Chromium chemical bond. Chromium - daily allowance, benefits and harms

Item #24. One of the hardest metals. It has high chemical resistance. One of essential metals used in the production of alloy steels. Most chromium compounds have a bright color, and the most different 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 from brightly colored chromium compounds one can obtain good paints. (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

AT earth's crust quite a lot of chromium - 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 - 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.

Young Soviet country in the first years of development, ferroalloys had to be imported 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 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 ores 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 mainly used 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.

High purity chromium (about 99.8%) is produced 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.

By 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.

Chromium has all the properties of a metal - it conducts heat and electric current well, has a characteristic metallic sheen. The main feature of chromium is its resistance to acids and oxygen.

For those who constantly deal with chromium, another of its features has become a byword: at a temperature of about 37 ° C, some physical properties of this metal sharply, abruptly change. At this temperature, there is a pronounced maximum of internal friction and a minimum of the modulus of elasticity. The electrical resistance, the coefficient of linear expansion, and the thermoelectromotive force change almost as sharply.

Scientists have yet to explain this anomaly.

Four natural isotopes of chromium are known. Their mass numbers are 50, 52, 53 and 54. The share of the most common isotope 52 Cr is about 84%

Chromium in alloys

It would probably be unnatural if the story of the use of chromium and its compounds began not with steel, but with something else. Chromium is one of the most important alloying elements used in the iron and steel industry. The addition of chromium to ordinary steels (up to 5% Cr) improves their physical properties and makes the metal more susceptible to heat treatment. Chromium is alloyed with spring, spring, tool, die and ball bearing steels. In them (except for ball-bearing steels), chromium is present together with manganese, molybdenum, nickel, vanadium. And ball bearing steels contain only chromium (about 1.5%) and carbon (about 1%). The latter forms with chromium carbides of exceptional hardness: Cr 3 C. Cr 7 C 3 and Cr 23 C 6 . They give ball bearing steel high wear resistance.

If the chromium content of the steel is increased to 10% or more, the steel becomes more resistant to oxidation and corrosion, but this is where a factor that can be called carbon limitation comes into play. The ability of carbon to bind large quantities chromium leads to depletion of steel with this element. Therefore, metallurgists face a dilemma: if you want to get corrosion resistance, reduce the carbon content and lose on wear resistance and hardness.

The most common grade of stainless steel contains 18% chromium and 8% nickel. The carbon content in it is very low - up to 0.1%. Stainless steels resist corrosion and oxidation well and retain their strength at high temperatures. From sheets of such steel, a sculptural group by V.I. Mukhina "Worker and Collective Farm Woman", which is installed in Moscow at the Northern entrance to the Exhibition of Achievements of the National Economy. Stainless steels are widely used in the chemical and petroleum industries.

High-chromium steels (containing 25...30% Cr) are particularly resistant to oxidation at high temperature. They are used for the manufacture of parts for heating furnaces.

Now a few words about chromium-based alloys. These are alloys containing more than 50% chromium. They have very high heat resistance. However, they have a very big drawback that negates all the advantages: these alloys are very sensitive to surface defects: it is enough to get a scratch, a microcrack, and the product will quickly collapse under load. For most alloys similar shortcomings are eliminated by thermomechanical treatment, but chromium-based alloys cannot be treated by such treatment. In addition, they are too fragile room temperature which also limits their applicability.

More valuable alloys of chromium with Nickel (they are often introduced as alloying additives and other elements). The most common alloys of this group - nichrome contain up to 20% chromium (the rest is nickel) and are used for the manufacture of heating elements. Nichromes have a large electrical resistance for metals; when current is passed, they heat up very much.

The addition of molybdenum and cobalt to chromium-nickel alloys makes it possible to obtain materials with high heat resistance and the ability to withstand heavy loads at 650...900°C. These alloys are used to make, for example, gas turbine blades.

Heat resistance is also possessed by chromium-cobalt alloys containing 25 ... 30% chromium. The industry also uses chromium as a material for anti-corrosion and decorative coatings.

...and other connections

The main chromium ore, chromite, is also used in the production of refractories. Magnesite-chromite bricks are chemically passive and heat-resistant, they can withstand repeated sharp temperature changes. Therefore, they are used in the construction of the arches of open-hearth furnaces. The resistance of magnesite-chromite vaults is 2...3 times greater than that of Dinas vaults*.

* Dinas is an acid refractory brick containing at least 93% silica. Dinas fire resistance is 1680...1730°C. In the 14th volume of the Great Soviet Encyclopedia (2nd edition), published in 1952, dynas is called an indispensable material for the arches of open-hearth furnaces. This statement should be considered obsolete, although dinas is still widely used as a refractory.

