Minimum oxidation state of chromium. Chromium is a refractory but very useful metal in construction. I. Repetition of the material of the previous lesson

DEFINITION

Chromium is the twenty-fourth element of the Periodic Table. Designation - Cr from the Latin "chromium". Located in the fourth period, VIB group. Refers to metals. The core charge is 24.

Chromium is found in earth's crust in the amount of 0.02% (mass.). In nature, it occurs mainly in the form of iron chromium FeO×Cr 2 O 3 .

Chromium is a solid shiny metal (Fig. 1), melting at 1890 o C; its density is 7.19 g / cm 3. At room temperature, chromium is resistant to both water and air. Dilute sulfuric and hydrochloric acids dissolve chromium, releasing hydrogen. In cold concentrated nitric acid, chromium is insoluble and becomes passive after treatment with it.

Rice. 1. Chrome. Appearance.

Atomic and molecular weight of chromium

DEFINITION

Relative molecular weight of a substance(M r) is a number showing how many times the mass of a given molecule is greater than 1/12 of the mass of a carbon atom, and relative atomic mass of an element(A r) - how many times the average mass of atoms of a chemical element is greater than 1/12 of the mass of a carbon atom.

Since chromium exists in the free state in the form of monatomic Cr molecules, the values of its atomic and molecular masses are the same. They are equal to 51.9962.

Isotopes of chromium

It is known that chromium can occur in nature in the form of four stable isotopes 50Cr, 52Cr, 53Cr, and 54Cr. Their mass numbers are 50, 52, 53, and 54, respectively. The nucleus of the atom of the chromium isotope 50 Cr contains twenty-four protons and twenty-six neutrons, and the remaining isotopes differ from it only in the number of neutrons.

There are artificial isotopes of chromium with mass numbers from 42 to 67, among which the most stable is 59 Cr with a half-life of 42.3 minutes, as well as one nuclear isotope.

Chromium ions

On the outer energy level of the chromium atom, there are six electrons that are valence:

1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1 .

As a result of chemical interaction, chromium gives up its valence electrons, i.e. is their donor, and turns into a positively charged ion:

Cr 0 -2e → Cr 2+;

Cr 0 -3e → Cr 3+;

Cr 0 -6e → Cr 6+.

Molecule and atom of chromium

In the free state, chromium exists in the form of monatomic Cr molecules. Here are some properties that characterize the atom and molecule of chromium:

Chromium alloys

Chromium metal is used for chromium plating, and also as one of the most important components of alloy steels. The introduction of chromium into steel increases its resistance to corrosion both in aqueous media at ordinary temperatures and in gases at elevated temperatures. In addition, chromium steels have increased hardness. Chromium is a part of stainless acid-resistant, heat-resistant steels.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

The task

Chromium oxide (VI) weighing 2 g was dissolved in water weighing 500 g. Calculate the mass fraction of chromic acid H 2 CrO 4 in the resulting solution.

Solution

Let's write the reaction equation for obtaining chromic acid from chromium (VI) oxide:

CrO 3 + H 2 O \u003d H 2 CrO 4.

Find the mass of the solution:

m solution \u003d m (CrO 3) + m (H 2 O) \u003d 2 + 500 \u003d 502 g.

n (CrO 3) \u003d m (CrO 3) / M (CrO 3);

n (CrO 3) \u003d 2/100 \u003d 0.02 mol.

According to the reaction equation n(CrO 3) :n(H 2 CrO 4) = 1:1, then

n (CrO 3) \u003d n (H 2 CrO 4) \u003d 0.02 mol.

Then the mass of chromic acid will be equal to (molar mass - 118 g / mol):

m (H 2 CrO 4) \u003d n (H 2 CrO 4) × M (H 2 CrO 4);

m (H 2 CrO 4) \u003d 0.02 × 118 \u003d 2.36 g.

The mass fraction of chromic acid in solution is:

ω = msolute / msolution × 100%;

ω (H 2 CrO 4) \u003d m solute (H 2 CrO 4) / m solution × 100%;

ω (H 2 CrO 4) \u003d 2.36 / 502 × 100% \u003d 0.47%.

Answer

The mass fraction of chromic acid is 0.47%.

Chromium

Item #24. One of the hardest metals. It has high chemical resistance. One of the most important metals used in the production of alloy steels. Most chromium compounds have a bright color, and a variety of colors. For this feature, the element was named chromium, which means “paint” in Greek.

How was it found

A mineral containing chromium was discovered near Yekaterinburg in 1766 by I.G. Lehmann and named "Siberian red lead". Now this mineral is called crocoite. Its composition is also known - РbCrО 4 . And at one time, "Siberian red lead" caused a lot of controversy among scientists. For thirty years they argued about its composition, until, finally, in 1797, the French chemist Louis Nicolas Vauquelin isolated a metal from it, which (also, by the way, after some disputes) was called chromium.

Vauquelin treated crocoite with K 2 CO 3 potash: lead chromate turned into potassium chromate. Then, with the help of hydrochloric acid, potassium chromate was converted into chromium oxide and water (chromic acid exists only in dilute solutions). By heating the green powder of chromium oxide in a graphite crucible with coal, Vauquelin obtained a new refractory metal.

The Paris Academy of Sciences in all its form witnessed the discovery. But, most likely, Vauquelin singled out not elemental chromium, but its carbides. This is evidenced by the needle-like shape of the light gray crystals obtained by Vauquelin.

The name "chrome" was suggested by Vauquelin's friends, but he did not like it - the metal did not differ in a special color. However, friends managed to persuade the chemist, referring to the fact that good paints can be obtained from brightly colored chromium compounds. (By the way, it was in the works of Vauquelin that the emerald color of some natural beryllium and aluminum silicates was first explained; as Vauquelin found out, they were colored by impurities of chromium compounds.) And this name was established for the new element.

