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. chemical reactions that flow with them have singularities. 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.

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.

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 main 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 non-metals and metals with oxidation states +5, +6, +7) form acidic oxides and hydroxides. Acid hydroxides are oxygen-containing acids, they are called hydroxides because there is a hydroxyl group in the 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 such oxides and hydroxides that exhibit both acidic and basic properties. This phenomenon is called amphoteric . Such oxides and hydroxides will be the focus of our attention in this article. All amphoteric oxides and hydroxides - solids, insoluble in water.

First, how do you determine if an oxide or hydroxide is amphoteric? There is a rule, a little conditional, 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.

• with interaction 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

• With interaction with alkalis it is a little more difficult. In these reactions, amphoteric compounds behave like acids, and the reaction products can be different, it all depends on the conditions.

Either the reaction takes place in solution, or the reactants are taken as solids and fused.

Interaction of basic compounds with amphoteric compounds during fusion.

Let's take zinc hydroxide as an example. As mentioned earlier, amphoteric compounds interacting with basic ones behave like acids. So we 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 alkali with 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 it). Only zinc hydroxide is written like this: H 2 ZnO 2 is not good. We write as usual Zn (OH) 2, but we mean (for our own convenience) that this is an "acid":

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

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

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

Pb (OH) 2 (solid.) + 2NaOH (solid.) (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) a little differently.

Let's take aluminum hydroxide as an example: 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 out the water from there:

H 3 AlO 3 - H 2 O → HAlO 2 + H 2 O.

Here we are working with this “acid” (HAlO 2):

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 down as usual, but we mean “acid” there:

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

The same is true for chromium hydroxide:

Cr(OH) 3 → H 3 CrO 3 → HCrO 2

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

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

With hydroxides containing four OH groups, it is exactly the same: we bring 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 form two amphoteric hydroxides each: 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	K 2 PbO 2	K2SnO3	K2PbO3
Salt name			metastannat	metablumbAT

The same principles as in the names of ordinary "salts", the element in the highest degree oxidation - 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 (by fusion) come into contact. That is, just like amphoteric hydroxides with alkalis, both amphoteric oxides and metal salts forming amphoteric oxides (salts of weak acids) will react. And instead of alkali, you can take a strongly basic oxide, and a salt of a metal that forms an alkali (salt of a weak acid).

Interactions:

Remember, the reactions below take place during fusion.

Amphoteric oxide with strongly 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 an alkali-forming metal:

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

Amphoteric hydroxide with strongly 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 an alkali-forming metal:

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

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

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

Below is information on salts of amphoteric hydroxides, the most common in the exam are marked in red.

	Hydroxide	Acid hydroxide	acid residue		Salt name
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	HFeO 2	FeO 2 -	KFeO 2	Metaferrate (BUT NOT FERRATE)
	Sn(OH)2	H2SnO2	SnO 2 2-	K2SnO2
	Pb(OH)2	H2PbO2	PbO 2 2-	K 2 PbO 2
SnO 2	Sn(OH)4	H2SnO3	SnO 3 2-	K2SnO3	MetastannAT (stannate)
PbO2	Pb(OH)4	H2PbO3	PbO 3 2-	K2PbO3	MetablumbAT (plumbat)
Cr2O3	Cr(OH)3	HCrO 2	CrO2 -	KCrO 2	Metachromat (BUT NOT CHROMATE)

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

In the Unified State Examination, this is called "the dissolution of aluminum hydroxide (zinc, beryllium, etc.) 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, hydroxozincate, 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 is surrounded six hydroxide ions.

Let's write down the abbreviated ionic equation these processes:

Al(OH) 3 + OH - → Al(OH) 4 -

The resulting ion is called the tetrahydroxoaluminate ion. The prefix "tetra" is added because there are four hydroxide ions. The tetrahydroxoaluminate ion has a - charge, since aluminum carries a 3+ charge, and four hydroxide ions 4-, in total it turns out -.

Al (OH) 3 + 3OH - → Al (OH) 6 3-

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

It is necessary to add a prefix indicating the amount of hydroxide ions. Because if you just write "hydroxoaluminate", it is not clear which ion you mean: Al (OH) 4 - or Al (OH) 6 3-.

