Zno decomposition. Amphoteric oxides

Amphoteric oxides (having dual properties) are in most cases metal oxides that have a small 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 ll (SnO), tin oxide lV (SnO2), lead oxide ll (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 main type. For example: ZnO (zinc oxide) + H2SO4 (hydrochloric acid) → ZnSO4 + H2O (water).

2. When interacting with strong alkalis, amphoteric oxides and hydroxides exhibit. At the same time, the duality of properties (that is, amphotericity) manifests itself in the formation of two salts.

In the melt, when reacting with alkali, an average salt is formed, for example:
ZnO (zinc oxide) + 2NaOH (sodium hydroxide) → Na2ZnO2 (common average 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 tetrahydroxoaluminate 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 (rarely) or 6.

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

What reactions prove the amphoteric nature of a metal?

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

Let us prove the amphotericity of the chemical element zinc (Zn) by the following chemical reactions:

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 dioxozincate) + H2O (water).
ZnO (zinc oxide) + 2NaOH (sodium hydroxide) = Na2ZnO2 (sodium dioxozincate) + H2O (water).

In the event that an element with dual properties in the compound has the following oxidation states, its dual (amphoteric) properties are most noticeable 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, basic and acidic properties are expressed approximately to the same extent, while in Cr +2, basic properties predominate, and in Cr +6, acidic ones. 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 of chemical elements with an oxidation state of +3 exist in the meta form. As an example, one can cite: aluminum metahydroxide (chemical formula AlO (OH) and iron metahydroxide (chemical formula FeO (OH)).

How are amphoteric oxides obtained?

1. The most convenient method of obtaining them is to precipitate from an aqueous solution using ammonia hydrate, that is, a weak base. For example:
Al (NO3) 3 (aluminum nitrate) + 3 (H2OxNH3) (aqueous hydrate) \u003d Al (OH) 3 (amphoteric oxide) + 3NH4NO3 (the reaction is performed at twenty degrees of heat).
Al(NO3)3 (aluminum nitrate) + 3(H2OxNH3) (ammonia hydrate aqueous solution) = AlO(OH) (amphoteric oxide) + 3NH4NO3 + H2O (reaction carried out at 80 °C)

In this case, in an exchange reaction of this type, in the case of an excess of alkalis, it will not precipitate. This is due to the fact that aluminum becomes an anion due to its dual properties: Al (OH) 3 (aluminum hydroxide) + OH- (excess alkali) = - (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 how 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 dioxozincate.

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 are 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 of 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 us write down the abbreviated ionic equation of 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 active metals interact with alkali solutions, 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.

Zinc is an element of a side subgroup of the second group, the fourth period of the periodic system of chemical elements of D. I. Mendeleev, with atomic number 30. It is denoted by the symbol Zn (lat. Zincum). A simple substance zinc under normal conditions is a brittle transition metal of a bluish-white color (it tarnishes 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 . Only electrons of the outer energy level participate in the formation of chemical bonds, since the d 10 configuration is very stable. In compounds, zinc has an oxidation state of +2.

Zinc is a reactive metal, has pronounced reducing properties, is inferior in activity to alkaline earth metals. Shows amphoteric properties.

Interaction of zinc with non-metals
When strongly heated in air, it burns with a bright bluish flame to form zinc oxide:
2Zn + O2 → 2ZnO.

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

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

Under the action of phosphorus vapor 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, 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.

The interaction of zinc with acids
In the electrochemical series of voltages of metals, zinc is 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 sulfuric and nitric acids to form a 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 an aqueous solution of ammonia, forming tetraamminzinc hydroxide:
Zn + 4NH 3 + 2H 2 O → (OH) 2 + H 2.

Interaction of zinc with oxides and salts
Zinc displaces metals in the stress row 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, 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 with 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, at temperatures above 125 ° C decomposes:
Zn(OH) 2 → ZnO + H 2 O.

Zinc hydroxide exhibits amphoteric properties, 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 readily soluble in aqueous ammonia to form tetraamminzinc 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.

Both main stages of pyrometallurgical processes - reduction with distillation and condensation of zinc - are of both theoretical and practical interest.