Incidentally, the syllable "chrome", precisely in the sense of "colored", is included in many scientific, technical and even musical terms. Widely known photographic films are "isopanchrome", "panchrome" and "orthochrome". The word "chromosome" in Greek means "the body that is colored." There is a "chromatic" scale (in music) and there is a harmonic "hromka".

Where is he located

There is quite a lot of chromium in the earth's crust - 0.02%. The main mineral from which industry obtains chromium is chromium spinel of variable composition with the general formula (Mg, Fe) O · (Cr, Al, Fe) 2 O 3 . Chrome ore is called chromites or chromium iron ore (because it almost always contains iron). There are deposits of chromium ores in many places. Our country has huge reserves of chromites. One of the largest deposits is located in Kazakhstan, in the Aktyubinsk region; it was discovered in 1936. Significant reserves of chrome ores are also in the Urals.

Chromites are mostly used for the smelting of ferrochromium. It is one of the most important ferroalloys and absolutely essential for the mass production of alloy steels.

Ferroalloys are alloys of iron with other elements used in the main rite for alloying and deoxidizing steel. Ferrochrome contains at least 60% Cr.

Tsarist Russia almost did not produce ferroalloys. Several blast furnaces of southern plants smelted low-percentage (for alloying metal) ferrosilicon and ferromanganese. Moreover, in 1910, a tiny factory was built on the Satka River, which flows in the Southern Urals, which smelted scanty amounts of ferromanganese and ferrochromium.

The young Soviet country in the first years of development had to import ferroalloys from abroad. Such dependence on the capitalist countries was unacceptable. Already in 1927 ... 1928. the construction of Soviet ferroalloy plants began. At the end of 1930, the first large ferroalloy furnace was built in Chelyabinsk, and in 1931 the Chelyabinsk plant, the firstborn of the USSR ferroalloy industry, was put into operation. In 1933, two more plants were launched - in Zaporozhye and Zestaponi. This made it possible to stop the import of ferroalloys. In just a few years, the production of many types of special steels was organized in the Soviet Union - ball-bearing, heat-resistant, stainless, automotive, high-speed ... All these steels include chromium.

At the 17th Party Congress, People's Commissar for Heavy Industry Sergo Ordzhonikidze said: “... if we did not have high-quality steels, we would not have an autotractor industry. The cost of high-quality steels we are currently using is estimated at over 400 million rubles. If it were necessary to import, it would be 400 million rubles. every year, damn it, you would be in bondage to the capitalists ... "

The plant on the basis of the Aktobe field was built later, during the years of the Great Patriotic War. He gave the first melting of ferrochromium on January 20, 1943. The workers of the city of Aktobe took part in the construction of the plant. The building was declared popular. The ferrochrome of the new plant was used to manufacture metal for tanks and cannons, for the needs of the front.

Years have passed. Now Aktobe Ferroalloy Plant is the largest enterprise producing ferrochromium of all grades. Highly qualified national cadres of metallurgists have grown up at the plant. From year to year, the plant and chromite mines are increasing their capacity, providing our ferrous metallurgy with high-quality ferrochromium.

Our country has a unique deposit of naturally alloyed iron ore rich in chromium and nickel. It is located in the Orenburg steppes. On the basis of this deposit, the Orsk-Khalilovsky metallurgical plant was built and operates. In the blast furnaces of the plant, naturally alloyed cast iron is smelted, which has a high heat resistance. Partly it is used in the form of casting, but most of it is sent for processing into nickel steel; chromium burns out when steel is smelted from cast iron.

Cuba, Yugoslavia, many countries of Asia and Africa have large reserves of chromites.

How to get it

Chromite is used mainly in three industries: metallurgy, chemistry and refractory production, and metallurgy consumes about two thirds of all chromite.

Steel alloyed with chromium has increased strength, resistance to corrosion in aggressive and oxidizing environments.

Obtaining pure chromium is an expensive and time-consuming process. Therefore, for alloying steel, mainly ferrochromium is used, which is obtained in electric arc furnaces directly from chromite. The reducing agent is coke. The content of chromium oxide in chromite should not be lower than 48%, and the ratio of Cr:Fe should not be less than 3:1.

Ferrochrome obtained in an electric furnace usually contains up to 80% chromium and 4 ... 7% carbon (the rest is iron).

But for alloying many high-quality steels, ferrochromium is needed, which contains little carbon (the reasons for this are discussed below, in the chapter “Chromium in Alloys”). Therefore, a part of high-carbon ferrochrome is subjected to special treatment in order to reduce the carbon content in it to tenths and hundredths of a percent.

Elemental, metallic chromium is also obtained from chromite. The production of commercially pure chromium (97...99%) is based on the aluminothermy method, discovered back in 1865 by the famous Russian chemist N.N. Beketov. The essence of the method is the reduction of aluminum oxides, the reaction is accompanied by a significant release of heat.

But first you need to get pure chromium oxide Cr 2 O 3. To do this, finely ground chromite is mixed with soda and limestone or iron oxide is added to this mixture. The whole mass is fired, and sodium chromate is formed:

2Cr 2 O 3 + 4Na 2 CO 3 + 3O 2 → 4Na 2 CrO 4 + 4CO 2.

Then sodium chromate is leached from the calcined mass with water; the lye is filtered, evaporated and treated with acid. The result is sodium dichromate Na 2 Cr 2 O 7 . By reducing it with sulfur or carbon when heated, green chromium oxide is obtained.

Chromium metal can be obtained by mixing pure chromium oxide with aluminum powder, heating this mixture in a crucible to 500...600°C and setting it on fire with barium peroxide. Aluminum takes away oxygen from chromium oxide. This reaction Cr 2 O 3 + 2Al → Al 2 O 3 + 2Cr is the basis of the industrial (aluminothermic) method for obtaining chromium, although, of course, the factory technology is much more complicated. Chromium, obtained aluminothermally, contains tenths of a percent of aluminum and iron, and hundredths of a percent of silicon, carbon and sulfur.

