Target: deepen students' knowledge on the topic of the lesson.
Tasks:
- characterize chromium as a simple substance;
- introduce students to chromium compounds of different oxidation states;
- show the dependence of the properties of compounds on the degree of oxidation;
- show the redox properties of chromium compounds;
- continue to develop students’ skills in writing down equations of chemical reactions in molecular and ionic form and creating an electronic balance;
- continue to develop the skills to observe a chemical experiment.
Lesson form: lecture with elements of independent work of students and observation of a chemical experiment.
Lesson progress
I. Repetition of material from the previous lesson.
1. Answer questions and complete tasks:
What elements belong to the chromium subgroup?
Write electronic formulas of atoms
What type of elements are they?
What oxidation states do the compounds exhibit?
How does the atomic radius and ionization energy change from chromium to tungsten?
You can ask students to complete the table using the tabulated values of atomic radii, ionization energies and draw conclusions.
Sample table:
2. Listen to a student’s report on the topic “Elements of the chromium subgroup in nature, preparation and application.”
II. Lecture.
Lecture outline:
- Chromium.
- Chromium compounds. (2)
- Chromium oxide; (2)
- Chromium hydroxide. (2)
- Chromium compounds. (3)
- Chromium oxide; (3)
- Chromium hydroxide. (3)
- Chromium compounds (6)
- Chromium oxide; (6)
- Chromic and dichromic acids.
- Dependence of the properties of chromium compounds on the degree of oxidation.
- Redox properties of chromium compounds.
1. Chrome.
Chrome is a white, shiny metal with a bluish tint, very hard (density 7.2 g/cm3), melting point 1890˚C.
Chemical properties: Chromium is an inactive metal under normal conditions. This is explained by the fact that its surface is covered with an oxide film (Cr 2 O 3). When heated, the oxide film is destroyed, and chromium reacts with simple substances at high temperatures:
- 4Сr +3О 2 = 2Сr 2 О 3
- 2Сr + 3S = Сr 2 S 3
- 2Сr + 3Cl 2 = 2СrСl 3
Exercise: draw up equations for the reactions of chromium with nitrogen, phosphorus, carbon and silicon; Compose an electronic balance for one of the equations, indicate the oxidizing agent and the reducing agent.
Interaction of chromium with complex substances:
At very high temperatures, chromium reacts with water:
- 2Сr + 3Н2О = Сr2О3 + 3Н2
Exercise:
Chromium reacts with dilute sulfuric and hydrochloric acids:
- Cr + H 2 SO 4 = CrSO 4 + H 2
- Cr + 2HCl = CrCl 2 + H 2
Exercise: draw up an electronic balance, indicate the oxidizing agent and reducing agent.
Concentrated sulfuric hydrochloric and nitric acids passivate chromium.
2. Chromium compounds. (2)
1. Chromium oxide (2)- CrO is a solid, bright red substance, a typical basic oxide (it corresponds to chromium (2) hydroxide - Cr(OH) 2), does not dissolve in water, but dissolves in acids:
- CrO + 2HCl = CrCl 2 + H 2 O
Exercise: draw up a reaction equation in molecular and ionic form for the interaction of chromium oxide (2) with sulfuric acid.
Chromium oxide (2) is easily oxidized in air:
- 4CrO+ O 2 = 2Cr 2 O 3
Exercise: draw up an electronic balance, indicate the oxidizing agent and reducing agent.
Chromium oxide (2) is formed by the oxidation of chromium amalgam with atmospheric oxygen:
2Сr (amalgam) + O 2 = 2СrО
2. Chromium hydroxide (2)- Cr(OH) 2 is a yellow substance, poorly soluble in water, with a pronounced basic character, therefore it interacts with acids:
- Cr(OH) 2 + H 2 SO 4 = CrSO 4 + 2H 2 O
Exercise: draw up reaction equations in molecular and ionic form for the interaction of chromium oxide (2) with hydrochloric acid.
Like chromium(2) oxide, chromium(2) hydroxide is oxidized:
- 4 Cr(OH) 2 + O 2 + 2H 2 O = 4Cr(OH) 3
Exercise: draw up an electronic balance, indicate the oxidizing agent and reducing agent.
Chromium hydroxide (2) can be obtained by the action of alkalis on chromium salts (2):
- CrCl 2 + 2KOH = Cr(OH) 2 ↓ + 2KCl
Exercise: write ionic equations.
3. Chromium compounds. (3)
1. Chromium oxide (3)- Cr 2 O 3 – dark green powder, insoluble in water, refractory, close in hardness to corundum (chromium hydroxide (3) – Cr(OH) 3) corresponds to it. Chromium oxide (3) is amphoteric in nature, but is poorly soluble in acids and alkalis. Reactions with alkalis occur during fusion:
- Cr 2 O 3 + 2KOH = 2KSrO 2 (chromite K)+ H 2 O
Exercise: draw up a reaction equation in molecular and ionic form for the interaction of chromium oxide (3) with lithium hydroxide.
It is difficult to interact with concentrated solutions of acids and alkalis:
- Cr 2 O 3 + 6 KOH + 3H 2 O = 2K 3 [Cr(OH) 6 ]
- Cr 2 O 3 + 6HCl = 2CrCl 3 + 3H 2 O
Exercise: draw up reaction equations in molecular and ionic form for the interaction of chromium oxide (3) with concentrated sulfuric acid and a concentrated solution of sodium hydroxide.
Chromium oxide (3) can be obtained from the decomposition of ammonium dichromate:
- (NН 4)2Сr 2 О 7 = N 2 + Сr 2 О 3 +4Н 2 О
2. Chromium hydroxide (3) Cr(OH) 3 is obtained by the action of alkalis on solutions of chromium salts (3):
- CrCl 3 + 3KOH = Cr(OH) 3 ↓ + 3KCl
Exercise: write ionic equations
Chromium hydroxide (3) is a gray-green precipitate, upon receipt of which the alkali must be taken in deficiency. The chromium hydroxide (3) obtained in this way, in contrast to the corresponding oxide, easily interacts with acids and alkalis, i.e. exhibits amphoteric properties:
- Cr(OH) 3 + 3HNO 3 = Cr(NO 3) 3 + 3H 2 O
- Cr(OH) 3 + 3KOH = K 3 [Cr(OH)6] (hexahydroxochromite K)
Exercise: draw up reaction equations in molecular and ionic form for the interaction of chromium hydroxide (3) with hydrochloric acid and sodium hydroxide.
When Cr(OH) 3 is fused with alkalis, metachromites and orthochromites are obtained:
- Cr(OH) 3 + KOH = KCrO 2 (metachromite K)+ 2H 2 O
- Cr(OH) 3 + KOH = K 3 CrO 3 (orthochromite K)+ 3H 2 O
4. Chromium compounds. (6)
1. Chromium oxide (6)- CrO 3 – dark red crystalline substance, highly soluble in water – a typical acidic oxide. This oxide corresponds to two acids:
- CrO 3 + H 2 O = H 2 CrO 4 (chromic acid – formed when there is excess water)
- CrO 3 + H 2 O =H 2 Cr 2 O 7 (dichromic acid - formed at a high concentration of chromium oxide (3)).
