Chromium - a general characteristic of the element, the chemical properties of chromium and its compounds. Chemical properties Coloring of chromium compounds
Chromium and its compounds are actively used in industrial production, in particular, in metallurgy, chemical and refractory industries.
Chromium Cr - a chemical element of the VI group of the periodic system of Mendeleev, atomic number 24, atomic mass 51.996, atomic radius 0.0125, Cr2+ ion radii - 0.0084; Cr3+ - 0.0064; Cr4+ - 6.0056.
Chromium exhibits oxidation states +2, +3, +6, respectively, has valencies II, III, VI.
Chromium is a hard, ductile, rather heavy, malleable steel-gray metal.
Boils at 2469 0 C, melts at 1878 ± 22 0 C. It has all the characteristic properties of metals - it conducts heat well, almost does not resist electric current, and has a luster inherent in most metals. And at the same time, it is resistant to corrosion in air and in water.
Impurities of oxygen, nitrogen and carbon, even in the smallest quantities, dramatically change the physical properties of chromium, for example, making it very brittle. But, unfortunately, it is very difficult to obtain chromium without these impurities.
The structure of the crystal lattice is body-centered cubic. A feature of chromium is a sharp change in its physical properties at a temperature of about 37°C.
6. Types of chromium compounds.
Chromium oxide (II) CrO (basic) is a strong reducing agent, extremely unstable in the presence of moisture and oxygen. Has no practical value.
Chromium oxide (III) Cr2O3 (amphoteric) is stable in air and in solutions.
Cr2O3 + H2SO4 = Cr2(SO4)3 + H2O
Cr2O3 + 2NaOH = Na2CrO4 + H2O
It is formed by heating some chromium (VI) compounds, for example:
4CrO3 2Cr2O3 + 3O2
(NH4)2Cr2O7 Cr2O3 + N2 + 4H2O
4Cr + 3O2 2Cr2O3
Chromium(III) oxide is used to reduce low purity chromium metal with aluminum (aluminothermy) or silicon (silicothermy):
Cr2O3 +2Al = Al2O3 +2Cr
2Cr2O3 + 3Si = 3SiO3 + 4Cr
Chromium oxide (VI) CrO3 (acidic) - dark crimson needle-like crystals.
Obtained by the action of an excess of concentrated H2SO4 on a saturated aqueous solution of potassium dichromate:
K2Cr2O7 + 2H2SO4 = 2CrO3 + 2KHSO4 + H2O
Chromium oxide (VI) is a strong oxidizing agent, one of the most toxic chromium compounds.
When CrO3 is dissolved in water, chromic acid H2CrO4 is formed
CrO3 + H2O = H2CrO4
Acid chromium oxide, reacting with alkalis, forms yellow chromates CrO42
CrO3 + 2KOH = K2CrO4 + H2O
2. Hydroxides
Chromium (III) hydroxide has amphoteric properties, dissolving both in
acids (behaves like a base), and in alkalis (behaves like an acid):
2Cr(OH)3 + 3H2SO4 = Cr2(SO4)3 + 6H2O
Cr(OH)3 + KOH = K
When calcining chromium (III) hydroxide, chromium (III) oxide Cr2O3 is formed.
Insoluble in water.
2Cr(OH)3 = Cr2O3 + 3H2O
3. Acids
Chromium acids corresponding to its +6 oxidation state and differing in the ratio of the number of CrO3 and H2O molecules exist only in the form of solutions. When the acid oxide CrO3 is dissolved, monochromic acid (simply chromic) H2CrO4 is formed.
CrO3 + H2O = H2CrO4
Acidification of a solution or an increase in CrO3 in it leads to acids of the general formula nCrO3 H2O
at n=2, 3, 4, these are, respectively, di, tri, tetrachromic acids.
The strongest of them is dichromic, that is, H2Cr2O7. Chromic acids and their salts are strong oxidizers and poisonous.
There are two types of salts: chromites and chromates.
Chromites with the general formula RCrO2 are salts of chromic acid HCrO2.
Cr(OH)3 + NaOH = NaCrO2 + 2H2O
Chromites vary in color from dark brown to completely black and are usually found in solid masses. Chromite is softer than many other minerals, the melting point of chromite depends on its composition 1545-1730 0 C.
Chromite has a metallic luster and is almost insoluble in acids.
Chromates are salts of chromic acids.
