And fat.
Scientists claim that cholesterol levels are affected by chromium. Element It is considered biogenic, that is, it is necessary for the body, not only for humans, but for all mammals.
With a lack of chromium, their growth slows down and cholesterol “jumps”. The norm is 6 milligrams of chromium from the total mass of a person.
Ions of the substance are found in all tissues of the body. You should be getting 9 micrograms per day.
You can take them from seafood, pearl barley, beets, liver and duck meat. While you are buying products, we will talk about other uses and properties of chromium.
Chromium Properties
Chromium is a chemical element pertaining to metals. The color of the substance is silver-blue.
The element is under the 24th ordinal, or, as they say, atomic number.
The number indicates the number of protons in the nucleus. As for the electrons rotating near it, they have a special property - to fall through.
This means that one or two particles can move from one sublevel to another.
As a result, the 24th element is able to half fill the 3rd sublevel. This results in a stable electronic configuration.
The failure of electrons is a rare phenomenon. In addition to chromium, perhaps only,, and are remembered.
Like the 24th substance, they are chemically inactive. Not then does the atom come to a stable state in order to react with everyone in a row.
Under normal conditions chromium is an element of the periodic table, which can only be "stirred up".
The latter, being the antipode of the 24th substance, is maximally active. The reaction produces fluoride chrome.
Element, properties which are discussed, does not oxidize, is not afraid of moisture and refractory materials.
The latter characteristic "delays" the reactions that are possible during heating. So, interaction with water vapor starts only at 600 degrees Celsius.
It turns out chromium oxide. The reaction with is also started, giving the nitride of the 24th element.
At 600 degrees, several compounds with and the formation of sulfide are also possible.
If you bring the temperature up to 2000, chromium will ignite on contact with oxygen. The result of combustion will be a dark green oxide.
This precipitate easily reacts with solutions and acids. The result of the interaction is chloride and chromium sulfide. All compounds of the 24th substance, as a rule, are brightly colored.
In its purest form, the main characteristics of the element chromium- toxicity. Metal dust irritates lung tissues.
Dermatitis, that is, allergic diseases, may appear. Accordingly, it is better not to exceed the norm of chromium for the body.
There is a norm for the content of the 24th element in the air. There should be 0.0015 milligrams per cubic meter of atmosphere. Exceeding the standard is considered pollution.
Chromium metal has a high density - more than 7 grams per cubic centimeter. This means that the substance is quite heavy.
The metal is also quite high. It depends on the electrolyte temperature and current density. In fungi and mold, this, apparently, commands respect.
If wood is impregnated with a chromium composition, microorganisms will not undertake to destroy it. Builders use it.
They are also satisfied with the fact that the treated wood burns worse, because chromium is a refractory metal. How and where else it can be applied, we will tell further.
Application of chromium
Chromium is an alloying element when smelted. Remember that under normal conditions, the 24th metal does not oxidize, does not rust?
The basis of steels -. It cannot boast of such properties. Therefore, chromium is added to increase corrosion resistance.
In addition, the addition of the 24th substance lowers the critical cooling rate point.
Silicothermal chromium is used for smelting. This is a duet of the 24th element with nickel.
Silicon, are used as additives. Nickel is responsible for ductility, while chromium is responsible for its oxidation resistance and hardness.
Connect chromium and with. It turns out superhard stellite. Additives to it - molybdenum and.
The composition is expensive, but necessary for surfacing machine parts in order to increase their wear resistance. Stellite is also sprayed onto working machines,.
In decorative corrosion-resistant coatings, as a rule, chromium compounds.
The bright range of their colors comes in handy. In cermets, color is not needed, therefore, powder chromium is used. It is added, for example, for strength to the lower layer of crowns for.
Chromium formula- component . This is a mineral from the group, but it does not have the usual color.
Uvarovite is a stone, and it is chromium that makes it so. It's no secret that they are used.
The green variety of stone is no exception, moreover, it is valued higher than the red one, because it is rare. Still, uvarovit a little standard.
This is also a plus, because mineral inserts are harder to scratch. The stone is faceted faceted, that is, forming corners, which increases the play of light.
Chromium mining
Extracting chromium from minerals is unprofitable. Most with the 24th element are used in their entirety.
In addition, the chromium content in, as a rule, is low. The substance is extracted, in the ground, from the ores.
One of them is associated opening chrome. It was found in Siberia. Crocoite was found there in the 18th century. It is red lead ore.
Its basis is, the second element is chromium. It was discovered by a German chemist named Lehman.
At the time of the discovery of the crocoite, he was visiting St. Petersburg, where he conducted experiments. Now, the 24th element is obtained by electrolysis of concentrated aqueous solutions of chromium oxide.