Recovery processes

Restoration is subjected to zinc agglomerate, which contains free oxide, ferrites, silicates and zinc aluminates, zinc sulfide and sulfate, and in addition, oxides and ferrites of other metals.
The processes of reduction of metal oxides proceed both in the solid phase (retorts and shaft furnaces) and in the liquid phase (electric furnaces). Reducing agents can be solid carbon, carbon monoxide, hydrogen and metallic iron. The most important are carbon monoxide CO and metallic iron.
There are two theories of the reduction of metal oxides with carbon monoxide "two-stage" A.A. Baikov and "adsorption-catalytic" G.I. Chufarov.
According to the first theory, the oxides first dissociate into metal and oxygen according to the reaction 2MeO=2Me+O2, and then the released oxygen combines with the reducing agent according to the equation O2+2CO=2CO2. Depending on the temperature, the oxide dissociation product can be a solid, liquid, or gaseous metal. Both stages of recovery proceed independently and tend to equilibrium. The overall result of the reactions depends on the conditions under which they take place.
A more modern theory of G.I. Chufarov assumes three stages of reduction: the adsorption of the reducing gas on the surface of the oxide, the reduction process itself, and the removal of the gaseous product from the reaction surface. In general, this theory can be described by the following equations:

It should be noted that according to both theories, the total reaction, expressing the stoichiometric ratio of the interacting substances, is the same:

Let us consider the behavior of individual components during the reduction of zinc agglomerate.
Zinc compounds. The agglomerate may contain ZnO, ZnO*Fe2O3, ZnO*SiO2, ZnO*Al2O3, ZnSO4 and ZnS.
Zinc oxide, depending on the conditions of heat treatment of the charge and its composition, can be reduced by various reducing agents.
Hydrogen, methane, and various hydrocarbons are formed in the wet mixture as a result of water decomposition and the release of volatile coal. Hydrogen and methane reduce ZnO by the reactions

The beginning of recovery is already noticeable at 450-550°. These reactions are not essential and proceed only in the initial period of distillation in horizontal retorts.
At temperatures above 600°, direct reduction of zinc oxide with solid carbon is possible. 2ZnO+G⇔2Zn+CO2. The intensity of the reaction is limited by the limited rate of diffusion of solids and, therefore, is of little practical importance. Above 1000°, the main reaction of zinc oxide reduction with carbon monoxide ZnO+CO⇔Zn+CO2 proceeds. The equilibrium constant of this reaction, under the condition of obtaining one metallic zinc only in the vapor state, can be found from the equation

It follows from the equation that the direction of flow depends on the ratio of the concentrations of CO and CO2 in the gas phase, which is determined by the well-known Boudouard curve. On fig. 12 shows a possible composition of the gas phase in the muffle of a distillation furnace. Above 1000°, carbon dioxide cannot exist in the presence of carbon and reacts with the latter in the reaction CO2+C=2CO.

Thus, for the successful reduction of ZnO with carbon monoxide, it is necessary to create favorable conditions for the occurrence of two reactions: ZnO + CO⇔Zn + CO2 and CO2 + C⇔2CO, namely: to have a high process temperature (at least 1000 °), a large excess of the reducing agent in charge and gas permeability of the charge sufficient for fast removal of gases and vapors of zinc.
When the reduction takes place in the melt at 1300-1400 ° (zinc electrothermal), the interaction of zinc oxide with metallic iron by the reaction ZnO + Fe = Zn + FeO becomes of great importance. Due to the possibility of this reaction, it is possible to obtain a high degree of sublimation of zinc and liquid slags with a low metal content . At the same time, the course of this reaction in horizontal retorts is undesirable due to the possible formation of low-melting ferrous compounds (matte and slag), which destroy the walls of the muffles.
Zinc ferrite at temperatures below 900° and with a lack of carbon is reduced with the formation of structurally free ZnO and Fe3O4. Under these conditions, ferrite can also be decomposed by oxides of other metals. At high temperatures, the reduction process proceeds rapidly with the formation of metallic zinc, metallic iron or ferrous oxide. The reduction of zinc ferrite does not cause any special difficulties in the practice of distillation.
Zinc silicates are also easily reduced by carbon and metallic iron. At a temperature of 1100-1200°, zinc is completely reduced from silicates.
Zinc aluminates or spinels are very refractory compounds. Unlike silicates, they are not reduced in retort furnaces.
Zinc sulfate, present in the agglomerate in small quantities, is reduced by carbon and carbon monoxide to sulfide and dissociates with the release of sulfur dioxide, while the following reactions occur:

The formation of zinc sulfide according to the last reaction occurs in the gas phase.
Zinc sulfide is practically not reduced during distillation in retorts and passes into rimming. In an electric furnace bath, zinc sulfide can be decomposed by iron at 1250-1300° by the reaction ZnS+Fe=Zn+FeS.
Compounds of lead and cadmium. In the agglomerate, lead is in the form of oxidized compounds: free oxide, silicates, ferrites, and partially in the form of sulfate. Lead from these compounds is easily reduced to metallic lead and sublimes to some extent, contaminating liquid zinc. The amount of sublimated lead depends on the process temperature. In retorts, the bulk of the lead remains in the rim. In shaft furnaces and electric furnaces, where the process temperature is higher, most of the lead is converted to zinc. The increased content of lead in the agglomerate has a destructive effect on the walls of the retorts. Therefore, it is necessary to increase the amount of coal in the charge to absorb molten lead.
Cadmium oxide is reduced at a temperature lower than zinc oxide. The vapor pressure of this metal is higher than that of zinc. In a batch process, cadmium sublimates at the beginning of distillation, so the first portions of condensed zinc are enriched in cadmium.
Impurities of lead and cadmium reduce the grade of finished zinc.
Arsenic and antimony compounds. Arsenic and antimony, due to their volatility, like lead and cadmium, pollute distillation products. The higher oxides As2Os and Sb2O5, arsenates and antimonates are reduced by carbon to the lower volatile oxides As2O3, Sb2O3 and to the metallic state. Some of them are trapped in the condenser along with zinc.
Copper compounds are easily reduced by carbonaceous reducing agents, but remain in solid or liquid distillation residues. If there is a certain amount of sulfur in the charge, copper passes into matte. In the absence of sulfur, copper forms cuprous cast iron with iron, significant quantities of which are obtained in electric furnaces.
iron compounds. The behavior of oxidized iron compounds in the reduction process is determined by the process conditions, temperature, and composition of the gas phase. In retorts and electric furnaces, a lot of metallic iron is obtained. In the shaft furnace, iron oxide is reduced to oxide and passes into slag.
Gold and silver do not sublimate under normal conditions and remain, depending on the nature of the process, in rimming or are distributed between cast iron, matte and slag. When chloride salts are added to the charge, part of the noble metals is sublimated and condensed in the distillation products.
Rare and scattered elements. In a reducing environment at high temperatures, most of the thallium, indium and selenium sublimes. Up to half of germanium and tellurium also passes into sublimates. A significant part of gallium remains in the distillation residues.
Silica, alumina, alkali metal oxides and sulfates interact with other charge compounds and form slag.

zinc condensation

The main difficulty in the practical implementation of the zinc vapor condensation process is that a significant part of the metal passes not into a liquid phase, but into a solid one, having the form of dust particles separated by oxide films. Therefore, the output of pig zinc does not exceed 70-75%.
The dependence of zinc vapor pressure on temperature, studied by K. Mayer, is shown by the curve in fig. 13. Above the curve lies the area of ​​supersaturated, and below - unsaturated vapors. The dew point of zinc vapor without admixture of other gases at a pressure of 1 atm is 906°. In practice, in the gases of muffle, electric and shaft furnaces, where zinc vapors are diluted with CO and CO2, the partial pressure of zinc vapors does not reach 0.5 atm. In retort gases in the initial period of distillation, it is about 300 mm Hg, and in the top gases of a shaft furnace - only 30-40 mm Hg. Art. Condensation of zinc from these gases will begin at temperatures of 820–830 and 650–660°, respectively.
For complete condensation, it is necessary that the temperature of the gases at the outlet of the condenser be close to the melting point of zinc, at which the equilibrium value of the vapor pressure is minimal. In practice, condensation ends at 500°. Under these conditions, the loss of zinc vapor with gases emitted into the atmosphere is approximately 0.4%.

However, compliance with the temperature regime in itself does not guarantee the receipt of all zinc in liquid form, and part of it, as mentioned above, is obtained in the form of dust. This is explained by various reasons. It has been noticed that the condensation of zinc vapors into the liquid phase proceeds more successfully on the convex surface of solids with a small radius of curvature and on surfaces wetted with liquid zinc. For successful condensation, it is also necessary that the ratio of the condenser surface to its volume does not exceed a certain value. Due to the fact that condensation begins mainly on the walls, it is necessary to ensure a certain residence time of the gases in the condenser and not to allow them to cool too abruptly. With a significant volume of gases saturated with zinc vapor, it is impossible to ensure effective condensation without special measures. which include bubbling gases through a bath of zinc and spraying them with molten zinc and lead.
The chemical conditions of condensation are also important. At a high content of CO2 in gases, the surface of the droplets is oxidized. Zinc, which prevents them from merging into a compact mass.
Thus, the rate and completeness of zinc vapor condensation are affected by: the partial pressure of zinc vapor, temperature, the speed of the gas mixture (no more than 5 cm / s), the presence of other gases and mechanical suspensions, the shape, size and material of the capacitor.

17.12.2019

The Far Cry series continues to please its players with stability. For so much time it becomes clear what you need to do in this game. Hunting, survival, capture...

16.12.2019

When creating the design of a living space, special attention should be paid to the interior of the living room - it will become the center of your "universe"....

15.12.2019

It is impossible to imagine building a house without the use of scaffolding. In other areas of economic activity, such structures are also used. FROM...

14.12.2019

As a method of permanent connection of metal products, welding appeared a little over a century ago. At the same time, its importance cannot be overestimated at the moment. IN...

14.12.2019

Optimizing the space around is extremely important for both small and large warehouses. This greatly simplifies the work and provides ...