The silicothermic method for obtaining commercially pure chromium is also used. In this case, chromium oxide is reduced by silicon according to the reaction

2Cr 2 O 3 + 3Si → 3SiO 2 + 4Cr.

This reaction takes place in arc furnaces. To bind silica, limestone is added to the mixture. The purity of silicothermal chromium is approximately the same as that of aluminothermic chromium, although, of course, the content of silicon in it is somewhat higher, and aluminum is somewhat lower. To obtain chromium, they tried to use other reducing agents - carbon, hydrogen, magnesium. However, these methods are not widely used.

Chromium high degree purity (about 99.8%) receive electrolytically.

Commercially pure and electrolytic chromium is used mainly for the production of complex chromium alloys.

Constants and properties of chromium

The atomic mass of chromium is 51.996. In the periodic table, he occupies a place in the sixth group. Its closest neighbors and analogues are molybdenum and tungsten. It is characteristic that the neighbors of chromium, as well as chromium itself, are widely used for alloying steels.

The melting point of chromium depends on its purity. Many researchers have tried to determine it and have obtained values from 1513 to 1920°C. Such a large "scatter" is primarily due to the amount and composition of impurities contained in chromium. It is now believed that chromium melts at about 1875°C. Boiling point 2199°C. The density of chromium is less than that of iron; it is equal to 7.19.

In terms of chemical properties, chromium is close to molybdenum and tungsten. Its highest oxide CrO 3 is acidic, it is chromic anhydride H 2 CrO 4. The mineral crocoite, from which we began our acquaintance with element No. 24, is a salt of this acid. In addition to chromic acid, dichromic acid H 2 Cr 2 O 7 is known, its salts, bichromates, are widely used in chemistry. The most common chromium oxide Cr 2 O 3 is amphoterene. In general, in different conditions chromium can exhibit valences from 2 to 6. Only compounds of tri- and hexavalent chromium are widely used.

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

Finding in nature.

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

It is found in Asia Minor, in the Urals, in North America, in southern Africa. The above-mentioned mineral crocoite - PbCrO 4 - is also of technical importance. Chromium oxide (3) and some of its other compounds are also found in nature. In the earth's crust, the chromium content in terms of metal is 0.03%. Chromium is found on the Sun, stars, meteorites.

Physical Properties.

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

Receipt.

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

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

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

Cr 2 O 3 + 2Al \u003d Al 2 O 3 + 2Cr

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

Chemical properties.

Thanks to a thin protective oxide film covering the surface of chromium, it is highly resistant to aggressive acids and alkalis. Chromium does not react with concentrated nitric and sulfuric acids, as well as with phosphoric acid. Chromium interacts with alkalis at t = 600-700 o C. However, chromium interacts with dilute sulfuric and hydrochloric acids, displacing hydrogen:

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

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

Hot chromium reacts with water vapor:

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

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

Cr + 2HF = CrF 2 + H 2
2Cr + N2 = 2CrN
2Cr + 3S = Cr2S3
Cr + Si = CrSi

The above physical and chemical properties of chromium have found their application in various fields of science and technology. For example, chromium and its alloys are used to obtain high-strength, corrosion-resistant coatings in mechanical engineering. Alloys in the form of ferrochrome are used as metal cutting tools. Chrome-plated alloys have found application in medical technology, in the manufacture of chemical process equipment.

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

Chromium heads the side subgroup of group VI of the periodic system of elements. Its electronic formula is as follows:

24 Cr IS 2 2S 2 2P 6 3S 2 3P 6 3d 5 4S 1

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

Divalent chromium compounds.

Chromium oxide (II) CrO - pyrophoric black powder (pyrophoric - the ability to ignite in air in a finely divided state). CrO dissolves in dilute hydrochloric acid:

CrO + 2HCl = CrCl 2 + H 2 O

In air, when heated above 100 0 C, CrO turns into Cr 2 O 3.

Divalent chromium salts are formed by dissolving chromium metal in acids. These reactions take place in an atmosphere of an inactive gas (for example, H 2), because in the presence of air, Cr(II) is easily oxidized to Cr(III).

Chromium hydroxide is obtained in the form of a yellow precipitate by the action of an alkali solution on chromium (II) chloride:

CrCl 2 + 2NaOH = Cr(OH) 2 + 2NaCl

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

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

Trivalent chromium compounds.

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

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

Cr 2 O 3 - amphoteric oxide, when fused with alkalis, forms chromites: Cr 2 O 3 + 2NaOH \u003d 2NaCrO 2 + H 2 O

Chromium hydroxide is also an amphoteric compound:

Cr(OH) 3 + HCl = CrCl 3 + 3H 2 O
Cr(OH) 3 + NaOH = NaCrO 2 + 2H 2 O

Anhydrous CrCl 3 has the appearance of dark purple leaves, is completely insoluble in cold water, and dissolves very slowly when boiled. Anhydrous chromium sulfate (III) Cr 2 (SO 4) 3 pink, also poorly soluble in water. In the presence of reducing agents, it forms purple chromium sulfate Cr 2 (SO 4) 3 *18H 2 O. Green chromium sulfate hydrates are also known, containing a smaller amount of water. Chrome alum KCr(SO 4) 2 *12H 2 O crystallizes from solutions containing violet chromium sulfate and potassium sulfate. A solution of chromic alum turns green when heated due to the formation of sulfates.

Reactions with chromium and its compounds

Almost all chromium compounds and their solutions are intensely colored. Having a colorless solution or a white precipitate, we can conclude with a high degree of probability that chromium is absent.

