The following oxides of the elements are amphoteric major subgroups: BeO, A1 2 O 3, Ga 2 O 3, GeO 2, SnO, SnO 2, PbO, Sb 2 O 3, PoO 2. Amphoteric hydroxides are the following hydroxides of the elements major subgroups: Be (OH) 2, A1 (OH) 3, Sc (OH) 3, Ga (OH) 3, In (OH) 3, Sn (OH) 2, SnO 2 nH 2 O, Pb (OH) 2 , PbO 2 nH 2 O.

The basic nature of the oxides and hydroxides of elements of one subgroup increases with increasing atomic number of the element (when comparing oxides and hydroxides of elements in the same oxidation state). For example, N 2 O 3, P 2 O 3, As 2 O 3 are acidic oxides, Sb 2 O 3 is an amphoteric oxide, Bi 2 O 3 is a basic oxide.

Let us consider the amphoteric properties of hydroxides using the example of beryllium and aluminum compounds.

Aluminum hydroxide exhibits amphoteric properties, reacts with both bases and acids and forms two series of salts:

1) in which the element A1 is in the form of a cation;

2A1 (OH) 3 + 6HC1 \u003d 2A1C1 3 + 6H 2 O A1 (OH) 3 + 3H + \u003d A1 3+ + 3H 2 O

In this reaction, A1(OH) 3 functions as a base, forming a salt in which aluminum is the A1 3+ cation;

2) in which the element A1 is part of the anion (aluminates).

A1 (OH) 3 + NaOH \u003d NaA1O 2 + 2H 2 O.

In this reaction, A1(OH) 3 acts as an acid, forming a salt in which aluminum is part of the AlO 2 - anion.

The formulas of dissolved aluminates are written in a simplified way, referring to the product formed during salt dehydration.

In the chemical literature, one can find different formulas of compounds formed by dissolving aluminum hydroxide in alkali: NaA1O 2 (sodium metaaluminate), Na tetrahydroxoaluminate sodium. These formulas do not contradict each other, since their difference is associated with different degrees of hydration of these compounds: NaA1O 2 2H 2 O is a different record of Na. When A1 (OH) 3 is dissolved in an excess of alkali, sodium tetrahydroxoaluminate is formed:

A1 (OH) 3 + NaOH \u003d Na.

During sintering of reagents, sodium metaaluminate is formed:

A1(OH) 3 + NaOH ==== NaA1O 2 + 2H 2 O.

Thus, we can say that in aqueous solutions there are simultaneously such ions as [A1 (OH) 4] - or [A1 (OH) 4 (H 2 O) 2] - (for the case when the reaction equation is drawn up taking into account the hydrate shells), and the notation A1O 2 is simplified.

Due to the ability to react with alkalis, aluminum hydroxide, as a rule, is not obtained by the action of alkali on solutions of aluminum salts, but an ammonia solution is used:

A1 2 (SO 4) 3 + 6 NH 3 H 2 O \u003d 2A1 (OH) 3 + 3(NH 4) 2 SO 4.

Among the hydroxides of elements of the second period, beryllium hydroxide exhibits amphoteric properties (beryllium itself exhibits a diagonal similarity to aluminum).

With acids:

Be (OH) 2 + 2HC1 \u003d BeC1 2 + 2H 2 O.

With bases:

Be (OH) 2 + 2NaOH \u003d Na 2 (sodium tetrahydroxoberyllate).

In a simplified form (if we represent Be (OH) 2 as an acid H 2 BeO 2)

Be (OH) 2 + 2NaOH (concentrated hot) \u003d Na 2 BeO 2 + 2H 2 O.

beryllate Na

Hydroxides of elements of secondary subgroups, corresponding to the highest oxidation states, most often have acidic properties: for example, Mn 2 O 7 - HMnO 4; CrO 3 - H 2 CrO 4. For lower oxides and hydroxides, the predominance of the main properties is characteristic: CrO - Cr (OH) 2; MnO - Mn (OH) 2; FeO - Fe (OH) 2. Intermediate compounds corresponding to oxidation states +3 and +4 often exhibit amphoteric properties: Cr 2 O 3 - Cr (OH) 3; Fe 2 O 3 - Fe (OH) 3. We illustrate this pattern on the example of chromium compounds (Table 9).

Table 9 - Dependence of the nature of oxides and their corresponding hydroxides on the degree of oxidation of the element

Interaction with acids leads to the formation of a salt in which the element chromium is in the form of a cation:

2Cr(OH) 3 + 3H 2 SO 4 = Cr 2 (SO 4) 3 + 6H 2 O.