Chemists obtain mainly potassium and sodium bichromates from chromite K 2 Cr 2 O 7 and Na 2 Cr 2 O 7 .

Phromates and chrome alums KCr(SO 4); used for tanning leather. Hence the name "chrome" boots. Leather. tanned with chromium compounds, it has a beautiful sheen, is durable and easy to use.

From lead chromate РbCrО 4 . manufacture various dyes. A solution of sodium dichromate is used to clean and pickle the surface of steel wire before galvanizing, and also brighten brass. Chromite and other chromium compounds are widely used as dyes for ceramic glazes and glass.

Finally, chromic acid is obtained from sodium dichromate, which is used as an electrolyte in chromium plating of metal parts.

What's next?

Chromium will retain its importance as an alloying addition to steel and as a material for metal coatings in the future; chromium compounds used in the chemical and refractory industries will not lose their value.

The situation is much more complicated with chromium-based alloys. Great brittleness and the exceptional complexity of machining do not yet allow these alloys to be widely used, although they can compete with any materials in terms of heat resistance and wear resistance. AT last years a new direction in the production of chromium-containing alloys has been outlined - alloying them with nitrogen. This gas, which is usually harmful in metallurgy, forms strong compounds with chromium - nitrides. Nitriding of chromium steels increases their wear resistance and reduces the content of deficient nickel in "stainless steels". Perhaps this method will also overcome the "machinability" of chromium-based alloys? Or others will come to the rescue here, until known methods? One way or another, one must think that in the future these alloys will take their rightful place among necessary equipment materials.

Three or six?

Since chromium resists air oxidation and acids well, it is often applied to the surface of other materials to protect them from corrosion. The application method has long been known - this is electrolytic deposition. However, at first, unexpected difficulties arose in the development of the electrolytic chromium plating process.

It is known that conventional electroplating is applied using electrolytes in which the ion of the applied element has a positive charge. With chromium, this did not work out: the coatings turned out to be porous and easily peeled off.

For almost three quarters of a century, scientists have been working on the problem of chromium plating, and only in the 20s of our century they found that the electrolyte of a chrome bath should contain not trivalent chromium, but chromic acid, i.e. hexavalent chromium. In industrial chromium plating, salts of sulfuric and hydrofluoric acids are added to the bath; free acid radicals catalyze the process of galvanic deposition of chromium.

Scientists have not yet come to a consensus on the mechanism of deposition of hexavalent chromium on the cathode of a galvanic bath. There is an assumption that hexavalent chromium passes first into trivalent, and then is reduced to metal. However, most experts agree that chromium at the cathode is restored immediately from the hexavalent state. Some scientists believe that atomic hydrogen is involved in this process, some that hexavalent chromium simply gains six electrons.

Decorative and solid

Chrome coatings are of two types: decorative and hard. More often you have to deal with decorative ones: on the clock, door handles and other items. Here, a layer of chromium is deposited on top of another metal, most commonly nickel or copper. Steel is protected from corrosion by this sublayer, and a thin (0.0002 ... 0.0005 mm) layer of chromium gives the product a formal look.

Solid surfaces are constructed differently. Chromium is applied to steel in a much thicker layer (up to 0.1 mm), but without sublayers. Such coatings increase the hardness and wear resistance of steel, as well as reduce the coefficient of friction.

Chrome plating without electrolyte

There is another way of applying chromium coatings - diffusion. This process takes place not in galvanic baths, but in furnaces.

The steel part is placed in chromium powder and heated in a reducing atmosphere. Within 4 hours at a temperature of 1300°C, a chromium-enriched layer 0.08 mm thick forms on the surface of the part. The hardness and corrosion resistance of this layer is much greater than the hardness of steel in the mass of the part. But this seemingly simple method had to be repeatedly improved. Chromium carbides formed on the surface of the steel, which prevented the diffusion of chromium into the steel. In addition, chromium powder sinters at a temperature of about a thousand degrees. To prevent this from happening, neutral refractory powder is mixed into it. Attempts to replace chromium powder with a mixture of chromium oxide and charcoal did not give positive results.

A more vital proposal was to use its volatile halide salts, such as CrCl 2 , as a carrier for chromium. Hot gas washes the chrome-plated product, and the following reaction occurs:

CrCl 2 + Fe ↔ FeCl 2 + Cr.

The use of volatile halide salts made it possible to lower the chromium plating temperature.

Chromium chloride (or iodide) is usually obtained in the chromium plating plant itself, by passing vapors of the corresponding hydrohalic acid through powdered chromium or ferrochromium. The resulting gaseous chloride washes the chrome-plated product.

