Chromium is an element of the secondary subgroup of the 6th group of the 4th period periodic table chemical elements of D.I. Mendeleev, with atomic number 24. Denoted by the symbol Cr (lat. Chromium). The simple substance chromium is a hard metal of a bluish-white color.
Chemical properties of chromium
Under normal conditions, chromium reacts only with fluorine. At high temperatures (above 600°C) it interacts with oxygen, halogens, nitrogen, silicon, boron, sulfur, phosphorus.
4Cr + 3O 2 – t° →2Cr 2 O 3
2Cr + 3Cl 2 – t° → 2CrCl 3
2Cr + N 2 – t° → 2CrN
2Cr + 3S – t° → Cr 2 S 3
When heated, it reacts with water vapor:
2Cr + 3H 2 O → Cr 2 O 3 + 3H 2
Chromium dissolves in dilute strong acids (HCl, H 2 SO 4)
In the absence of air, Cr 2+ salts are formed, and in air, Cr 3+ salts are formed.
Cr + 2HCl → CrCl 2 + H 2
2Cr + 6HCl + O 2 → 2CrCl 3 + 2H 2 O + H 2
The presence of a protective oxide film on the surface of the metal explains its passivity in relation to concentrated solutions of acids - oxidizers.
Chromium compounds
Chromium(II) oxide and chromium(II) hydroxide are basic in nature.
Cr(OH) 2 + 2HCl → CrCl 2 + 2H 2 O
Chromium (II) compounds are strong reducing agents; transform into chromium (III) compounds under the influence of atmospheric oxygen.
2CrCl 2 + 2HCl → 2CrCl 3 + H 2
4Cr(OH) 2 + O 2 + 2H 2 O → 4Cr(OH) 3
Chromium oxide (III) Cr 2 O 3 is a green, water-insoluble powder. Can be obtained by calcination of chromium(III) hydroxide or potassium and ammonium dichromates:
2Cr(OH) 3 – t° → Cr 2 O 3 + 3H 2 O
4K 2 Cr 2 O 7 – t° → 2Cr 2 O 3 + 4K 2 CrO 4 + 3O 2
(NH 4) 2 Cr 2 O 7 – t° → Cr 2 O 3 + N 2 + 4H 2 O (volcano reaction)
Amphoteric oxide. When Cr 2 O 3 is fused with alkalis, soda and acid salts, chromium compounds with an oxidation state of (+3) are obtained:
Cr 2 O 3 + 2NaOH → 2NaCrO 2 + H 2 O
Cr 2 O 3 + Na 2 CO 3 → 2NaCrO 2 + CO 2
When fused with a mixture of alkali and oxidizing agent, chromium compounds are obtained in the oxidation state (+6):
Cr 2 O 3 + 4KOH + KClO 3 → 2K 2 CrO 4 + KCl + 2H 2 O
Chromium (III) hydroxide C r (OH) 3 . Amphoteric hydroxide. Gray-green, decomposes when heated, losing water and forming green metahydroxide CrO(OH). Does not dissolve in water. Precipitates from solution as a gray-blue and bluish-green hydrate. Reacts with acids and alkalis, does not interact with ammonia hydrate.
It has amphoteric properties - it dissolves in both acids and alkalis:
2Cr(OH) 3 + 3H 2 SO 4 → Cr 2 (SO 4) 3 + 6H 2 O Cr(OH) 3 + ZH + = Cr 3+ + 3H 2 O
Cr(OH) 3 + KOH → K, Cr(OH) 3 + ZON - (conc.) = [Cr(OH) 6 ] 3-
Cr(OH) 3 + KOH → KCrO 2 + 2H 2 O Cr(OH) 3 + MOH = MSrO 2 (green) + 2H 2 O (300-400 °C, M = Li, Na)
Cr(OH) 3 →(120 o C – H 2 O) CrO(OH) →(430-1000 0 C –H 2 O) Cr2O3
2Cr(OH) 3 + 4NaOH (conc.) + ZN 2 O 2 (conc.) = 2Na 2 CrO 4 + 8H 2 0
Receipt: precipitation with ammonia hydrate from a solution of chromium(III) salts:
Cr 3+ + 3(NH 3 H 2 O) = WITHr(OH) 3 ↓+ ЗNН 4+
Cr 2 (SO 4) 3 + 6NaOH → 2Cr(OH) 3 ↓+ 3Na 2 SO 4 (in excess alkali - the precipitate dissolves)
Chromium (III) salts have a purple or dark green color. Their chemical properties resemble colorless aluminum salts.
