Amphoteric oxides (having dual properties) are in most cases metal oxides that have low electronegativity. Depending on external conditions, they exhibit either acidic or oxide properties. These oxides are formed which usually exhibit the following oxidation states: ll, lll, lV.

Examples of amphoteric oxides: zinc oxide (ZnO), chromium oxide lll (Cr2O3), aluminum oxide (Al2O3), tin oxide lll (SnO), tin oxide lV (SnO2), lead oxide lll (PbO), lead oxide lV (PbO2) , titanium oxide lV (TiO2), manganese oxide lV (MnO2), iron oxide lll (Fe2O3), beryllium oxide (BeO).

Reactions characteristic of amphoteric oxides:

1. These oxides can react with strong acids. In this case, salts of the same acids are formed. Reactions of this type are a manifestation of the properties of the basic type. For example: ZnO (zinc oxide) + H2SO4 (hydrochloric acid) → ZnSO4 + H2O (water).

2. When interacting with strong alkalis, amphoteric oxides and hydroxides exhibit dual properties (that is, amphotericity) manifested in the formation of two salts.

In the melt, when reacting with an alkali, an average ordinary salt is formed, for example:
ZnO (zinc oxide) + 2NaOH (sodium hydroxide) → Na2ZnO2 (common salt) + H2O (water).
Al2O3 (aluminum oxide) + 2NaOH (sodium hydroxide) = 2NaAlO2 + H2O (water).
2Al(OH)3 (aluminum hydroxide) + 3SO3 (sulfur oxide) = Al2(SO4)3 (aluminum sulfate) + 3H2O (water).

In solution, amphoteric oxides react with alkali to form a complex salt, for example: Al2O3 (aluminum oxide) + 2NaOH (sodium hydroxide) + 3H2O (water) + 2Na(Al(OH)4) (sodium tetrahydroxyaluminate complex salt).

3. Each metal of any amphoteric oxide has its own coordination number. For example: for zinc (Zn) - 4, for aluminum (Al) - 4 or 6, for chromium (Cr) - 4 (rare) or 6.

4. Amphoteric oxide does not react with water and does not dissolve in it.

What reactions prove that a metal is amphoteric?

Relatively speaking, an amphoteric element can exhibit properties of both metals and non-metals. Similar characteristic feature present in A-group elements: Be (beryllium), Ga (gallium), Ge (germanium), Sn (tin), Pb, Sb (antimony), Bi (bismuth) and some others, as well as many B-group elements - these are Cr (chromium), Mn (manganese), Fe (iron), Zn (zinc), Cd (cadmium) and others.

Let's prove it as follows: chemical reactions amphotericity of the chemical element zinc (Zn):

1. Zn(OH)2 + N2O5 (dianitrogen pentoxide) = Zn(NO3)2 (zinc nitrate) + H2O (water).
ZnO (zinc oxide) + 2HNO3 = Zn(NO3)2 (zinc nitrate) + H2O (water).

b) Zn(OH)2 (zinc hydroxide) + Na2O (sodium oxide) = Na2ZnO2 (sodium dioxocincate) + H2O (water).
ZnO (zinc oxide) + 2NaOH (sodium hydroxide) = Na2ZnO2 (sodium dioxinate) + H2O (water).

In the event that an element with dual properties in a compound has the following oxidation states, its dual (amphoteric) properties are most noticeably manifested in the intermediate stage of oxidation.

An example is chromium (Cr). This element has the following oxidation states: 3+, 2+, 6+. In the case of +3, the basic and acidic properties are expressed to approximately the same extent, while in Cr +2 the basic properties predominate, and in Cr +6 the acidic properties predominate. Here are the reactions that prove this statement:

Cr+2 → CrO (chromium oxide +2), Cr(OH)2 → CrSO4;
Cr+3 → Cr2O3 (chromium oxide +3), Cr(OH)3 (chromium hydroxide) → KCrO2 or chromium sulfate Cr2(SO4)3;
Cr+6 → CrO3 (chromium oxide +6), H2CrO4 → K2CrO4.

In most cases, amphoteric oxides chemical elements with oxidation state +3 exist in meta form. As an example, we can cite: aluminum metahydroxide (chemical formula AlO(OH) and iron metahydroxide (chemical formula FeO(OH)).

How are amphoteric oxides prepared?

1. The most convenient method for their preparation is precipitation from an aqueous solution using ammonia hydrate, that is, a weak base. For example:
Al(NO3)3 (aluminum nitrate) + 3(H2OxNH3) (aqueous hydrate) = Al(OH)3 (amphoteric oxide) + 3NH4NO3 (reaction performed at twenty degrees Celsius).
Al(NO3)3 (aluminum nitrate) + 3(H2OxNH3) (aqueous ammonium hydrate) = AlO(OH) (amphoteric oxide) + 3NH4NO3 + H2O (reaction carried out at 80 °C)

Moreover, in an exchange reaction of this type, in the case of an excess of alkalis, no alkalis will be deposited. This is due to the fact that aluminum becomes an anion due to its dual properties: Al(OH)3 (aluminum hydroxide) + OH− (excess alkalis) = − (aluminum hydroxide anion).

Examples of reactions of this type:
Al(NO3)3 (aluminum nitrate) + 4NaOH(excess sodium hydroxide) = 3NaNO3 + Na(Al(OH)4).
ZnSO4 (zinc sulfate) + 4NaOH (excess sodium hydroxide) = Na2SO4 + Na2(Zn(OH)4).