Cr(III) sulfate

Reaction with bases leads to the formation of salt, in which the element chromium is part of the anion:

Cr (OH) 3 + 3NaOH \u003d Na 3 + 3H 2 O.

hexahydroxochromate(III) Na

Zinc oxide and hydroxide ZnO, Zn(OH) 2 are typically amphoteric compounds, Zn(OH) 2 easily dissolves in acid and alkali solutions.

Interaction with acids leads to the formation of a salt in which the element zinc is in the form of a cation:

Zn(OH) 2 + 2HC1 = ZnCl 2 + 2H 2 O.

Interaction with bases leads to the formation of a salt in which the zinc element is in the anion. When interacting with alkalis in solutions tetrahydroxozincates are formed, when fused- zincates:

Zn(OH) 2 + 2NaOH \u003d Na 2.

Or when fusing:

Zn (OH) 2 + 2NaOH \u003d Na 2 ZnO 2 + 2H 2 O.

Zinc hydroxide is obtained similarly to aluminum hydroxide.

Bases, amphoteric hydroxides

Bases are complex substances consisting of metal atoms and one or more hydroxo groups (-OH). The general formula is Me + y (OH) y, where y is the number of hydroxo groups equal to the oxidation state of the metal Me. The table shows the classification of bases.

Properties of alkali hydroxides of alkali and alkaline earth metals

1. Aqueous solutions of alkalis are soapy to the touch, change the color of indicators: litmus - blue, phenolphthalein - raspberry.

2. Aqueous solutions dissociate:

3. Interact with acids, entering into an exchange reaction:

Polyacid bases can give intermediate and basic salts:

4. Interact with acid oxides, forming medium and acid salts, depending on the basicity of the acid corresponding to this oxide:

5. Interact with amphoteric oxides and hydroxides:

a) fusion:

b) in solutions:

6. React with water-soluble salts if a precipitate or gas is formed:

Insoluble bases (Cr (OH) 2, Mn (OH) 2, etc.) interact with acids and decompose when heated:

Amphoteric hydroxides

Compounds are called amphoteric, which, depending on the conditions, can be both donors of hydrogen cations and exhibit acidic properties, and their acceptors, i.e., exhibit basic properties.

Chemical properties of amphoteric compounds

1. Interacting with strong acids, they reveal the main properties:

Zn(OH) 2 + 2HCl = ZnCl 2 + 2H 2 O

2. Interacting with alkalis - strong bases, they exhibit acidic properties:

Zn (OH) 2 + 2NaOH \u003d Na 2 ( complex salt)

Al (OH) 3 + NaOH \u003d Na ( complex salt)

Compounds are called complex in which at least one covalent bond was formed by the donor-acceptor mechanism.

The general method for obtaining bases is based on exchange reactions, by which both insoluble and soluble bases can be obtained.

CuSO 4 + 2KOH \u003d Cu (OH) 2 ↓ + K 2 SO 4

K 2 CO 3 + Ba (OH) 2 \u003d 2 KOH + BaCO 3 ↓

When soluble bases are obtained by this method, an insoluble salt precipitates.

When obtaining water-insoluble bases with amphoteric properties, an excess of alkali should be avoided, since dissolution of the amphoteric base may occur, for example:

AlCl 3 + 4KOH \u003d K [Al (OH) 4] + 3KSl

In such cases, ammonium hydroxide is used to obtain hydroxides, in which amphoteric hydroxides do not dissolve:

AlCl 3 + 3NH 3 + ZH 2 O \u003d Al (OH) 3 ↓ + 3NH 4 Cl

Hydroxides of silver and mercury decompose so easily that when you try to obtain them by an exchange reaction, instead of hydroxides, oxides precipitate:

2AgNO 3 + 2KOH \u003d Ag 2 O ↓ + H 2 O + 2KNO 3

In industry, alkalis are usually obtained by electrolysis of aqueous solutions of chlorides.

2NaCl + 2H 2 O → ϟ → 2NaOH + H 2 + Cl 2

Alkalis can also be obtained by reacting alkali and alkaline earth metals or their oxides with water.

2Li + 2H 2 O \u003d 2LiOH + H 2

SrO + H 2 O \u003d Sr (OH) 2

acids

Acids are called complex substances, the molecules of which consist of hydrogen atoms that can be replaced by metal atoms, and acid residues. Under normal conditions, acids can be solid (phosphoric H 3 PO 4; silicon H 2 SiO 3) and liquid (sulfuric acid H 2 SO 4 will be a pure liquid).

Gases such as hydrogen chloride HCl, hydrogen bromide HBr, hydrogen sulfide H 2 S form the corresponding acids in aqueous solutions. The number of hydrogen ions formed by each acid molecule during dissociation determines the charge of the acid residue (anion) and the basicity of the acid.