The process takes a long time - several hours. The layer applied in this way is much more strongly bonded to the base material than the galvanically applied one.

It all started with washing dishes...

In any analytical laboratory there is a large bottle with a dark liquid. This is a "chromium mixture" - a mixture of a saturated solution of potassium bichromate with concentrated sulfuric acid. Why is she needed?

On the fingers of a person there is always fatty contamination, which easily transfers to glass. It is these deposits that the chromium mixture is designed to wash off. It oxidizes fat and removes its residues. But this substance must be handled with care. A few drops of a chromium mixture that fell on a suit can turn it into a kind of sieve: there are two substances in the mixture, and both are "robbers" - a strong acid and a strong oxidizing agent.

Chrome and wood

Even in our age of glass, aluminium, concrete and plastics, wood cannot but be recognized as excellent. building material. Its main advantage is ease of processing, and its main disadvantages are fire hazard, susceptibility to destruction by fungi, bacteria, and insects. Wood can be made more resistant by impregnating it with special solutions, which necessarily include chromates and dichromates plus zinc chloride, copper sulfate, sodium arsenate and some other substances. Impregnation greatly increases the resistance of wood to the action of fungi, insects, flames.

Looking at a drawing

Illustrations in printed publications are made with cliches - metal plates on which this pattern (or rather, its mirror image) is engraved by chemical means or manually. Before the invention of photography, clichés were only engraved by hand; it is laborious work that requires great skill.

But back in 1839 there was a discovery that seemed to have nothing to do with printing. It was found that paper impregnated with sodium or potassium dichromate, after illumination bright light suddenly turns brown. Then it turned out that bichromate coatings on paper, after exposure, do not dissolve in water, but, when wetted, acquire a bluish tint. This property was used by printers. The desired pattern was photographed on a plate with a colloidal coating containing bichromate. The illuminated areas did not dissolve during washing, but the non-exposed ones dissolved, and a pattern remained on the plate from which it was possible to print.

Now other photosensitive materials are used in printing, the use of bichromate gels is declining. But do not forget that chromium helped the "pioneers" of the photomechanical method in printing.

The content of the article

CHROMIUM– (Chromium) Cr, chemical element 6(VIb) of group of the Periodic system. Atomic number 24, atomic mass 51,996. There are 24 known isotopes of chromium from 42 Cr to 66 Cr. Isotopes 52 Cr, 53 Cr, 54 Cr are stable. The 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, a professor of chemistry 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 Leman brought samples of the mineral to St. Petersburg. After treating the crystals with hydrochloric acid, he obtained a white precipitate, in which he found lead. Leman called the mineral Siberian red lead (plomb rouge de Sibérie), now it is known that it was crocoite (from the Greek "krokos" - saffron) - natural lead chromate PbCrO 4.

The German traveler and naturalist Peter Simon Pallas (1741-1811) led the 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 Lehman, became interested in crocoite. Pallas wrote: “This amazing red lead mineral is not found in any other deposit. Turns yellow when ground into powder and can be used in miniature art. 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 matter 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 samples of Siberian red lead were added to the collections of many mineralogical cabinets in Europe.

In 1796, a sample of crocoite came to Nicolas-Louis Vauquelin (1763–1829), professor of chemistry at the Paris Mineralogical School, who analyzed the mineral, but found nothing in it except oxides of lead, iron, and aluminum. Continuing the study of Siberian red lead, Vauquelin 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 it was treated with a lead salt, a yellow precipitate formed, with a mercury salt, a red one, and when tin chloride was added, the solution turned green. 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 this was chromic anhydride). Having calcined them with coal in a graphite crucible, after the reaction, he discovered a lot of intergrown gray needle-shaped crystals of a metal unknown until that time. Vauquelin stated the high refractoriness of the metal and its resistance to acids.

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

Most likely, Vauquelin obtained not pure metal, but its carbides, as evidenced by the needle-like shape of the crystals obtained, 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

Chromium is a chemical element with atomic number 24. It is a hard, shiny, steel-gray metal that polishes well and does not tarnish. Used in alloys such as stainless steel, and as a cover. The human body requires small amounts of trivalent chromium to metabolize sugar, but Cr(VI) is highly toxic.

Various chromium compounds, such as chromium(III) oxide and lead chromate, are brightly colored and are used in paints and pigments. The red color of a ruby is due to the presence of this chemical element. Some substances, especially sodium, are oxidizing agents used to oxidize organic compounds and (together with sulfuric acid) to clean glassware. In addition, chromium oxide (VI) is used in the production of magnetic tape.

