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

Topic: Introduction

Lesson: Amphoteric oxides and hydroxides

Rice. 1. Substances exhibiting amphoteric properties

Basic oxides react with acidic oxides, and acidic oxides react 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 given in Table 1.

Let us consider the amphoteric properties of zinc and aluminum oxides. Using 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. Exhibits the properties of a 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 reactions considered occur when heated, during fusion. If we take solutions of substances, the reactions will proceed somewhat differently.

ZnO + 2NaOH + H 2 O → Na 2 (sodium tetrahydroxoaluminate) 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. Aluminum Oxide Minerals

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

Reactions of zinc and aluminum hydroxides with sodium oxide occur during fusion, because these hydroxides are solid and are not part of 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 are characterized by their acidic properties. These reactions can be carried out both by fusion of solids and in solutions. But this will result in different substances, i.e. The reaction products depend on the reaction conditions: in a melt or in a solution.

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

Al(OH) 3 + NaOH solid. 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.

Whether it turns out to be sodium tetrahydroxoaluminate or sodium hexahydroxoaluminate depends on how much alkali we took. In the last reaction, a lot of alkali 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 (diluted) → ZnSO 4 + H 2

2Al + 3H 2 SO 4 (dil.) → 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 upon 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 occur with them have their own characteristics. You have looked at examples of amphoteric oxides and hydroxides .

1. Rudzitis G.E. Inorganic and organic chemistry. 8th grade: textbook for general education institutions: basic level / G. E. Rudzitis, F.G. Feldman.M.: Enlightenment. 2011 176 p.: ill.

2. Popel P.P. Chemistry: 8th grade: textbook for general education institutions / P.P. Popel, L.S. Krivlya. -K.: IC “Academy”, 2008.-240 p.: ill.

3. Gabrielyan O.S. Chemistry. 9th grade. Textbook. Publisher: Bustard: 2001. 224s.

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

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

3. Sodium hydroxide solution was gradually added to the aluminum sulfate solution until there was excess. What did you observe? Write the reaction equations.

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 produced by the combustion reaction of the corresponding metal in oxygen, for example:

2Al + 3/2O2 = Al2O3

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

ZnSO 4 + NaOH = Zn(OH) 2 + Na 2 SO 4

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

ZnSO 4 + 4NaOH excess = 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 = R-CH(NH 3 + Cl -) = 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 = 2AlCl 3 + 3H 2 O

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

Zn(OH) 2 + NaOH= Na 2

NH 2 –CH 2 -COOH + HCl = Cl

Specific properties of amphoteric organic compounds

When amino acids are dissolved in water, the amino group and the carboxyl group react 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, a water molecule is split off and a product is formed in which fragments of the molecule are linked to each other by a peptide bond (-CO-NH-). For example:

Also, amino acids have all the chemical properties of carboxylic acids (by the carboxyl group) and amines (by 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
Solution	a) 2Al + 6H 2 O = 2Al(OH) 3 + 3H 2 Al(OH) 3 + 3HCl = AlCl 3 + 3H 2 O AlCl 3 + 4NaOH ex = Na + 3NaCl b) 2Al + 3/2O 2 = Al 2 O 3 Al 2 O 3 + NaOH+ 3H 2 O= 2Na 2Na + H 2 SO 4 = 2Al(OH) 3 + Na 2 SO 4 + 2H 2 O 2Al(OH) 3 = Al 2 O 3 + 3H 2 O 2Al 2 O 3 = 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 12% alkali solution will be required for this?
Solution	Let's write the reaction equation: NH 2 –CH 2 -COOH + NaOH= NH 2 –CH 2 -COONa + H 2 O Let's calculate the mass of the acid that reacted: m(NH 2 –CH 2 -COOH) = ώ k - you ×m p - pa m(NH 2 –CH 2 -COOH) = 0.05 × 150 = 7.5 g

The following oxides of elements are amphoteric main 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 main 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 character of the oxides and hydroxides of elements of the same 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 element A1 is in the form of a cation;

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

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

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

A1(OH) 3 + NaOH = 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 manner, meaning the product formed during the dehydration of salt.

In the chemical literature you can find different formulas of compounds formed when aluminum hydroxide is dissolved in alkali: NaA1O 2 (sodium metaaluminate), Na sodium tetrahydroxyaluminate. 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 notation for Na. When A1(OH) 3 is dissolved in excess alkali, sodium tetrahydroxyaluminate is formed:

A1(OH) 3 + NaOH = Na.

