The maximum oxidation state of chromium in the compound. The oxidation state of chromium. I. Repetition of the material of the previous lesson

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 are used in paints and pigments. The red color of a ruby ​​is due to the presence of this chemical element. Some substances, especially sodium, are oxidizing agents used to oxidize organic compounds and (along with sulfuric acid) for cleaning laboratory glassware. In addition, chromium oxide (VI) is used in the production of magnetic tape.

Discovery and etymology

The history of the discovery of the chemical element chromium is as follows. In 1761, Johann Gottlob Lehmann found 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 croconte mineral.

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

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

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

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

Mining and production

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

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

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

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

Main characteristics

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

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

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

isotopes

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

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

Chemical element chromium: properties, formula of compounds

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

Potassium bichromate

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

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

Chromic acid

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

Lead(II) chromate

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

Cr and pentavalent bond

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

Application

Chromium is a chemical element whose properties have provided it with many different uses, some of which are listed below.

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

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

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

Role in biology

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

Precautionary measures

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

Since chromium compounds are used in dyes and leather tanning, they are often found in the soil and groundwater of abandoned industrial sites that require environmental cleanup and remediation. Primer containing Cr(VI) is still widely used in the aerospace and automotive industries.

Element properties

Main physical properties chrome are as follows:

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

Chromium oxide(II) and chromium(II) hydroxide are basic

Cr(OH)+2HCl→CrCl+2HO

Chromium(II) compounds are strong reducing agents; transform into a chromium(III) compound under the action of atmospheric oxygen.

2CrCl+ 2HCl → 2CrCl+ H

4Cr(OH)+O+ 2HO→4Cr(OH)

Chromium oxide(III) CrO is a green, water-insoluble powder. It can be obtained by calcining chromium(III) hydroxide or potassium and ammonium dichromates:

2Cr(OH)-→CrO+ 3HO

4KCrO-→ 2CrO + 4KCrO + 3O

(NH)CrO-→ CrO+ N+ HO

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

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

Cr 2 O 3 + 6HCl \u003d 2CrCl 3 + 3H 2 O

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

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

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

Cr (OH) 3 + 3HNO 3 \u003d Cr (NO 3) 3 + 3H 2 O

Cr(OH) 3 + 3KOH = K 3 [Cr(OH)6] (hexahydroxochromite K)

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

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

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

Chromium compounds(VI).

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

CrO 3 + H 2 O \u003d H 2 CrO 4 (chromic acid - formed with excess water)

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

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

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

It also oxidizes iodine, sulfur, phosphorus, coal:

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

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

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

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

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

Chromic and dichromic acids.

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

Chromate - CrO 4 2- ions and dichromate - Cr2O 7 2- ions easily pass into each other when the solution environment changes

IN acidic environment chromate solution is converted to dichromate:

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

In an alkaline environment, dichromates turn into chromates:

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

When diluted, dichromic acid becomes chromic acid:

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

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

Redox properties of chromium compounds.

Reactions in an acid medium.

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

K 2 Cr 2 O 7 + 3H 2 S + 4H 2 SO 4 \u003d 3S + Cr 2 (SO 4) 3 + K 2 SO 4 + 7H 2 O

S-2 – 2e → S 0

2Cr +6 + 6e → 2Cr +3

Reactions in an alkaline medium.

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

2KCrO 2 +3 Br2 +8NaOH \u003d 2Na 2 CrO 4 + 2KBr + 4NaBr + 4H 2 O

Cr +3 - 3e → Cr +6

Redox properties of chromium compounds with different 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 +2, +3 and +6.

With an increase in the degree of oxidation, acidic and oxidizing properties increase. Chromium Cr2+ derivatives are very strong reducing agents. The Cr2+ ion is formed at the first stage of Chromium dissolution in acids or during the reduction of Cr3+ in an acidic solution with zinc. Nitrous oxide Cr(OH)2 passes into Cr2O3 upon dehydration. 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. Cr(OH)3 hydroxide (more precisely, Cr2O3 nH2O) is an amphoteric compound that forms salts with the Cr3+ cation or salts of chromic 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 agents.salts.