Salts of monochromic acid H2CrO4 are called monochromates (chromates) R2CrO4, salts of dichromic acid H2Cr2O7 dichromates (bichromates) - R2Cr2O7. Monochromats are usually colored yellow. They are stable only in an alkaline environment, and upon acidification they turn into orange-red dichromates:
2Na2CrO4 + H2SO4 = Na2Cr2O7 + Na2SO4 + H2O
"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 swamp 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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Chemistry Tutor
Continuation. See in No. 22/2005; 1, 2, 3, 5, 6, 8, 9, 11, 13, 15, 16, 18, 22/2006;
3, 4, 7, 10, 11, 21/2007;
2, 7, 11, 18/2008
ACTIVITY 25
10th grade(first year of study)
Chromium and its compounds
1. Position in the table of D.I. Mendeleev, the structure of the atom.
2. Origin of the name.
3. Physical properties.
4. Chemical properties.
5. Being in nature.
6. Basic methods of obtaining.
7. The most important chromium compounds:
a) chromium(II) oxide and hydroxide;
b) chromium(III) oxide and hydroxide, their amphoteric properties;
c) chromium(VI) oxide, chromic and dichromic acid, chromates and dichromates.
9. Redox properties of chromium compounds.
Chromium is located in the secondary subgroup of group VI of the table of D.I. Mendeleev. When compiling the electronic formula of chromium, it must be remembered that due to the greater stability of configuration 3 d 5 at the chromium atom, an electron slip is observed and the electronic formula has the form: 1 s 2 2s 2 p 6 3s 2 p 6 4s 1 3d 5 . In compounds, chromium can exhibit oxidation states +2, +3, and +6 (the +3 oxidation state is the most stable):
Chrome gets its name from the Greek word chroma(color, paint) due to the bright varied color of its compounds.
Chrome is a white shiny metal, very hard, brittle, refractory. Resistant to corrosion. In air, it becomes covered with an oxide film, due to which the surface becomes dull.
Chemical properties
Under normal conditions, chromium is an inactive metal and only reacts with fluorine. But when heated, the oxide film of chromium is destroyed, and chromium reacts with many simple and complex substances (similar to Al).
4Cr + 3O 2 2Cr 2 O 3 .
Metals (-).
Nonmetals (+):
2Cr + 3Cl 2 2CrCl 3 ,
2Cr + 3F 2 \u003d 2CrF 3,
2Cr + 3SCr 2 S 3 ,
H 2 O (+/-): *
2Cr + 3H 2 O (steam) Cr 2 O 3 + 3H 2.
Basic oxides (-).
Acid oxides (-).
Bases (+/-):
2Cr + 6NaOH + 6H 2 O \u003d 2Na 3 + 3H 2.
Non-oxidizing acids (+).
Cr + 2HCl \u003d CrCl 2 + H 2.
Oxidizing acids (-). Passivation.
Salts (+/-):
2Cr + 3CuSO 4 \u003d Cr 2 (SO 4) 3 + 3Cu,
Cr + CaCl 2 no reaction.
In nature, the element chromium is represented by four isotopes with mass numbers 50, 52, 53 and 54. In nature, chromium occurs only in the form of compounds, the most important of which are chromium iron ore, or chromite (FeOzhCr 2 O 3) and lead red ore (PbCrO 4).
Metal chromium is obtained: 1) from its oxide using aluminothermy:
Cr 2 O 3 + 2Al 2Cr + Al 2 O 3,
2) electrolysis of aqueous solutions or melts of its salts:
From chromium iron ore in industry, an alloy of iron with chromium is obtained - ferrochromium, which is widely used in metallurgy:
FeO Cr 2 O 3 + 4CFe + 2Cr + 4CO.
The most important chromium compounds
Chromium forms three oxides and their corresponding hydroxides, the nature of which naturally changes with an increase in the oxidation state of chromium:
Chromium oxide(II) (CrO) is a solid, bright red or brown-red substance, insoluble under normal conditions in water, a typical basic oxide. Chromium(II) oxide is easily oxidized in air when heated, and is reduced to pure chromium.
CrO + 2HCl \u003d CrCl 2 + H 2 O,
4CrO + O 2 2Cr 2 O 3,
CrO + H 2 Cr + H 2 O.
Chromium(II) oxide is obtained by direct oxidation of chromium:
2Cr + O 2 2CrO.
