3. Oxides

Oxides - complex substances, consisting of two elements, one of them is Oxygen. Practically all elements (excepting three noble gases - He, Ar and Ne) may form oxides.

Classification of oxides:

Non-Salifiables	non-salts forming (CO, N2O, NO)
Salifiables	salts forming, subdivided into:
	Basic - it is a metal oxides in which metals display low oxidation number +1, +2 (Na2O, MgO, CuO);
	Amphoteric (for some metals with oxidation number +3, +4, sometimes +2) - ZnO, Al2O3, Cr2O3, SnO, PbO, MnO2;
	Acidic - it is oxides of non-metals and some metals with oxidation number from +5 to +7 (SO2, SO3, P2O5, Mn2O7, CrO3).

Preparation

1. Interaction of simple and complex substances with Oxygen:

2Mg + O₂ = 2MgO;

4Al + 3O₂ = 2Al₂O₃;

S + O₂ = SO₂;

2H₂S + 3O₂ = 2SO₂ + 2H₂O;

2C₂H₂ + 5O₂ = 4CO₂ + 2H₂O;

4FeS + 7O₂ = 2Fe₂O₃ + 4SO₂.

2. Decomposition of some substances containing Oxygen (oxides, bases, acids, salts) on heating:

4CrO₃ 2Cr₂O₃ + 3O₂;

Cu(OH)₂ CuO + H₂O;

2Fe(OH)₃ Fe₂O₃ + 3H₂O;

H₂CO₃ CO₂ + H₂O;

H₂SiO₃ SiO₂ + H₂O;

CaCO₃ CaO + CO₂;

2AgNO₃ 2Ag + 2NO₂ + O₂.

Chemical properties

Basic oxides

1. Interaction with water. Oxides of alkaline (Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O) and alkaline-earth (CaO, SrO, BaO) metals form soluble bases - alkalis:

Li₂O + H₂O = 2LiOH;

Rb₂O + H₂O = 2RbOH;

CaO + H₂O = Ca(OH)_2;

BaO + H₂O = Ba(OH)₂.

2. Interaction with acidic oxides forming the salts:

K₂O + SO₂ = K₂SO₃;

SrO + N₂O₅ = Sr(NO₃)₂;

CuO + SO₃ = CuSO₄;

3BaO + P₂O₅ = Ba₃(PO₄)₂.

3. Interaction with acids forming the salts and water:

Na₂O + 2HNO₃ = 2NaNO₃ + H₂O

NiO + H₂SO₄ = NiSO₄ + H₂O

3MgO + 2H₃PO₄ = Mg₃(PO₄)₂ + 3H₂O.

Other basic oxides do not react with water directly.

Acidic oxides

1. The most of acidic oxides reacts with water forming the acids (except a few oxides like SiO₂, MoO₃, WO₃):

SO₃ + H₂O = H₂SO₄;

N₂O₅ + H₂O = 2HNO₃;

P₂O₅ + 3H₂O = 2H₃PO₄;

4NO₂ + O₂ + 2H₂O = 4HNO₃.

2. Interaction with basic acids with forming the salts:

N₂O₃ + Na₂O = 2NaNO₂;

P₂O₅ + 3CaO = 2Ca₃(PO₄)₂;

SO₃ + BaO = BaSO₄.

3. Interaction with bases forming the salts and water:

SO₃ + Ba(OН)₂ = BaSO₄ + H₂O;

N₂O₅ + 2КOН = 2KNO₃ + H₂O;

SiO₂+2NaOН = Na₂SiO₃ + H₂O.

Amphoteric oxides

Term Amphoteric (from Greek) means double-sided, mutual. Therefore amphoteric oxides occupy a position between basic and acidic oxides and display corresponding chemical properties.

1. Interaction with basic and acidic oxides forming the salts:

PbO + SO₃ = PbSO₄;

PbO + N₂O₅ = Pb(NO₃)₂;

Cr₂O₃ + 3SO₃ = Cr₂(SO₄)₃;

Cr₂O₃ + 3N₂O₅ = 2Cr(NO₃)₃;

Li₂O + PbO = Li₂PbO₂;

CaO + PbO = CaPbO₂;

3Li₂O + Cr₂O₃ = 2Li₃CrO₃;

Li₂O + Cr₂O₃ = 2LiCrO₂;

CaO + Cr₂O₃ = Ca₃(CrO₃)₂;

CaO + Cr₂O₃ = Ca(CrO₂)₂.

2. Interaction with acids forming the salts and water:

PbO+ 2 HCl = PbCl₂ + H₂O;

Al₂O₃ + 2 H₃PO₄ = 2AlPO₄ + 3H₂O.

3. Interaction with alkalis forming the salts and water:

Cr₂O₃ + 6KOH = 2K₃CrO₃ + 3H₂O;

MnO₂ + 2NaOH = Na₂MnO₃ + H₂O.

Peroxides may be shown as a peculiar group of Oxygen-contained compounds. Frequently peroxides are considered as salts of Hydrogen peroxide H₂O₂, where O₂^-2 (-O-O-) is an analogue of acid residue. For example, BaO₂ - Barium peroxide includes Ba²⁺ and formally O^-.