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

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

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

What exactly (hexa- or tetra-) salt you write as a product does not matter. Even in the USE answers it is written: “... K 3 (the formation of K is acceptable". The main thing is not to forget to make sure that all indices are correctly affixed. 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 equalize. It is necessary to add water to the left side, because 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 especially interact with alkali solutions. active metals which form amphoteric compounds. Namely, it is: aluminum, zinc and beryllium. To equalize, the left also needs water. 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 exam:

Amphoteric substance		Salt name
Al2O3 Al(OH)3		Sodium tetrahydroxoaluminate	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 hexahydroxoaluminate	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
	K4	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, what is in solution. Very often, tasks on this issue come across in B parts.

Before discussing the chemical properties of bases and amphoteric hydroxides, let's clearly define what it is?

1) Bases or basic hydroxides include metal hydroxides in the oxidation state +1 or +2, i.e. the formulas of which are written either as MeOH or as Me(OH) 2 . However, there are exceptions. So, the hydroxides Zn (OH) 2, Be (OH) 2, Pb (OH) 2, Sn (OH) 2 do not belong to the bases.

2) Amphoteric hydroxides include metal hydroxides in the oxidation state +3, +4, and, as exceptions, hydroxides Zn (OH) 2, Be (OH) 2, Pb (OH) 2, Sn (OH) 2. Metal hydroxides in the oxidation state +4, in USE assignments do not meet, therefore will not be considered.

Chemical properties of bases

All bases are divided into:

Recall that beryllium and magnesium are not alkaline earth metals.

In addition to being soluble in water, alkalis also dissociate very well in aqueous solutions, while insoluble bases have low degree dissociation.

This difference in solubility and ability to dissociate between alkalis and insoluble hydroxides leads, in turn, to noticeable differences in their chemical properties. So, in particular, alkalis are more chemically active compounds and are often capable of entering into those reactions that insoluble bases do not enter into.

Reaction of bases with acids

Alkalis react with absolutely all acids, even very weak and insoluble ones. For example:

Insoluble bases react with almost all soluble acids, do not react with insoluble silicic acid:

It should be noted that both strong and weak bases with general formula species Me (OH) 2 can form basic salts with a lack of acid, for example:

Interaction with acid oxides

Alkalis react with all acidic oxides to form salts and often water:

Insoluble bases are able to react with all higher acid oxides corresponding to stable acids, for example, P 2 O 5, SO 3, N 2 O 5, with the formation of medium salts:

Insoluble bases of the form Me (OH) 2 react in the presence of water with carbon dioxide exclusively with the formation of basic salts. For example:

Cu(OH) 2 + CO 2 = (CuOH) 2 CO 3 + H 2 O

With silicon dioxide, due to its exceptional inertness, only the most strong bases- alkalis. In this case, normal salts are formed. The reaction does not proceed with insoluble bases. For example:

Interaction of bases with amphoteric oxides and hydroxides

All alkalis react with amphoteric oxides and hydroxides. If the reaction is carried out by fusing an amphoteric oxide or hydroxide with a solid alkali, such a reaction leads to the formation of hydrogen-free salts:

If aqueous solutions of alkalis are used, then hydroxo complex salts are formed:

In the case of aluminum, under the action of an excess of concentrated alkali, instead of the Na salt, the Na 3 salt is formed:

The interaction of bases with salts

Any base reacts with any salt only if two conditions are met simultaneously:

1) solubility of starting compounds;

2) the presence of a precipitate or gas among the reaction products

For example:

Thermal stability of bases

All alkalis, except Ca(OH) 2 , are resistant to heat and melt without decomposition.