Chromium hydroxide(II) (Cr(OH) 2) - water-insoluble yellow substance, weak electrolyte, exhibits basic properties, readily soluble in concentrated acids; easily oxidized in the presence of moisture by atmospheric oxygen; when calcined in air, it decomposes to form chromium(III) oxide:
Cr(OH) 2 + 2HCl = CrCl 2 + 2H 2 O,
4Cr(OH) 2 + O 2 2Cr 2 O 3 + 4H 2 O.
Chromium(II) hydroxide is obtained by the exchange reaction between a chromium(II) salt and an alkali solution in the absence of oxygen:
CrCl 2 + 2NaOH \u003d Cr (OH) 2 + 2NaCl.
Chromium oxide(III) (Cr 2 O 3) exhibits amphoteric properties. It is a refractory (hardness comparable to corundum) green powder, insoluble in water. Carcinogen! It is obtained by decomposition of ammonium dichromate, chromium (III) hydroxide, reduction of potassium dichromate or direct oxidation of chromium:
(NH 4) 2 Cr 2 O 7 N 2 + Cr 2 O 3 + 4H 2 O,
2Cr (OH) 3 Cr 2 O 3 + 3H 2 O,
2K 2 Cr 2 O 7 + 3С2Cr 2 O 3 + 2K 2 CO 3 + CO 2,
4Cr + 3O 2 2Cr 2 O 3 .
Under normal conditions, chromium(III) oxide is poorly soluble in acids and alkalis; it exhibits amphoteric properties when fused with alkalis or with alkali metal carbonates (forming chromites); at high temperatures, chromium(III) oxide can be reduced to a pure metal:
Cr 2 O 3 + 2KOH 2KCrO 2 + H 2 O,
Cr 2 O 3 + Na 2 CO 3 2NaCrO 2 + CO 2,
Cr 2 O 3 + 6HCl \u003d 2CrCl 3 + 3H 2 O,
2Cr 2 O 3 + 3C4Cr + 3CO 2.
Chromium hydroxide(III) (Cr (OH) 3) is precipitated by the action of alkalis on salts of trivalent chromium (gray-green precipitate):
CrCl 3 + 3NaOH (lack) = Cr(OH) 3 + 3NaCl.
It exhibits amphoteric properties, dissolving both in acids and in an excess of alkalis; thermally unstable:
Cr(OH) 3 + 3HCl = CrCl 3 + 3H 2 O,
Cr(OH) 3 + 3KOH \u003d K 3,
Cr(OH) 3 + KOH KCrO 2 + 2H 2 O,
2Cr(OH)3Cr2O3 + 3H2O.
Chromium oxide(VI) (CrO 3) - a dark red crystalline substance, poisonous, exhibits acidic properties. Let's well dissolve in water, at dissolution of this oxide in water chromic acids are formed; how the acid oxide CrO 3 interacts with basic oxides and with alkalis; thermally unstable; is the strongest oxidizing agent
CrO 3 + H 2 O =
2CrO 3 + H 2 O =
CrO 3 + K 2 OK 2 CrO 4 ,
CrO 3 + 2NaOH \u003d Na 2 CrO 4 + H 2 O,
4CrO 3 2Cr 2 O 3 + 3O 2,
This oxide is obtained by the interaction of dry chromates and dichromates with concentrated sulfuric acid:
K 2 Cr 2 O 7 + H 2 SO 4 (conc.) 2CrO 3 + K 2 SO 4 + H 2 O,
K 2 CrO 4 + H 2 SO 4 (conc.) CrO 3 + K 2 SO 4 + H 2 O.
Chrome and dichromic acid exist only in aqueous solutions, but form stable salts - chromates and dichromates. Chromates and their solutions are yellow, and dichromates are orange. Chromate ions and dichromate ions easily pass into each other when the solution medium changes. AT acidic environment chromates turn into dichromates, the solution acquires an orange color; in an alkaline environment dichromates turn into chromates, the solution turns yellow:
2K 2 CrO 4 + H 2 SO 4) K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O,
K 2 Cr 2 O 7 + 2KOH)2K 2 CrO 4 + H 2 O.
The ion is stable in an alkaline medium, but in an acidic one.
Oxidation–reduction properties
chromium compound
Of all chromium compounds, the most stable are compounds with a chromium oxidation state of +3. Chromium compounds with an oxidation state of +2 are strong reducing agents and are easily oxidized to +3:
4Cr(OH) 2 + O 2 + 2H 2 O = 4Cr(OH) 3,
4CrCl 2 + 4HCl + O 2 = 4CrCl 3 + 2H 2 O.
