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Group 16: Oxygen - Simple Oxides

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Group 16: Oxygen - Simple Oxides - Lesson Summary

A binary compound of oxygen with another element is called an oxide. On the basis of their oxygen content, oxides can be broadly classified into simple oxides and mixed oxides.


Simple oxides:
Simple oxides are oxides that contain just as much oxygen as permitted by the normal valency of its metal.

EX: H2O, MgO & Al2O3.


Mixed oxides:
Mixed oxides are oxides that are made up of two simpler oxides.

Ex: Red lead (Pb3O4) is a mixed oxide of lead monoxide (PbO) &lead dioxide (PbO2).

Ferroso-ferric oxide (Fe3O4) is a mixed oxide of ferrous oxide (FeO) and ferric oxide (Fe2O3).

On the basis of their chemical behaviour, simple oxides can be further classified into acidic oxides, basic oxides, amphoteric oxides and neutral oxides.


Acidic oxide:
An oxide that combines with water to give an acid is termed an acidic oxide. Ex: Oxides of non-metals, such as CO2, SO2, SO3, P2O5, Cl2O7 & N2O5, or metallic oxides of high oxidation states, such as Mn2O7, CrO3 &V2O5 are acidic nature.

When sulphur dioxide dissolves in water to give sulphurous acid.

         SO2              +  H2O      →     H2SO3
Sulphur dioxide          Water            Sulphurous acid

When chromic anhydride combines with water give chromic acid.

         Cr2O3                +  H2O        →      H2Cr2O4
Chromic anhydride          Water            Chromic acid

Basic oxide:
An oxide that gives a base with water is known as a basic oxide.

EX: Oxides of most metals, such asNa2O, CaO, BaO, are basic in nature.

When calcium oxide combines with water to give calcium hydroxide, which is a base.

      CaO             +      H2O        → Ca(OH)2
Calcium oxide           Water             Calcium hydroxide

Amphoteric oxides:
A few metallic oxides exhibit dual behaviour. They show the characteristics of both acidic as well as basic oxides. Such oxides are known as amphoteric oxides. They react with acids as well as alkalis.

EX: Zinc oxide behaves as an acidic oxide when treated with concentrated sodium hydroxide, and as a basic oxide when treated with hydrochloric acid.

     ZnO          +   2H2O    +  2NaOH           →  Na3Zn[OH]4     +       H2
Zinc oxide          Water         NaOH(conc)          Sodium Zincate   Hydrogen

      ZnO       +      2HCl                    →         ZnCl2      +        H2O
Zinc oxide        Hydrochloric acid             Zinc chloride         Water

Aluminium oxide reacts with acids as well as alkalis.

Al2O3 (s) +  6HCl (aq) + 9H2O (l)  → 2[Al(H2O)6]3+ (aq) + 6Cl- (aq)
Al2O3 (s) + 6NaOH (aq) + 3H2O (l) → 2Na3[Al(OH)6] (aq)

Neutral oxides:
These oxides do not show any tendency to form salts when treated with acids or bases.

EX: Carbon monoxide and nitrous oxide are neutral oxides.

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