It is used as wear-resistant and beautiful galvanic coatings (chrome plating). Chromium is used for the production of alloys: chromium-30 and chromium-90, indispensable for the production of nozzles of 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:

CrO3 + 2NaOH = Na2CrO4 + H2O

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, they are easily oxidized

4(5rC12 + O2 + 4HCI = 4CrC13 + 2H2O

Chromium compounds (!!!) are characterized by reducing properties. Under the action of oxidizing agents, they pass:

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 names for chromites).

Chromium compounds are characterized by reducing properties. Under the action of oxidizing agents, they pass:

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

Target: deepen students' knowledge of the topic.

Tasks:

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

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

Lesson progress

I. Repetition of the material of the previous lesson.

1. Answer questions and complete tasks:

What elements belong to the chromium subgroup?

Write electronic formulas atoms

What type of elements are they?

What are the oxidation states in the compounds?

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

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

Sample table:

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

II. Lecture.

Lecture plan:

1. Chromium.
2. Chromium compounds. (2)
• Chromium oxide; (2)
• Chromium hydroxide. (2)
1. Chromium compounds. (3)
• Chromium oxide; (3)
• Chromium hydroxide. (3)
1. Chromium compounds (6)
• Chromium oxide; (6)
• Chromic and dichromic acids.
1. Dependence of the properties of chromium compounds on the degree of oxidation.
2. Redox properties of chromium compounds.

1. Chrome.

Chromium is a white lustrous metal with a bluish tint, very hard (density 7.2 g/cm3), melting point 1890˚С.

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

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

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

The interaction of chromium with complex substances:

At very high temperatures, chromium reacts with water:

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

The task:

Chromium reacts with dilute sulfuric and hydrochloric acids:

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

The task: 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 - 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

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

Chromium oxide (2) is easily oxidized in air:

• 4CrO + O 2 \u003d 2Cr 2 O 3

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

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

2Сr (amalgam) + О 2 = 2СrО

2. Chromium hydroxide (2)- Cr (OH) 2 - 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

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

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

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

The task: 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

The task: write ionic equations.

3. Chromium compounds. (3)

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

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

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

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

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

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

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

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

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

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

The task: write ionic equations

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

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

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

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

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

4. Chromium compounds. (6)

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

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

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

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

It also oxidizes iodine, sulfur, phosphorus, coal:

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

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

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

• 4CrO 3 \u003d 2Cr 2 O 3 + 3O 2

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

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

2. Chromic and dichromic acids.

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

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

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

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

In an alkaline environment, dichromates turn into chromates:

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

When diluted, dichromic acid becomes chromic acid:

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

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

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

6. Redox properties of chromium compounds.

Reactions in an acid medium.

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

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

The task:

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

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

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

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

Reactions in an alkaline medium.

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

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

The task:

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

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

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

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

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

To the lecture method:

1. To enhance the cognitive activity of students and maintain interest, it is advisable to conduct a demonstration experiment during the lecture. Depending on the capabilities of the educational laboratory, students can demonstrate the following experiments:
• obtaining chromium oxide (2) and chromium hydroxide (2), proof of their basic properties;
• obtaining chromium oxide (3) and chromium hydroxide (3), proof of their amphoteric properties;
• obtaining chromium oxide (6) and dissolving it in water (obtaining chromic and dichromic acids);
• the transition of chromates to dichromates, dichromates to chromates.
1. Tasks of independent work can be differentiated taking into account the real learning opportunities of students.
2. You can complete the lecture by completing the following tasks: write the equations of chemical reactions with which you can carry out the following transformations:

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

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2. Egorov A.S. Chemistry tutor. - Rostov-on-Don: "Phoenix", 2006.-765 p.
3. Kudryavtsev A.A. Compilation of chemical equations. - M., "Higher School", 1979. - 295 p.
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5. Ushkalova V.N. Chemistry: competitive tasks and answers. - M.: "Enlightenment", 2000. - 223 p.

Task number 1

The +2 oxidation state in all compounds exhibits

Answer: 4

Explanation:

Of all the proposed options, the +2 oxidation state in complex compounds is shown only by zinc, being an element of the secondary subgroup of the second group, where the maximum oxidation state is equal to the group number.

Tin is an element of the main subgroup of group IV, a metal, which exhibits oxidation states 0 (in simple matter), +2, +4 (group number).

Phosphorus is an element of the main subgroup of the main group, being a non-metal, it exhibits oxidation states from -3 (group number - 8) to +5 (group number).

Iron is a metal, the element is located in a secondary subgroup of the main group. For iron, oxidation states are characteristic: 0, +2, +3, +6.