When the reagents are sintered, sodium metaaluminate is formed:

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

Thus, we can say that in aqueous solutions there are simultaneously ions such 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 hydration shell), 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 using an ammonia solution:

A1 2 (SO 4) 3 + 6 NH 3 H 2 O = 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 = BeC1 2 + 2H 2 O.

With reasons:

Be(OH) 2 + 2NaOH = Na 2 (sodium tetrahydroxoberyllate).

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

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

beryllate Na

Hydroxides of elements of side subgroups, corresponding to higher oxidation states, most often have acidic properties: for example, Mn 2 O 7 - HMnO 4; CrO 3 – H 2 CrO 4. Lower oxides and hydroxides are characterized by a predominance of basic properties: CrO – Cr(OH) 2; МnО – 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 О 3 – Fe(OH) 3. Let us illustrate this pattern using 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 chromium element 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

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

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

Na hexahydroxochromate(III)

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

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 element zinc is part of the anion. When interacting with alkalis in solutions tetrahydroxycinates are formed, during fusion– zincates:

Zn(OH) 2 + 2NaOH = Na 2.

Or when fusing:

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

Zinc hydroxide is prepared similarly to aluminum hydroxide.

Amphoteric metals are represented by non-complex elements, which are a kind of analogue of a group of metal-type components. The similarity can be seen in a number of physical and chemical properties. Moreover, the substances themselves have not been shown to exhibit amphoteric properties, while various compounds are quite capable of exhibiting them.

For example, we can consider hydroxides with oxides. They clearly have a dual chemical nature. It is expressed in the fact that, depending on the conditions, the above-mentioned compounds can have the properties of either alkalis or acids. The concept of amphotericity appeared quite a long time ago; it has been familiar to science since 1814. The term "amphotericity" expressed the ability of a chemical substance to behave in a certain way when carrying out an acidic (main) reaction. The resulting properties depend on the type of reagents present, the type of solvent, and the conditions under which the reaction is carried out.

What are amphoteric metals?

The list of amphoteric metals includes many items. Some of them can be confidently called amphoteric, some - presumably, others - conditionally. If we consider the issue on a large scale, then for brevity we can simply name the serial numbers of the above mentioned metals. These numbers are: 4.13, from 22 to 32, from 40 to 51, from 72 to 84, from 104 to 109. But there are metals that can be called basic. These include chromium, iron, aluminum and zinc. Strontium and beryllium complete the main group. The most common of all listed at the moment is aluminum. Its alloys have been used for many centuries in a wide variety of fields and applications. The metal has excellent anti-corrosion resistance and is easy to cast and various types of machining. In addition, the popularity of aluminum is complemented by such advantages as high thermal conductivity and good electrical conductivity.

Aluminum is an amphoteric metal, which tends to exhibit chemical activity. The durability of this metal is determined by a strong oxide film and, under normal environmental conditions, during chemical reactions, aluminum acts as a reducing element. Such an amphoteric substance is capable of interacting with oxygen in the event of fragmentation of the metal into small particles. Such interaction requires the influence of high temperature conditions. A chemical reaction upon contact with an oxygen mass is accompanied by a huge release of thermal energy. At temperatures above 200 degrees, the interaction of reactions when combined with a substance such as sulfur forms aluminum sulfide. Amphoteric aluminum is not able to directly interact with hydrogen, and when this metal is mixed with other metal components, various alloys containing intermetallic compounds arise.

Iron is an amphoteric metal, which is one of the side subgroups of group 4 of the period in the system of elements of the chemical type. This element stands out as the most common component of the group of metallic substances in the components of the earth's crust. Iron is classified as a simple substance, among the distinctive properties of which are its malleability and silvery-white color. Such a metal has the ability to provoke an increased chemical reaction and quickly goes into the stage of corrosion when exposed to high temperatures. Iron placed in pure oxygen burns out completely, and when brought to a finely dispersed state it can spontaneously ignite in plain air. When exposed to air, a metallic substance quickly oxidizes due to excessive humidity, that is, it rusts. When burning in an oxygen mass, a kind of scale is formed, which is called iron oxide.

Basic properties of amphoteric metals

Properties of amphoteric metals is a basic concept in amphotericity. Let's look at what they are. In the standard state, every metal is a solid. Therefore, they are considered to be weak electrolytes. In addition, no metal can dissolve in water. Bases are obtained through a special reaction. During this reaction, the metal salt is combined with a small dose of alkali. The rules require the entire process to be carried out carefully, carefully and rather slowly.