The salts that are formed in this case belong to They include the following complex anions: (Al(OH)4)− and also (Zn(OH)4)2−. This is what these salts are called: Na(Al(OH)4) - sodium tetrahydroxoaluminate, Na2(Zn(OH)4) - sodium tetrahydroxozincate. The products of the interaction of aluminum or zinc oxides with solid alkali are called differently: NaAlO2 - sodium dioxoaluminate and Na2ZnO2 - sodium dioxoaluminate.

Recovery processes

Zinc condensation

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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 (sol.) + 2NaOH (sol.) (t, fusion) → 2H 2 O + Na 2 BeO 2 (sodium metaberylate, or beryllate)

Pb(OH) 2(solv.) + 2NaOH (solv.) (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”:

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, just like 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 (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 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.

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 must 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:

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 it! 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.

Zinc - element side subgroup the second group, the fourth period of the periodic system of chemical elements by D.I. Mendeleev, with atomic number 30. Denoted by the symbol Zn (lat. Zincum). The simple substance zinc normal conditions- brittle, bluish transition metal white(fades in air, becoming covered with a thin layer of zinc oxide).

In the fourth period, zinc is the last d element, its valence electrons 3d 10 4s 2 . In education chemical bonds only electrons from the outer energy level are involved, since the d 10 configuration is very stable. In compounds, zinc has an oxidation state of +2.

Zinc is a chemically active metal, has pronounced reducing properties, and is inferior in activity to alkaline earth metals. Exhibits amphoteric properties.

Interaction of zinc with nonmetals
When heated strongly in air, it burns with a bright bluish flame to form zinc oxide:
2Zn + O 2 → 2ZnO.

When ignited, it reacts vigorously with sulfur:
Zn + S → ZnS.

Reacts with halogens under normal conditions in the presence of water vapor as a catalyst:
Zn + Cl 2 → ZnCl 2 .

When phosphorus vapor acts on zinc, phosphides are formed:
Zn + 2P → ZnP 2 or 3Zn + 2P → Zn 3 P 2.

Zinc does not interact with hydrogen, nitrogen, boron, silicon, or carbon.

Interaction of zinc with water
Reacts with water vapor at red heat to form zinc oxide and hydrogen:
Zn + H 2 O → ZnO + H 2 .

Interaction of zinc with acids
In the electrochemical voltage series of metals, zinc is located before hydrogen and displaces it from non-oxidizing acids:
Zn + 2HCl → ZnCl 2 + H 2 ;
Zn + H 2 SO 4 → ZnSO 4 + H 2 .

Reacts with dilute nitric acid to form zinc nitrate and ammonium nitrate:
4Zn + 10HNO 3 → 4Zn(NO 3) 2 + NH 4 NO 3 + 3H 2 O.

Reacts with concentrated sulfur and nitric acids with the formation of zinc salt and acid reduction products:
Zn + 2H 2 SO 4 → ZnSO 4 + SO 2 + 2H 2 O;
Zn + 4HNO 3 → Zn(NO 3) 2 + 2NO 2 + 2H 2 O

Interaction of zinc with alkalis
Reacts with alkali solutions to form hydroxo complexes:
Zn + 2NaOH + 2H 2 O → Na 2 + H 2

when fused, it forms zincates:
Zn + 2KOH → K 2 ZnO 2 + H 2 .

Interaction with ammonia
With gaseous ammonia at 550–600°C it forms zinc nitride:
3Zn + 2NH 3 → Zn 3 N 2 + 3H 2;
dissolves in aqueous solution ammonia, forming tetraamminium zinc hydroxide:
Zn + 4NH 3 + 2H 2 O → (OH) 2 + H 2 .

Interaction of zinc with oxides and salts
Zinc displaces metals located in the voltage series to the right of it from solutions of salts and oxides:
Zn + CuSO 4 → Cu + ZnSO 4 ;
Zn + CuO → Cu + ZnO.

Zinc(II) oxide ZnO – white crystals, when heated they acquire a yellow color. Density 5.7 g/cm 3, sublimation temperature 1800°C. At temperatures above 1000°C it is reduced to metallic zinc by carbon, carbon monoxide and hydrogen:
ZnO + C → Zn + CO;
ZnO + CO → Zn + CO 2;
ZnO + H 2 → Zn + H 2 O.

Does not interact with water. Shows amphoteric properties, reacts with solutions of acids and alkalis:
ZnO + 2HCl → ZnCl 2 + H 2 O;
ZnO + 2NaOH + H 2 O → Na 2.

When fused with metal oxides, it forms zincates:
ZnO + CoO → CoZnO 2 .

When interacting with non-metal oxides, it forms salts, where it is a cation:
2ZnO + SiO 2 → Zn 2 SiO 4,
ZnO + B 2 O 3 → Zn(BO 2) 2.

Zinc (II) hydroxide Zn(OH) 2 – a colorless crystalline or amorphous substance. Density 3.05 g/cm 3, decomposes at temperatures above 125°C:
Zn(OH) 2 → ZnO + H 2 O.

Zinc hydroxide exhibits amphoteric properties and is easily soluble in acids and alkalis:
Zn(OH) 2 + H 2 SO 4 → ZnSO 4 + 2H 2 O;
Zn(OH) 2 + 2NaOH → Na 2;

also easily dissolves in an aqueous solution of ammonia to form tetraamminium zinc hydroxide:
Zn(OH) 2 + 4NH 3 → (OH) 2.

It is obtained in the form of a white precipitate when zinc salts react with alkalis:
ZnCl 2 + 2NaOH → Zn(OH) 2 + 2NaCl.