Task number 2

The compound of the composition KEO 4 forms each of the two elements:

1) phosphorus and chlorine

2) fluorine and manganese

3) chlorine and manganese

4) silicon and bromine

Answer: 3

Explanation:

The salt of the KEO 4 composition contains the acid residue EO 4 - , where oxygen has an oxidation state of -2, therefore, the oxidation state of the element E in this acid residue is +7. Of the proposed options, chlorine and manganese are suitable - elements of the main and secondary subgroups of group VII, respectively.

Fluorine is also an element of the main subgroup of group VII, however, being the most electronegative element, it does not show positive oxidation states (0 and -1).

Boron, silicon and phosphorus are elements of the main subgroups of groups 3, 4 and 5, respectively, therefore, in salts they show the corresponding maximum oxidation states of +3, +4, +5.

Task number 3

• 1. Zn and Cr
• 2. Si and B
• 3. Fe and Mn
• 4.P and As

Answer: 4

Explanation:

The same highest oxidation state in the compounds, equal to the group number (+5), is shown by P and As. These elements are located in the main subgroup of group V.

Zn and Cr are elements of secondary subgroups of groups II and VI, respectively. In compounds, zinc exhibits the highest oxidation state +2, chromium - +6.

Fe and Mn are elements of the secondary subgroups of groups VIII and VII, respectively. The highest oxidation state for iron is +6, for manganese - +7.

Task number 4

The same highest oxidation state in compounds exhibit

• 1. Hg and Cr
• 2. Si and Al
• 3.F and Mn
• 4. P and N

Answer: 4

Explanation:

P and N show the same highest oxidation state in compounds, equal to the group number (+5). These elements are located in the main subgroup of group V.

Hg and Cr are elements of secondary subgroups of groups II and VI, respectively. In compounds, mercury exhibits the highest oxidation state +2, chromium - +6.

Si and Al are elements of the main subgroup IV and III groups respectively. Therefore, for silicon, the maximum oxidation state in complex compounds is +4 (the group number where silicon is located), for aluminum - +3 (the group number where aluminum is located).

F and Mn are elements of the main and secondary subgroups of groups VII, respectively. However, fluorine, being the most electronegative element Periodic system chemical elements, does not show positive oxidation states: in complex compounds, its oxidation state is −1 (group number −8). The highest oxidation state of manganese is +7.

Task number 5

The +3 oxidation state nitrogen exhibits in each of two substances:

• 1. HNO 2 and NH 3
• 2. NH 4 Cl and N 2 O 3
• 3. NaNO 2 and NF 3
• 4. HNO 3 and N 2

Answer: 3

Explanation:

In nitrous acid HNO 2, the oxidation state of oxygen in the acid residue is -2, for hydrogen - +1, therefore, in order for the molecule to remain electrically neutral, the oxidation state of nitrogen is +3. In ammonia, NH 3, nitrogen is a more electronegative element, therefore it pulls the electron pair of a covalent polar bond onto itself and has a negative oxidation state of -3, the oxidation state of hydrogen in ammonia is +1.

Ammonium chloride NH 4 Cl is an ammonium salt, so the oxidation state of nitrogen is the same as in ammonia, i.e. equals -3. In oxides, the oxidation state of oxygen is always -2, so for nitrogen it is +3.

In sodium nitrite NaNO 2 (salts of nitrous acid), the degree of oxidation of nitrogen is the same as in nitrogen in nitrous acid, because is +3. In nitrogen fluoride, the oxidation state of nitrogen is +3, since fluorine is the most electronegative element in the Periodic Table and in complex compounds it exhibits a negative oxidation state of -1. This answer option satisfies the condition of the task.

IN nitric acid nitrogen has the highest oxidation state, equal to the group number (+5). Nitrogen as a simple compound (since it consists of atoms of one chemical element) has an oxidation state of 0.

Task number 6

The highest oxide of an element of group VI corresponds to the formula

• 1. E 4 O 6
• 2.EO 4
• 3. EO 2
• 4. EO 3

Answer: 4

Explanation:

Supreme oxide The element is the oxide of the element with its maximum oxidation state. In a group, the highest oxidation state of an element is equal to the group number, therefore, in group VI, the maximum oxidation state of an element is +6. In oxides, oxygen exhibits an oxidation state of -2. The numbers below the element symbol are called indices and indicate the number of atoms of this element in the molecule.