Chromium oxide (6) is a very strong oxidizing agent, therefore it energetically interacts with organic substances:
- C 2 H 5 OH + 4CrO 3 = 2CO 2 + 2Cr 2 O 3 + 3H 2 O
Also oxidizes iodine, sulfur, phosphorus, coal:
- 3S + 4CrO 3 = 3SO 2 + 2Cr 2 O 3
Exercise: draw up equations of chemical reactions of chromium oxide (6) with iodine, phosphorus, coal; create an electronic balance for one of the equations, indicate the oxidizing agent and reducing agent
When heated to 250 0 C, chromium oxide (6) decomposes:
- 4CrO3 = 2Cr2O3 + 3O2
Chromium oxide (6) can be obtained by the action of concentrated sulfuric acid on solid chromates and dichromates:
- K 2 Cr 2 O 7 + H 2 SO 4 = K 2 SO 4 + 2CrO 3 + H 2 O
2. Chromic and dichromic acids.
Chromic and dichromic acids exist only in aqueous solutions and form stable salts, chromates and dichromates, respectively. Chromates and their solutions are yellow in color, dichromates are orange.
Chromate - CrO 4 2- ions and dichromate - Cr 2O 7 2- ions easily transform into each other when the solution environment changes
In an acidic solution, chromates transform into dichromates:
- 2K 2 CrO 4 + H 2 SO 4 = K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O
In an alkaline environment, dichromates turn into chromates:
- K 2 Cr 2 O 7 + 2 KOH = 2 K 2 CrO 4 + H 2 O
When diluted, dichromic acid turns into chromic acid:
- H 2 Cr 2 O 7 + H 2 O = 2H 2 CrO 4
5. Dependence of the properties of chromium compounds on the degree of oxidation.
| Oxidation state | +2 | +3 | +6 |
| Oxide | CrO | Cr 2 O 3 | СrО 3 |
| Character of the oxide | basic | amphoteric | acid |
| Hydroxide | Cr(OH) 2 | Cr(OH) 3 – H 3 CrO 3 | H 2 CrO 4 |
| Nature of the hydroxide | basic | amphoteric | acid |
|
→ weakening of basic properties and strengthening of acidic properties→ |
|||
6. Redox properties of chromium compounds.
Reactions in an acidic environment.
In an acidic environment, Cr +6 compounds transform into Cr +3 compounds under the action of reducing agents: H 2 S, SO 2, FeSO 4
- K 2 Cr 2 O 7 + 3H 2 S + 4H 2 SO 4 = 3S + Cr 2 (SO 4) 3 + K 2 SO 4 + 7H 2 O
- S -2 – 2e → S 0
- 2Cr +6 + 6e → 2Cr +3
Exercise:
1. Equalize the reaction equation using the electronic balance method, indicate the oxidizing agent and reducing agent:
- Na 2 CrO 4 + K 2 S + H 2 SO 4 = S + Cr 2 (SO 4) 3 + K 2 SO 4 + Na 2 SO 4 + H 2 O
2. Add the reaction products, equalize the equation using the electronic balance method, indicate the oxidizing agent and reducing agent:
- K 2 Cr 2 O 7 + SO 2 + H 2 SO 4 =? +? +H 2 O
Reactions in an alkaline environment.
In an alkaline environment, chromium compounds Cr +3 transform into compounds Cr +6 under the action of oxidizing agents: J2, Br2, Cl2, Ag2O, KClO3, H2O2, KMnO4:
- 2KCrO 2 +3 Br 2 +8NaOH =2Na 2 CrO 4 + 2KBr +4NaBr + 4H 2 O
- Cr +3 - 3e → Cr +6
- Br2 0 +2e → 2Br -
Exercise:
Equalize the reaction equation using the electronic balance method, indicate the oxidizing agent and reducing agent:
- NaCrO 2 + J 2 + NaOH = Na 2 CrO 4 + NaJ + H 2 O
Add the reaction products, equalize the equation using the electronic balance method, indicate the oxidizing agent and reducing agent:
- Cr(OH) 3 + Ag 2 O + NaOH = Ag + ? + ?
Thus, the oxidizing properties consistently increase with a change in oxidation states in the series: Cr +2 → Cr +3 → Cr +6. Chromium compounds (2) are strong reducing agents and are easily oxidized, turning into chromium compounds (3). Chromium compounds (6) are strong oxidizing agents and are easily reduced to chromium compounds (3). Chromium compounds (3) when interacting with strong reducing agents exhibit oxidizing properties, turning into chromium compounds (2), and when interacting with strong oxidizing agents they exhibit reducing properties, turning into chromium compounds (6)
To the lecture methodology:
- To enhance students’ cognitive activity and maintain interest, it is advisable to conduct a demonstration experiment during the lecture. Depending on the capabilities of the educational laboratory, the following experiments can be demonstrated to students:
- obtaining chromium oxide (2) and chromium hydroxide (2), proof of their basic properties;
- obtaining chromium oxide (3) and chromium hydroxide (3), proving their amphoteric properties;
- obtaining chromium oxide (6) and dissolving it in water (preparation of chromic and dichromic acids);
- transition of chromates to dichromates, dichromates to chromates.
- Independent work tasks can be differentiated taking into account the real learning capabilities of students.
- You can complete the lecture by completing the following tasks: write equations of chemical reactions that can be used to carry out the following transformations:
.III. Homework: improve the lecture (add the equations of chemical reactions)
- Vasilyeva Z.G. Laboratory work in general and inorganic chemistry. -M.: “Chemistry”, 1979 – 450 p.
- Egorov A.S. Chemistry tutor. – Rostov-on-Don: “Phoenix”, 2006.-765 p.
- Kudryavtsev A.A. Writing chemical equations. - M., “Higher School”, 1979. - 295 p.
- Petrov M.M. Inorganic chemistry. – Leningrad: “Chemistry”, 1989. – 543 p.
- Ushkalova V.N. Chemistry: competition tasks and answers. - M.: “Enlightenment”, 2000. – 223 p.
17.doc
Chromium. Chromium (II), (III) and (VI) oxides. Hydroxides and salts of chromium (II) and (III). Chromates and dichromates. Chromium(III) complex compounds
17.1. Brief characteristics of elements of the chromium subgroup
The chromium subgroup is a side subgroup of group VI of the periodic table of elements D.I. Mendeleev. The subgroup includes chromium Cr, molybdenum Mo, tungsten W.
These elements are also transition metals, because they build up the d-sublevel of the pre-external layer. In the outer layer of the atoms of these elements there is one (for chromium and molybdenum) or two (for tungsten) electrons. Thus, the atoms of elements of the chromium subgroup have six valence electrons that can participate in the formation of a chemical bond (see Table 30).
Chromium, molybdenum, and tungsten are similar in many physical and chemical properties: thus, in the form of simple substances, they are all refractory silver-white metals that have great hardness and a number of valuable mechanical properties - the ability to roll, draw, and form. povke.
From a chemical point of view, all metals of the chromium subgroup are resistant to air and water (under normal conditions); when heated, they all interact with oxygen, halogens, phosphorus, and carbon.
Under the influence of concentrated acids (HNO 3, H 2 SO 4) at ordinary temperatures, the metals of the chromium subgroup are passivated.
For all elements of the chromium subgroup, the most typical compounds are where their oxidation states are +2, +3, +6 (although there are compounds where their degrees can also be +4 and +5, and for chromium +1). Elements of the chromium subgroup do not have a negative oxidation state, and they do not form volatile hydrogen compounds. Solid hydrides such as CrH 3 are known only for chromium. Compounds of divalent elements are unstable and are easily oxidized to higher oxidation states.
With an increase in the degree of oxidation, the acidic nature of the oxides increases; with a maximum oxidation state of +6, oxides of the RO 3 type are formed, which correspond to the acids H 2 RO 4. The strength of acids naturally decreases from chromium to tungsten. Most salts of these acids are slightly soluble in water; only alkali metal and ammonium salts dissolve well.
As in other cases, the metallic properties of elements of the chromium subgroup increase with increasing atomic number.