Compounds containing chromium in the +6 oxidation state are strong oxidizers, while chromium is reduced from +6 to +3:
K 2 Cr 2 O 7 + 3H 2 S + 4H 2 SO 4 = 3S + Cr 2 (SO 4) 3 + K 2 SO 4 + 7H 2 O.
To detect alcohol in exhaled air, a reaction based on the oxidizing ability of chromium(VI) oxide is used:
4CrO 3 + 3С 2 H 5 OH 2Cr 2 O 3 + 3CH 3 COOH + 3H 2 O.
A solution of potassium dichromate in concentrated sulfuric acid is called chrome mixture and is used to clean chemical glassware.
Test on the topic "Chromium and its compounds"
1. Some element forms all three types of oxides (basic, amphoteric, and acidic). The oxidation state of an element in an amphoteric oxide will be:
a) minimum;
b) maximum;
c) intermediate between the minimum and maximum;
d) can be anyone.
2. When a freshly prepared precipitate of chromium (III) hydroxide reacts with an excess of an alkali solution, the following is formed:
a) medium salt; b) basic salt;
c) double salt; d) complex salt.
3. The total number of electrons in the preexternal level of the chromium atom is:
a) 12; b) 13; in 1; d) 2.
4. Which of the metal oxides is acidic?
a) Copper(II) oxide; b) chromium(VI) oxide;
c) chromium(III) oxide; d) iron(III) oxide.
5. What mass of potassium dichromate (in g) is required to oxidize 11.2 g of iron in sulfuric acid solution?
a) 58.8; b) 14.7; c) 294; d) 29.4.
6. What mass of water (in g) must be evaporated from 150 g of a 10% solution of chromium(III) chloride to obtain a 30% solution of this salt?
a) 100; b) 20; c) 50; d) 40.
7. The molar concentration of sulfuric acid in the solution is 11.7 mol/l, and the density of the solution is 1.62 g/ml. The mass fraction of sulfuric acid in this solution is (in%):
a) 35.4; b) 98; c) 70.8; d) 11.7.
8. The number of oxygen atoms in 19.4 g of potassium chromate is:
a) 0.602 10 23; b) 2.408 10 23;
c) 2.78 10 23; d) 6.02 10 23 .
9. Litmus will show a red color in an aqueous solution (multiple correct answers are possible):
a) chromium(III) chloride; b) chromium(II) chloride;
c) potassium chloride; d) hydrochloric acid.
10. The transition of chromate to dichromate occurs in ... environment and is accompanied by the process:
a) acidic, recovery process;
b) acidic, there is no change in oxidation states;
c) alkaline, recovery process;
d) alkaline, there is no change in oxidation states.
Key to the test
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| in | G | b | b | G | a | in | b | a, b, d | b |
Qualitative tasks for the identification of substances 1. An aqueous solution of some salt is divided into two parts. One of them was treated with an excess of alkali and heated, the released gas changed the color of red litmus to blue. The other part was treated with hydrochloric acid, the released gas caused the lime water to become cloudy. What salt was analyzed? Support your answer with reaction equations.
Answer. ammonium carbonate.
2. When ammonia, sodium sulfide and silver nitrate are added (separately) to an aqueous solution of substance A, white precipitates are formed, two of which are of the same composition. What is Substance A? Write reaction equations.
Solution
Substance A - AlCl 3.
AlCl 3 + 3NH 4 OH \u003d Al (OH) 3 + 3NH 4 Cl,
2AlCl 3 + 3Na 2 S + 6H 2 O 2Al(OH) 3 + 3H 2 S + 6NaCl,
AlCl 3 + 3AgNO 3 \u003d 3AgCl + Al (NO 3) 3.
Answer. aluminum chloride.
3. When a colorless gas A with a pungent characteristic odor is burned in the presence of oxygen, another gas B is formed, without color and odor, which reacts with lithium at room temperature to form a solid substance C. Identify the substances, write the reaction equations.
Solution
Substance A - NH 3,
substance B - N 2,
substance C - Li 3 N.
4NH 3 + 3O 2 2N 2 + 6H 2 O,
N 2 + 6Li = 2Li 3 N.
Answer. NH 3 , N 2 , Li 3 N.
4. Colorless gas A, with a characteristic pungent odor, reacts with another colorless gas, B, which has the smell of rotten eggs. As a result of the reaction, a simple C and a complex substance are formed. Substance C reacts with copper to form a black salt. Identify substances, give reaction equations.