We strongly heat in the flame of a burner on a porcelain cup such an amount of potassium dichromate that will fit on the tip of a knife. Salt will not release water of crystallization, but will melt at a temperature of about 400 0 C with the formation of a dark liquid. Let's heat it for a few more minutes on a strong flame. After cooling, a green precipitate forms on the shard. Part of it is soluble in water (it turns yellow), and the other part is left on the shard. The salt decomposed when heated, resulting in the formation of soluble yellow potassium chromate K 2 CrO 4 and green Cr 2 O 3 .
Dissolve 3g of powdered potassium dichromate in 50ml of water. To one part add some potassium carbonate. It will dissolve with the release of CO 2 , and the color of the solution will become light yellow. Chromate is formed from potassium bichromate. If we now add a 50% solution of sulfuric acid in portions, then the red-yellow color of the bichromate will appear again.
Pour into a test tube 5 ml. potassium dichromate solution, boil with 3 ml of concentrated hydrochloric acid under draft. Yellow-green poisonous gaseous chlorine is released from the solution, because chromate will oxidize HCl to Cl 2 and H 2 O. The chromate itself will turn into green trivalent chromium chloride. It can be isolated by evaporating the solution, and then, fusing with soda and nitrate, converted to chromate.
When a solution of lead nitrate is added, yellow lead chromate precipitates; when interacting with a solution of silver nitrate, a red-brown precipitate of silver chromate is formed.
Add hydrogen peroxide to a solution of potassium bichromate and acidify the solution with sulfuric acid. The solution becomes deep blue color due to the formation of chromium peroxide. Peroxide, when shaken with some ether, will turn into an organic solvent and turn it blue. This reaction is specific for chromium and is very sensitive. It can be used to detect chromium in metals and alloys. First of all, it is necessary to dissolve the metal. With prolonged boiling with 30% sulfuric acid (hydrochloric acid can also be added), chromium and many steels partially dissolve. The resulting solution contains chromium (III) sulfate. To be able to conduct a detection reaction, we first neutralize it with caustic soda. Gray-green chromium (III) hydroxide precipitates, which dissolves in excess NaOH and forms green sodium chromite. Filter the solution and add 30% hydrogen peroxide. When heated, the solution will turn yellow, as chromite is oxidized to chromate. Acidification will result in a blue color of the solution. The colored compound can be extracted by shaking with ether.

Analytical reactions for chromium ions.

To 3-4 drops of a solution of chromium chloride CrCl 3 add a 2M solution of NaOH until the initial precipitate dissolves. Note the color of the sodium chromite formed. Heat the resulting solution in a water bath. What is happening?
To 2-3 drops of CrCl 3 solution add an equal volume of 8M NaOH solution and 3-4 drops of 3% H 2 O 2 solution. Heat the reaction mixture in a water bath. What is happening? What precipitate is formed if the resulting colored solution is neutralized, CH 3 COOH is added to it, and then Pb (NO 3) 2 ?
Pour 4-5 drops of solutions of chromium sulfate Cr 2 (SO 4) 3, IMH 2 SO 4 and KMnO 4 into a test tube. Heat the reaction site for several minutes on a water bath. Note the change in color of the solution. What caused it?
To 3-4 drops of acidified nitric acid K 2 Cr 2 O 7 solution add 2-3 drops of H 2 O 2 solution and mix. The blue color of the solution that appears is due to the appearance of perchromic acid H 2 CrO 6:

Cr 2 O 7 2- + 4H 2 O 2 + 2H + = 2H 2 CrO 6 + 3H 2 O

Pay attention to the rapid decomposition of H 2 CrO 6:

2H 2 CrO 6 + 8H+ = 2Cr 3+ + 3O 2 + 6H 2 O
blue color green color

Perchromic acid is much more stable in organic solvents.

To 3-4 drops of K 2 Cr 2 O 7 solution acidified with nitric acid, add 5 drops of isoamyl alcohol, 2-3 drops of H 2 O 2 solution and shake the reaction mixture. The layer of organic solvent that floats to the top is colored bright blue. The color fades very slowly. Compare the stability of H 2 CrO 6 in organic and aqueous phases.
When CrO 4 2- and Ba 2+ ions interact, a yellow precipitate of barium chromate BaCrO 4 precipitates.
Silver nitrate forms brick red precipitate of silver chromate with CrO 4 2 ions.
Take three test tubes. Place 5-6 drops of K 2 Cr 2 O 7 solution in one of them, the same volume of K 2 CrO 4 solution in the second, and three drops of both solutions in the third. Then add three drops of potassium iodide solution to each tube. Explain the result. Acidify the solution in the second tube. What is happening? Why?

Entertaining experiments with chromium compounds

A mixture of CuSO 4 and K 2 Cr 2 O 7 turns green when alkali is added, and turns yellow in the presence of acid. By heating 2 mg of glycerol with a small amount of (NH 4) 2 Cr 2 O 7 and then adding alcohol, a bright green solution is obtained after filtration, which turns yellow when an acid is added, and turns green in a neutral or alkaline medium.
Place in the center of the can with thermite "ruby mixture" - thoroughly ground and placed in aluminum foil Al 2 O 3 (4.75 g) with the addition of Cr 2 O 3 (0.25 g). So that the jar does not cool down longer, it is necessary to bury it under the upper edge in the sand, and after the thermite is ignited and the reaction begins, cover it with an iron sheet and cover it with sand. Bank to dig out in a day. The result is a red-ruby powder.
10 g of potassium bichromate is triturated with 5 g of sodium or potassium nitrate and 10 g of sugar. The mixture is moistened and mixed with collodion. If the powder is compressed in a glass tube, and then the stick is pushed out and set on fire from the end, then a “snake” will begin to crawl out, first black, and after cooling - green. A stick with a diameter of 4 mm burns at a speed of about 2 mm per second and lengthens 10 times.
If you mix solutions of copper sulfate and potassium dichromate and add a little ammonia solution, then an amorphous brown precipitate of the composition 4СuCrO 4 * 3NH 3 * 5H 2 O will fall out, which dissolves in hydrochloric acid to form a yellow solution, and in excess of ammonia a green solution is obtained. If further alcohol is added to this solution, a green precipitate will form, which, after filtration, becomes blue, and after drying, blue-violet with red sparkles, clearly visible in strong light.
The chromium oxide left after the “volcano” or “pharaoh snake” experiments can be regenerated. To do this, it is necessary to fuse 8 g of Cr 2 O 3 and 2 g of Na 2 CO 3 and 2.5 g of KNO 3 and treat the cooled alloy with boiling water. Soluble chromate is obtained, which can also be converted into other Cr(II) and Cr(VI) compounds, including the original ammonium dichromate.