Cr(III) compounds can exhibit both oxidizing and reducing properties:
Zn + 2Cr +3 Cl 3 → 2Cr +2 Cl 2 + ZnCl 2
2Cr +3 Cl 3 + 16NaOH + 3Br 2 → 6NaBr + 6NaCl + 8H 2 O + 2Na 2 Cr +6 O 4
Hexavalent chromium compounds
Chromium(VI) oxide CrO 3 - bright red crystals, soluble in water.
Obtained from potassium chromate (or dichromate) and H 2 SO 4 (conc.).
K 2 CrO 4 + H 2 SO 4 → CrO 3 + K 2 SO 4 + H 2 O
K 2 Cr 2 O 7 + H 2 SO 4 → 2CrO 3 + K 2 SO 4 + H 2 O
CrO 3 is an acidic oxide, with alkalis it forms yellow chromates CrO 4 2-:
CrO 3 + 2KOH → K 2 CrO 4 + H 2 O
In an acidic environment, chromates turn into orange dichromates Cr 2 O 7 2-:
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, this reaction proceeds in the opposite direction:
K 2 Cr 2 O 7 + 2KOH → 2K 2 CrO 4 + H 2 O
Potassium dichromate is an oxidizing agent in an acidic environment:
K 2 Cr 2 O 7 + 4H 2 SO 4 + 3Na 2 SO 3 = Cr 2 (SO 4) 3 + 3Na 2 SO 4 + K 2 SO 4 + 4H 2 O
K 2 Cr 2 O 7 + 4H 2 SO 4 + 3NaNO 2 = Cr 2 (SO 4) 3 + 3NaNO 3 + K 2 SO 4 + 4H 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 + 7H 2 SO 4 + 6FeSO 4 = Cr 2 (SO 4) 3 + 3Fe 2 (SO 4) 3 + K 2 SO 4 + 7H 2 O
Potassium chromate K 2 Cr O 4 . Oxosol. Yellow, non-hygroscopic. Melts without decomposition, thermally stable. Very soluble in water ( yellow the color of the solution corresponds to the CrO 4 2- ion), slightly hydrolyzes the anion. In an acidic environment it turns into K 2 Cr 2 O 7 . Oxidizing agent (weaker than K 2 Cr 2 O 7). Enters into ion exchange reactions.
Qualitative reaction on the CrO 4 2- ion - the precipitation of a yellow precipitate of barium chromate, which decomposes in a strongly acidic environment. It is used as a mordant for dyeing fabrics, a leather tanning agent, a selective oxidizing agent, and a reagent in analytical chemistry.
Equations of the most important reactions:
2K 2 CrO 4 +H 2 SO 4(30%)= K 2 Cr 2 O 7 +K 2 SO 4 +H 2 O
2K 2 CrO 4 (t) +16HCl (concentration, horizon) = 2CrCl 3 +3Cl 2 +8H 2 O+4KCl
2K 2 CrO 4 +2H 2 O+3H 2 S=2Cr(OH) 3 ↓+3S↓+4KOH
2K 2 CrO 4 +8H 2 O+3K 2 S=2K[Cr(OH) 6 ]+3S↓+4KOH
2K 2 CrO 4 +2AgNO 3 =KNO 3 +Ag 2 CrO 4(red) ↓
Qualitative reaction:
K 2 CrO 4 + BaCl 2 = 2KCl + BaCrO 4 ↓
2BaCrO 4 (t) + 2HCl (dil.) = BaCr 2 O 7 (p) + BaC1 2 + H 2 O
Receipt: sintering of chromite with potash in air:
4(Cr 2 Fe ‖‖)O 4 + 8K 2 CO 3 + 7O 2 = 8K 2 CrO 4 + 2Fe 2 O 3 + 8СO 2 (1000 °C)
Potassium dichromate K 2 Cr 2 O 7 . Oxosol. Technical name chrome peak. Orange-red, non-hygroscopic. Melts without decomposition, and decomposes upon further heating. Very soluble in water ( orange The color of the solution corresponds to the Cr 2 O 7 2- ion. In an alkaline environment it forms K 2 CrO 4 . A typical oxidizing agent in solution and during fusion. Enters into ion exchange reactions.