Discovery and etymology

The history of the discovery of the chemical element chromium is as follows. In 1761, Johann Gottlob Lehmann found in Ural mountains orange-red mineral and called it "Siberian red lead". Although it was erroneously identified as a compound of lead with selenium and iron, the material was actually lead chromate with chemical formula PbCrO 4 . Today it is known as the croconte mineral.

In 1770, Peter Simon Pallas visited the place where Leman found a red lead mineral that had very useful pigment properties in paints. The use of Siberian red lead as a paint fast development. In addition, bright yellow from croconte has become fashionable.

In 1797, Nicolas-Louis Vauquelin obtained samples of red By mixing croconte with hydrochloric acid, he obtained the oxide CrO 3 . Chromium as a chemical element was isolated in 1798. Vauquelin obtained it by heating the oxide with charcoal. He was also able to detect traces of chromium in precious stones such as ruby and emerald.

In the 1800s, Cr was mainly used in paints and leather salts. Today, 85% of the metal is used in alloys. The rest is used in the chemical industry, the production of refractory materials and the foundry industry.

The pronunciation of the chemical element chromium corresponds to the Greek χρῶμα, which means "color", because of the many colored compounds that can be obtained from it.

Mining and production

The element is made from chromite (FeCr 2 O 4). Approximately half of this ore in the world is mined in South Africa. In addition, Kazakhstan, India and Turkey are its major producers. There are enough explored deposits of chromite, but geographically they are concentrated in Kazakhstan and southern Africa.

Deposits of native chromium metal are rare, but they do exist. For example, it is mined at the Udachnaya mine in Russia. It is rich in diamonds, and the reducing environment helped form pure chromium and diamonds.

For the industrial production of metal, chromite ores are treated with molten alkali (caustic soda, NaOH). In this case, sodium chromate (Na 2 CrO 4) is formed, which is reduced by carbon to Cr 2 O 3 oxide. The metal is obtained by heating the oxide in the presence of aluminum or silicon.

In 2000, approximately 15 Mt of chromite ore was mined and processed into 4 Mt of ferrochromium, 70% chromium-iron, with an estimated market value of US$2.5 billion.

Main characteristics

The characteristic of the chemical element chromium is due to the fact that it is a transition metal of the fourth period of the periodic table and is located between vanadium and manganese. Included in the VI group. It melts at a temperature of 1907 °C. In the presence of oxygen, chromium quickly forms a thin layer of oxide, which protects the metal from further interaction with oxygen.

As a transition element, it reacts with substances in various proportions. Thus, it forms compounds in which it has various oxidation states. Chromium is a chemical element with ground states +2, +3 and +6, of which +3 is the most stable. In addition, states +1, +4 and +5 are observed in rare cases. Chromium compounds in the +6 oxidation state are strong oxidizing agents.

What color is chrome? The chemical element imparts a ruby hue. The Cr 2 O 3 used for is also used as a pigment called "chrome green". Its salts color glass in an emerald green color. Chromium is a chemical element whose presence makes a ruby red. Therefore, it is used in the production of synthetic rubies.

isotopes

Isotopes of chromium have atomic weights from 43 to 67. Typically, this chemical element consists of three stable forms: 52 Cr, 53 Cr and 54 Cr. Of these, 52 Cr is the most common (83.8% of all natural chromium). In addition, 19 radioisotopes have been described, of which 50 Cr is the most stable, with a half-life exceeding 1.8 x 10 17 years. 51 Cr has a half-life of 27.7 days, and for all other radioactive isotopes it does not exceed 24 hours, and for most of them it lasts less than one minute. The element also has two metastates.

Chromium isotopes in the earth's crust, as a rule, accompany manganese isotopes, which finds application in geology. 53 Cr is formed during the radioactive decay of 53 Mn. The Mn/Cr isotope ratio reinforces other information about the early history of the solar system. Changes in the ratios of 53 Cr/ 52 Cr and Mn/Cr from different meteorites prove that new atomic nuclei were created just before the formation of the solar system.

Chemical element chromium: properties, formula of compounds

Chromium oxide (III) Cr 2 O 3, also known as sesquioxide, is one of the four oxides of this chemical element. It is obtained from chromite. The green compound is commonly referred to as "chrome green" when used as a pigment for enamel and glass painting. The oxide can dissolve in acids, forming salts, and in molten alkali, chromites.