Stva. The chemical activity of metals in the chromium - molybdenum - tungsten series decreases noticeably.
All metals of the chromium subgroup are widely used in modern technology, especially in the metallurgical industry for the production of special steels.
17.2. Chromium
Being in nature
Chromium is a fairly common element; its content in the earth's crust is approximately 0.02% (22nd place). Chromium is found exclusively in compounds; the main minerals are chromite FeCr 2 O 4 (or FeO Cr 2 O 3), or chromium iron ore, and crocoite PbCtO 4 (or PbO CrO 3). The color of many elements is due to the presence of chromium in them. So, for example, the golden-green tone of an emerald or the red tone of a ruby is given by an admixture of chromium oxide Cr 2 O 3.
Receipt
The raw material for the industrial production of chromium is chromium iron ore. Its chemical processing leads to Cr 2 O 3. Reduction of Cr 2 O 3 with aluminum or silicon produces metallic chromium of low purity:
Cr 2 O 3 +Al=Al 2 O 3 +2Cr
2Cr 2 O 3 +3Si=3SiO 2 +4Cr
Purer metal is obtained by electrolysis of concentrated solutions of chromium compounds.
^ Physical properties
Chrome is a steel-gray metal, hard, rather heavy. ( = 7.19 g/cm 3), plastic, malleable, melts at 1890°C, boils at 2480°C. It occurs in nature as a mixture of four stable isotopes with mass numbers 50, 52, 53 and 54. The most common isotope is 52 Cr (83.76%).
Chemical properties
The arrangement of electrons in the 3d and 4s orbitals of the chromium atom can be represented by the diagram:
This shows that chromium can exhibit different oxidation states in compounds from +1 to +6; Of these, the most stable are chromium compounds with oxidation states +2, +3, +6. Thus, not only the s-electron of the outer level, but also five d-electrons of the pre-external level participate in the formation of chemical bonds.
Under normal conditions, chromium is resistant to oxygen, water, and some other chemicals. At high temperatures, chromium burns in oxygen:
4Cr+3O 2 =2Cr 2 O 3
When heated, it reacts with water vapor:
2Cr+3H 2 O=Cr 2 O 3 +3H 2
When heated, chromium metal also reacts with halogens, sulfur, nitrogen, phosphorus, carbon, silicon and boron. For example: 2Cr+N 2 =2CrN 2Cr+3S=Cr 2 S 3 Cr+2Si=CrSi 2
The metal dissolves at ordinary temperature in dilute acids (HCl, H 2 SO 4) with the release of hydrogen. In these cases, in the absence of air, chromium (II) salts are formed:
Cr+2HCl=CrCl2+H 2 And in air - chromium (III) salts: 4Cr+12НCl+3О 2 =4CrСl+6Н 2 O
If the metal is immersed for some time in nitric acid (concentrated or diluted), then it ceases to dissolve in HCl and H 2 SO 4, does not change when heated with halogens, etc. This phenomenon - passivation - is explained by the formation of a protective layer on the metal surface - a very dense and mechanically strong (albeit very thin) film of chromium oxide Cr 2 O 3.
Application
The main consumer of chromium is metallurgy. With the addition of chromium, steel becomes much more resistant to chemical reagents; Such important properties of steel as strength, hardness and wear resistance also increase. Electrolytic coating of iron products with chromium (chrome plating) also gives them corrosion resistance.
The family of chromium alloys is very large. Nichrome (alloys with nickel) and lame (with aluminum and iron) are resistant
Chivas have high resistance and are used to make heaters in electric resistance furnaces. Stellite - an alloy of chromium (20-25%), cobalt (45-60%), tungsten (5-20%), iron (1-3%) - very hard, resistant to wear and corrosion; used in the metal processing industry for the manufacture of cutting tools. Chrome-molybdenum steels are used to create aircraft fusels.
^ 17.3. Chromium (II), (III) and (VI) oxides
Chromium forms three oxides: CrO, Cr 2 O 3, CrO 3.
Chromium (II) oxide CrO is a pyrophoric black powder. Has basic properties.
In redox reactions it behaves as a reducing agent:
CrO is obtained by decomposition of chromium carbonyl Cr(CO) 6 in vacuum at 300°C.
Chromium (III) oxide Cr 2 O 3 is a refractory green powder. It is close to corundum in hardness, which is why it is included in polishing agents. Formed by the interaction of Cr and O 2 at high temperature. In the laboratory, chromium(III) oxide can be prepared by heating ammonium dichromate:
(N -3 H 4) 2 Cr +6 2 O 7 =Cr +3 2 O 3 +N 0 2 +4H 2 O
Chromium(III) oxide has amphoteric properties. When interacting with acids, chromium (III) salts are formed: Cr 2 O 3 +3H 2 SO 4 =Cr 2 (SO 4) 3 +3H 2 O
When interacting with alkalis in the melt, chromium (III) compounds are formed - chromites (in the absence of oxygen): Cr 2 O 3 + 2NaOH = 2NaCrO 2 + H 2 O
Chromium(III) oxide is insoluble in water.
In redox reactions, chromium(III) oxide behaves as a reducing agent:
Chromium (VI) oxide CrO 3 - chromic anhydride, is a dark red needle-shaped crystals. When heated to about 200°C, it decomposes:
4CrO 3 =2Cr 2 O 3 +3O 2
Easily dissolves in water, being acidic in nature, it forms chromic acids. With excess water, chromic acid H 2 CrO 4 is formed:
CrO 3 +H 2 O=H 2 CrO 4
At a high concentration of CrO 3, dichromic acid H 2 Cr 2 O 7 is formed:
2CrO 3 +H 2 O=H 2 Cr 2 O 7
Which, when diluted, turns into chromic acid:
H 2 Cr 2 O 7 +H 2 O=2H 2 CrO 4
Chromic acids exist only in aqueous solution; none of these acids are isolated in a free state. However, their salts are very stable.
Chromium(VI) oxide is a strong oxidizing agent:
3S+4CrO 3 =3SO 2 +2Cr 2 O 3
Oxidizes iodine, sulfur, phosphorus, coal, turning into Cr 2 O 3. CrO 3 is obtained by the action of an excess of concentrated sulfuric acid on a saturated aqueous solution of sodium dichromate: Na 2 Cr 2 O 7 +2H 2 SO 4 =2CrO 3 +2NaHSO 4 +H 2 O It should be noted that chromium (VI) oxide is highly toxic.
^ 17.4. Hydroxides and salts of chromium (II) and (III). Chromium(III) complex compounds
Chromium (II) hydroxide Cr(OH) 2 is obtained in the form of a yellow precipitate by treating solutions of chromium (II) salts with alkalis in the absence of oxygen:
CrСl 2 +2NaOH=Cr(OH) 2 +2NaCl
Cr(OH) 2 has typical basic properties and is a strong reducing agent:
2Cr(OH) 2 +H 2 O+1/2O 2 =2Cr(OH) 3
Aqueous solutions of chromium (II) salts are obtained without access to air by dissolving chromium metal in dilute acids in a hydrogen atmosphere or by reducing trivalent chromium salts with zinc in an acidic environment. Anhydrous salts of chromium (II) are white, and aqueous solutions and crystalline hydrates are blue.
In their chemical properties, chromium (II) salts are similar to divalent iron salts, but differ from the latter in more pronounced reducing properties, i.e. are more easily oxidized than the corresponding ferrous iron compounds. This is why it is very difficult to obtain and store divalent chromium compounds.