When amphoteric substances combine with acidic oxides or acids themselves, the former give a reaction characteristic of bases. If such bases are combined with bases, the properties of acids appear. Strong heating of amphoteric hydroxides leads to their decomposition. As a result of decomposition, water and the corresponding amphoteric oxide are formed. As can be seen from the examples given, the properties are quite extensive and require careful analysis, which can be carried out during chemical reactions.

The chemical properties of amphoteric metals can be compared to those of regular metals to draw parallels or see differences. All metals have a fairly low ionization potential, due to which they act as reducing agents in chemical reactions. It is also worth noting that the electronegativity of non-metals is higher than that of metals.

Amphoteric metals exhibit both reducing and oxidizing properties. But at the same time, amphoteric metals have compounds characterized by a negative oxidation state. All metals have the ability to form basic hydroxides and oxides. Depending on the increase in the serial number in the periodic ranking, a decrease in the basicity of the metal was observed. It should also be noted that metals, for the most part, can only be oxidized by certain acids. Thus, metals react differently with nitric acid.

Amphoteric non-metals, which are simple substances, have a clear difference in their structure and individual characteristics regarding physical and chemical manifestations. The type of some of these substances is easy to determine visually. For example, copper is a simple amphoteric metal, while bromine is classified as a non-metal.

In order not to be mistaken in determining the variety of simple substances, it is necessary to clearly know all the signs that distinguish metals from non-metals. The main difference between metals and non-metals is the ability of the former to donate electrons located in the external energy sector. Nonmetals, on the contrary, attract electrons to the external energy storage zone. All metals have the property of transmitting energetic brilliance, which makes them good conductors of thermal and electrical energy, while non-metals cannot be used as a conductor of electricity and heat.

Amphoteric compounds

Chemistry is always a unity of opposites.

Look at the periodic table.

Some elements (almost all metals exhibiting oxidation states +1 and +2) form basic oxides and hydroxides. For example, potassium forms the oxide K 2 O, and the hydroxide KOH. They exhibit basic properties, such as interacting with acids.

K2O + HCl → KCl + H2O

Some elements (most nonmetals and metals with oxidation states +5, +6, +7) form acidic oxides and hydroxides. Acid hydroxides are oxygen-containing acids, they are called hydroxides because they have a hydroxyl group in their structure, for example, sulfur forms acid oxide SO 3 and acid hydroxide H 2 SO 4 (sulfuric acid):

Such compounds exhibit acidic properties, for example they react with bases:

H2SO4 + 2KOH → K2SO4 + 2H2O

And there are elements that form oxides and hydroxides that exhibit both acidic and basic properties. This phenomenon is called amphoteric . It is these oxides and hydroxides that will focus our attention in this article. All amphoteric oxides and hydroxides are solids insoluble in water.

First, how can we determine whether an oxide or hydroxide is amphoteric? There is a rule, a little arbitrary, but you can still use it:

Amphoteric hydroxides and oxides are formed by metals in oxidation states +3 and +4, For example (Al 2 O 3 , Al(OH) 3 , Fe 2 O 3 , Fe(OH) 3)

And four exceptions:metalsZn , Be , Pb , Sn form the following oxides and hydroxides:ZnO , Zn ( OH ) 2 , BeO , Be ( OH ) 2 , PbO , Pb ( OH ) 2 , SnO , Sn ( OH ) 2 , in which they exhibit an oxidation state of +2, but despite this, these compounds exhibit amphoteric properties .

The most common amphoteric oxides (and their corresponding hydroxides): ZnO, Zn(OH) 2, BeO, Be(OH) 2, PbO, Pb(OH) 2, SnO, Sn(OH) 2, Al 2 O 3, Al (OH) 3, Fe 2 O 3, Fe(OH) 3, Cr 2 O 3, Cr(OH) 3.

The properties of amphoteric compounds are not difficult to remember: they interact with acids and alkalis.

• When interacting with acids, everything is simple; in these reactions, amphoteric compounds behave like basic ones:

Al 2 O 3 + 6HCl → 2AlCl 3 + 3H 2 O

ZnO + H 2 SO 4 → ZnSO 4 + H 2 O

BeO + HNO 3 → Be(NO 3 ) 2 + H 2 O

Hydroxides react in the same way:

Fe(OH) 3 + 3HCl → FeCl 3 + 3H 2 O

Pb(OH) 2 + 2HCl → PbCl 2 + 2H 2 O

• Interacting with alkalis is a little more complicated. In these reactions, amphoteric compounds behave like acids, and the reaction products can be different, depending on the conditions.