Potassium bichromate

K 2 Cr 2 O 7 is a powerful oxidizing agent and is preferred as a cleaning agent for laboratory glassware from organics. For this, its saturated solution is used. Sometimes, however, it is replaced with sodium dichromate, based on the higher solubility of the latter. In addition, it can regulate the process of oxidation of organic compounds, converting primary alcohol into aldehyde, and then into carbon dioxide.

Potassium dichromate can cause chromium dermatitis. Chromium is probably the cause of the sensitization leading to the development of dermatitis, especially of the hands and forearms, which is chronic and difficult to treat. Like other Cr(VI) compounds, potassium bichromate is carcinogenic. It must be handled with gloves and appropriate protective equipment.

Chromic acid

The compound has the hypothetical structure H 2 CrO 4 . Neither chromic nor dichromic acids are found in nature, but their anions are found in various substances. "Chromic acid", which can be found on sale, is actually its acid anhydride - CrO 3 trioxide.

Lead(II) chromate

PbCrO 4 has a bright yellow color and is practically insoluble in water. For this reason, it has found application as a coloring pigment under the name "yellow crown".

Cr and pentavalent bond

Chromium is distinguished by its ability to form pentavalent bonds. The compound is created by Cr(I) and a hydrocarbon radical. A pentavalent bond is formed between two chromium atoms. Its formula can be written as Ar-Cr-Cr-Ar where Ar is a specific aromatic group.

Application

Chromium is a chemical element whose properties provided it with many various options applications, some of which are listed below.

It gives metals resistance to corrosion and a glossy surface. Therefore, chromium is included in alloys such as stainless steel, used in cutlery, for example. It is also used for chrome plating.

Chromium is a catalyst for various reactions. It is used to make molds for firing bricks. Its salts tan the skin. Potassium bichromate is used to oxidize organic compounds such as alcohols and aldehydes, as well as to clean laboratory glassware. It serves as a fixing agent for dyeing fabric and is also used in photography and photo printing.

CrO 3 is used to make magnetic tapes (for example, for audio recording), which have the best performance than iron oxide films.

Role in biology

Trivalent chromium is a chemical element essential for the metabolism of sugar in the human body. In contrast, hexavalent Cr is highly toxic.

Precautionary measures

Chromium metal and Cr(III) compounds are not generally considered hazardous to health, but substances containing Cr(VI) can be toxic if ingested or inhaled. Most of these substances are irritating to the eyes, skin and mucous membranes. With chronic exposure, chromium(VI) compounds can cause eye damage if not properly treated. In addition, it is a recognized carcinogen. Lethal dose of this chemical element - about half a teaspoon. According to the recommendations of the World Health Organization, the maximum allowable concentration of Cr (VI) in drinking water is 0.05 mg per litre.

Since chromium compounds are used in dyes and for tanning leather, they are often found in soil and groundwater abandoned industrial facilities requiring environmental cleanup and restoration. Primer containing Cr(VI) is still widely used in the aerospace and automotive industries.

Element properties

The main physical properties of chromium are as follows:

Atomic number: 24.
Atomic weight: 51.996.
Melting point: 1890 °C.
Boiling point: 2482 °C.
Oxidation state: +2, +3, +6.
Electron configuration: 3d 5 4s 1 .

Chromium is a trace mineral that is used in different forms. In bioadditives, this is usually its chloride or picolinate (salt better absorbed by the intestines). The complex present in yeast, known as the glucose tolerance factor and including chromium, and three amino acids - glutamine, glycine and cysteine, is well absorbed.

Beneficial features chromium and its role in the body

Chromium is essential for insulin to work. This hormone is responsible for transporting glucose from the blood to the cells, where it is “burned” to release energy. Insulin is effective and helps maintain normal blood sugar levels only if the body has enough chromium. This metal increases the number of insulin receptors by cell membrane. By increasing our glucose tolerance (the ability to tolerate its consumption without negative health consequences) by increasing the effectiveness of insulin, chromium inhibits its production, and as a result inhibits the conversion of sugar into fats. This leads to a decrease in blood levels of cholesterol (especially "bad", i.e., low-density lipoproteins) and triglycerides.

Prevention

Chromium supplements reduce the risk diabetes in insulin resistant people. They produce enough insulin, but the sensitivity of the cells to it is reduced. As a result, to maintain normal blood glucose levels, the pancreas has to secrete increased amounts of this enzyme. However, even they may not be enough, and then type II diabetes (non-insulin dependent) develops with excess blood sugar, which is usually accompanied by obesity and hypercholesterolemia (high cholesterol) with all the ensuing consequences. This risk is reduced by the prophylactic use of chromium, which reduces insulin resistance and thus increases glucose tolerance.

