Oxygen: Oxides Classification

Oxides

Generally all the elements react with dioxygen to form oxides. Oxides are binary compounds of oxygen. Oxides may be classified depending on their structure (or) their chemical properties.

i)  Acidic oxides

The oxides of non-metals are usually covalent and acidic. They have low melting and boiling points, though some B2O3 and SiO2 form infinite "giant molecules" and have high melting points. They are all acidic. Some oxides dissolve in water and thus forming acids. Hence they are called as acid anhydrides

B₂O₃ + 3H₂O -- > 2H₃BO₃

N2O5 + H2O -- > 2HNO3

P4O10 + 6H2O -- > 4H3PO4

SO3 + H2O -- > H2SO4

others which do not react with water such as SiO2 reacts with NaOH and shows acidic properties.

ii) Basic oxides

Metallic oxides are generally basic. Most metal oxides are ionic and contain the O^2- ion. Some oxides dissolve in water and form alkaline solution.

Na₂O + H₂O  -- >  2NaOH

BaO + H2O  -- > Ba(OH)2

Many metal oxides with formula M2O3 and MO2, though ionic, do not react with water.

Examples : Tl₂O₃, Bi₂O₃ and ThO₂.

These oxides react with water to form salts and hence they are bases.

CaO + 2HCl  -- > CaCl₂ + H₂O

If a metal exists in more than one oxidation state and they form more than one oxide

eg. CrO, Cr2O3, CrO3, PbO, PbO2

iii) Amphoteric oxides

The oxides which react with both strong acids and strong bases are called as amphoteric oxides.

ZnO + 2NaOH  -- > Na2ZnO2 + H2O

Sodium zincate

ZnO + 2HCl  -- > ZnCl2 + H2O

iv) Peroxides

These oxides contain more oxygen than would be expelled from the oxidation number of M. Some are ionic and contains the peroxide ion O₂^2-. The metal belonging to the group I and II (Na₂O₂, BaO₂) contain O₂^2- ion. Others are covalently bound and contain -O-O- in the structure.

Oxides such as PbO₂ react with acids liberate Cl₂

PbO₂ + 4HCl  -- > PbCl₂ + 2H₂O + Cl₂

v) Compound oxides

Some oxides behave as if they are compounds of the two oxides.

Ex. Ferrous-ferric oxide (Fe₃O₄). This is considered to be the mixture of FeO and Fe₂O₃.

They react with acids and forms a mixture of ferrous and ferric salts.

Fe₃O₄ + 8HCl  -- >  FeCl₂ + 2FeCl₃ + 4H₂O

vi) Neutral oxides

A few covalent oxides have no acidic (or) basic properties (N₂O, NO, CO).

vii) Dioxides

They also contain higher proportion of O₂ than expected. But they do not liberate H₂O₂ with acid.

Ex. NO₂, SO₂