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Chapter: 11th 12th std standard Class Organic Inorganic Physical Chemistry Higher secondary school College Notes

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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.

 

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 O2- 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 O22-. The metal belonging to the group I and II (Na2O2, BaO2) contain O22- 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

 

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