The benefits of chromium

Stress, infection, increased physical activity accelerate the “burning” of glucose, and as a result, the mobilization of chromium, which is more intensively excreted in the urine. The same is observed in hyperglycemic exacerbations in diabetic patients. The intake of chromium from food is usually barely up to standard, so in such situations it is advisable to take its supplements.

Indications and uses of chromium, recommended daily allowance, contraindications, food sources of chromium

There are no recommended daily allowances for chromium, but it is believed that chromium deficiency in adults can be prevented by doses of 50 to 200 micrograms per day. It should be noted that even with a varied, healthy diet, getting 200 micrograms of chromium per day from food is almost impossible. The standard menu usually gives us 40-50 mcg / day, and a starvation diet (for example, when losing weight), of course, less.

- Flaw. Chromium deficiency is fraught with irritability, weight gain and impaired sensitivity of the limbs, as well as exacerbation of non-insulin dependent diabetes.

Excess. Chromium supplements appear to be harmless. However, their high doses make it difficult to digest and.

Indications for the use of chromium

Difficulty digesting proteins, fats or carbohydrates.

Elevated blood glucose levels (insulin resistance, type II diabetes).

Elevated blood levels of "bad" cholesterol (low-density lipoprotein) and triglycerides.

Contraindications

Diabetic patients should only take chromium after consulting their doctor. They may need to adjust their doses of insulin and/or other medications already prescribed for their illness.

Application methods

Doses

Usually, chromium in additives is combined with other minerals, so it is necessary to specify its amount in the preparation according to the inscription on the package. In one tablet or capsule it should be from 25 to 200 mcg (more is dangerous). Such dietary supplements are taken as a general tonic, as well as when losing weight with a starvation diet and to increase the effectiveness of insulin.

Target: deepen students' knowledge of the topic.

Tasks:

characterize chromium as a simple substance;
introduce students to chromium compounds varying degrees oxidation;
show the dependence of the properties of compounds on the degree of oxidation;
show redox properties of chromium compounds;
to continue the formation of students' skills to write down the equations of chemical reactions in molecular and ionic form, to draw up an electronic balance;
continue the formation of skills to observe a chemical experiment.

Lesson form: lecture with elements independent work students and observation of a chemical experiment.

Lesson progress

I. Repetition of the material of the previous lesson.

1. Answer questions and complete tasks:

What elements belong to the chromium subgroup?

Write electronic formulas of atoms

What type of elements are they?

What are the oxidation states in the compounds?

How do the atomic radius and ionization energy change from chromium to tungsten?

You can offer students to fill out a table using the tabular values of the radii of atoms, ionization energies and draw conclusions.

Sample table:

2. Listen to the student's message on the topic "Elements of the chromium subgroup in nature, obtaining and using."

II. Lecture.

Lecture plan:

Chromium.
Chromium compounds. (2)

Chromium oxide; (2)
Chromium hydroxide. (2)

Chromium compounds. (3)

Chromium oxide; (3)
Chromium hydroxide. (3)

Chromium compounds (6)

Chromium oxide; (6)
Chromic and dichromic acids.

Dependence of the properties of chromium compounds on the degree of oxidation.
Redox properties of chromium compounds.

1. Chrome.

Chrome is white with a bluish tint shiny metal, very hard (density 7.2 g / cm 3), melting point 1890˚С.

Chemical properties: Chromium is an inactive metal under normal conditions. This is due to the fact that its surface is covered with an oxide film (Cr 2 O 3). When heated, the oxide film is destroyed, and chromium reacts with simple substances at high temperature:

4Cr + 3O 2 \u003d 2Cr 2 O 3
2Cr + 3S = Cr 2 S 3
2Cr + 3Cl 2 = 2CrCl 3

Exercise: write equations for the reactions of chromium with nitrogen, phosphorus, carbon and silicon; to one of the equations, draw up an electronic balance, indicate the oxidizing agent and reducing agent.

The interaction of chromium with complex substances:

At very high temperatures, chromium reacts with water:

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

Exercise:

Chromium reacts with dilute sulfuric and hydrochloric acids:

Cr + H 2 SO 4 = CrSO 4 + H 2
Cr + 2HCl \u003d CrCl 2 + H 2

Exercise: draw up an electronic balance, indicate the oxidizing agent and reducing agent.

Concentrated sulfuric hydrochloric and nitric acid passivate chromium.

2. Chromium compounds. (2)

1. Chromium oxide (2)- CrO - a solid bright red substance, a typical basic oxide (it corresponds to chromium (2) hydroxide - Cr (OH) 2), does not dissolve in water, but dissolves in acids:

CrO + 2HCl = CrCl 2 + H 2 O

Exercise: draw up a reaction equation in the molecular and ionic form of the interaction of chromium oxide (2) with sulfuric acid.