Qualitative reactions- blue color of an ethereal solution in the presence of H 2 O 2, blue color of an aqueous solution under the action of atomic hydrogen.
It is used as a leather tanning agent, a mordant for dyeing fabrics, a component of pyrotechnic compositions, a reagent in analytical chemistry, a metal corrosion inhibitor, in a mixture with H 2 SO 4 (conc.) - for washing chemical dishes.
Equations of the most important reactions:
4K 2 Cr 2 O 7 =4K 2 CrO 4 +2Cr 2 O 3 +3O 2 (500-600 o C)
K 2 Cr 2 O 7 (t) +14HCl (conc) = 2CrCl 3 +3Cl 2 +7H 2 O+2KCl (boiling)
K 2 Cr 2 O 7 (t) +2H 2 SO 4(96%) ⇌2KHSO 4 +2CrO 3 +H 2 O (“chromium mixture”)
K 2 Cr 2 O 7 +KOH (conc) =H 2 O+2K 2 CrO 4
Cr 2 O 7 2- +14H + +6I - =2Cr 3+ +3I 2 ↓+7H 2 O
Cr 2 O 7 2- +2H + +3SO 2 (g) = 2Cr 3+ +3SO 4 2- +H 2 O
Cr 2 O 7 2- +H 2 O +3H 2 S (g) =3S↓+2OH - +2Cr 2 (OH) 3 ↓
Cr 2 O 7 2- (conc.) +2Ag + (dil.) =Ag 2 Cr 2 O 7 (red) ↓
Cr 2 O 7 2- (dil.) +H 2 O +Pb 2+ =2H + + 2PbCrO 4 (red) ↓
K 2 Cr 2 O 7(t) +6HCl+8H 0 (Zn)=2CrCl 2(syn) +7H 2 O+2KCl
Receipt: treatment of K 2 CrO 4 with sulfuric acid:
2K 2 CrO 4 + H 2 SO 4 (30%) = K 2Cr 2 O 7 + K 2 SO 4 + H 2 O
DEFINITION
Chromium located in the fourth period of group VI of the secondary (B) subgroup of the Periodic table. Designation – Cr. In the form of a simple substance - a grayish-white shiny metal.
Chrome has a body-centered cubic lattice structure. Density - 7.2 g/cm3. The melting and boiling points are 1890 o C and 2680 o C, respectively.
Oxidation state of chromium in compounds
Chromium can exist in the form of a simple substance - a metal, and the oxidation state of metals in the elemental state is equal to zero, since the distribution of electron density in them is uniform.
Oxidation states (+2) And (+3) chromium appears in oxides (Cr +2 O, Cr +3 2 O 3), hydroxides (Cr +2 (OH) 2, Cr +3 (OH) 3), halides (Cr +2 Cl 2, Cr +3 Cl 3 ), sulfates (Cr +2 SO 4, Cr +3 2 (SO 4) 3) and other compounds.
Chromium is also characterized by its oxidation state (+6) : Cr +6 O 3, H 2 Cr +6 O 4, H 2 Cr +6 2 O 7, K 2 Cr +6 2 O 7, etc.
Examples of problem solving
EXAMPLE 1
EXAMPLE 2
| Exercise | Phosphorus has the same oxidation state in the following compounds: a) Ca 3 P 2 and H 3 PO 3; b) KH 2 PO 4 and KPO 3; c) P 4 O 6 and P 4 O 10; d) H 3 PO 4 and H 3 PO 3. |
| Solution | In order to give the correct answer to the question posed, we will alternately determine the degree of oxidation of phosphorus in each pair of proposed compounds. a) The oxidation state of calcium is (+2), oxygen and hydrogen - (-2) and (+1), respectively. Let us take the value of the oxidation state of phosphorus as “x” and “y” in the proposed compounds: 3 ×2 + x ×2 = 0; 3 + y + 3×(-2) = 0; The answer is incorrect. b) The oxidation state of potassium is (+1), oxygen and hydrogen are (-2) and (+1), respectively. Let us take the value of the oxidation state of chlorine as “x” and “y” in the proposed compounds: 1 + 2×1 +x + (-2)×4 = 0; 1 + y + (-2)×3 = 0; The answer is correct. |
| Answer | Option (b). |
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.