Examples of redox transitions involving chromium and its compounds

1. Cr 2 O 7 2- -- Cr 2 O 3 -- CrO 2 - -- CrO 4 2- -- Cr 2 O 7 2-

a) (NH 4) 2 Cr 2 O 7 = Cr 2 O 3 + N 2 + 4H 2 O b) Cr 2 O 3 + 2NaOH \u003d 2NaCrO 2 + H 2 O
c) 2NaCrO 2 + 3Br 2 + 8NaOH = 6NaBr + 2Na 2 CrO 4 + 4H 2 O
d) 2Na 2 CrO 4 + 2HCl = Na 2 Cr 2 O 7 + 2NaCl + H 2 O

2. Cr(OH) 2 -- Cr(OH) 3 -- CrCl 3 -- Cr 2 O 7 2- -- CrO 4 2-

a) 2Cr(OH) 2 + 1/2O 2 + H 2 O = 2Cr(OH) 3
b) Cr(OH) 3 + 3HCl = CrCl 3 + 3H 2 O
c) 2CrCl 3 + 2KMnO 4 + 3H 2 O = K 2 Cr 2 O 7 + 2Mn(OH) 2 + 6HCl
d) K 2 Cr 2 O 7 + 2KOH = 2K 2 CrO 4 + H 2 O

3. CrO - Cr (OH) 2 - Cr (OH) 3 - Cr (NO 3) 3 - Cr 2 O 3 - CrO - 2
Cr2+

a) CrO + 2HCl = CrCl 2 + H 2 O
b) CrO + H 2 O \u003d Cr (OH) 2
c) Cr(OH) 2 + 1/2O 2 + H 2 O = 2Cr(OH) 3
d) Cr(OH) 3 + 3HNO 3 = Cr(NO 3) 3 + 3H 2 O
e) 4Cr (NO 3) 3 \u003d 2Cr 2 O 3 + 12NO 2 + O 2
f) Cr 2 O 3 + 2 NaOH = 2NaCrO 2 + H 2 O

Chrome element as an artist

Chemists quite often turned to the problem of creating artificial pigments for painting. In the 18th-19th centuries, the technology for obtaining many pictorial materials was developed. Louis Nicolas Vauquelin in 1797, who discovered the previously unknown element chromium in Siberian red ore, prepared a new, remarkably stable paint - chrome green. Its chromophore is aqueous chromium (III) oxide. Under the name "emerald green" it began to be produced in 1837. Later, L. Vauquelen proposed several new paints: barite, zinc and chrome yellow. Over time, they were replaced by more persistent yellow, orange pigments based on cadmium.

Chrome green is the most durable and lightfast paint that is not affected by atmospheric gases. Rubbed in oil, chrome green has great hiding power and is capable of drying quickly, therefore, since the 19th century. it is widely used in painting. It is of great importance in porcelain painting. The fact is that porcelain products can be decorated with both underglaze and overglaze painting. In the first case, paints are applied to the surface of only a slightly fired product, which is then covered with a layer of glaze. This is followed by the main, high-temperature firing: for sintering the porcelain mass and melting the glaze, the products are heated to 1350 - 1450 0 C. Very few paints can withstand such a high temperature without chemical changes, and in the old days there were only two of them - cobalt and chromium. Black oxide of cobalt, applied to the surface of a porcelain item, fuses with the glaze during firing, chemically interacting with it. As a result, bright blue cobalt silicates are formed. This cobalt blue chinaware is well known to everyone. Chromium oxide (III) does not interact chemically with the components of the glaze and simply lies between the porcelain shards and the transparent glaze with a "deaf" layer.

In addition to chrome green, artists use paints derived from Volkonskoite. This mineral from the group of montmorillonites (a clay mineral of the subclass of complex silicates Na (Mo, Al), Si 4 O 10 (OH) 2) was discovered in 1830 by the Russian mineralogist Kemmerer and named after M.N. Volkonskaya, the daughter of the hero of the Battle of Borodino, General N N. Raevsky, wife of the Decembrist S. G. Volkonsky. Volkonskoite is a clay containing up to 24% chromium oxide, as well as oxides of aluminum and iron (III). The variability of the composition of the mineral found in the Urals, in the Perm and Kirov regions determines its diverse coloration - from the color of a darkened winter fir to the bright green color of a marsh frog.

Pablo Picasso turned to the geologists of our country with a request to study the reserves of Volkonskoite, which gives the paint a uniquely fresh tone. At present, a method has been developed for obtaining artificial wolkonskoite. It is interesting to note that, according to modern research, Russian icon painters used paints from this material as early as the Middle Ages, long before its “official” discovery. Guinier green (created in 1837), whose chromoform is a hydrate of chromium oxide Cr 2 O 3 * (2-3) H 2 O, where part of the water is chemically bound, and part is adsorbed, was also known among artists. This pigment gives the paint an emerald hue.

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Chrome is a silvery white, hard, shiny, but at the same time rather brittle metal. Previously, it was believed that chromium has practically no plastic properties. But in the 70s of the last century, by remelting it with an electron beam in a vacuum, a very plastic metal was obtained, stretching into a thin wire. Chemistry course, part 2. Special for engineering and transport universities / G.P. Luchinsky [i dr.]. - M.: Higher School, 1972. - P.101.