According to protolytic theory of acids and bases, proposed simultaneously by the Danish chemist Bronsted and the English chemist Lowry, an acid is a substance splitting off with this reaction protons, a basis- a substance capable of receive protons.

acid → base + H +

Based on these ideas, it is clear basic properties of ammonia, which, due to the presence of a lone electron pair at the nitrogen atom, effectively accepts a proton when interacting with acids, forming an ammonium ion through a donor-acceptor bond.

HNO 3 + NH 3 ⇆ NH 4 + + NO 3 -

acid base acid base

A more general definition of acids and bases proposed by the American chemist G. Lewis. He suggested that acid-base interactions are quite do not necessarily occur with protone transfer. In the determination of acids and bases according to Lewis, the main role in chemical reactions is given to electronic steam.

Cations, anions, or neutral molecules that can accept one or more pairs of electrons are called Lewis acids.

For example, aluminum fluoride AlF 3 is an acid, since it is able to accept an electron pair when interacting with ammonia.

AlF 3 + :NH 3 ⇆ :

Cations, anions or neutral molecules capable of donating electron pairs are called Lewis bases (ammonia is a base).

The Lewis definition covers all acid-base processes that have been considered by the previously proposed theories. The table compares the definitions of acids and bases currently in use.

Nomenclature of acids

Since there are different definitions of acids, their classification and nomenclature are rather arbitrary.

According to the number of hydrogen atoms capable of splitting off in an aqueous solution, acids are divided into monobasic(e.g. HF, HNO 2), dibasic(H 2 CO 3 , H 2 SO 4) and tribasic(H 3 RO 4).

According to the composition of the acid is divided into anoxic(HCl, H 2 S) and oxygen-containing(HClO 4 , HNO 3).

Usually names of oxygenated acids derived from the name of a non-metal with the addition of the endings -kai, -way, if the oxidation state of the non-metal is equal to the group number. As the oxidation state decreases, the suffixes change (in order of decreasing metal oxidation state): - oval, ististaya, - ovate:

If we consider the polarity of the hydrogen-non-metal bond within a period, we can easily relate the polarity of this bond to the position of the element in the Periodic Table. From metal atoms that easily lose valence electrons, hydrogen atoms accept these electrons, forming a stable two-electron shell like the shell of a helium atom, and give ionic metal hydrides.

In hydrogen compounds of elements of groups III-IV of the Periodic system, boron, aluminum, carbon, silicon form covalent, weakly polar bonds with hydrogen atoms that are not prone to dissociation. For elements of groups V-VII of the Periodic system, within a period, the polarity of the non-metal-hydrogen bond increases with the charge of the atom, but the distribution of charges in the resulting dipole is different than in hydrogen compounds of elements that tend to donate electrons. Atoms of non-metals, in which several electrons are needed to complete the electron shell, pull towards themselves (polarize) a pair of bond electrons the stronger, the greater the charge of the nucleus. Therefore, in the series CH 4 - NH 3 - H 2 O - HF or SiH 4 - PH 3 - H 2 S - Hcl, bonds with hydrogen atoms, while remaining covalent, become more polar, and the hydrogen atom in the dipole of the element-hydrogen bond becomes more electropositive. If polar molecules are in a polar solvent, the process of electrolytic dissociation can occur.

Let us discuss the behavior of oxygen-containing acids in aqueous solutions. These acids have an H-O-E bond and, naturally, the O-E bond affects the polarity of the H-O bond. Therefore, these acids dissociate, as a rule, more easily than water.

H 2 SO 3 + H 2 O ⇆ H s O + + HSO 3

HNO 3 + H 2 O ⇆ H s O + + NO 3

Let's look at a few examples properties of oxygenated acids, formed by elements that are capable of exhibiting different oxidation states. It is known that hypochlorous acid HClO very weak hydrochloric acid HClO 2 also weak but stronger than hypochlorous, hypochlorous acid HclO 3 strong. Perchloric acid HClO 4 is one of the the strongest inorganic acids.

Dissociation according to the acidic type (with the elimination of the H ion) requires breaking the O-H bond. How can one explain the decrease in the strength of this bond in the series HClO - HClO 2 - HClO 3 - HClO 4? In this series, the number of oxygen atoms associated with the central chlorine atom increases. Each time a new bond of oxygen with chlorine is formed, an electron density is drawn away from the chlorine atom, and hence from the single O-Cl bond. As a result, the electron density partially leaves the О-Н bond, which is weakened because of this.