Progress of the lesson
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.
Chromium (Cr), a chemical element of group VI of the periodic system of Mendeleev. It is a transition metal with atomic number 24 and atomic mass 51.996. Translated from Greek, the name of the metal means “color”. The metal owes its name to the variety of colors that are inherent in its various compounds.
Physical characteristics of chromium
The metal has sufficient hardness and brittleness at the same time. On the Mohs scale, the hardness of chromium is rated at 5.5. This indicator means that chromium has the maximum hardness of all metals known today, after uranium, iridium, tungsten and beryllium. The simple substance chromium is characterized by a bluish-white color.
Metal is not a rare element. Its concentration in the earth's crust reaches 0.02% by mass. shares Chromium is never found in its pure form. It is found in minerals and ores, which are the main source of metal extraction. Chromite (chromium iron ore, FeO*Cr 2 O 3) is considered the main chromium compound. Another fairly common, but less important mineral is crocoite PbCrO 4 .
The metal can be easily melted at a temperature of 1907 0 C (2180 0 K or 3465 0 F). At a temperature of 2672 0 C it boils. The atomic mass of the metal is 51.996 g/mol.
Chromium is a unique metal due to its magnetic properties. At room temperature, it exhibits antiferromagnetic ordering, while other metals exhibit it at extremely low temperatures. However, if chromium is heated above 37 0 C, physical properties chrome change. Thus, the electrical resistance and linear expansion coefficient change significantly, the elastic modulus reaches a minimum value, and internal friction increases significantly. This phenomenon is associated with the passage of the Néel point, at which the antiferromagnetic properties of the material can change to paramagnetic. This means that the first level has been passed, and the substance has sharply increased in volume.
The structure of chromium is a body-centered lattice, due to which the metal is characterized by the temperature of the brittle-ductile period. However, in the case of this metal, the degree of purity is of great importance, therefore, the value is in the range -50 0 C - +350 0 C. As practice shows, crystallized metal does not have any ductility, but soft annealing and molding make it malleable.
Chemical properties of chromium
The atom has the following external configuration: 3d 5 4s 1. As a rule, in compounds chromium has the following oxidation states: +2, +3, +6, among which Cr 3+ exhibits the greatest stability. In addition, there are other compounds in which chromium exhibits a completely different oxidation state, namely: +1 , +4, +5.
The metal is not particularly chemically reactive. When chromium is exposed to normal conditions, the metal exhibits resistance to moisture and oxygen. However, this characteristic does not apply to the compound of chromium and fluorine - CrF 3, which, when exposed to temperatures exceeding 600 0 C, interacts with water vapor, forming Cr 2 O 3 as a result of the reaction, as well as nitrogen, carbon and sulfur.
When chromium metal is heated, it reacts with halogens, sulfur, silicon, boron, carbon, and some other elements, resulting in the following chemical reactions of chromium:
Cr + 2F 2 = CrF 4 (with an admixture of CrF 5)
2Cr + 3Cl2 = 2CrCl3
2Cr + 3S = Cr 2 S 3
Chromates can be obtained by heating chromium with molten soda in air, nitrates or chlorates of alkali metals:
2Cr + 2Na 2 CO 3 + 3O 2 = 2Na 2 CrO 4 + 2CO 2.
Chromium is not toxic, which cannot be said about some of its compounds. As is known, dust from this metal, if it enters the body, can irritate the lungs; it is not absorbed through the skin. But, since it does not occur in its pure form, its entry into the human body is impossible.
Trivalent chromium enters environment during mining and processing of chrome ore. Chromium is likely introduced into the human body in the form of a dietary supplement used in weight loss programs. Chromium, with a valence of +3, is an active participant in glucose synthesis. Scientists have found that excessive consumption of chromium does not cause any particular harm to the human body, since it is not absorbed, however, it can accumulate in the body.
Compounds involving hexavalent metal are extremely toxic. The likelihood of them entering the human body appears during the production of chromates, chrome plating of objects, and during some welding work. The ingestion of such chromium into the body is fraught with serious consequences, since compounds in which the hexavalent element is present are strong oxidizing agents. Therefore, they can cause bleeding in the stomach and intestines, sometimes with perforation of the intestine. When such compounds come into contact with the skin, strong chemical reactions occur in the form of burns, inflammation, and ulcers.