Electrolysis of sulfate is also possible. These are 2 ways to get the cleanest chrome. Molecule oxide or sulfate is destroyed in the crucible, where the original compounds are ignited.
The 24th element is separated, the rest goes to slag. It remains to smelt chromium in an arc. This is how the purest metal is extracted.
There are other ways to get chromium element, for example, reduction of its oxide with silicon.
But, this method gives a metal with a large amount of impurities and, moreover, is more expensive than electrolysis.
Chrome price
In 2016, the price of chromium is still declining. January began with 7450 dollars per ton.
By mid-summer, only 7,100 conventional units are asked for per 1,000 kilograms of metal. Data provided by Infogeo.ru.
That is, Russian prices are considered. The world price of chromium reached almost $9,000 per ton.
The lowest mark of summer differs from the Russian one by only 25 dollars upwards.
If not the industrial sector is considered, for example, metallurgy, but the benefits of chromium for the body, you can study the offers of pharmacies.
So, "Picolinate" of the 24th substance costs about 200 rubles. For "Kartnitin Chrome Forte" they ask for 320 rubles. This is the price tag for a pack of 30 tablets.
Turamine Chromium can also make up for the deficiency of the 24th element. Its cost is 136 rubles.
Chromium, by the way, is part of the tests for the detection of drugs, in particular, marijuana. One test costs 40-45 rubles.
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.
The main physical properties of 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 ( a)* 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 chromium ore is the FeCr 2 O 4 chromite mineral, 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 is 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 ( a= 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.
Chromium(0) is almost never found in its pure form. 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 hence 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 (chromium green) 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 the physicochemical properties of chromium and the element interacting with it increases, 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 AB 2 type, 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 rate of dissolution of chromium is greatly influenced by 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 have been studied. 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.
"National Research Tomsk Polytechnic University"
Institute of Natural Resources Geoecology and Geochemistry
Chromium
By discipline:
Chemistry
Completed:
student of group 2G41 Tkacheva Anastasia Vladimirovna 10/29/2014
Checked:
teacher Stas Nikolay Fedorovich
Position in the periodic system
Chromium- an element of a side subgroup of the 6th group of the 4th period of the periodic system of chemical elements of D. I. Mendeleev with atomic number 24. It is indicated by the symbol Cr(lat. Chromium). simple substance chromium- hard bluish-white metal. Chromium is sometimes referred to as a ferrous metal.
The structure of the atom
17 Cl) 2) 8) 7 - diagram of the structure of the atom
1s2s2p3s3p - electronic formula
The atom is located in period III, and has three energy levels
The atom is located in VII in the group, in the main subgroup - at the external energy level of 7 electrons
Element properties
Physical properties
Chromium is a white shiny metal with a cubic body-centered lattice, a = 0.28845 nm, characterized by hardness and brittleness, with a density of 7.2 g / cm 3, one of the hardest pure metals (second only to beryllium, tungsten and uranium), with a melting point of 1903 degrees. And with a boiling point of about 2570 degrees. C. In air, the surface of chromium is covered with an oxide film, which protects it from further oxidation. The addition of carbon to chromium further increases its hardness.
Chemical properties
Chromium under normal conditions is an inert metal, when heated it becomes quite active.
Interaction with non-metals
When heated above 600°C, chromium burns in oxygen:
4Cr + 3O 2 \u003d 2Cr 2 O 3.
It reacts with fluorine at 350°C, with chlorine at 300°C, with bromine at a red heat temperature, forming chromium (III) halides:
2Cr + 3Cl 2 = 2CrCl 3 .
It reacts with nitrogen at temperatures above 1000°C to form nitrides:
2Cr + N 2 = 2CrN
or 4Cr + N 2 = 2Cr 2 N.
2Cr + 3S = Cr 2 S 3 .
Reacts with boron, carbon and silicon to form borides, carbides and silicides:
Cr + 2B = CrB 2 (the formation of Cr 2 B, CrB, Cr 3 B 4, CrB 4 is possible),
2Cr + 3C \u003d Cr 2 C 3 (the formation of Cr 23 C 6, Cr 7 B 3 is possible),
Cr + 2Si = CrSi 2 (possible formation of Cr 3 Si, Cr 5 Si 3, CrSi).
It does not interact directly with hydrogen.
Interaction with water
In a finely ground hot state, chromium reacts with water, forming chromium (III) oxide and hydrogen:
2Cr + 3H 2 O \u003d Cr 2 O 3 + 3H 2
Interaction with acids
In the electrochemical series of voltages of metals, chromium is before hydrogen, it displaces hydrogen from solutions of non-oxidizing acids:
Cr + 2HCl \u003d CrCl 2 + H 2;
Cr + H 2 SO 4 \u003d CrSO 4 + H 2.