The first option is incorrect, because the element has an oxidation state of 0-(-2)⋅6/4 = +3.

In the second version, the element has an oxidation state of 0-(-2) ⋅ 4 = +8.

In the third variant, the oxidation state of the element E: 0-(-2) ⋅ 2 = +4.

In the fourth variant, the oxidation state of the element E: 0-(-2) ⋅ 3 = +6, i.e. this is the desired answer.

Task number 7

The oxidation state of chromium in ammonium dichromate (NH 4) 2 Cr 2 O 7 is

• 1. +6
• 2. +2
• 3. +3
• 4. +7

Answer: 1

Explanation:

In ammonium dichromate (NH 4) 2 Cr 2 O 7 in the ammonium cation NH 4 + nitrogen, as a more electronegative element, has a lower oxidation state of -3, hydrogen is positively charged +1. Therefore, the entire cation has a charge of +1, but since there are 2 of these cations, the total charge is +2.

In order for the molecule to remain electrically neutral, the acid residue Cr 2 O 7 2− must have a charge of -2. Oxygen in the acid residues of acids and salts always has a charge of -2, therefore, 7 oxygen atoms that make up the ammonium dichromate molecule are charged -14. Chromium atoms Cr into molecules 2, therefore, if the charge of chromium is denoted by x, then we have:

2x + 7 ⋅ (-2) = -2 where x = +6. The charge of chromium in the ammonium dichromate molecule is +6.

Task number 8

An oxidation state of +5 is possible for each of the two elements:

1) oxygen and phosphorus

2) carbon and bromine

3) chlorine and phosphorus

4) sulfur and silicon

Answer: 3

Explanation:

In the first proposed answer, only phosphorus, as an element of the main subgroup of group V, can exhibit an oxidation state of +5, which is the maximum for it. Oxygen (an element of the main subgroup of group VI), being an element with high electronegativity, in oxides exhibits an oxidation state of -2, as a simple substance - 0, and in combination with fluorine OF 2 - +1. The +5 oxidation state is not typical for it.

Carbon and bromine are elements of the main subgroups of groups IV and VII, respectively. Carbon is characterized by a maximum oxidation state of +4 (equal to the group number), and bromine exhibits oxidation states of -1, 0 (in a simple compound Br 2), +1, +3, +5 and +7.

Chlorine and phosphorus are elements of the main subgroups of groups VII and V, respectively. Phosphorus exhibits a maximum oxidation state of +5 (equal to the group number), for chlorine, similarly to bromine, oxidation states of -1, 0 (in a simple compound Cl 2), +1, +3, +5, +7 are characteristic.

Sulfur and silicon are elements of the main subgroups of groups VI and IV, respectively. Sulfur exhibits a wide range of oxidation states from -2 (group number - 8) to +6 (group number). For silicon, the maximum oxidation state is +4 (group number).

Task number 9

• 1. NaNO3
• 2. NaNO2
• 3.NH4Cl
• 4. NO

Answer: 1

Explanation:

In sodium nitrate NaNO 3, sodium has an oxidation state of +1 (group I element), there are 3 oxygen atoms in the acid residue, each of which has an oxidation state of −2, therefore, in order for the molecule to remain electrically neutral, nitrogen must have an oxidation state of: 0 − (+ 1) − (−2) 3 = +5.

In sodium nitrite NaNO 2, the sodium atom also has an oxidation state of +1 (group I element), there are 2 oxygen atoms in the acid residue, each of which has an oxidation state of −2, therefore, in order for the molecule to remain electrically neutral, nitrogen must have an oxidation state: 0 − (+1) − (−2) 2 = +3.

NH 4 Cl - ammonium chloride. In chlorides, chlorine atoms have an oxidation state of −1, hydrogen atoms, of which there are 4 in the molecule, are positively charged, therefore, in order for the molecule to remain electrically neutral, the nitrogen oxidation state is: 0 − (−1) − 4 (+1) = −3. In ammonia and cations of ammonium salts, nitrogen has a minimum oxidation state of −3 (the number of the group in which the element is located is −8).

In the nitric oxide NO molecule, oxygen exhibits a minimum oxidation state of −2, as in all oxides, therefore, the oxidation state of nitrogen is +2.