Depending on the quality of chromium that needs to be obtained at the output, there are several methods for producing the metal: electrolysis of concentrated aqueous solutions of chromium oxide, electrolysis of sulfates, and reduction with silicon oxide. However, the latter method is not very popular, since it produces chromium with a huge amount of impurities. Moreover, it is also not economically viable.
| Oxidation state | Oxide | Hydroxide | Character | Predominant forms in solutions | Notes |
| +2 | CrO (black) | Cr(OH)2 (yellow) | Basic | Cr2+ (blue salts) | Very strong reducing agent |
| Cr2O3 (green) | Cr(OH)3 (grey-green) | Amphoteric |
Cr3+ (green or purple salts) |
||
| +4 | CrO2 | does not exist | Non-salt-forming | - |
Rarely encountered, uncharacteristic |
| +6 | CrO3 (red) |
H2CrO4 |
Acid |
CrO42- (chromates, yellow) |
The transition depends on the pH of the environment. A strong oxidizing agent, hygroscopic, very toxic. |
Redox properties of chromium compounds with varying degrees of oxidation.
Chromium. The structure of the atom. Possible oxidation states. Acid-base properties. Application.
Cr +24)2)8)13)1
Chromium has oxidation states of +2, +3 and +6.
As the degree of oxidation increases, the acidic and oxidizing properties increase. Chromium Cr2+ derivatives are very strong reducing agents. The Cr2+ ion is formed at the first stage of the dissolution of Chromium in acids or during the reduction of Cr3+ in an acidic solution with zinc. When dehydrated, hydroxide Cr(OH)2 turns into Cr2O3. Cr3+ compounds are stable in air. They can be both reducing and oxidizing agents. Cr3+ can be reduced in an acidic solution with zinc to Cr2+ or oxidized in an alkaline solution to CrO42- with bromine and other oxidizing agents. Hydroxide Cr(OH)3 (or rather Cr2O3 nH2O) is an amphoteric compound that forms salts with the Cr3+ cation or salts of chromous acid HCrO2 - chromites (for example, KSrO2, NaCrO2). Cr6+ compounds: chromic anhydride CrO3, chromic acids and their salts, among which the most important are chromates and dichromates - strong oxidizing salts.
Used as wear-resistant and beautiful galvanic coatings (chrome plating). Chromium is used for the production of alloys: chromium-30 and chromium-90, which are indispensable for the production of nozzles for powerful plasma torches and in the aerospace industry.
Chromium is chemically inactive. Under normal conditions, it reacts only with fluorine (from non-metals), forming a mixture of fluorides.
Chromates and dichromates
Chromates are formed by the interaction of CrO3, or solutions of chromic acids with alkalis:
СгО3 + 2NaOH = Na2CrO4 + Н2О
Dichromates are obtained by the action of acids on chromates:
2 Na2Cr2O4 + H2SO4 = Na2Cr2O7 + Na2SO4 + H2O
Chromium compounds are characterized by redox reactions.
Chromium (II) compounds are strong reducing agents and are easily oxidized
4(5gCl2 + O2 + 4HCI = 4CrCl3 + 2H2O
Chromium compounds (!!!) are characterized by reducing properties. Under the influence of oxidizing agents they go:
to chromates - in an alkaline environment,
in dichromates - in an acidic environment.
Cr(OH)3. CrOH + HCl = CrCl + H2O, 3CrOH + 2NaOH = Cr3Na2O3 + 3H2O
Chromates(III) (old name: chromites).
Chromium compounds are characterized by reducing properties. Under the influence of oxidizing agents they go:
to chromates - in an alkaline environment,
in dichromates - in an acidic environment.
2Na3 [Cr(OH)6] + 3Br2 + 4NaOH = 2Na2CrO4 + 6NaBr + 8H2O
5Cr2(SO4)3 + 6KMnO4 + 11H2O = 3K2Cr2O7 + 2H2Cr2O7 + 6MnSO4 + 9H2SO4
Salts of chromic acids in an acidic environment are strong oxidizing agents:
3Na2SO3 + K2Cr2O7 + 4H2SO4 = 3Na2SO4 + Cr2(SO4)3 + K2SO4 + 4H2O