Such a pattern - enhancement of acidic properties with an increase in the degree of oxidation of the central atom - characteristic not only for chlorine, but also for other elements. For example, nitric acid HNO 3 , in which the nitrogen oxidation state is +5, is stronger than nitrous acid HNO 2 (nitrogen oxidation state is +3); sulfuric acid H 2 SO 4 (S +6) is stronger than sulfurous acid H 2 SO 3 (S +4).

Obtaining acids

1. Anoxic acids can be obtained in the direct combination of non-metals with hydrogen.

H 2 + Cl 2 → 2HCl,

H 2 + S ⇆ H 2 S

2. Some oxygenated acids can be obtained interaction of acid oxides with water.

3. Both anoxic and oxygenated acids can be obtained according to exchange reactions between salts and other acids.

BaBr 2 + H 2 SO 4 \u003d BaSO 4 ↓ + 2HBr

CuSO 4 + H 2 S \u003d H 2 SO 4 + CuS ↓

FeS + H 2 SO 4 (pa zb) \u003d H 2 S + FeSO 4

NaCl (T) + H 2 SO 4 (conc) = HCl + NaHSO 4

AgNO 3 + HCl = AgCl↓ + HNO 3

CaCO 3 + 2HBr \u003d CaBr 2 + CO 2 + H 2 O

4. Some acids can be obtained using redox reactions.

H 2 O 2 + SO 2 \u003d H 2 SO 4

3P + 5HNO 3 + 2H 2 O \u003d ZH 3 RO 4 + 5NO 2

Sour taste, action on indicators, electrical conductivity, interaction with metals, basic and amphoteric oxides, bases and salts, formation of esters with alcohols - these properties are common to inorganic and organic acids.

can be divided into two types of reactions:

1) general for acids the reactions are associated with the formation of hydronium ion H 3 O + in aqueous solutions;

2) specific(i.e. characteristic) reactions specific acids.

The hydrogen ion can enter into redox reactions, reducing to hydrogen, as well as in a compound reaction with negatively charged or neutral particles having lone pairs of electrons, i.e. in acid-base reactions.

The general properties of acids include the reactions of acids with metals in the series of voltages up to hydrogen, for example:

Zn + 2Н + = Zn 2+ + Н 2

Acid-base reactions include reactions with basic oxides and bases, as well as with medium, basic, and sometimes acidic salts.

2 CO 3 + 4HBr \u003d 2CuBr 2 + CO 2 + 3H 2 O

Mg (HCO 3) 2 + 2HCl \u003d MgCl 2 + 2CO 2 + 2H 2 O

2KHSO 3 + H 2 SO 4 \u003d K 2 SO 4 + 2SO 2 + 2H 2 O

Note that polybasic acids dissociate stepwise, and at each next step, dissociation is more difficult, therefore, with an excess of acid, acidic salts are most often formed, rather than medium ones.

Ca 3 (PO 4) 2 + 4H 3 PO 4 \u003d 3Ca (H 2 PO 4) 2

Na 2 S + H 3 PO 4 = Na 2 HPO 4 + H 2 S

NaOH + H 3 PO 4 = NaH 2 PO 4 + H 2 O

KOH + H 2 S \u003d KHS + H 2 O

At first glance, the formation of acidic salts may seem surprising. monobasic hydrofluoric (hydrofluoric) acid. However, this fact can be explained. Unlike all other hydrohalic acids, hydrofluoric acid is partially polymerized in solutions (due to the formation of hydrogen bonds) and different particles (HF) X can be present in it, namely H 2 F 2, H 3 F 3, etc.

A special case of acid-base balance - reactions of acids and bases with indicators that change color depending on the acidity of the solution. Indicators are used in qualitative analysis to detect acids and bases in solutions.

The most commonly used indicators are litmus(in neutral environment purple, in sour - red, in alkaline - blue), methyl orange(in sour environment red, in neutral - orange, in alkaline - yellow), phenolphthalein(in highly alkaline environment crimson red, in neutral and acidic - colorless).

Specific properties different acids can be of two types: first, the reactions leading to the formation insoluble salts, and, secondly, redox transformations. If the reactions associated with the presence of an H + ion in them are common to all acids (qualitative reactions for detecting acids), specific reactions are used as qualitative reactions for individual acids:

Ag + + Cl - = AgCl (white precipitate)

Ba 2+ + SO 4 2- \u003d BaSO 4 (white precipitate)

3Ag + + PO 4 3 - = Ag 3 PO 4 (yellow precipitate)

Some specific reactions of acids are due to their redox properties.

Anoxic acids in aqueous solution can only oxidize.

