KCrO 2 + 2K 2 O + 3H 2.

Reacts with alkaline melts of oxidizing agents, for example potassium chlorate, and chromium is converted 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 to form green chromium(III) oxide Cr 2 O 3, which has amphoteric properties.

Compounds of chromium with boron (borides Cr 2 B, CrB, Cr 3 B 4, CrB 2, CrB 4 and Cr 5 B 3), with carbon (carbides Cr 23 C 6, Cr 7 C 3 and Cr 3 C 2), were synthesized. 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 medium (“hydrogen at the time of release”):

All these Cr 2+ salts are strong reducing agents, to the point that when standing, they displace hydrogen from water. Oxygen in the air, especially in an acidic environment, oxidizes Cr 2+, as a result of which the blue solution quickly turns green.

Brown or yellow hydroxide Cr(OH) 2 precipitates when alkalis are added to solutions of chromium(II) salts.

Chromium dihalides CrF 2, CrCl 2, CrBr 2 and CrI 2 were synthesized

Cr(+3) compounds

The oxidation state +3 corresponds to the amphoteric oxide Cr 2 O 3 and hydroxide Cr (OH) 3 (both green). This is the most stable oxidation state of chromium. Chromium compounds in this oxidation state range in color from dirty purple (3+ ion) 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 reacting ammonia with solutions of chromium (III) salts:

Cr+3NH+3H2O→Cr(OH)↓+3NH

You can use alkali solutions, but in their excess a soluble hydroxo complex is formed:

Cr+3OH→Cr(OH)↓

Cr(OH)+3OH→

By fusing Cr 2 O 3 with alkalis, chromites are obtained:

Cr2O3+2NaOH→2NaCrO2+H2O

Uncalcined chromium(III) oxide dissolves in alkaline solutions and acids:

Cr2O3+6HCl→2CrCl3+3H2O

When chromium(III) compounds are oxidized in an alkaline medium, chromium(VI) compounds are formed:

2Na+3HO→2NaCrO+2NaOH+8HO

The same thing happens when chromium (III) oxide is fused with alkali and oxidizing agents, or with alkali in air (the melt acquires a yellow color):

2Cr2O3+8NaOH+3O2→4Na2CrO4+4H2O

Chromium compounds (+4)[

By careful decomposition of chromium(VI) oxide CrO 3 under hydrothermal conditions, chromium(IV) oxide CrO 2 is obtained, which is ferromagnetic and has metallic conductivity.

Among chromium tetrahalides, CrF 4 is stable, chromium tetrachloride CrCl 4 exists only in vapors.

Chromium compounds (+6)

The oxidation state +6 corresponds to the acidic chromium (VI) oxide CrO 3 and a number of acids, between which there is an equilibrium. The simplest of them are chromium H 2 CrO 4 and dichromium H 2 Cr 2 O 7 . They form two series of salts: yellow chromates and orange dichromates, respectively.

Chromium (VI) oxide CrO 3 is formed by the interaction of concentrated sulfuric acid with solutions of dichromates. A typical acidic oxide, when interacting with water it forms strong unstable chromic acids: chromic H 2 CrO 4, dichromic H 2 Cr 2 O 7 and other 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 the orange solution of K 2 Cr 2 O 7, the color turns yellow again as K 2 CrO 4 chromate is formed again:

Cr2O+2OH→2CrO+HO

It does not reach a high degree of polymerization, as occurs with 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 blood-red, slightly soluble silver chromate is used to detect silver in alloys using assay acid.

Chromium pentafluoride CrF 5 and low-stable chromium hexafluoride CrF 6 are known. Volatile chromium oxyhalides CrO 2 F 2 and CrO 2 Cl 2 (chromyl chloride) were also obtained.

Chromium(VI) compounds are strong oxidizing agents, for example:

K2Cr2O7+14HCl→2CrCl3+2KCl+3Cl2+7H2O

The addition of hydrogen peroxide, sulfuric acid and an organic solvent (ether) to 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.