Chromium (III) hydroxide Cr(OH) 3 is a gelatinous precipitate of gray-green color, it is obtained by the action of alkalis on solutions of chromium (III) salts:
Cr 2 (SO 4) 3 +6NaOH=2Cr(OH) 3 +3Na 2 SO 4
Chromium (III) hydroxide has amphoteric properties, dissolving both in acids to form chromium (III) salts:
2Cr(OH) 3 +3H 2 SO 4 =Cr 2 (SO 4) 3 +6H 2 O and in alkalis with the formation of hydroxychromites: Cr(OH) 3 +NaOH=Na 3
When Cr(OH) 3 is fused with alkalis, metachromites and orthochromites are formed:
Cr(OH) 3 +NaOH=NaCrO 2 +2H 2 O Cr(OH) 3 +3NaOH=Na 3 CrO 3 +3H 2 O
When chromium (III) hydroxide is calcined, chromium (III) oxide is formed:
2Cr(OH) 3 =Cr 2 O 3 +3H 2 O
Salts of trivalent chromium, both in the solid state and in aqueous solutions, are colored. For example, anhydrous chromium (III) sulfate Cr 2 (SO 4) 3 is violet-red in color; aqueous solutions of chromium (III) sulfate, depending on conditions, can change color from purple to green. This is explained by the fact that in aqueous solutions the Cr 3+ cation exists only in the form of a hydrated 3+ ion due to the tendency of trivalent chromium to form complex compounds. The purple color of aqueous solutions of chromium (III) salts is due precisely to the 3+ cation. When heated, chromium(III) complex salts can
Partially lose water, forming salts of various colors, even green.
Trivalent chromium salts are similar to aluminum salts in composition, crystal lattice structure, and solubility; Thus, for chromium (III), as well as for aluminum, the formation of chromium-potassium alum KCr(SO 4) 2 12H 2 O is typical; they are used for tanning leather and as a mordant in textiles.
Chromium salts (III) Cr 2 (SO 4) 3, CrCl 3, etc. stable when stored in air, but subject to hydrolysis in solutions:
Cr 3+ +3Сl - +ННCr(ОН) 2+ +3Сl - +Н +
Hydrolysis occurs at stage I, but there are salts that are completely hydrolyzed:
Cr 2 S 3 +H 2 O=Cr(OH) 3 +H 2 S
In redox reactions in an alkaline environment, chromium (III) salts behave as reducing agents:
It should be noted that in the series of chromium hydroxides of various oxidation states Cr(OH) 2 - Cr(OH) 3 - H 2 CrO 4, the basic properties are naturally weakened and the acidic properties are strengthened. This change in properties is due to an increase in the degree of oxidation and a decrease in the ionic radii of chromium. In the same series, the oxidizing properties are consistently enhanced. Cr (II) compounds are strong reducing agents and are easily oxidized, turning into chromium (III) compounds. Chromium(VI) compounds are strong oxidizing agents and are easily reduced to chromium(III) compounds. Compounds with an intermediate oxidation state, i.e. chromium (III) compounds can, when interacting with strong reducing agents, exhibit oxidizing properties, turning into chromium (II) compounds, and when interacting with strong oxidizing agents, exhibit reducing properties, turning into chromium (VI) compounds.
^ 17.5. Chromates and dichromates
Chromic acids form two series of compounds: chromates - the so-called salts of chromic acid, and dichromates - the so-called salts of dichromic acid. Chromates are colored yellow (the color of the chromate ion CrO 2-4), dichromates are colored orange (the color of the dichromate ion Cr 2 O 2-7) .
Chromates and dichromates dissociate, forming chromate and dichromate ions, respectively:
K 2 CrO 4 2K + +CrO 2- 4
K 2 Cr 2 O 7 2K + +Cr 2 O 2- 7
Chromates are obtained by reacting CrO 3 with alkalis:
CrO 3 +2NaOH=Na 2 CrO 4 +H 2 O
Dichromates are formed by adding acids to chromates:
2Na 2 CrO 4 +H 2 SO 4 =Na 2 Cr 2 O 7 +Na 2 SO 4 +H 2 O
The reverse transition is also possible when adding alkalis to solutions of dichromates:
Na 2 Cr 2 O 7 +2NaOH=2Na 2 CrO 4 +H 2 O
Thus, in acidic solutions, dichromates predominantly exist (they color the solution orange), and in alkaline solutions, chromates (yellow solutions) exist. The equilibrium in the chromate-dichromate system can be represented by the following equation in reduced ionic form:
2CrO 2- 4 +2H + Cr 2 O 2- 7 +H 2 O Cr 2 O 2- 7 +2OH - 2CrO 2- 4 +H 2 O
Salts of chromic acids in an acidic environment are strong oxidizing agents. They are usually reduced to chromium(III) compounds, for example:
Application
Chromium (VI) compounds are highly toxic: they affect the skin, respiratory tract, and cause inflammation of the eyes. In laboratories, a chromium mixture is often used for washing chemical glassware.
Which consists of equal volumes of a saturated aqueous solution of K 2 Cr 2 O 7 and concentrated H 2 SO 4.
Water-soluble sodium and potassium chromates are used in textile and leather production as wood preservatives. Insoluble chromates of some metals are excellent artistic paints. These are yellow crowns (PbCrO 4, |ZnCrO 4, SrCrO 4), and red lead-molybdenum crowns (contains PbCrO 4 and MoCrO 4) and many others. SnCrO 4 is famous for its richness of shades - from pink-red to violet, used in porcelain painting.
1) Chromium (III) oxide.
Chromium oxide can be obtained:
Thermal decomposition of ammonium dichromate:
(NH 4) 2 C 2 O 7 Cr 2 O 3 + N 2 + 4H 2 O
Reduction of potassium dichromate with carbon (coke) or sulfur:
2K 2 Cr 2 O 7 + 3C 2Cr 2 O 3 + 2K 2 CO 3 + CO 2
K 2 Cr 2 O 7 + S Cr 2 O 3 + K 2 SO 4
Chromium(III) oxide has amphoteric properties.
Chromium (III) oxide forms salts with acids:
Cr 2 O 3 + 6HCl = 2CrCl 3 + 3H 2 O
When chromium (III) oxide is fused with oxides, hydroxides and carbonates of alkali and alkaline earth metals, chromates (III) (chromites) are formed:
Сr 2 O 3 + Ba(OH) 2 Ba(CrO 2) 2 + H 2 O
Сr 2 O 3 + Na 2 CO 3 2NaCrO 2 + CO 2
With alkaline melts of oxidizing agents – chromates (VI) (chromates)
Cr 2 O 3 + 3KNO 3 + 4KOH = 2K 2 CrO 4 + 3KNO 2 + 2H 2 O
Cr 2 O 3 + 3Br 2 + 10NaOH = 2Na 2 CrO 4 + 6NaBr + 5H 2 O
Cr 2 O 3 + O 3 + 4KOH = 2K 2 CrO 4 + 2H 2 O
Cr 2 O 3 + 3O 2 + 4Na 2 CO 3 = 2Na 2 CrO 4 + 4CO 2
Сr 2 O 3 + 3NaNO 3 + 2Na 2 CO 3 2Na 2 CrO 4 + 2CO 2 + 3NaNO 2
Cr 2 O 3 + KClO 3 + 2Na 2 CO 3 = 2Na 2 CrO 4 + KCl + 2CO 2
2) Chromium(III) hydroxide
Chromium(III) hydroxide has amphoteric properties.