2KMnO 4 + 16HCl \u003d 5Cl 2 + 2KCl + 2MnCl 2 + 8H 2 O

H 2 S + Br 2 \u003d S + 2HBg

Oxygen-containing acids can only be oxidized if the central atom in them is in a lower or intermediate oxidation state, as, for example, in sulfurous acid:

H 2 SO 3 + Cl 2 + H 2 O \u003d H 2 SO 4 + 2HCl

Many oxygen-containing acids, in which the central atom has the maximum oxidation state (S +6, N +5, Cr +6), exhibit the properties of strong oxidizing agents. Concentrated H 2 SO 4 is a strong oxidizing agent.

Cu + 2H 2 SO 4 (conc) = CuSO 4 + SO 2 + 2H 2 O

Pb + 4HNO 3 \u003d Pb (NO 3) 2 + 2NO 2 + 2H 2 O

C + 2H 2 SO 4 (conc) = CO 2 + 2SO 2 + 2H 2 O

It should be remembered that:

• Acid solutions react with metals that are in the electrochemical series of voltages to the left of hydrogen, subject to a number of conditions, the most important of which is the formation of a soluble salt as a result of the reaction. The interaction of HNO 3 and H 2 SO 4 (conc.) with metals proceeds differently.

Concentrated sulfuric acid in the cold passivates aluminum, iron, chromium.

• In water, acids dissociate into hydrogen cations and anions of acid residues, for example:

• Inorganic and organic acids interact with basic and amphoteric oxides, provided that a soluble salt is formed:

• Both those and other acids react with bases. Polybasic acids can form both medium and acidic salts (these are neutralization reactions):

• The reaction between acids and salts occurs only if a precipitate or gas is formed:

The interaction of H 3 PO 4 with limestone will stop due to the formation of the last insoluble precipitate Ca 3 (PO 4) 2 on the surface.

The features of the properties of nitric HNO 3 and concentrated sulfuric H 2 SO 4 (conc.) acids are due to the fact that when they interact with simple substances (metals and non-metals), not H + cations, but nitrate and sulfate ions will act as oxidizing agents. It is logical to expect that as a result of such reactions, not hydrogen H 2 is formed, but other substances are obtained: necessarily salt and water, as well as one of the products of the reduction of nitrate or sulfate ions, depending on the concentration of acids, the position of the metal in a series of voltages and reaction conditions (temperature, metal fineness, etc.).

These features of the chemical behavior of HNO 3 and H 2 SO 4 (conc.) clearly illustrate the thesis of the theory of chemical structure about the mutual influence of atoms in the molecules of substances.

The concepts of volatility and stability (stability) are often confused. Volatile acids are called acids, the molecules of which easily pass into a gaseous state, that is, they evaporate. For example, hydrochloric acid is a volatile but persistent, stable acid. The volatility of unstable acids cannot be judged. For example, non-volatile, insoluble silicic acid decomposes into water and SiO 2 . Aqueous solutions of hydrochloric, nitric, sulfuric, phosphoric and a number of other acids are colorless. An aqueous solution of chromic acid H 2 CrO 4 is yellow, permanganic acid HMnO 4 is raspberry.

Reference material for passing the test:

periodic table

Solubility table

Class: 8

Lesson Objectives:
-formation of the concept of "amphoteric", application of knowledge about the acid-base properties of compounds.

Lesson objectives:
- to ensure the assimilation of the properties of amphoteric compounds;
- to summarize information about the characteristic properties of oxides, acids and bases, to prepare for the implementation of practical work;
- consolidate the skill of drawing up reaction equations;
- to develop the ability to analyze information, highlight cause-and-effect relationships;
- improve the ability to find common features and differences in the composition and properties of substances;
- maintain self-confidence;
- to cultivate teamwork skills and an attentive attitude to the opinion of another person.

Lesson type:
A combined lesson of learning new knowledge and applying knowledge, skills and abilities.

Lesson steps:

I.Organization of the beginning of the lesson.

Teacher: Guys, today we have to prepare for practical work on the characteristic properties of the studied substances (oxides, acids and bases). In addition, we will get acquainted with substances that have both acidic and basic properties, showing them depending on what they react with. You have serious individual and group work to do, and as assistants we use color symbol system and scheme reflecting the chemical properties of substances.
The system of color symbols is based on the ability of a person to memorize concepts and terms, associating them with color (for example, the name of subway stations is often associated with the color of a branch on a diagram).