Physical properties Silver-white metal The hardest metal Brittle, with a density of 7.2 g/cm 3 Melting temperature C

Chemical properties of chromium 1. Reacts with non-metals (when heated) A) 4Cr + 3O 2 = 2Cr 2 O 3 B) 2Cr + N 2 = 2CrN C) 2Cr + 3S = Cr 2 S 3 2. Reacts with water vapor (in hot state) 2Cr + 3H 2 O=Cr 2 O 3 + 3H 2 3. Reacts with acids Cr + H 2 SO 4 = CrSO 4 + H 2 4. Reacts with salts of less active metals Cr + CuSO 4 = CrSO 4 + Cu

Chromium compounds Chromium(II) compounds Chromium(III) compounds Chromium(VI) compounds CrO - basic oxide Cr(OH) 2 - base CrO 3 - acidic oxide H 2 CrO 4 - chromic (H 2 Cr 2 O 7) - dichromic acid Cr 2 O 3 - amphoteric oxide Cr(OH) 3 - amphoteric compound

Chromium(III) compounds Cr 2 O 3 – under normal conditions does not react with solutions of acids and alkalis. Cr 2 O 3 -reacts only upon fusion Cr 2 O 3 +Ba(OH) 2 = Ba(CrO 2) 2 +H 2 O Reacts with more active metals Cr 2 O 3 + 2Al= Al 2 O 3 + 2Cr 1 .Reacts with acids Cr(OH) 3 + 3HCL= =CrCL H 2 O 2. Reacts with alkalis Cr(OH) 3 +3NaOH= =Na 3 (Cr(OH) 6) 3. When heated, 2Cr(OH) 3 decomposes =Cr 2 O 3 + 3H 2 O

• Designation - Cr (Chromium);
• Period - IV;
• Group - 6 (VIb);
• Atomic mass - 51.9961;
• Atomic number - 24;
• Atomic radius = 130 pm;
• Covalent radius = 118 pm;
• Electron distribution - 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1 ;
• melting temperature = 1857°C;
• boiling point = 2672°C;
• Electronegativity (according to Pauling/according to Alpred and Rochow) = 1.66/1.56;
• Oxidation state: +6, +3, +2, 0;
• Density (no.) = 7.19 g/cm3;
• Molar volume = 7.23 cm 3 /mol.

Chromium (color, paint) was first found at the Berezovsky gold deposit (Middle Urals), the first mentions date back to 1763; in his work “The First Foundations of Metallurgy” M.V. Lomonosov calls it “red lead ore”.

Rice. Structure of the chromium atom.

The electronic configuration of the chromium atom is 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1 (see Electronic structure of atoms). In the formation of chemical bonds with other elements, 1 electron located on the outer 4s level + 5 electrons of the 3d sublevel (6 electrons in total) can participate, therefore, in compounds, chromium can take oxidation states from +6 to +1 (the most common are +6 , +3, +2). Chromium is a chemically inactive metal; it reacts with simple substances only at high temperatures.

Physical properties of chromium:

• bluish-white metal;
• very hard metal (in the presence of impurities);
• fragile when n. y.;
• plastic (in its pure form).

Chemical properties of chromium

• at t=300°C reacts with oxygen:
  4Cr + 3O 2 = 2Cr 2 O 3;
• at t>300°C reacts with halogens, forming mixtures of halides;
• at t>400°C reacts with sulfur to form sulfides:
  Cr + S = CrS;
• at t=1000°C finely ground chromium reacts with nitrogen, forming chromium nitride (a semiconductor with high chemical stability):
  2Cr + N 2 = 2CrN;
• reacts with dilute hydrochloric and sulfuric acids to release hydrogen:
  Cr + 2HCl = CrCl 2 + H 2;
  Cr + H 2 SO 4 = CrSO 4 + H 2;
• warm concentrated nitric and sulfuric acids dissolve chromium.

With concentrated sulfuric and nitric acid at no. chromium does not react, and chromium also does not dissolve in aqua regia; it is noteworthy that pure chromium does not react even with dilute sulfuric acid; the reason for this phenomenon has not yet been established. During long-term storage in concentrated nitric acid, chromium becomes covered with a very dense oxide film (passivates) and stops reacting with dilute acids.

Chromium compounds

It was already said above that the “favorite” oxidation states of chromium are +2 (CrO, Cr(OH) 2), +3 (Cr 2 O 3, Cr(OH) 3), +6 (CrO 3, H 2 CrO 4 ).

Chrome is chromophore, i.e., an element that gives color to the substance in which it is contained. For example, in the oxidation state +3, chromium gives a purple-red or green color (ruby, spinel, emerald, garnet); in the oxidation state +6 - yellow-orange color (crocoite).

In addition to chromium, chromophores also include iron, nickel, titanium, vanadium, manganese, cobalt, copper - all these are d-elements.