Task number 10

The highest degree nitrogen oxidation exhibits in a compound whose formula is

• 1. Fe(NO 3) 3
• 2. NaNO2
• 3. (NH 4) 2 SO 4
• 4 NO 2

Answer: 1

Explanation:

Nitrogen is an element of the main subgroup of group V, therefore, it can exhibit a maximum oxidation state equal to the group number, i.e. +5.

One structural unit of iron nitrate Fe(NO 3) 3 consists of one Fe 3+ ion and three nitrate ions. In nitrate ions, nitrogen atoms, regardless of the type of counterion, have an oxidation state of +5.

In sodium nitrite NaNO 2, sodium has an oxidation state of +1 (an element of the main subgroup of group I), there are 2 oxygen atoms in the acid residue, each of which has an oxidation state of −2, therefore, in order for the molecule to remain electrically neutral, nitrogen must have an oxidation state of 0 − ( +1) − (−2)⋅2 ​​= +3.

(NH 4) 2 SO 4 - ammonium sulfate. In sulfuric acid salts, the SO 4 2− anion has a charge of 2−, therefore, each ammonium cation is charged with 1+. On hydrogen, the charge is +1, therefore on nitrogen -3 (nitrogen is more electronegative, therefore it pulls the common electron pair of the N−H bond). In ammonia and cations of ammonium salts, nitrogen has a minimum oxidation state of −3 (the number of the group in which the element is located is −8).

In the nitric oxide NO 2 molecule, oxygen exhibits a minimum oxidation state of −2, as in all oxides, therefore, the oxidation state of nitrogen is +4.

Task number 11

28910E

In compounds of the composition Fe(NO 3) 3 and CF 4, the degree of oxidation of nitrogen and carbon is, respectively,

Answer: 4

Explanation:

One structural unit of iron (III) nitrate Fe(NO 3) 3 consists of one iron ion Fe 3+ and three nitrate ions NO 3 − . In nitrate ions, nitrogen always has an oxidation state of +5.

In carbon fluoride CF 4, fluorine is a more electronegative element and attracts a common electron pair C-F connections, showing an oxidation state of -1. Therefore, carbon C has an oxidation state of +4.

Task number 12

A32B0B

The oxidation state +7 chlorine exhibits in each of the two compounds:

• 1. Ca(OCl) 2 and Cl 2 O 7
• 2. KClO 3 and ClO 2
• 3. BaCl 2 and HClO 4
• 4. Mg(ClO 4) 2 and Cl 2 O 7

Answer: 4

Explanation:

In the first variant, chlorine atoms have oxidation states +1 and +7, respectively. One structural unit of calcium hypochlorite Ca(OCl) 2 consists of one calcium ion Ca 2+ (Ca is an element of the main subgroup of group II) and two hypochlorite ions OCl − , each of which has a charge of 1−. In complex compounds, except for OF 2 and various peroxides, oxygen always has an oxidation state of −2, so it is obvious that chlorine has a charge of +1. In chlorine oxide Cl 2 O 7, as in all oxides, oxygen has an oxidation state of −2, therefore, chlorine in this compound has an oxidation state of +7.

In potassium chlorate KClO 3, the potassium atom has an oxidation state of +1, and oxygen - -2. In order for the molecule to remain electrically neutral, chlorine must exhibit an oxidation state of +5. In chlorine oxide ClO 2, oxygen, as in any other oxide, has an oxidation state of −2, therefore, for chlorine, its oxidation state is +4.

In the third version, the barium cation in the complex compound is charged +2, therefore, a negative charge of −1 is concentrated on each chlorine anion in the BaCl 2 salt. In perchloric acid HClO 4, the total charge of 4 oxygen atoms is -2⋅4 = -8, on the hydrogen cation the charge is +1. For the molecule to remain electrically neutral, the charge of chlorine must be +7.

In the fourth variant, in the magnesium perchlorate molecule Mg (ClO 4) 2, the charge of magnesium is +2 (in all complex compounds, magnesium exhibits an oxidation state of +2), therefore, each ClO 4 - anion has a charge of 1 -. In total, 4 oxygen ions, where each exhibits an oxidation state of -2, have a charge of -8. Therefore, for the total charge of the anion to be 1−, the charge on chlorine must be +7. In chlorine oxide Cl 2 O 7 , as explained above, the charge of chlorine is +7.