2Cr(OH) 3 = Cr 2 O 3 + 3H 2 O
2Cr(OH) 3 + 3Br 2 + 10KOH = 2K 2 CrO 4 + 6KBr + 8H 2 O
3) Chromium(III) salts
2CrCl 3 + 3Br 2 + 16KOH = 2K 2 CrO 4 + 6KBr + 6KCl + 8H 2 O
2CrCl 3 + 3H 2 O 2 + 10NaOH = 2Na 2 CrO 4 + 6NaCl + 8H 2 O
Cr 2 (SO 4) 3 + 3H 2 O 2 + 10NaOH = 2Na 2 CrO 4 + 3Na 2 SO 4 + 8H 2 O
Cr 2 (SO 4) 3 + 3Br 2 + 16NaOH = 2Na 2 CrO 4 + 6NaBr + 3Na 2 SO 4 + 8H 2 O
Cr 2 (SO 4) 3 + 6KMnO 4 + 16KOH = 2K 2 CrO 4 + 6K 2 MnO 4 + 3K 2 SO 4 + 8H 2 O.
2Na 3 + 3Br 2 + 4NaOH = 2Na 2 CrO 4 + 6NaBr + 8H 2 O
2K 3 + 3Br 2 + 4KOH = 2K 2 CrO 4 + 6KBr + 8H 2 O
2KCrO2 + 3PbO2 + 8KOH = 2K2CrO4 + 3K2PbO2 + 4H2O
Cr 2 S 3 + 30HNO 3 (conc.) = 2Cr(NO 3) 3 + 3H 2 SO 4 + 24NO 2 + 12H 2 O
2CrCl 3 + Zn = 2CrCl 2 + ZnCl 2
Chromates (III) easily react with acids:
NaCrO 2 + HCl (deficiency) + H 2 O = Cr(OH) 3 + NaCl
NaCrO 2 + 4HCl (excess) = CrCl 3 + NaCl + 2H 2 O
K 3 + 3CO 2 = Cr(OH) 3 ↓ + 3NaHCO 3
In solution they undergo complete hydrolysis
NaCrO 2 + 2H 2 O = Cr(OH) 3 ↓ + NaOH
Most chromium salts are highly soluble in water, but are easily hydrolyzed:
Cr 3+ + HOH ↔ CrOH 2+ + H +
СrCl 3 + HOH ↔ CrOHCl 2 + HCl
Salts formed by chromium (III) cations and a weak or volatile acid anion are completely hydrolyzed in aqueous solutions:
Cr 2 S 3 + 6H 2 O = 2Cr(OH) 3 ↓ + 3H 2 S
Chromium(VI) compounds
1) Chromium (VI) oxide.
Chromium(VI) oxide. Highly poisonous!
Chromium(VI) oxide can be prepared by the action of concentrated sulfuric acid on dry chromates or dichromates:
Na 2 Cr 2 O 7 + 2H 2 SO 4 = 2CrO 3 + 2NaHSO 4 + H 2 O
Acidic oxide that interacts with basic oxides, bases, water:
CrO 3 + Li 2 O → Li 2 CrO 4
CrO 3 + 2KOH → K 2 CrO 4 + H 2 O
CrO 3 + H 2 O = H 2 CrO 4
2CrO 3 + H 2 O = H 2 Cr 2 O 7
Chromium (VI) oxide is a strong oxidizing agent: it oxidizes carbon, sulfur, iodine, phosphorus, turning into chromium (III) oxide
4CrO 3 → 2Cr 2 O 3 + 3O 2.
4CrO 3 + 3S = 2Cr 2 O 3 + 3SO 2
Oxidation of salts:
2CrO 3 + 3K 2 SO 3 + 3H 2 SO 4 = 3K 2 SO 4 + Cr 2 (SO 4) 3 + 3H 2 O
Oxidation of organic compounds:
4CrO 3 + C 2 H 5 OH + 6H 2 SO 4 = 2Cr 2 (SO 4) 2 + 2CO 2 + 9H 2 O
Strong oxidizing agents are salts of chromic acids - chromates and dichromates. The reduction products of which are chromium (III) derivatives.
In a neutral environment, chromium (III) hydroxide is formed:
K 2 Cr 2 O 7 + 3Na 2 SO 3 + 4H 2 O = 2Cr(OH) 3 ↓ + 3Na 2 SO 4 + 2KOH
2K 2 CrO 4 + 3(NH 4) 2 S + 2H 2 O = 2Cr(OH) 3 ↓ + 3S↓ + 6NH 3 + 4KOH
In alkaline – hydroxochromates (III):
2K 2 CrO 4 + 3NH 4 HS + 5H 2 O + 2KOH = 3S + 2K 3 + 3NH 3 H 2 O
2Na 2 CrO 4 + 3SO 2 + 2H 2 O + 8NaOH = 2Na 3 + 3Na 2 SO 4
2Na 2 CrO 4 + 3Na 2 S + 8H 2 O = 3S + 2Na 3 + 4NaOH
In acidic – chromium (III) salts:
3H 2 S + K 2 Cr 2 O 7 + 4H 2 SO 4 = K 2 SO 4 + Cr 2 (SO 4) 3 + 3S + 7H 2 O
K 2 Cr 2 O 7 + 7H 2 SO 4 + 6KI = Cr 2 (SO 4) 3 + 3I 2 + 4K 2 SO 4 + 7H 2 O
K 2 Cr 2 O 7 + 3H 2 S + 4H 2 SO 4 = K 2 SO 4 + Cr 2 (SO 4) 3 + 3S + 7H 2 O
8K 2 Cr 2 O 7 + 3Ca 3 P 2 + 64HCl = 3Ca 3 (PO 4) 2 + 16CrCl 3 + 16KCl + 32H 2 O
K 2 Cr 2 O 7 + 7H 2 SO 4 + 6FeSO 4 = Cr 2 (SO 4) 3 + 3Fe 2 (SO 4) 3 + K 2 SO 4 + 7H 2 O
K 2 Cr 2 O 7 + 4H 2 SO 4 + 3KNO 2 = Cr 2 (SO 4) 3 + 3KNO 3 + K 2 SO 4 + 4H 2 O
K 2 Cr 2 O 7 + 14HCl = 3Cl 2 + 2CrCl 3 + 7H 2 O + 2KCl
K 2 Cr 2 O 7 + 3SO 2 + 8HCl = 2KCl + 2CrCl 3 + 3H 2 SO 4 + H 2 O
2K 2 CrO 4 + 16HCl = 3Cl 2 + 2CrCl 3 + 8H 2 O + 4KCl
The recovery product in various environments can be represented schematically:
H 2 O Cr(OH) 3 gray-green precipitate
K 2 CrO 4 (CrO 4 2–)
OH – 3 – emerald green solution
K 2 Cr 2 O 7 (Cr 2 O 7 2–) H + Cr 3+ blue-violet solution
Salts of chromic acid - chromates - are yellow, and salts of dichromic acid - dichromates - are orange. By changing the reaction of the solution, it is possible to carry out the mutual conversion of chromates into dichromates:
2K 2 CrO 4 + 2HCl (diluted) = K 2 Cr 2 O 7 + 2KCl + H 2 O
2K 2 CrO 4 + H 2 O + CO 2 = K 2 Cr 2 O 7 + KHCO 3
acidic environment
2СrO 4 2 – + 2H + Cr 2 O 7 2– + H 2 O
alkaline environment
Chromium. Chromium compounds.
1. Chromium (III) sulfide was treated with water, gas was released and an insoluble substance remained. A solution of sodium hydroxide was added to this substance and chlorine gas was passed through, and the solution acquired a yellow color. The solution was acidified with sulfuric acid, as a result the color changed to orange; The gas released when the sulfide was treated with water was passed through the resulting solution, and the color of the solution changed to green. Write the equations for the reactions described.