All insoluble bases, as well as slightly soluble Ca (OH) 2, decompose when heated. The highest decomposition temperature for calcium hydroxide is about 1000 o C:

Insoluble hydroxides have much lower decomposition temperatures. So, for example, copper (II) hydroxide decomposes already at temperatures above 70 o C:

Chemical properties of amphoteric hydroxides

Interaction of amphoteric hydroxides with acids

Amphoteric hydroxides react with strong acids:

Amphoteric metal hydroxides in the +3 oxidation state, i.e. type Me (OH) 3, do not react with acids such as H 2 S, H 2 SO 3 and H 2 CO 3 due to the fact that salts that could be formed as a result of such reactions are subject to irreversible hydrolysis to the original amphoteric hydroxide and corresponding acid:

Interaction of amphoteric hydroxides with acid oxides

Amphoteric hydroxides react with higher oxides, which correspond to stable acids (SO 3, P 2 O 5, N 2 O 5):

Amphoteric metal hydroxides in the +3 oxidation state, i.e. type Me (OH) 3, do not react with acid oxides SO 2 and CO 2.

Interaction of amphoteric hydroxides with bases

Of the bases, amphoteric hydroxides react only with alkalis. However, if used water solution alkalis, then hydroxo complex salts are formed:

And when amphoteric hydroxides are fused with solid alkalis, their anhydrous analogues are obtained:

Interaction of amphoteric hydroxides with basic oxides

Amphoteric hydroxides react when fused with oxides of alkali and alkaline earth metals:

Thermal decomposition of amphoteric hydroxides

All amphoteric hydroxides are insoluble in water and, like any insoluble hydroxides, decompose when heated to the corresponding oxide and water.

Amphoteric metals are simple substances, which are structurally, chemically and similar to the metal group of elements. By themselves, metals cannot exhibit amphoteric properties, as opposed to their compounds. For example, the oxides and hydroxides of some metals have a dual chemical nature - in some conditions they behave like acids, while in others they have the properties of alkalis.

The main amphoteric metals are aluminum, zinc, chromium, and iron. Beryllium and strontium can be attributed to the same group of elements.

amphoteric?

For the first time this property was discovered quite a long time ago. And the term "amphoteric elements" was introduced into science in 1814 by the famous chemists L. Tenard and J. Gay-Lussac. In those times chemical compounds it was customary to divide into groups that corresponded to their main properties during the reactions.

However, the group of oxides and bases had dual abilities. Under some conditions, such substances behaved like alkalis, while in others, on the contrary, they acted like acids. This is how the term "amphoteric" was born. For such, the behavior during the acid-base reaction depends on the conditions of its implementation, the nature of the reagents involved, and also on the properties of the solvent.

Interestingly, under natural conditions, amphoteric metals can interact with both alkali and acid. For example, during the reaction of aluminum with aluminum sulfate is formed. And when the same metal reacts with concentrated alkali, a complex salt is formed.

Amphoteric bases and their main properties

Under normal conditions, these are solids. They are practically insoluble in water and are considered rather weak electrolytes.

The main method for obtaining such bases is the reaction of a metal salt with a small amount of alkali. The precipitation reaction must be carried out slowly and carefully. For example, when receiving zinc hydroxide, caustic soda is carefully added in drops to a test tube with zinc chloride. Each time you need to gently shake the container to see the white precipitate of metal at the bottom of the dish.

With acids and amphoteric substances react as bases. For example, when zinc hydroxide reacts with hydrochloric acid zinc chloride is formed.

But during reactions with bases, amphoteric bases behave like acids.

In addition, with strong heating, they decompose with the formation of the corresponding amphoteric oxide and water.

The most common amphoteric metals are: a brief description of

Zinc belongs to the group of amphoteric elements. And although alloys of this substance were widely used even in ancient civilizations, they were able to isolate it in its pure form only in 1746.

Pure metal is a rather brittle bluish substance. Zinc rapidly oxidizes in air - its surface tarnishes and becomes covered with a thin film of oxide.

In nature, zinc exists mainly in the form of minerals - zincites, smithsonites, calamites. The most famous substance is zinc blende, which consists of zinc sulfide. Most large deposits of this mineral are found in Bolivia and Australia.

Aluminum Today it is considered the most common metal on the planet. Its alloys have been used for many centuries, and in 1825 the substance was isolated in its pure form.

Pure aluminum is a light, silver-colored metal. It is easy to machine and cast. This element has high electrical and thermal conductivity. In addition, this metal is resistant to corrosion. The fact is that its surface is covered with a thin, but very resistant oxide film.

Today, aluminum is widely used in industry.