Answer. SO 2 , H 2 S, S.
5. Colorless gas A with a pungent characteristic odor, lighter than air, reacts with strong acid B to form salt C, an aqueous solution of which does not form precipitates with either barium chloride or silver nitrate. Identify substances, give reaction equations (one of the possible options).
Answer. NH 3 , HNO 3 , NH 4 NO 3 .
6. A simple substance A, formed by atoms of the second most common element of the earth's crust, reacts when heated with iron (II) oxide, resulting in the formation of compound B, which is insoluble in aqueous solutions of alkalis and acids (except hydrofluoric). Substance B, when fused with quicklime, forms an insoluble salt C. Identify the substances, give the reaction equations (one of the possible options).
Answer. Si, SiO 2 , CaSiO 3 .
7. Water-insoluble brown compound A decomposes upon heating to form two oxides, one of which is water. The other oxide, B, is reduced by carbon to form metal C, the second most abundant metal in nature. Identify substances, write reaction equations.
Answer. Fe (OH) 3, Fe 2 O 3, Fe.
8. Substance A, which is part of one of the most common minerals, forms gas B when treated with hydrochloric acid. When substance B reacts when heated with simple substance C, only one compound is formed - a combustible gas without color and odor. Identify substances, give reaction equations.
Answer. CaCO 3 , CO 2 , C.
9. Light metal A, which reacts with dilute sulfuric acid, but does not react in cold with concentrated sulfuric acid, interacts with a solution of sodium hydroxide, and gas and salt B are formed. When hydrochloric acid is added to substance B, salt C is formed. Identify substances, give equations reactions.
Answer. Al, NaAlO 2 , NaCl.
10. Substance A is a soft, well-cut silver-white metal, lighter than water. When substance A interacts with a simple substance B, compound C is formed, which is soluble in water to form an alkaline solution. When substance C is treated with hydrochloric acid, a gas with an unpleasant odor is released and salt is formed, which turns the flame of the burner purple. Identify substances, give reaction equations.
Answer. K, S, K2S.
11. Colorless gas A with a pungent characteristic odor is oxidized by oxygen in the presence of a catalyst to compound B, which is a volatile liquid. Substance B, reacting with quicklime, forms salt C. Identify the substances, give the reaction equations.
Answer. SO 2 , SO 3 , CaSO 4 .
12. A simple substance A, liquid at room temperature, reacts with a silvery-white light metal B, forming salt C, which, when treated with an alkali solution, gives a white precipitate that dissolves in an excess of alkali. Identify substances, give reaction equations.
Answer. Br 2 , Al, AlBr 3 .
13. A yellow simple solid substance A reacts with a silvery-white light metal B, resulting in the formation of salt C, which is completely hydrolyzed in an aqueous solution with the formation of a white precipitate and a poisonous gas with an unpleasant odor. Identify substances, give reaction equations.
Answer. S, Al, Al 2 S 3 .
14. A simple unstable gaseous substance A turns into another simple substance B, in the atmosphere of which metal C burns; The product of this reaction is an oxide in which the metal is in two oxidation states. Identify substances, give reaction equations.
Answer. O 3 , O 2 , Fe.
15. A dark purple crystalline substance A, when heated, decomposes to form a simple gaseous substance B, in the atmosphere of which a simple substance C burns, forming a colorless, odorless gas, which is part of the air in small quantities. Identify substances, give reaction equations.
Answer. KMnO 4 , O 2 , C.
16. A simple substance A, which is a semiconductor, reacts with a simple gaseous substance B to form compound C, which is insoluble in water. When fused with alkalis, substance C forms compounds called soluble glasses. Identify substances, give reaction equations (one of the possible options).
Answer. Si, O 2 , SiO 2 .
17. Toxic, colorless gas A with an unpleasant odor decomposes when heated into simple substances, one of which B is a yellow solid. When substance B is burned, a colorless gas C with an unpleasant odor is formed, which decolorizes many organic paints. Identify substances, give reaction equations.
Answer. H2S, S, SO2.
18. Volatile hydrogen compound A burns in air to form substance B, which is soluble in hydrofluoric acid. When substance B is fused with sodium oxide, a water-soluble salt C is formed. Identify the substances, give the reaction equations.
Answer. SiH 4 , SiO 2 , Na 2 SiO 3 .