2. After briefly heating an unknown powdery substance of an orange substance, an orange-colored substance begins a spontaneous reaction, which is accompanied by a change in color to green, the release of gas and sparks. The solid residue was mixed with potassium hydroxide and heated, the resulting substance was added to a dilute solution of hydrochloric acid, and a green precipitate was formed, which dissolves in excess acid. Write the equations for the reactions described.
3. Two salts turn the flame purple. One of them is colorless, and when it is slightly heated with concentrated sulfuric acid, the liquid in which copper dissolves is distilled off; the latter transformation is accompanied by the release of brown gas. When a second salt of a sulfuric acid solution is added to the solution, the yellow color of the solution changes to orange, and when the resulting solution is neutralized with alkali, the original color is restored. Write the equations for the reactions described.
4. Trivalent chromium hydroxide was treated with hydrochloric acid. Potash was added to the resulting solution, the precipitate that formed was separated and added to a concentrated solution of potassium hydroxide, as a result of which the precipitate dissolved. After adding excess hydrochloric acid, a green solution was obtained. Write the equations for the reactions described.
5. When dilute hydrochloric acid was added to the solution of a yellow salt, which colors the flame violet, the color changed to orange-red. After neutralizing the solution with concentrated alkali, the color of the solution returned to its original color. When barium chloride is added to the resulting mixture, a yellow precipitate forms. The precipitate was filtered and a solution of silver nitrate was added to the filtrate. Write the equations for the reactions described.
6. Soda ash was added to the solution of trivalent chromium sulfate. The resulting precipitate was separated, transferred to a solution of sodium hydroxide, bromine was added and heated. After neutralizing the reaction products with sulfuric acid, the solution acquires an orange color, which disappears after passing sulfur dioxide through the solution. Write the equations for the reactions described.
7) Chromium (III) sulfide powder was treated with water. The resulting gray-green precipitate was treated with chlorine water in the presence of potassium hydroxide. A solution of potassium sulfite was added to the resulting yellow solution, and a gray-green precipitate formed again, which was calcined until the mass was constant. Write the equations for the reactions described.
8) Chromium (III) sulfide powder was dissolved in sulfuric acid. At the same time, gas was released and a solution was formed. An excess of ammonia solution was added to the resulting solution, and the gas was passed through a lead nitrate solution. The resulting black precipitate turned white after treatment with hydrogen peroxide. Write the equations for the reactions described.
9) Ammonium dichromate decomposed when heated. The solid decomposition product was dissolved in sulfuric acid. A solution of sodium hydroxide was added to the resulting solution until a precipitate formed. Upon further addition of sodium hydroxide to the precipitate, it dissolved. Write the equations for the reactions described.
10) Chromium (VI) oxide reacted with potassium hydroxide. The resulting substance was treated with sulfuric acid, and an orange salt was isolated from the resulting solution. This salt was treated with hydrobromic acid. The resulting simple substance reacted with hydrogen sulfide. Write the equations for the reactions described.
11. Chrome was burned in chlorine. The resulting salt reacted with a solution containing hydrogen peroxide and sodium hydroxide. Excess sulfuric acid was added to the resulting yellow solution, and the color of the solution changed to orange. When copper(I) oxide reacted with this solution, the color of the solution turned blue-green. Write the equations for the reactions described.
12. Sodium nitrate was fused with chromium(III) oxide in the presence of sodium carbonate. The gas released reacted with an excess of barium hydroxide solution, forming a white precipitate. The precipitate was dissolved in an excess of hydrochloric acid solution and silver nitrate was added to the resulting solution until the precipitation stopped. Write the equations for the reactions described.
13. Potassium was fused with sulfur. The resulting salt was treated with hydrochloric acid. The gas released was passed through a solution of potassium dichromate in sulfuric acid. the precipitated yellow substance was filtered and fused with aluminum. Write the equations for the reactions described.
14. Chrome was burned in a chlorine atmosphere. Potassium hydroxide was added dropwise to the resulting salt until the precipitation stopped. The resulting precipitate was oxidized with hydrogen peroxide in sodium hydroxide and evaporated. An excess of a hot solution of concentrated hydrochloric acid was added to the resulting solid residue. Write the equations for the reactions described.
Chromium. Chromium compounds.
1) Cr 2 S 3 + 6H 2 O = 2Cr(OH) 3 ↓ + 3H 2 S
2Cr(OH) 3 + 3Cl 2 + 10NaOH = 2Na 2 CrO 4 + 6NaCl + 8H 2 O
Na 2 Cr 2 O 7 + 4H 2 SO 4 + 3H 2 S = Cr 2 (SO 4) 3 + Na 2 SO 4 + 3S↓ + 7H 2 O
2) (NH 4) 2 Cr 2 O 7 Cr 2 O 3 + N 2 + 4H 2 O
Cr 2 O 3 + 2KOH 2KCrO 2 + H 2 O
KCrO 2 + H 2 O + HCl = KCl + Cr(OH) 3 ↓
Cr(OH) 3 + 3HCl = CrCl 3 + 3H 2 O
3) KNO 3 (tv.) + H 2 SO 4 (conc.) HNO 3 + KHSO 4
4HNO 3 + Cu = Cu(NO 3) 2 + 2NO 2 + 2H 2 O
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
4) Cr(OH) 3 + 3HCl = CrCl 3 + 3H 2 O
2CrCl 3 + 3K 2 CO 3 + 3H 2 O = 2Cr(OH) 3 ↓ + 3CO 2 + 6KCl
Cr(OH) 3 + 3KOH = K 3
K 3 + 6HCl = CrCl 3 + 3KCl + 6H 2 O
5) 2K 2 CrO 4 + 2HCl = K 2 Cr 2 O 7 + 2KCl + H 2 O
K 2 Cr 2 O 7 + 2KOH = 2K 2 CrO 4 + H 2 O
K 2 CrO 4 + BaCl 2 = BaCrO 4 ↓ + 2 KCl
KCl + AgNO 3 = AgCl↓ + KNO 3
6) Cr 2 (SO 4) 3 + 3Na 2 CO 3 + 6H 2 O = 2Cr(OH) 3 ↓ + 3CO 2 + 3K 2 SO 4
2Cr(OH) 3 + 3Br 2 + 10NaOH = 2Na 2 CrO 4 + 6NaBr + 8H 2 O
2Na 2 CrO 4 + H 2 SO 4 = Na 2 Cr 2 O 7 + Na 2 SO 4 + H 2 O
Na 2 Cr 2 O 7 + H 2 SO 4 + 3SO 2 = Cr 2 (SO 4) 3 + Na 2 SO 4 + H 2 O
7) Cr 2 S 3 + 6H 2 O = 2Cr(OH) 3 ↓ + 3H 2 S
2Cr(OH) 3 + 3Cl 2 + 10KOH = 2K 2 CrO 4 + 6KCl + 8H 2 O
2K 2 CrO 4 + 3K 2 SO 3 + 5H 2 O = 2Cr(OH) 2 + 3K 2 SO 4 + 4KOH
2Cr(OH) 3 Cr 2 O 3 + 3H 2 O
8) Cr 2 S 3 + 3H 2 SO 4 = Cr 2 (SO 4) 3 + 3H 2 S
Cr 2 (SO 4) 3 + 6NH 3 + 6H 2 O = 2Cr(OH) 3 ↓ + 3(NH 4) 2 SO 4
H 2 S + Pb(NO 3) 2 = PbS + 2HNO 3
PbS + 4H 2 O 2 = PbSO 4 + 4H 2 O
9) (NH 4) 2 Cr 2 O 7 Cr 2 O 3 + N 2 + 4H 2 O
Cr 2 O 3 + 3H 2 SO 4 = Cr 2 (SO 4) 3 + 3H 2 O
Cr 2 (SO 4) 3 + 6NaOH = 2Cr(OH) 3 ↓ + 3Na 2 SO 4
Cr(OH) 3 + 3NaOH = Na 3
10) CrO 3 + 2KOH = K 2 CrO 4 + H 2 O
2K 2 CrO 4 + H 2 SO 4 (diluted) = K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O
K 2 Cr 2 O 7 + 14HBr = 3Br 2 + 2CrBr 3 + 7H 2 O + 2KBr
Br 2 + H 2 S = S + 2HBr
11) 2Cr + 3Cl 2 = 2CrCl 3
2CrCl 3 + 10NaOH + 3H 2 O 2 = 2Na 2 CrO 4 + 6NaCl + 8H 2 O
2Na 2 CrO 4 + H 2 SO 4 = Na 2 Cr 2 O 7 + Na 2 SO 4 + H 2 O
Na 2 Cr 2 O 7 + 3Cu 2 O + 10H 2 SO 4 = 6CuSO 4 + Cr 2 (SO 4) 3 + Na 2 SO 4 + 10H 2 O
12) 3NaNO 3 + Cr 2 O 3 + 2Na 2 CO 3 = 2Na 2 CrO 4 + 3NaNO 2 + 2CO 2
CO 2 + Ba(OH) 2 = BaCO 3 ↓ + H 2 O
BaCO 3 + 2HCl = BaCl 2 + CO 2 + H 2 O
BaCl 2 + 2AgNO 3 = 2AgCl↓ + Ba(NO 3) 2
13) 2K + S = K 2 S
K 2 S + 2HCl = 2KCl + H 2 S
3H 2 S + K 2 Cr 2 O 7 + 4H 2 SO 4 = 3S + Cr 2 (SO 4) 3 + K 2 SO 4 + 7H 2 O
3S + 2Al = Al 2 S 3
14) 2Cr + 3Cl 2 = 2CrCl 3
CrCl 3 + 3KOH = 3KCl + Cr(OH) 3 ↓
2Cr(OH) 3 + 3H 2 O 2 + 4KOH = 2K 2 CrO 4 + 8H 2 O
2K 2 CrO 4 + 16HCl = 2CrCl 3 + 4KCl + 3Cl 2 + 8H 2 O
Non-metals.