The color of common compounds that include chromium:

• chromium in oxidation state +2:
  • chromium oxide CrO - red;
  • chromium fluoride CrF 2 - blue-green;
  • chromium chloride CrCl 2 - has no color;
  • chromium bromide CrBr 2 - has no color;
  • Chromium iodide CrI 2 - red-brown.
• chromium in oxidation state +3:
  • Cr 2 O 3 - green;
  • CrF 3 - light green;
  • CrCl 3 - violet-red;
  • CrBr 3 - dark green;
  • CrI 3 - black.
• chromium in oxidation state +6:
  • CrO 3 - red;
  • potassium chromate K 2 CrO 4 - lemon yellow;
  • ammonium chromate (NH 4) 2 CrO 4 - golden yellow;
  • calcium chromate CaCrO 4 - yellow;
  • Lead chromate PbCrO 4 - light brown-yellow.

Chromium oxides:

• Cr +2 O - basic oxide;
• Cr 2 +3 O 3 - amphoteric oxide;
• Cr +6 O 3 - acidic oxide.

Chromium hydroxides:

• ".

  Application of chromium

  • as a alloying additive in the smelting of heat-resistant and corrosion-resistant alloys;
  • for chrome plating of metal products in order to give them high corrosion resistance, abrasion resistance and a beautiful appearance;
  • chromium-30 and chromium-90 alloys are used in plasma torch nozzles and in the aviation industry.

Chromium is an element of the 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 hard metal of a bluish-white color.

Chemical properties of chromium

Under normal conditions, chromium reacts only with fluorine. At high temperatures (above 600°C) it interacts with oxygen, halogens, nitrogen, silicon, boron, sulfur, phosphorus.

4Cr + 3O 2 – t° →2Cr 2 O 3

2Cr + 3Cl 2 – t° → 2CrCl 3

2Cr + N 2 – t° → 2CrN

2Cr + 3S – t° → Cr 2 S 3

When heated, 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 - oxidizers.

Chromium compounds

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

Cr(OH) 2 + 2HCl → CrCl 2 + 2H 2 O

Chromium (II) compounds are strong reducing agents; transform into chromium (III) compounds under the influence 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. Can be obtained by calcination of 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 with an oxidation state of (+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 fused with a mixture of alkali and 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. Gray-green, decomposes when heated, losing water and forming green metahydroxide CrO(OH). Does not dissolve in water. 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 + ZN + = Cr 3+ + 3H 2 O

Cr(OH) 3 + KOH → K, Cr(OH) 3 + ZON - (conc.) = [Cr(OH) 6 ] 3-

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

Cr(OH) 3 →(120 o C – H 2 O) CrO(OH) →(430-1000 0 C –H 2 O) Cr2O3

2Cr(OH) 3 + 4NaOH (conc.) + ZN 2 O 2 (conc.) = 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 ↓+ ЗNН 4+

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

Chromium (III) salts have a purple or dark green color. Their chemical properties 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.

Obtained 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 is an acidic oxide, with alkalis it forms yellow chromates CrO 4 2-:

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 = 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 O 4 . Oxosol. Yellow, non-hygroscopic. Melts without decomposition, thermally stable. Very 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 turns 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 CrO 4 2- ion - the precipitation of a yellow precipitate of barium chromate, which decomposes 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 (concentration, horizon) = 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[Cr(OH) 6 ]+3S↓+4KOH

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

Qualitative reaction:

K 2 CrO 4 + BaCl 2 = 2KCl + BaCrO 4 ↓

2BaCrO 4 (t) + 2HCl (dil.) = BaCr 2 O 7 (p) + BaC1 2 + H 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 °C)

Potassium dichromate K 2 Cr 2 O 7 . Oxosol. Technical name chrome peak. Orange-red, non-hygroscopic. Melts without decomposition, and decomposes upon further heating. Very soluble in water ( orange The color of the solution corresponds to the Cr 2 O 7 2- ion. In an alkaline environment it forms K 2 CrO 4 . A typical oxidizing agent in solution and during fusion. Enters into ion exchange reactions.

Qualitative reactions- blue color of an ethereal solution in the presence of H 2 O 2, blue color 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, 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 =4K 2 CrO 4 +2Cr 2 O 3 +3O 2 (500-600 o C)

K 2 Cr 2 O 7 (t) +14HCl (conc) = 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) =H 2 O+2K 2 CrO 4

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

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

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

Cr 2 O 7 2- (conc.) +2Ag + (dil.) =Ag 2 Cr 2 O 7 (red) ↓

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

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