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
ZnO + 2HCl -- > ZnCl2 + H2O
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).
They also contain higher proportion of O2 than expected. But they do not liberate H2O2 with acid.
Ex. NO2, SO2
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