19. Compound A, sparingly soluble in water, of white color, as a result of calcination at high temperature with coal and sand in the absence of oxygen, forms a simple substance B, which exists in several allotropic modifications. When this substance is burned in air, compound C is formed, which dissolves in water to form an acid capable of forming three series of salts. Identify substances, write reaction equations.
Answer. Ca 3 (PO 4) 2, P, P 2 O 5.
* The +/– sign means that this reaction does not proceed with all reagents or under specific conditions.
To be continued
DEFINITION
Chromium is the twenty-fourth element of the Periodic Table. Designation - Cr from the Latin "chromium". Located in the fourth period, VIB group. Refers to metals. The core charge is 24.
Chromium is contained in the earth's crust in an amount of 0.02% (wt.). In nature, it occurs mainly in the form of iron chromium FeO×Cr 2 O 3 .
Chromium is a solid shiny metal (Fig. 1), melting at 1890 o C; its density is 7.19 g / cm 3. At room temperature, chromium is resistant to both water and air. Dilute sulfuric and hydrochloric acids dissolve chromium, releasing hydrogen. In cold concentrated nitric acid, chromium is insoluble and becomes passive after treatment with it.
Rice. 1. Chrome. Appearance.
Atomic and molecular weight of chromium
DEFINITION
Relative molecular weight of a substance(M r) is a number showing how many times the mass of a given molecule is greater than 1/12 of the mass of a carbon atom, and relative atomic mass of an element(A r) - how many times the average mass of atoms of a chemical element is greater than 1/12 of the mass of a carbon atom.
Since chromium exists in the free state in the form of monatomic Cr molecules, the values of its atomic and molecular masses are the same. They are equal to 51.9962.
Isotopes of chromium
It is known that chromium can occur in nature in the form of four stable isotopes 50Cr, 52Cr, 53Cr, and 54Cr. Their mass numbers are 50, 52, 53, and 54, respectively. The nucleus of the atom of the chromium isotope 50 Cr contains twenty-four protons and twenty-six neutrons, and the remaining isotopes differ from it only in the number of neutrons.
There are artificial isotopes of chromium with mass numbers from 42 to 67, among which the most stable is 59 Cr with a half-life of 42.3 minutes, as well as one nuclear isotope.
Chromium ions
On the outer energy level of the chromium atom, there are six electrons that are valence:
1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1 .
As a result of chemical interaction, chromium gives up its valence electrons, i.e. is their donor, and turns into a positively charged ion:
Cr 0 -2e → Cr 2+;
Cr 0 -3e → Cr 3+;
Cr 0 -6e → Cr 6+.
Molecule and atom of chromium
In the free state, chromium exists in the form of monatomic Cr molecules. Here are some properties that characterize the atom and molecule of chromium:
Chromium alloys
Chromium metal is used for chromium plating, and also as one of the most important components of alloy steels. The introduction of chromium into steel increases its resistance to corrosion both in aqueous media at ordinary temperatures and in gases at elevated temperatures. In addition, chromium steels have increased hardness. Chromium is a part of stainless acid-resistant, heat-resistant steels.
Examples of problem solving
EXAMPLE 1
EXAMPLE 2
| Exercise | Chromium oxide (VI) weighing 2 g was dissolved in water weighing 500 g. Calculate the mass fraction of chromic acid H 2 CrO 4 in the resulting solution. |
| Solution | Let's write the reaction equation for obtaining chromic acid from chromium (VI) oxide: CrO 3 + H 2 O \u003d H 2 CrO 4. Find the mass of the solution: m solution \u003d m (CrO 3) + m (H 2 O) \u003d 2 + 500 \u003d 502 g. n (CrO 3) \u003d m (CrO 3) / M (CrO 3); n (CrO 3) \u003d 2/100 \u003d 0.02 mol. According to the reaction equation n(CrO 3) :n(H 2 CrO 4) = 1:1, then n (CrO 3) \u003d n (H 2 CrO 4) \u003d 0.02 mol. Then the mass of chromic acid will be equal to (molar mass - 118 g / mol): m (H 2 CrO 4) \u003d n (H 2 CrO 4) × M (H 2 CrO 4); m (H 2 CrO 4) \u003d 0.02 × 118 \u003d 2.36 g. The mass fraction of chromic acid in solution is: ω = msolute / msolution × 100%; ω (H 2 CrO 4) \u003d m solute (H 2 CrO 4) / m solution × 100%; ω (H 2 CrO 4) \u003d 2.36 / 502 × 100% \u003d 0.47%. |
| Answer | The mass fraction of chromic acid is 0.47%. |