Main physical properties chromium are given below: Lavrukhina A.K. Analytical chemistry of chromium / A.K. Lavrukhina, L.V. Yukina. - M.: Nauka, 1979. - S.9-10.

Atomic mass 51.996

atomic volume, cm 3 /g-atom 7,23

Atomic radius E

covalent 1.18

metallic 1.27

Steam pressure (1560°K), atm 1,50 10 -6

The lattice period ( but)* I , B 2.8829

Density, g/cm 3

x-ray 7.194

pycnometric 7.160

Ionization potentials

I 1 \u003d 6.764 I 4 \u003d (51)

I 2 \u003d 16.49 I 5 \u003d 73

I 3 \u003d 31 I 6 \u003d 90.6

Brinell hardness (20°), MPa 1120* 2

Melting point, °K 2176.0

Boiling point, °K 2840.0

heat of melting, cal/mol 3300,0* 3

heat of sublimation, kcal/mol 94,8* 3

Thermal conductivity, w/m deg 88,6

Specific electronic 1.40

heat capacity g, mj (mol deg)

atomization energy, kcal/mol

Entropy S° T (298° K)

gaseous Cr, cal/(g-atom deg) 41,64

metal Cr, cal/(mol deg) 5,70

The main ore of chromium is the mineral chromite FeCr 2 O 4 , which has a spinel structure in which Cr (III) atoms occupy octahedral positions and Fe (II) occupy tetrahedral positions. Cotton F. Fundamentals of inorganic chemistry / F. Cotton, J. Wilkinson. - M.: Mir, 1979. - P. 458.

Chromite is reduced with carbon, and to obtain ferrochromium, the content of chromium oxide in the ore must be at least 48%. During the melting process, the following reaction takes place:

FeO Cr 2 O 3 + 4C > Fe + 2Cr + 4CO^

In addition, chromium is a part of many minerals, in particular, crocoite PbCrO 4 ; other minerals containing chromium include finicite, menachloite or phenicohloite 3PbO*2Cr 2 O 3 , berezovite, trapakalite, magnochromite, etc. Properties of the elements: reference ed./M.E. Dritz [et al.]. - M: Metallurgy, 1985. - P.368.

Other impurities also significantly affect the physical and chemical properties of chromium. For example, in the presence of Al, Cu, Ni, Fe, Co, Si, W, Mo impurities (up to ~1%), the brittleness threshold of chromium sharply increases; impurities of hydrogen, oxygen and nitrogen have very little effect. A.K. Lavrukhin. Decree. op. - p.9.

Chromium of technical purity is obtained by aluminothermic, silicothermal, electrolytic and other methods from chromium oxide, which is obtained from chromium iron ore. M.E. Dritz. Decree. op. - P.368.

If you want to get pure chromium, then chromite is first alloyed with NaOH and oxidized with oxygen to convert Cr (III) to CrO 4 2-. The alloy is dissolved in water, sodium dichromate is precipitated from it, which is then reduced with carbon:

Na 2 Cr 2 O 7 + 2C > Cr 2 O 3 + Na 2 CO 3 + CO ^

The resulting oxide is reduced to metallic chromium:

Cr 2 O 3 + 2Al > Al 2 O 3 + 2Cr F. Cotton. Decree. op. - P.458.

The purest chromium for laboratory research obtained by the iodide method. This process is based on the formation of volatile chromium iodides (at 700-900°C) and their dissociation on a heated surface (at 1000-1100°C). Chromium metal after iodide refining is ductile in the cast state (tensile elongation 9-18%). M.E. Dritz. Decree. op. - P.368-369.

For metallic chromium, polymorphic modifications are known, one of which is stable - this is b-chromium. in-chrome is a less stable modification, obtained by electrolytic deposition. The crystal lattices of b-chromium and b-chromium are shown below in the figure. G.P. Luchinsky. Decree. op. - P.101-102.

Under nonequilibrium conditions, the formation of chromium crystals with a different structure is possible; condensation of chromium vapor resulted in a variety with a primitive cubic lattice ( but= 4.581E), close to structural type in-W. Chromium has a complex magnetic structure; it is characterized by three magnetic transformations: at 120, 310, 473°K. A.K. Lavrukhin. Decree. op. - p.9.

As mentioned above, chromium is an element of group VIB of the fourth period.

If we exclude the stoichiometry of the compounds, chromium resembles the elements of group VIB (sulfur group) only in that it forms an acid oxide, and CrO 2 Cl 2 has a covalent nature and is easily hydrolyzed. F. Cotton. Decree. op. - P.458.

The electronic structure of its atoms is 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1. Chromium belongs to the group of transition elements, in which the d-orbitals are only partially filled. This determines the ability of chromium to form paramagnetic compounds, its variable valence and the color of many compounds.

A characteristic feature of chromium as a transition element of the d-group is the ability to form numerous complex compounds with different structures, valencies, and types of bonds. The formation of complex compounds with neutral molecules leads to the stabilization of the lower oxidation states of the d-elements. As a consequence, there are chromium compounds in the oxidation state 0 (system d 6). Monovalent chromium is reliably known only in the form of complexes K 3 , ClO 4 (where Dip is 2,2?-dipyridyl). A.K. Lavrukhin. Decree. op. - P.12.

Most often, chromium compounds have the following spatial structure:

> Octahedral structures like in 2+ or 3+

> Tetrahedral structures, as in Cr(O-tert-C 4 H 9) 4

> Tetrahedral structures, as in CrO 4 3- , CrO 4 2- , CrO 3 F. Cotton. Decree. op. - P.459.

Chromium, being a reducing agent, can donate from 2 to 6 electrons.

Therefore, the following oxidation states are typical for chromium: from -2 to +6. In compounds, chromium often exhibits degrees +2, +3, +6, less often +1, +4, +5. M.E. Dritz. Decree. op. - P.373.