In the presence of atmospheric oxygen, chromium (III) salts are formed:
4Cr + 12HCl + 3O 2 = 4CrCl 3 + 6H 2 O.
Concentrated nitric and sulfuric acids passivate chromium. Chromium can dissolve in them only with strong heating, chromium (III) salts and acid reduction products are formed:
2Cr + 6H 2 SO 4 = Cr 2 (SO 4) 3 + 3SO 2 + 6H 2 O;
Cr + 6HNO 3 \u003d Cr (NO 3) 3 + 3NO 2 + 3H 2 O.
Interaction with alkaline reagents
In aqueous solutions of alkalis, chromium does not dissolve; it slowly reacts with alkali melts to form chromites and release hydrogen:
2Cr + 6KOH \u003d 2KCrO 2 + 2K 2 O + 3H 2.
Reacts with alkaline melts of oxidizing agents, such as potassium chlorate, while chromium passes into potassium chromate:
Cr + KClO 3 + 2KOH = K 2 CrO 4 + KCl + H 2 O.
Recovery of metals from oxides and salts
Chromium is an active metal, capable of displacing metals from solutions of their salts: 2Cr + 3CuCl 2 = 2CrCl 3 + 3Cu.
Properties of a simple substance
Stable in air due to passivation. For the same reason, it does not react with sulfuric and nitric acids. At 2000 °C, it burns out with the formation of green chromium (III) oxide Cr 2 O 3, which has amphoteric properties.
Synthesized compounds of chromium with boron (borides Cr 2 B, CrB, Cr 3 B 4, CrB 2, CrB 4 and Cr 5 B 3), with carbon (carbides Cr 23 C 6, Cr 7 C 3 and Cr 3 C 2), with silicon (silicides Cr 3 Si, Cr 5 Si 3 and CrSi) and nitrogen (nitrides CrN and Cr 2 N).
Cr(+2) compounds
The oxidation state +2 corresponds to the basic oxide CrO (black). Cr 2+ salts (blue solutions) are obtained by reducing Cr 3+ salts or dichromates with zinc in an acidic environment (“hydrogen at the time of isolation”):
All these Cr 2+ salts are strong reducing agents, to the extent that they displace hydrogen from water upon standing. Oxygen in the air, especially in an acidic environment, oxidizes Cr 2+, as a result of which the blue solution quickly turns green.
Brown or yellow Cr(OH) 2 hydroxide precipitates when alkalis are added to solutions of chromium(II) salts.
Chromium dihalides CrF 2 , CrCl 2 , CrBr 2 and CrI 2 were synthesized
Cr(+3) compounds
The +3 oxidation state corresponds to the amphoteric oxide Cr 2 O 3 and the hydroxide Cr (OH) 3 (both green). This is the most stable oxidation state of chromium. Chromium compounds in this oxidation state have a color from dirty purple (ion 3+) to green (anions are present in the coordination sphere).
Cr 3+ is prone to the formation of double sulfates of the form M I Cr (SO 4) 2 12H 2 O (alum)
Chromium (III) hydroxide is obtained by acting with ammonia on solutions of chromium (III) salts:
Cr+3NH+3H2O→Cr(OH)↓+3NH
Alkali solutions can be used, but in their excess a soluble hydroxo complex is formed:
Cr+3OH→Cr(OH)↓
Cr(OH)+3OH→
By fusing Cr 2 O 3 with alkalis, chromites are obtained:
Cr2O3+2NaOH→2NaCrO2+H2O
Uncalcined chromium (III) oxide dissolves in alkaline solutions and in acids:
Cr2O3+6HCl→2CrCl3+3H2O
When chromium(III) compounds are oxidized in an alkaline medium, chromium(VI) compounds are formed:
2Na+3HO→2NaCrO+2NaOH+8HO
The same thing happens when chromium (III) oxide is fused with alkali and oxidizing agents, or with alkali in air (the melt becomes yellow in this case):
2Cr2O3+8NaOH+3O2→4Na2CrO4+4H2O
Chromium compounds (+4)[
With careful decomposition of chromium oxide (VI) CrO 3 under hydrothermal conditions, chromium oxide (IV) CrO 2 is obtained, which is ferromagnetic and has metallic conductivity.
Among chromium tetrahalides, CrF 4 is stable, chromium tetrachloride CrCl 4 exists only in vapor.
Chromium compounds (+6)
The +6 oxidation state corresponds to acidic chromium oxide (VI) CrO 3 and a number of acids between which there is an equilibrium. The simplest of them are chromic H 2 CrO 4 and two-chrome H 2 Cr 2 O 7 . They form two series of salts: yellow chromates and orange dichromates, respectively.