Either the reaction occurs in solution, or the reacting substances are taken as solids and fused.

Interaction of basic compounds with amphoteric ones during fusion.

Let's look at the example of zinc hydroxide. As mentioned earlier, amphoteric compounds interact with basic compounds and behave like acids. So let’s write zinc hydroxide Zn (OH) 2 as an acid. The acid has hydrogen in front, let's take it out: H 2 ZnO 2 . And the reaction of the alkali with the hydroxide will proceed as if it were an acid. “Acid residue” ZnO 2 2-divalent:

2K OH(TV) + H 2 ZnO 2(solid) (t, fusion)→ K 2 ZnO 2 + 2 H 2 O

The resulting substance K 2 ZnO 2 is called potassium metazincate (or simply potassium zincate). This substance is a salt of potassium and the hypothetical “zinc acid” H 2 ZnO 2 (it is not entirely correct to call such compounds salts, but for our own convenience we will forget about that). Just write zinc hydroxide like this: H 2 ZnO 2 - not good. We write Zn (OH) 2 as usual, but we mean (for our own convenience) that it is an “acid”:

2KOH (solid) + Zn (OH) 2(solid) (t, fusion) → K 2 ZnO 2 + 2H 2 O

With hydroxides, which have 2 OH groups, everything will be the same as with zinc:

Be(OH) 2(tv.) + 2NaOH (tv.) (t, fusion)→ 2H 2 O + Na 2 BeO 2 (sodium metaberyllate, or beryllate)

Pb(OH) 2 (sol.) + 2NaOH (sol.) (t, fusion) → 2H 2 O + Na 2 PbO 2 (sodium metaplumbate, or plumbate)

With amphoteric hydroxides with three OH groups (Al (OH) 3, Cr (OH) 3, Fe (OH) 3) it is a little different.

Let's look at the example of aluminum hydroxide: Al (OH) 3, write it in the form of an acid: H 3 AlO 3, but we don’t leave it in this form, but take the water out of there:

H 3 AlO 3 – H 2 O → HAlO 2 + H 2 O.

It is this “acid” (HAlO 2) that we work with:

HAlO 2 + KOH → H 2 O + KAlO 2 (potassium metaaluminate, or simply aluminate)

But aluminum hydroxide cannot be written like this HAlO 2, we write it as usual, but we mean “acid” there:

Al(OH) 3(solv.) + KOH (solv.) (t, fusion)→ 2H 2 O + KAlO 2 (potassium metaaluminate)

The same goes for chromium hydroxide:

Cr(OH) 3 → H 3 CrO 3 → HCrO 2

Cr(OH) 3(tv.) + KOH (tv.) (t, fusion)→ 2H 2 O + KCrO 2 (potassium metachromate,

BUT NOT CHROMATE, chromates are salts of chromic acid).

It’s the same with hydroxides containing four OH groups: we move hydrogen forward and remove water:

Sn(OH) 4 → H 4 SnO 4 → H 2 SnO 3

Pb(OH) 4 → H 4 PbO 4 → H 2 PbO 3

It should be remembered that lead and tin each form two amphoteric hydroxides: with an oxidation state of +2 (Sn (OH) 2, Pb (OH) 2), and +4 (Sn (OH) 4, Pb (OH) 4).

And these hydroxides will form different “salts”:

Oxidation state
Hydroxide formula	Sn(OH)2	Pb(OH)2	Sn(OH)4	Pb(OH)4
Formula of hydroxide as acid	H2SnO2	H2PbO2	H2SnO3	H2PbO3
Salt (potassium)	K2SnO2	K2PbO2	K2SNO3	K2PbO3
Name of salt			metastannAT	metablumbAT

The same principles as in the names of ordinary “salts”, the element in the highest oxidation state is the suffix AT, in the intermediate - IT.

Such “salts” (metachromates, metaaluminates, metaberyllates, metazincates, etc.) are obtained not only as a result of the interaction of alkalis and amphoteric hydroxides. These compounds are always formed when a strongly basic “world” and an amphoteric one (during fusion) come into contact. That is, in the same way as amphoteric hydroxides, amphoteric oxides and metal salts that form amphoteric oxides (salts of weak acids) will react with alkalis. And instead of an alkali, you can take a strong basic oxide and a salt of the metal that forms the alkali (a salt of a weak acid).

Interactions:

Remember, the reactions below occur during fusion.