Chromium oxide (2) is easily oxidized in air:

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

Exercise: draw up an electronic balance, indicate the oxidizing agent and reducing agent.

Chromium oxide (2) is formed during the oxidation of chromium amalgam with atmospheric oxygen:

2Сr (amalgam) + О 2 = 2СrО

2. Chromium hydroxide (2)- Cr (OH) 2 - substance yellow color, poorly soluble in water, with a pronounced basic character, therefore interacts with acids:

Cr(OH) 2 + H 2 SO 4 = CrSO 4 + 2H 2 O

Exercise: compose reaction equations in the molecular and ionic form of the interaction of chromium oxide (2) with hydrochloric acid.

Like chromium(2) oxide, chromium(2) hydroxide oxidizes:

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

Exercise: draw up an electronic balance, indicate the oxidizing agent and reducing agent.

Chromium hydroxide (2) can be obtained by the action of alkalis on chromium salts (2):

CrCl 2 + 2KOH = Cr(OH) 2 ↓ + 2KCl

Exercise: write ionic equations.

3. Chromium compounds. (3)

1. Chromium oxide (3)- Cr 2 O 3 - dark green powder, insoluble in water, refractory, close to corundum in hardness (it corresponds to chromium hydroxide (3) - Cr (OH) 3). Chromium oxide (3) is amphoteric in nature, however, it is poorly soluble in acids and alkalis. Reactions with alkalis occur during fusion:

Cr 2 O 3 + 2KOH = 2KSrO 2 (chromite K)+ H 2 O

Exercise: draw up a reaction equation in the molecular and ionic form of the interaction of chromium oxide (3) with lithium hydroxide.

It is difficult to interact with concentrated solutions of acids and alkalis:

Cr 2 O 3 + 6 KOH + 3H 2 O \u003d 2K 3 [Cr (OH) 6]
Cr 2 O 3 + 6HCl \u003d 2CrCl 3 + 3H 2 O

Exercise: compose reaction equations in the molecular and ionic form of the interaction of chromium oxide (3) with concentrated sulfuric acid and concentrated sodium hydroxide solution.

Chromium oxide (3) can be obtained by decomposition of ammonium dichromate:

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

2. Chromium hydroxide (3) Cr (OH) 3 is obtained by the action of alkalis on solutions of chromium salts (3):

CrCl 3 + 3KOH \u003d Cr (OH) 3 ↓ + 3KSl

Exercise: write ionic equations

Chromium hydroxide (3) is a gray-green precipitate, upon receipt of which, alkali must be taken in short supply. Chromium (3) hydroxide obtained in this way, unlike the corresponding oxide, easily interacts with acids and alkalis, i.e. exhibits amphoteric properties:

Cr (OH) 3 + 3HNO 3 \u003d Cr (NO 3) 3 + 3H 2 O
Cr(OH) 3 + 3KOH = K 3 [Cr(OH)6] (hexahydroxochromite K)

Exercise: compose reaction equations in the molecular and ionic form of the interaction of chromium hydroxide (3) with hydrochloric acid and sodium hydroxide.

When Cr (OH) 3 is fused with alkalis, metachromites and orthochromites are obtained:

Cr(OH) 3 + KOH = KCrO 2 (metachromite K)+ 2H2O
Cr(OH) 3 + KOH = K 3 CrO 3 (orthochromite K)+ 3H2O

4. Chromium compounds. (6)

1. Chromium oxide (6)- CrO 3 - dark red crystalline substance, highly soluble in water - typical acid oxide. This oxide corresponds to two acids:

CrO 3 + H 2 O \u003d H 2 CrO 4 (chromic acid - formed with excess water)
CrO 3 + H 2 O \u003d H 2 Cr 2 O 7 (dichromic acid - is formed at a high concentration of chromium oxide (3)).

Chromium oxide (6) is a very strong oxidizing agent, therefore it interacts vigorously with organic substances:

C 2 H 5 OH + 4CrO 3 \u003d 2CO 2 + 2Cr 2 O 3 + 3H 2 O

It also oxidizes iodine, sulfur, phosphorus, coal:

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

Exercise: write equations chemical reactions chromium oxide (6) with iodine, phosphorus, coal; to one of the equations, draw up an electronic balance, indicate the oxidizing agent and reducing agent

When heated to 250 0 C, chromium oxide (6) decomposes:

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

Chromium oxide (6) can be obtained by the action of concentrated sulfuric acid on solid chromates and dichromates:

K 2 Cr 2 O 7 + H 2 SO 4 \u003d K 2 SO 4 + 2CrO 3 + H 2 O

2. Chromic and dichromic acids.

Chromic and dichromic acids exist only in aqueous solutions, they form stable salts, respectively chromates and dichromates. Chromates and their solutions are yellow, dichromates are orange.