IV A group (carbon, silicon).
Carbon. Carbon compounds.
I. Carbon.
Carbon can exhibit both reducing and oxidizing properties. Carbon exhibits reducing properties with simple substances formed by nonmetals with a higher electronegativity value compared to it (halogens, oxygen, sulfur, nitrogen), as well as with metal oxides, water and other oxidizing agents.
When heated with excess air, graphite burns to form carbon monoxide (IV):
when there is a lack of oxygen, you can get CO
Amorphous carbon reacts with fluorine already at room temperature.
C + 2F 2 = CF 4
When heated with chlorine:
C + 2Cl 2 = CCl 4
With stronger heating, carbon reacts with sulfur and silicon:
Under the action of an electric discharge, carbon combines with nitrogen, forming diacine:
2C + N 2 → N ≡ C – C ≡ N
In the presence of a catalyst (nickel) and upon heating, carbon reacts with hydrogen:
C + 2H 2 = CH 4
With water, hot coke forms a mixture of gases:
C + H 2 O = CO + H 2
The reducing properties of carbon are used in pyrometallurgy:
C + CuO = Cu + CO
When heated with oxides of active metals, carbon forms carbides:
3C + CaO = CaC 2 + CO
9C + 2Al 2 O 3 = Al 4 C 3 + 6CO
2C + Na 2 SO 4 = Na 2 S + CO 2
2C + Na 2 CO 3 = 2Na + 3CO
Carbon is oxidized by such strong oxidizing agents as concentrated sulfuric and nitric acids, and other oxidizing agents:
C + 4HNO 3 (conc.) = CO 2 + 4NO 2 + 2H 2 O
C + 2H 2 SO 4 (conc.) = 2SO 2 + CO 2 + 2H 2 O
3C + 8H 2 SO 4 + 2K 2 Cr 2 O 7 = 2Cr 2 (SO 4) 3 + 2K 2 SO 4 + 3CO 2 + 8H 2 O
In reactions with active metals, carbon exhibits the properties of an oxidizing agent. In this case, carbides are formed:
4C + 3Al = Al 4 C 3
Carbides undergo hydrolysis, forming hydrocarbons:
Al 4 C 3 + 12H 2 O = 4Al(OH) 3 + 3CH 4
CaC 2 + 2H 2 O = Ca(OH) 2 + C 2 H 2
Chromium forms three oxides: CrO, Cr 2 O 3, CrO 3.
Chromium (II) oxide CrO is a pyrophoric black powder. Has basic properties.
In redox reactions it behaves as a reducing agent:
CrO is obtained by decomposition of chromium carbonyl Cr(CO) 6 in vacuum at 300°C.
Chromium (III) oxide Cr 2 O 3 is a refractory green powder. It is close to corundum in hardness, which is why it is included in polishing agents. Formed by the interaction of Cr and O 2 at high temperatures. In the laboratory, chromium(III) oxide can be prepared by heating ammonium dichromate:
(N -3 H 4) 2 Cr +6 2 O 7 =Cr +3 2 O 3 +N 0 2 +4H 2 O
Chromium(III) oxide has amphoteric properties. When interacting with acids, chromium (III) salts are formed: Cr 2 O 3 +3H 2 SO 4 =Cr 2 (SO 4) 3 +3H 2 O
When interacting with alkalis in the melt, chromium (III) compounds are formed - chromites (in the absence of oxygen): Cr 2 O 3 + 2NaOH = 2NaCrO 2 + H 2 O
Chromium(III) oxide is insoluble in water.
In redox reactions, chromium(III) oxide behaves as a reducing agent:
Chromium (VI) oxide CrO 3 - chromic anhydride, is a dark red needle-shaped crystals. When heated to about 200°C, it decomposes:
4CrO 3 =2Cr 2 O 3 +3O 2
Easily dissolves in water, being acidic in nature, it forms chromic acids. With excess water, chromic acid H 2 CrO 4 is formed:
CrO 3 +H 2 O=H 2 CrO 4
At a high concentration of CrO 3, dichromic acid H 2 Cr 2 O 7 is formed:
2CrO 3 +H 2 O=H 2 Cr 2 O 7
which, when diluted, turns into chromic acid:
H 2 Cr 2 O 7 +H 2 O=2H 2 CrO 4
Chromic acids exist only in aqueous solution; none of these acids are isolated in a free state. However, their salts are very stable.
Chromium(VI) oxide is a strong oxidizing agent:
3S+4CrO 3 =3SO 2 +2Cr 2 O 3
Oxidizes iodine, sulfur, phosphorus, coal, turning into Cr 2 O 3. CrO 3 is obtained by the action of an excess of concentrated sulfuric acid on a saturated aqueous solution of sodium dichromate: Na 2 Cr 2 O 7 +2H 2 SO 4 =2CrO 3 +2NaHSO 4 +H 2 O It should be noted that chromium (VI) oxide is highly toxic.
Chromium is a chemical element with atomic number 24. It is a hard, shiny, steel-gray metal that polishes well and does not tarnish. Used in alloys such as stainless steel and as a coating. The human body requires small amounts of trivalent chromium to metabolize sugar, but Cr(VI) is highly toxic.
Various chromium compounds, such as chromium(III) oxide and lead chromate, are brightly colored and used in paints and pigments. The red color of ruby is due to the presence of this chemical element. Some substances, especially sodium, are oxidizing agents used to oxidize organic compounds and (together with sulfuric acid) to clean laboratory glassware. In addition, chromium (VI) oxide is used in the production of magnetic tape.