Amphoteric oxide with strong basic oxide:

ZnO (solid) + K 2 O (solid) (t, fusion) → K 2 ZnO 2 (potassium metazincate, or simply potassium zincate)

Amphoteric oxide with alkali:

ZnO (solid) + 2KOH (solid) (t, fusion) → K 2 ZnO 2 + H 2 O

Amphoteric oxide with a salt of a weak acid and a metal that forms an alkali:

ZnO (sol.) + K 2 CO 3 (sol.) (t, fusion) → K 2 ZnO 2 + CO 2

Amphoteric hydroxide with strong basic oxide:

Zn(OH) 2 (solid) + K 2 O (solid) (t, fusion) → K 2 ZnO 2 + H 2 O

Amphoteric hydroxide with alkali:

Zn (OH) 2 (solid) + 2KOH (solid) (t, fusion) → K 2 ZnO 2 + 2H 2 O

Amphoteric hydroxide with a salt of a weak acid and a metal that forms an alkali:

Zn (OH) 2(tv.) + K 2 CO 3(tv.) (t, fusion)→ K 2 ZnO 2 + CO 2 + H 2 O

Salts of a weak acid and a metal forming an amphoteric compound with a strong basic oxide:

ZnCO 3 (solid) + K 2 O (solid) (t, fusion) → K 2 ZnO 2 + CO 2

Salts of a weak acid and a metal that forms an amphoteric compound with an alkali:

ZnCO 3 (solid) + 2KOH (solid) (t, fusion) → K 2 ZnO 2 + CO 2 + H 2 O

Salts of a weak acid and a metal forming an amphoteric compound with a salt of a weak acid and a metal forming an alkali:

ZnCO 3(tv.) + K 2 CO 3(tv.) (t, fusion)→ K 2 ZnO 2 + 2CO 2

Below is information on salts of amphoteric hydroxides; the most common ones in the Unified State Examination are marked in red.

	Hydroxide	Hydroxide as acid	Acid residue		Name of salt
BeO	Be(OH) 2	H 2 BeO 2	BeO 2 2-	K 2 BeO 2	Metaberyllate (beryllate)
ZnO	Zn(OH) 2	H 2 ZnO 2	ZnO 2 2-	K 2 ZnO 2	Metazincate (zincate)
Al 2 O 3	Al(OH) 3	HAlO 2	AlO 2 —	KAlO 2	Metaaluminate (aluminate)
Fe2O3	Fe(OH) 3	HFeO2	FeO2 -	KFeO2	Metaferrate (BUT NOT FERRATE)
	Sn(OH)2	H2SnO2	SnO 2 2-	K2SnO2
	Pb(OH)2	H2PbO2	PbO 2 2-	K2PbO2
SnO2	Sn(OH)4	H2SnO3	SnO 3 2-	K2SNO3	MetastannAT (stannate)
PbO2	Pb(OH)4	H2PbO3	PbO 3 2-	K2PbO3	MetablumAT (plumbat)
Cr2O3	Cr(OH)3	HCrO2	CrO2 -	KCrO2	Metachromat (BUT NOT CHROMATE)

Interaction of amphoteric compounds with solutions of ALKALI (here only alkalis).

In the Unified State Examination this is called “dissolution of aluminum hydroxide (zinc, beryllium, etc.) with alkali.” This is due to the ability of metals in the composition of amphoteric hydroxides in the presence of an excess of hydroxide ions (in an alkaline medium) to attach these ions to themselves. A particle is formed with a metal (aluminum, beryllium, etc.) in the center, which is surrounded by hydroxide ions. This particle becomes negatively charged (anion) due to hydroxide ions, and this ion will be called hydroxoaluminate, hydroxyzincate, hydroxoberyllate, etc. Moreover, the process can proceed in different ways; the metal can be surrounded by a different number of hydroxide ions.

We will consider two cases: when the metal is surrounded four hydroxide ions, and when it's surrounded six hydroxide ions.

Let us write down the abbreviated ionic equation for these processes:

Al(OH) 3 + OH — → Al(OH) 4 —

The resulting ion is called Tetrahydroxoaluminate ion. The prefix “tetra-” is added because there are four hydroxide ions. The tetrahydroxyaluminate ion has a charge -, since aluminum carries a charge of 3+, and four hydroxide ions have a charge of 4-, the total is -.

Al(OH) 3 + 3OH - → Al(OH) 6 3-

The ion formed in this reaction is called hexahydroxoaluminate ion. The prefix “hexo-” is added because there are six hydroxide ions.

It is necessary to add a prefix indicating the number of hydroxide ions. Because if you simply write “hydroxyaluminate”, it is not clear which ion you mean: Al (OH) 4 - or Al (OH) 6 3-.