II. Checking the assimilation of the previous material.

Teacher: For execution 1st task on your tables are cards of red and blue colors, on each card is the formula of a complex substance. Substances are different, but belong to the same class, which one?
Students find out that these are oxides ( formulas for acid oxides should be written on red cards, and formulas for basic oxides on blue).
Teacher: We will work in pairs, you need to write the reaction equations for the interaction of substances recorded on the cards with water. Each mini-group should make 2 equations. Two students will work individually on the board, their task is to write the reaction of the interaction of oxide with water and draw up a diagram of the rule for such an interaction from separate words. (The student who writes the equation with acidic oxide is invited to work with a red marker or chalk, and the one with the basic oxide is blue).

As the assignment progresses, discuss:
-composition of basic oxides;
-composition of acid oxides;
- the result of the interaction of oxides with water;
- what acidic and basic oxides do not interact with water;
-composition and rules for formulating bases and acids.

There should be a note on the board:

After completing the task, you need to discuss:
-which oxides we marked in red and which in blue;
- how in practical work students will be able to prove that the resulting substance is an acid or a base;
What are indicators and how do they change color?

III. Preparing students for the conscious assimilation of new knowledge.

Teacher: We discussed with you how you can experimentally prove the presence of the resulting acid or alkali, but today our work is theoretical and we have to perform 2nd task. Now, on the spread of the board, rule schemes are written ( in the same colors decisions), and you try to find examples of reaction equations. We work in groups, then 2 people do the task at the blackboard.

This scheme once again reminds us of the rule:
The most typical for compounds are reactions of interaction with substances opposite in properties.

Teacher: It is no coincidence that the central part of the board is empty for now. There was room for special compounds, their name comes from the Greek word amphoteros, meaning "both." The word amphibian is the same root to it, let's remember what it means?

IV. Learning new material.

Amphoteric - the ability of compounds to exhibit either acidic or basic properties, depending on what they react with.
There are quite a few amphoteric compounds. Of the oxides, zinc oxide, aluminum oxide, copper oxides, tin oxides, lead oxides, iron (III) oxide, etc. have dual properties. ( On the board you can write the formulas of amphoteric oxides)
Let's replace the signs in our schemes "basic oxide" and "acid oxide" on the plate "amphoteric oxide" and get new rules. To complete the 3rd task, we use the schemes written on the board.
3 task: Knowing that zinc oxide is amphoteric, write the equations for the reactions of its interaction with hydrochloric acid and sodium hydroxide.

Teacher: Amphoteric oxides do not react with water. However, water itself is a classic example of an amphoteric oxide, because reacts with both acidic and basic oxides.

V. Primary comprehension of knowledge.

Teacher: How can you tell if a compound is amphoteric?
Oxides and hydroxides of most transition elements and many elements of secondary subgroups are amphoteric in nature.
For the convenience of determining the nature of the compounds, some variants of the table of D.I. Mendeleev are equipped with colored icons similar to those that we used today. I will sign the blue badge, and you yourself will sign the other two.

Remember that oxides and hydroxides of active metals are always basic,
Compounds of non-metals are usually acidic in nature.

VI. Consolidation of knowledge.

Teacher: Your 4th task is the most difficult, but if you remember the chemical properties of bases and acids, then you can handle it.
4th task: Write down the reaction equations for the interaction of amphoteric zinc hydroxide with acid and alkali. Before you begin your independent work on this task, I will help you a little.
Let's write the formula for zinc hydroxide Zn(OH)2 together. In this form, we are used to writing bases, but the same substance can also be represented as an acid, it is enough to open the brackets and move hydrogen to the first place: H2ZnO2. Such an acid exists, it is called zincic, and its salts are zincates.

VII. Control and self-examination of knowledge.

When analyzing the 4th task, it is worth paying attention to:
-chemical properties of acids and bases;
- naming salts;
- the duality of the properties of amphoteric compounds.
Students who quickly completed the task can be asked to complete the task from the textbook after the paragraph.

VIII. Generalization and systematization of knowledge.

Teacher: In order to help yourself remember the rules for writing reaction products, there are many different schemes. I will give an example for oxides, and you can try to make similar schemes for acids, bases and amphoteric hydroxides.

IX. Information about homework, summing up the lesson.

As homework, it is proposed to prepare for practical work

DEFINITION

Amphoteric compounds- compounds that, depending on the reaction conditions, can exhibit both the properties of acids and bases, i.e. can both donate and accept a proton (H+).

Amphoteric inorganic compounds include oxides and hydroxides of the following metals - Al, Zn, Be, Cr (in the oxidation state +3) and Ti (in the oxidation state +4). Amphoteric organic compounds are amino acids - NH 2 -CH (R) -COOH.