For chromium, the most stable oxidation state is +3 (d 3- system; half filling t 2g orbitals in octahedral coordination). Compounds with a formal oxidation state of -2 are also known. In the +6 oxidation state, chromium somewhat resembles vanadium (+5). Anorganicum/G. Blumenthal [i dr.]. - M.: Mir, 1984. - S.617-618.

The solubility of chromium compounds varies mainly depending on the degree of oxidation.

The most prevalent are the 3-valent and 6-valent states of chromium. Registration numbers assigned by the Chemical Abstracts Service (CAS) for 3-valent and 6-valent chromium are 16065-83-3 and 18540-29-9, respectively. Wilbur S, Abadin H, Fay M, et al.

Chromium Chemical Abstracts Service (CAS) registration number is 7440-47-3. Hygienic criteria for the state of the environment. Chromium. A modern edition of the United Nations Environment Programme. Geneva. 1990. Table No. 1.

IN pure form chromium(0) is practically non-existent. Nevertheless, there is a relatively unstable chromium in the 2-valent state, which, under the influence of the environment, is easily oxidized to chromium (III).

Chromium compounds are more stable in the 3-valent state, more stable in the environment and occur naturally in ores such as ferrochromates (FeCr 2 O 4). Chromium 6 is second in stability, but it is found in rare minerals such as crocoite (PbCrO 4). The 6-valent chromium compounds are primarily the result of human activity. Wilbur S, Abadin H, Fay M, et al.

The relationship between the 3-valent and 6-valent states of chromium is described by the equation:

Cr 2 6+ O 7 2- + 14H + + 6h > 2Cr (III) + 7H 2 O + 1.33V

The differences in electron charge between the two states reflect the strong oxidizing properties of 6-valent chromium and thus the energy required to oxidize the 3-valent form to the 6-valent form. Hygienic criteria for the state of the environment. Chromium. A modern edition of the United Nations Environment Programme. Geneva. 1990.

In a series of voltages, chromium is among the electronegative elements and relatively active metals that can go into solution with the formation of positive ions (chromium is between zinc and iron: Zn¦Zn 2+ - 0.762; Cr¦Cr 3+ - 0.71; Fe ¦ Fe 2+ - 044). Mikhailenko Ya.I. Course of General and Inorganic Chemistry / Ya.I. Mikhailenko. - M.: Higher school, 1966. - S.320. However, in air and in oxidizing environments, chromium is easily passivated and acquires the properties of noble metals.

In air, chromium deposits retain their luster and color. This is explained by the fact that the passive film on the surface of chromium, which is characterized by a small thickness and high transparency, well protects the coating from tarnishing. When the temperature rises to 400-500°C, the oxidizability of chromium increases slightly. The temperature of the rapid oxidation of chromium is about 1100°C or more. Cherkez M.B. Chrome plating/M.B. Cherkez. - L .: Mashinostroenie, 1971. - P. 31.

The most common oxide is Cr 2 O 3 (31.6 O), which is a green refractory substance (green chromium) used for the preparation of glue and oil paints. The highest chromium oxide CrO 3 - dark red needle-shaped crystals is a chromic anhydride, we will dissolve well in water. M.E. Dritz. Decree. op. - P.374.

Fluoride CrF 2 - bluish-green crystals, slightly soluble in water; in air they are oxidized to Cr 2 O 3 . Get CrF 2 passing gas. HF over chromium metal powder at red heat. Known double fluorides with cations NH 4+ and K + composition M I CrF 3

Chromium(III) fluoride exists in anhydrous and hydrated forms. Greenish needles of CrF 3 are insoluble in water, alcohol, ammonia, poorly soluble in acids. The hydrated form is insoluble in ethanol, slightly soluble in water. A.K. Lavrukhin. Decree. op. - P.19-20.

When heated, it combines directly with other halogens, as well as with nitrogen, silicon, boron and some metals:

2Cr + 3Cl 2 > 2CrCl 3

Cr + 2Si > CrSi 2

Two chromium nitrides Cr 2 N and CrN are known. The latter is obtained by passing a stream of nitrogen over a thin powder of pyrophoric chromium heated at 600-900 ° C: A.K. Lavrukhin. Decree. op. - P.21.

2Cr + N 2 > 2CrN

Chloride CrCl 2 is a colorless crystalline hygroscopic compound, soluble in water. Get CrCl 2 passing gas. HCl over powdered chromium at red heat.

Chromium(III) chloride is produced in many ways. Anhydrous CrCl 3 - red-violet crystals, poorly soluble in water, however, in the presence of traces of reducing agents, its solubility increases. Insoluble in absolute ethanol and methanol, acetaldehyde, acetone, diethyl ether.

CrBr 2 bromide, a yellowish-white compound, is obtained by reacting Cr metal and dry HBr at high temperature. Let's dissolve in water with formation of blue solution and in ethanol.

Bromide CrBr 3 is a black crystalline compound, which is obtained by the action of bromine on heated chromium. Soluble in hot water.

Iodide CrJ 2 pale gray compound, obtained by synthesis from Cr and J 2 at 800°C; soluble in water. Black CrJ 3 is obtained by heating iodine with chromium at 500° C. in an evacuated tube. Difficult to dissolve in water. A.K. Lavrukhin. Decree. op. - P.20-21.

In 1926, Weisselfelder succeeded in obtaining chromium hydride CrH 3 for the first time. ME AND. Mikhailenko. Decree. op. - P.320. CrH hydride is also known, these hydrides differ in crystal structure and properties. They are not stable and decompose when heated. Chromium absorbs significant amounts of hydrogen, especially during its electrolytic separation from solutions containing sugar as a reducing agent. The hydrogen content in the resulting solid solution can reach up to 5 at. %. G.P. Luchinsky. Decree. op. - P.103.