Chromate - CrO 4 2- ions and dichromate - Cr 2O 7 2- ions easily pass into each other when the solution environment changes

In the acidic environment of the solution, chromates turn into dichromates:

2K 2 CrO 4 + H 2 SO 4 = K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O

In an alkaline environment, dichromates turn into chromates:

K 2 Cr 2 O 7 + 2KOH \u003d 2K 2 CrO 4 + H 2 O

When diluted, dichromic acid becomes chromic acid:

H 2 Cr 2 O 7 + H 2 O \u003d 2H 2 CrO 4

5. Dependence of the properties of chromium compounds on the degree of oxidation.

Oxidation state	+2	+3	+6
Oxide	CrO	Cr 2 O 3	CrO 3
The nature of the oxide	basic	amphoteric	acid
Hydroxide	Cr(OH) 2	Cr(OH) 3 - H 3 CrO 3	H 2 CrO 4
The nature of the hydroxide	basic	amphoteric	acid
→ weakening of basic properties and strengthening of acidic→

6. Redox properties of chromium compounds.

Reactions in an acid medium.

In an acidic environment, Cr +6 compounds turn into Cr +3 compounds under the action of reducing agents: H 2 S, SO 2, FeSO 4

K 2 Cr 2 O 7 + 3H 2 S + 4H 2 SO 4 \u003d 3S + Cr 2 (SO 4) 3 + K 2 SO 4 + 7H 2 O
S-2 – 2e → S 0
2Cr +6 + 6e → 2Cr +3

Exercise:

1. Equalize the reaction equation using the electron balance method, indicate the oxidizing agent and reducing agent:

Na 2 CrO 4 + K 2 S + H 2 SO 4 = S + Cr 2 (SO 4) 3 + K 2 SO 4 + Na 2 SO 4 + H 2 O

2. Add the reaction products, equate the equation using the electron balance method, indicate the oxidizing agent and reducing agent:

K 2 Cr 2 O 7 + SO 2 + H 2 SO 4 \u003d? +? +H 2 O

Reactions in an alkaline medium.

In an alkaline environment, Cr +3 chromium compounds are converted into Cr +6 compounds under the action of oxidizing agents: J2, Br2, Cl2, Ag2O, KClO3, H2O2, KMnO4:

2KCrO 2 +3 Br 2 +8NaOH \u003d 2Na 2 CrO 4 + 2KBr + 4NaBr + 4H 2 O
Cr +3 - 3e → Cr +6
Br2 0 +2e → 2Br -

Exercise:

Equalize the reaction equation using the electron balance method, indicate the oxidizing agent and reducing agent:

NaCrO 2 + J 2 + NaOH = Na 2 CrO 4 + NaJ + H 2 O

Add the reaction products, equate the equation using the electron balance method, indicate the oxidizing agent and reducing agent:

Cr(OH) 3 + Ag 2 O + NaOH = Ag + ? +?

In this way, oxidizing properties sequentially increase with a change in the oxidation states in the series: Cr +2 → Cr +3 → Cr +6. Chromium compounds (2) are strong reducing agents, they are easily oxidized, turning into chromium compounds (3). Chromium compounds (6) are strong oxidizers, easily reduced to chromium compounds (3). Chromium (3) compounds, when interacting with strong reducing agents, exhibit oxidizing properties, turning into chromium (2) compounds, and when interacting with strong oxidizing agents, they exhibit reducing properties, turning into chromium compounds (6)

To the lecture method:

To activate cognitive activity students and maintaining interest, it is advisable to conduct a demonstration experiment during the lecture. Depending on the capabilities of the educational laboratory, students can demonstrate the following experiments:

obtaining chromium oxide (2) and chromium hydroxide (2), proof of their basic properties;
obtaining chromium oxide (3) and chromium hydroxide (3), proof of their amphoteric properties;
obtaining chromium oxide (6) and dissolving it in water (obtaining chromic and dichromic acids);
the transition of chromates to dichromates, dichromates to chromates.

Tasks of independent work can be differentiated taking into account the real learning opportunities of students.
You can complete the lecture by completing the following tasks: write the equations of chemical reactions with which you can carry out the following transformations:

.III. Homework: finalize the lecture (add the equations of chemical reactions)

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