Preparation of amphoteric compounds

Amphoteric oxides are obtained by the combustion reaction of the corresponding metal in oxygen, for example:

2Al + 3/2O 2 = Al 2 O 3

Amphoteric hydroxides are obtained by an exchange reaction between an alkali and a salt containing an "amphoteric" metal:

ZnSO 4 + NaOH \u003d Zn (OH) 2 + Na 2 SO 4

If alkali is present in excess, then there is a possibility of obtaining a complex compound:

ZnSO 4 + 4NaOH g = Na 2 + Na 2 SO 4

Organic amphoteric compounds - amino acids are obtained by replacing a halogen with an amino group in halogen-substituted carboxylic acids. In general, the reaction equation will look like this:

R-CH (Cl) -COOH + NH 3 \u003d R-CH (NH 3 + Cl -) \u003d NH 2 -CH (R) -COOH

Chemical amphoteric compounds

The main chemical property of amphoteric compounds is their ability to react with acids and alkalis:

Al 2 O 3 + 6HCl \u003d 2AlCl 3 + 3H 2 O

Zn(OH) 2 + 2HNO 3 = Zn(NO 3) 2 + 2H 2 O

Zn (OH) 2 + NaOH \u003d Na 2

NH 2 -CH 2 -COOH + HCl \u003d Cl

Specific properties of amphoteric organic compounds

When amino acids are dissolved in water, the amino group and the carboxyl group interact with each other to form compounds called internal salts:

NH 2 –CH 2 -COOH ↔ + H 3 N–CH 2 -COO -

The internal salt molecule is called a bipolar ion.

Two amino acid molecules can interact with each other. In this case, the water molecule is cleaved and a product is formed in which the fragments of the molecule are interconnected by a peptide bond (-CO-NH-). For example:

Also, amino acids are characterized by all the chemical properties of carboxylic acids (according to the carboxyl group) and amines (according to the amino group).

Examples of problem solving

EXAMPLE 1

Exercise	Carry out a series of transformations: a) Al → Al(OH) 3 → AlCl 3 → Na; b) Al → Al 2 O 3 → Na → Al(OH) 3 → Al 2 O 3 → Al
Decision	a) 2Al + 6H 2 O \u003d 2Al (OH) 3 + 3H 2 Al(OH) 3 + 3HCl = AlCl 3 + 3H 2 O AlCl 3 + 4NaOH g = Na + 3NaCl b) 2Al + 3/2O 2 = Al 2 O 3 Al 2 O 3 + NaOH + 3H 2 O \u003d 2Na 2Na + H 2 SO 4 \u003d 2Al (OH) 3 + Na 2 SO 4 + 2H 2 O 2Al(OH) 3 \u003d Al 2 O 3 + 3H 2 O 2Al 2 O 3 \u003d 4Al + 3O 2

EXAMPLE 2

Exercise	Calculate the mass of salt that can be obtained by reacting 150 g of a 5% solution of aminoacetic acid with the required amount of sodium hydroxide. How many grams of a 12% alkali solution will be required for this?
Decision	Let's write the reaction equation: NH 2 -CH 2 -COOH + NaOH \u003d NH 2 -CH 2 -COONa + H 2 O Calculate the mass of the acid that reacted: m (NH 2 -CH 2 -COOH) \u003d ώ to - you × m p - ra m (NH 2 -CH 2 -COOH) \u003d 0.05 × 150 \u003d 7.5 g

We will devote this lesson to the study of amphoteric oxides and hydroxides. On it we will talk about substances that have amphoteric (dual) properties, and the features of the chemical reactions that occur with them. But first, let's repeat what acidic and basic oxides react with. After we consider examples of amphoteric oxides and hydroxides.

Subject: Introduction

Lesson: Amphoteric oxides and hydroxides

Rice. 1. Substances exhibiting amphoteric properties

Basic oxides react with acidic oxides, and acidic oxides with bases. But there are substances whose oxides and hydroxides, depending on the conditions, will react with both acids and bases. Such properties are called amphoteric.

Substances with amphoteric properties are shown in Fig. 1. These are compounds formed by beryllium, zinc, chromium, arsenic, aluminum, germanium, lead, manganese, iron, tin.

Examples of their amphoteric oxides are shown in Table 1.

Consider the amphoteric properties of zinc and aluminum oxides. On the example of their interaction with basic and acidic oxides, with acid and alkali.

ZnO + Na 2 O → Na 2 ZnO 2 (sodium zincate). Zinc oxide behaves like an acid.

ZnO + 2NaOH → Na 2 ZnO 2 + H 2 O

3ZnO + P 2 O 5 → Zn 3 (PO 4) 2 (zinc phosphate)

ZnO + 2HCl → ZnCl 2 + H 2 O

Aluminum oxide behaves similarly to zinc oxide:

Interaction with basic oxides and bases:

Al 2 O 3 + Na 2 O → 2NaAlO 2 (sodium metaaluminate). Aluminum oxide behaves like an acid.