Chromium oxide (VI) CrO 3 is formed by the interaction of concentrated sulfuric acid with solutions of dichromates. A typical acid oxide, when interacting with water, it forms strong unstable chromic acids: chromic H 2 CrO 4, dichromic H 2 Cr 2 O 7 and other isopoly acids with the general formula H 2 Cr n O 3n+1. An increase in the degree of polymerization occurs with a decrease in pH, that is, an increase in acidity:
2CrO+2H→Cr2O+H2O
But if an alkali solution is added to an orange solution of K 2 Cr 2 O 7, how does the color turn yellow again, since chromate K 2 CrO 4 is formed again:
Cr2O+2OH→2CrO+HO
It does not reach a high degree of polymerization, as occurs in tungsten and molybdenum, since polychromic acid decomposes into chromium (VI) oxide and water:
H2CrnO3n+1→H2O+nCrO3
The solubility of chromates roughly corresponds to the solubility of sulfates. In particular, yellow barium chromate BaCrO 4 precipitates when barium salts are added to both chromate and dichromate solutions:
Ba+CrO→BaCrO↓
2Ba+CrO+H2O→2BaCrO↓+2H
The formation of a blood-red, poorly soluble silver chromate is used to detect silver in alloys using assay acid.
Chromium pentafluoride CrF 5 and unstable chromium hexafluoride CrF 6 are known. Volatile chromium oxyhalides CrO 2 F 2 and CrO 2 Cl 2 (chromyl chloride) have also been obtained.
Chromium(VI) compounds are strong oxidizing agents, for example:
K2Cr2O7+14HCl→2CrCl3+2KCl+3Cl2+7H2O
The addition of hydrogen peroxide, sulfuric acid, and an organic solvent (ether) to dichromates leads to the formation of blue chromium peroxide CrO 5 L (L is a solvent molecule), which is extracted into the organic layer; this reaction is used as an analytical one.
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. In 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 dichromate. 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 acquires a 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 K 2 Cr 2 O 7 solution acidified with nitric acid, 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 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 fill 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). 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's 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 adsorbed, was also popular with artists. This pigment gives the paint an emerald hue.
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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 - РbCrО4. 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 K2 CO3 potash: lead chromate was converted to 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 O3. 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 didn’t have high-quality steels, we wouldn’t 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 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 Cr2 O3. 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:
2Cr2 O3 + 4Na2 CO3 + 3O2 → 4Na2 CrO4 + 4CO2 .
Then sodium chromate is leached from the calcined mass with water; the lye is filtered, evaporated and treated with acid. The result is sodium bichromate Na2 Cr2 O7. 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 Cr2 O3 + 2Al → Al2 O3 + 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
2Cr2 O3 + 3Si → 3SiO2 + 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.
In terms of chemical properties, chromium is close to molybdenum and tungsten. Its highest oxide CrO3 is acidic, it is chromic anhydride H2 CrO4. 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 H2 Cr2 O7 is known, and its salts, bichromates, are widely used in chemistry. The most common chromium oxide Cr2 O3 is amphoterene. In general, under different conditions, chromium can exhibit valencies 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 of the physical properties of this metal change abruptly, abruptly. 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: Cr3 C. Cr7 C3 and Cr23 C6. 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 amounts of chromium leads to depletion of steel in 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 temperatures. 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, such shortcomings are eliminated by thermomechanical treatment, but chromium-based alloys cannot be treated in this way. In addition, they are too brittle at room temperature, which also limits their application.
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.
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 arches is 2...3 times greater than that of Dinas ones.
Dinas is an acid refractory brick containing at least 93% silica. Dinas fire resistance is 1680...1730°C. In the 14th volume of the Big Soviet Encyclopedia(2nd edition) Dinas is called an indispensable material for the vaults 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 K2 Cr2 O7 and Na2 Cr2 O7.
Phromates and chrome alums KCr(SO4); 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 PbCrO4. 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.
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 fragility and exceptional complexity machining do not yet allow the wide use of these alloys, although in terms of heat resistance and wear resistance they can compete with any materials. 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 will other, yet unknown methods come to the rescue here? One way or another, one must think that in the future these alloys will take their rightful place among the materials needed by technology.
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 watches, 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.
More vital was the proposal to use its volatile halide salts, for example, CrCl2, as a carrier of chromium. Hot gas washes the chrome-plated product, and the following reaction occurs:
CrCl2 + Fe ↔ FeCl2 + 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 this age of glass, aluminium, concrete and plastics, it is impossible not to recognize wood as an 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 from cliches - metal plates on which this pattern (or rather, its mirror image) is engraved chemically 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 has been found that paper impregnated with sodium or potassium dichromate, after being illuminated with a 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.