As a result of the interaction of metals with carbon at high temperatures, carbides of various compositions are formed. The most studied are Cr 4 C, Cr 2 C 3 , Cr 3 C 2 . G.P. Luchinsky. Decree. op. - P.103.

With sulfur, chromium forms sulfides CrS (38.1% S), Cr 2 S 3 (47.9% S), Cr 3 S 4 (45.1% S). Sulfide CrS is unstable at room temperature and decomposes with the release of pure chromium. M.E. Dritz. Decree. op. - P.374.

Sulfides are obtained by 24-hour heating in an electric furnace at 1000°C mixtures of appropriate equivalent amounts of electrolytic chromium and purified sulfur in sealed quartz ampoules.

Only diphosphide CrP 2 , which is formed during synthesis from elements at high temperatures, monophosphide CrP, which is formed during synthesis from elements by passing phosphine over chromium powder heated to 850 ° C, and subphosphide Cr 3 P, have been reliably studied. Lavrukhin. Decree. op. - P.22.

The closest analogues of chromium are molybdenum and tungsten, with which it forms continuous solid solutions. As the difference in physical and chemical properties chromium and the element interacting with it, the solubility decreases, and is absent in the limit. Elements of the IA subgroup - lithium, sodium, potassium, rubidium and cesium - do not interact with chromium under normal conditions due to the large difference in the sizes of atomic diameters. Gold, copper and silver are extremely sparingly soluble in chromium. chromium chemical oxide metal

Beryllium forms limited solid solutions with chromium with a variable temperature solubility, as well as the metal compound CrBe 2 . There is no information on the interaction of chromium with magnesium, calcium, strontium and barium. The possibility of the formation of solid solutions of these elements in chromium is extremely limited due to the large difference in the atomic diameters of chromium and these elements.

The propensity of chromium to interact with metals of subgroup IIB - zinc, cadmium and mercury - is also extremely weakly expressed. With elements IIIA of the subgroup - yttrium and lanthanum - chromium forms limited solid solutions and metal compounds - borides and aluminides; some of them, such as CrB, are of practical interest in the development of alloys with special properties.

With elements of the IVA subgroup - titanium, zirconium and hafnium - chromium forms limited solid solutions and compounds of the type AB 2, related by their crystal chemical nature to Laves phases. These TiCr 2 , ZrCr 2 , and HfCr 2 phases have a structure of the MgCu 2 type at room temperature, and upon heating undergo a polymorphic transformation MgCu 2 - MgZn 2 .

With silicon, chromium forms silicides: Cr 3 Si, Cr 3 Si 2, Cr 5 Si 3, CrSi, CrSi 2.

Chromium interacts with elements of the VA subgroup in different ways. With vanadium, chromium forms continuous solid solutions, and with niobium and tantalum, metal compounds of the Laves phase type - NbCr 2 and TaCr 2 .

With manganese and rhenium, the interaction of chromium is practically the same - limited solid solutions of great extent are formed on the chromium side and intermediate compounds of the y-phase type.

With elements of group VIII, chromium forms limited solid solutions, and with some of them (cobalt, iron, platinum, palladium, iridium and ruthenium), in addition, metal compounds. Metal compounds of chromium with platinum, iridium, ruthenium have a crystal lattice of the β-tungsten type. In chromium-iron and chromium-cobalt systems, there is a y-phase, which contributes to an increase in hardness and embrittlement of alloys. M.E. Dritz. Decree. op. - S.374-375.

Chromium is corrosion resistant to many acids, alkalis and salts. M.B. Cherkez. Decree op. - P.31. Some acids, for example, concentrated nitric, phosphoric, chloric, chloric, form an oxide film on chromium, leading to its passivation. In this state, chromium has exceptionally high corrosion resistance and is unaffected by dilute mineral acids. Chromium is electronegative with respect to the most practically important metals and alloys, and if it forms a galvanic couple with them, it accelerates their corrosion. M.E. Dritz. Decree. op. - P.373.

At the same time, as mentioned above, chromium is resistant to corrosion, so it is used as a protective coating that is applied by electrolysis. F. Cotton. Decree. op. - P.458.

In hydrochloric and hot, concentrated sulfuric acid, chromium dissolves vigorously:

Cr + 2HCl > CrCl 2 + H 2 ^

Cr + H 2 SO 4 > CrSO 4 + H 2 ^

However, the dissolution rate of chromium big influence exerts the temperature of the electrolyte during its deposition. M.B. Cherkez. Decree op. - P.31.

A greater number of simple and complex compounds of Cr (II) and Cr (III) with organic acids are known. So chromium (II) acetate is one of the most common and stable compounds of divalent chromium; salts of carboxylic acids are known. Chromium (III) forms complexes with oxalic acid: +, 0 (where OAc is an acetate ion), -, 3-.

The complex formation reactions of Cr (III) with malonic and succinic acids have been studied; complexes of composition 1:1, 1:2, 1:3 were obtained. Similar compositions of the complexes were obtained by the interaction of Cr (III) and phthalic acid. Complexes of Cr (III) with adipyric acid (Ad) have compositions 0 and - . Complexes of Cr (III) with ascorbic acid and alizarinsulfonic acids. Complexes of Cr (II) and Cr (III) with picolinic acid of compositions CrА + and CrА 2+ have been studied. An exceptionally sharp decrease in the reducing properties of Cr (II) in the CrA + complex has been established; its oxidation does not occur even in a stream of oxygen at 20°C.

Chromium (III) forms complexes with ethylenediaminetetraacetic acid (H 4 Y) and its derivatives very slowly; this process is accelerated by heating. In aqueous solutions at different pH there are four different complexes: violet H and -, blue 2- and in a strongly alkaline solution - green 3-. With nitrilotriacetic acid (H 3 X) in alkaline solutions of Cr (III) forms hydrocomplexes - (violet) and 2- (green). A.K. Lavrukhin. Decree. op. - P.34-25.