Al 2 O 3 + 2NaOH → 2NaAlO 2 + H 2 O

Interaction with acid oxides and acids. Shows the properties of the basic oxide.

Al 2 O 3 + P 2 O 5 → 2AlPO 4 (aluminum phosphate)

Al 2 O 3 + 6HCl → 2AlCl 3 + 3H 2 O

The considered reactions occur during heating, during fusion. If we take solutions of substances, then the reactions will go a little differently.

ZnO + 2NaOH + H 2 O → Na 2 (sodium tetrahydroxozincate) Al 2 O 3 + 2NaOH + 3H 2 O → 2Na (sodium tetrahydroxoaluminate)

As a result of these reactions, salts are obtained that are complex.

Rice. 2. Minerals based on aluminum oxide

Aluminium oxide.

Aluminum oxide is an extremely common substance on Earth. It forms the basis of clay, bauxite, corundum and other minerals. Fig.2.

As a result of the interaction of these substances with sulfuric acid, zinc sulfate or aluminum sulfate is obtained.

ZnO + H 2 SO 4 → ZnSO 4 + H 2 O

Al 2 O 3 + 3H 2 SO 4 → Al 2 (SO 4) 3 + 3H 2 O

The reactions of zinc and aluminum hydroxides with sodium oxide occur during fusion, because these hydroxides are solid and do not enter into solutions.

Zn (OH) 2 + Na 2 O → Na 2 ZnO 2 + H 2 O salt is called sodium zincate.

2Al(OH) 3 + Na 2 O → 2NaAlO 2 + 3H 2 O salt is called sodium metaaluminate.

Rice. 3. Aluminum hydroxide

The reactions of amphoteric bases with alkalis characterize their acidic properties. These reactions can be carried out both in the fusion of solids and in solutions. But in this case, different substances will be obtained, i.e. the reaction products depend on the reaction conditions: in the melt or in solution.

Zn(OH) 2 + 2NaOH solid. Na 2 ZnO 2 + 2H 2 O

Al(OH) 3 + NaOH tv. NaAlO 2 + 2H 2 O

Zn (OH) 2 + 2NaOH solution → Na 2 Al (OH) 3 + NaOH solution → Na sodium tetrahydroxoaluminate Al (OH) 3 + 3NaOH solution → Na 3 sodium hexahydroxoaluminate.

It turns out sodium tetrahydroxoaluminate or sodium hexahydroxoaluminate depends on how much alkali we took. In the last alkali reaction, a lot is taken and sodium hexahydroxoaluminate is formed.

Elements that form amphoteric compounds may themselves exhibit amphoteric properties.

Zn + 2NaOH + 2H 2 O → Na 2 + H 2 (sodium tetrahydroxozincate)

2Al + 2NaOH + 6H 2 O → 2Na + 3H 2 ((sodium tetrahydroxoaluminate)

Zn + H 2 SO 4 (decomposed) → ZnSO 4 + H 2

2Al + 3H 2 SO 4 (diff.) → Al 2 (SO 4) 3 + 3H 2

Recall that amphoteric hydroxides are insoluble bases. And when heated, they decompose, forming oxide and water.

Decomposition of amphoteric bases on heating.

2Al(OH) 3 Al 2 O 3 + 3H 2 O

Zn(OH) 2 ZnO + H 2 O

Summing up the lesson.

You learned the properties of amphoteric oxides and hydroxides. These substances have amphoteric (dual) properties. The chemical reactions that take place with them have features. You have looked at examples of amphoteric oxides and hydroxides .

1. Rudzitis G.E. Inorganic and organic chemistry. Grade 8: textbook for educational institutions: basic level / G. E. Rudzitis, F.G. Feldman.M.: Enlightenment. 2011 176 pp.: ill.

2. Popel P.P. Chemistry: 8th class: a textbook for general educational institutions / P.P. Popel, L.S. Krivlya. -K.: IC "Academy", 2008.-240 p.: ill.

3. Gabrielyan O.S. Chemistry. Grade 9 Textbook. Publisher: Drofa.: 2001. 224s.

1. No. 6,10 (p. 130) Rudzitis G.E. Inorganic and organic chemistry. Grade 9: textbook for educational institutions: basic level / G. E. Rudzitis, F.G. Feldman.M.: Enlightenment. 2008 170s.: ill.

2. Write the formula of sodium hexahydroxoaluminate. How is this substance obtained?

3. A solution of sodium hydroxide was gradually added to a solution of aluminum sulfate to an excess. What did you observe? Write reaction equations.