When an alkali reacts with an amphoteric hydroxide, a salt is formed in the solution. The cation of which is an alkali cation, and the anion is a complex ion, the formation of which we discussed earlier. The anion is square brackets.

Al(OH)3 + KOH → K (potassium tetrahydroxoaluminate)

Al (OH) 3 + 3KOH → K 3 (potassium hexahydroxoaluminate)

What kind of salt (hexa- or tetra-) you write as a product does not matter. Even in the Unified State Examination answers it is written: “... K 3 (the formation of K is permissible." The main thing is not to forget to ensure that all indices are entered correctly. Keep track of the charges, and keep in mind that their sum should be equal to zero.

In addition to amphoteric hydroxides, amphoteric oxides react with alkalis. The product will be the same. Only if you write the reaction like this:

Al 2 O 3 + NaOH → Na

Al 2 O 3 + NaOH → Na 3

But these reactions will not be equalized for you. You need to add water to the left side, because the interaction occurs in solution, there is enough water there, and everything will equalize:

Al 2 O 3 + 2NaOH + 3H 2 O → 2Na

Al 2 O 3 + 6NaOH + 3H 2 O → 2Na 3

In addition to amphoteric oxides and hydroxides, some particularly active metals that form amphoteric compounds interact with alkali solutions. Namely this: aluminum, zinc and beryllium. To equalize, water is also needed on the left. And, in addition, the main difference between these processes is the release of hydrogen:

2Al + 2NaOH + 6H 2 O → 2Na + 3H 2

2Al + 6NaOH + 6H 2 O → 2Na 3 + 3H 2

The table below shows the most common examples of the properties of amphoteric compounds in the Unified State Examination:

Amphoteric substance		Name of salt
Al2O3 Al(OH) 3		Sodium tetrahydroxyaluminate	Al(OH) 3 + NaOH → Na Al 2 O 3 + 2NaOH + 3H 2 O → 2Na 2Al + 2NaOH + 6H 2 O → 2Na + 3H 2
	Na 3	Sodium hexahydroxyaluminate	Al(OH) 3 + 3NaOH → Na 3 Al 2 O 3 + 6NaOH + 3H 2 O → 2Na 3 2Al + 6NaOH + 6H 2 O → 2Na 3 + 3H 2
Zn(OH)2	K2	Sodium tetrahydroxozincate	Zn(OH) 2 + 2NaOH → Na 2 ZnO + 2NaOH + H 2 O → Na 2 Zn + 2NaOH + 2H 2 O → Na 2 +H 2
	K 4	Sodium hexahydroxozincate	Zn(OH) 2 + 4NaOH → Na 4 ZnO + 4NaOH + H 2 O → Na 4 Zn + 4NaOH + 2H 2 O → Na 4 +H 2
Be(OH)2	Li 2	Lithium tetrahydroxoberyllate	Be(OH) 2 + 2LiOH → Li 2 BeO + 2LiOH + H 2 O → Li 2 Be + 2LiOH + 2H 2 O → Li 2 +H 2
	Li 4	Lithium hexahydroxoberyllate	Be(OH) 2 + 4LiOH → Li 4 BeO + 4LiOH + H 2 O → Li 4 Be + 4LiOH + 2H 2 O → Li 4 +H 2
Cr2O3 Cr(OH)3		Sodium tetrahydroxochromate	Cr(OH) 3 + NaOH → Na Cr 2 O 3 + 2NaOH + 3H 2 O → 2Na
	Na 3	Sodium hexahydroxochromate	Cr(OH) 3 + 3NaOH → Na 3 Cr 2 O 3 + 6NaOH + 3H 2 O → 2Na 3
Fe2O3 Fe(OH) 3		Sodium tetrahydroxoferrate	Fe(OH) 3 + NaOH → Na Fe 2 O 3 + 2NaOH + 3H 2 O → 2Na
	Na 3	Sodium hexahydroxoferrate	Fe(OH) 3 + 3NaOH → Na 3 Fe 2 O 3 + 6NaOH + 3H 2 O → 2Na 3

The salts obtained in these interactions react with acids, forming two other salts (salts of a given acid and two metals):

2Na 3 + 6H 2 SO 4 → 3Na 2 SO 4 + Al 2 (SO 4 ) 3 +12H 2 O

That's all! Nothing complicated. The main thing is not to confuse, remember what is formed during fusion and what is in solution. Very often, assignments on this issue come across B parts.