Discovery and etymology
The history of the discovery of the chemical element chromium is as follows. In 1761, Johann Gottlob Lehmann found an orange-red mineral in the Ural Mountains and named it “Siberian red lead.” Although it was erroneously identified as a compound of lead with selenium and iron, the material was actually lead chromate with the chemical formula PbCrO 4 . Today it is known as the mineral croconte.
In 1770, Peter Simon Pallas visited the site where Lehmann found the red lead mineral, which had very useful properties as a pigment in paints. The use of Siberian red lead as paint developed rapidly. In addition, the bright yellow color of crocont has become fashionable.
In 1797, Nicolas-Louis Vauquelin obtained samples of red. By mixing croconte with hydrochloric acid, he obtained CrO 3 oxide. Chromium was isolated as a chemical element in 1798. Vauquelin obtained it by heating the oxide with charcoal. He was also able to detect traces of chromium in gemstones such as ruby and emerald.
In the 1800s, Cr was primarily used in dyes and tanning salts. Today, 85% of the metal is used in alloys. The remainder is used in the chemical, refractory and foundry industries.
The pronunciation of the chemical element chromium corresponds to the Greek χρῶμα, meaning "color", due to the variety of colored compounds that can be obtained from it.
Mining and production
The element is produced from chromite (FeCr 2 O 4). About half of the world's ore is mined in South Africa. In addition, Kazakhstan, India and Türkiye are its major producers. There are enough explored deposits of chromite, but geographically they are concentrated in Kazakhstan and southern Africa.
Deposits of native chromium metal are rare, but they do exist. For example, it is mined at the Udachnaya mine in Russia. It is rich in diamonds, and the reducing environment helped produce pure chromium and diamonds.
For industrial metal production, chromite ores are treated with molten alkali (caustic soda, NaOH). In this case, sodium chromate (Na 2 CrO 4) is formed, which is reduced by carbon to the oxide Cr 2 O 3. The metal is produced by heating the oxide in the presence of aluminum or silicon.
In 2000, approximately 15 million tons of chromite ore were mined and processed into 4 million tons of ferrochrome, a 70% chromium-iron alloy, with an approximate market value of US$2.5 billion.
Main characteristics
The characteristics of the chemical element chromium are due to the fact that it is a transition metal of the fourth period of the periodic table and is located between vanadium and manganese. Included in group VI. Melts at a temperature of 1907 °C. In the presence of oxygen, chromium quickly forms a thin layer of oxide, which protects the metal from further interaction with oxygen.
As a transition element, it reacts with substances in different proportions. Thus, it forms compounds in which it has different oxidation states. Chromium is a chemical element with the basic states +2, +3 and +6, of which +3 is the most stable. In addition, in rare cases conditions +1, +4 and +5 are observed. Chromium compounds in the +6 oxidation state are strong oxidizing agents.
What color is chrome? The chemical element gives the ruby hue. The Cr 2 O 3 used for is also used as a pigment called chrome green. Its salts color glass emerald green. Chromium is the chemical element whose presence makes rubies red. Therefore, it is used in the production of synthetic rubies.
isotopes
Isotopes of chromium have atomic weights ranging from 43 to 67. Typically, this chemical element consists of three stable forms: 52 Cr, 53 Cr and 54 Cr. Of these, 52 Cr is the most common (83.8% of all natural chromium). In addition, 19 radioisotopes have been described, of which the most stable is 50 Cr with a half-life exceeding 1.8x10 17 years. 51 Cr has a half-life of 27.7 days, and for all other radioactive isotopes it does not exceed 24 hours, and for most of them it lasts less than one minute. The element also has two meta states.
Isotopes of chromium in the earth's crust, as a rule, accompany isotopes of manganese, which is used in geology. 53 Cr is formed during the radioactive decay of 53 Mn. The Mn/Cr isotope ratio reinforces other clues about the early history of the Solar System. Changes in the 53 Cr/ 52 Cr and Mn/Cr ratios from different meteorites prove that new atomic nuclei were created just before the formation of the Solar System.
Chemical element chromium: properties, formula of compounds
Chromium(III) oxide Cr 2 O 3, also known as sesquioxide, is one of the four oxides of this chemical element. It is obtained from chromite. The green color compound is commonly called "chrome green" when used as a pigment for enamel and glass painting. The oxide can dissolve in acids, forming salts, and in molten alkali - chromites.
Potassium dichromate
K 2 Cr 2 O 7 is a powerful oxidizing agent and is preferred as a means for cleaning laboratory glassware from organic matter. For this purpose, its saturated solution is used. Sometimes, however, it is replaced with sodium bichromate, based on the higher solubility of the latter. In addition, it can regulate the oxidation process of organic compounds, converting primary alcohol into aldehyde and then into carbon dioxide.
Potassium dichromate can cause chrome dermatitis. Chromium is likely to cause sensitization leading to the development of dermatitis, especially of the hands and forearms, which is chronic and difficult to cure. Like other Cr(VI) compounds, potassium bichromate is carcinogenic. It must be handled with gloves and appropriate protective equipment.
Chromic acid
The compound has the hypothetical structure H 2 CrO 4 . Neither chromic nor dichromic acids occur in nature, but their anions are found in various substances. The “chromic acid” that can be found on sale is actually its acid anhydride - CrO 3 trioxide.
Lead(II) chromate
PbCrO 4 has a bright yellow color and is practically insoluble in water. For this reason, it has found use as a coloring pigment called crown yellow.
Cr and pentavalent bond
Chromium is distinguished by its ability to form pentavalent bonds. The compound is created by Cr(I) and a hydrocarbon radical. A pentavalent bond is formed between two chromium atoms. Its formula can be written as Ar-Cr-Cr-Ar, where Ar represents a specific aromatic group.
Application
Chromium is a chemical element whose properties have given it many different uses, some of which are listed below.
It gives metals corrosion resistance and a glossy surface. Therefore, chromium is included in alloys such as stainless steel, used, for example, in cutlery. It is also used for chrome plating.
Chromium is a catalyst for various reactions. It is used to make molds for firing bricks. Its salts are used to tan leather. Potassium bichromate is used for the oxidation of organic compounds such as alcohols and aldehydes, as well as for cleaning laboratory glassware. It serves as a fixing agent for fabric dyeing and is also used in photography and photo printing.
CrO 3 is used to make magnetic tapes (for example, for audio recording), which have better characteristics than films with iron oxide.
Role in biology
Trivalent chromium is a chemical element necessary for the metabolism of sugar in the human body. In contrast, hexavalent Cr is highly toxic.
Precautionary measures
Chromium metal and Cr(III) compounds are generally not considered a health hazard, but substances containing Cr(VI) can be toxic if ingested or inhaled. Most of these substances are irritating to the eyes, skin and mucous membranes. With chronic exposure, chromium(VI) compounds can cause eye damage if not treated properly. In addition, it is a recognized carcinogen. The lethal dose of this chemical element is about half a teaspoon. According to the recommendations of the World Health Organization, the maximum permissible concentration of Cr (VI) in drinking water is 0.05 mg per liter.
Because chromium compounds are used in dyes and to tan leather, they are often found in soil and groundwater from abandoned industrial sites requiring environmental cleanup and remediation. Primer containing Cr(VI) is still widely used in the aerospace and automotive industries.
Element properties
The main physical properties of chromium are as follows:
- Atomic number: 24.
- Atomic weight: 51.996.
- Melting point: 1890 °C.
- Boiling point: 2482 °C.
- Oxidation state: +2, +3, +6.
- Electron configuration: 3d 5 4s 1.
