Oxides are binary compounds formed when elements combine with oxygen. Binary compounds are compounds containing two elements only. Oxides are classified into several groups, namely, acidic oxides, basic oxides, amphoteric oxides, neutral oxides and higher oxides.
1) Acidic oxides: These are oxides formed between a non-metal and oxygen.
They react with water to form acids.
With alkalis, they form a salt and water only.
Nitrogen (IV) oxide forms two salts with sodium hydroxide, since it is a mixed anhydride.
2) Basic oxides: These are oxides formed between a metal and oxygen.
They react with acids to form salt and water only.
Most basic oxides are not soluble in water, but those that do, form alkalis, e.g.
3) Amphoteric oxides: Amphoteric oxides are oxides of metals which can behave both as basic oxides and as acidic oxides, i.e. they can react with both acids and alkalis to form salts and water only. The behaviour of amphoteric oxides is determined by the conditions of the reaction.
With acids, amphoteric oxides behave as bases, forming a salt and water only.
With alkalis, they behave like acids and exhibit acidic properties by forming complex salts.
Amphoteric oxides include the oxides of aluminium, zinc, lead and tin.
4) Neutral oxides: These are oxides which show neither acidic nor basic properties. Hence, they are neutral to litmus. Carbon (II) oxide, CO, dinitrogen (I) oxide, N2O and water, H2O are examples of neutral oxides.
5) Higher oxides: Higher oxides are oxides containing a higher proportion of oxygen than the ordinary oxides. These higher oxides are classified into three groups namely the peroxides, the dioxides and the mixed oxides.
a) Peroxides: They contain the peroxide ion, (O – O)2- and react with dilute acid to yield hydrogen peroxide, H2O2. Examples of peroxides inlcude sodium peroxide, Na2O2, potassium peroxide, K2O2, calcium peroxide, CaO2 and barium peroxide, BaO2.
Peroxides are generally unstable, e.g. sodium peroxide readily reacts with moisture from air to give sodium hydroxide and oxygen.
b) Dioxides: These are higher oxides which do not produce hydrogen peroxide, when they react with dilute acid. They release oxygen on heating and hence, they are oxidising agents.
Examples are lead (IV) oxide, PbO2and manganese (IV) oxide; MnO2.
c) Mixed oxides: These are higher oxides commonly found in nature. For example, trilead tetraoxide is prepared by heating lead (II) oxide for some time, in the presence of oxygen at a temperature of 400°C.
This compound, Pb3O4, in many chemical reactions acts as if it consists of lead (II) oxide and lead (IV) oxide. For example, action of dilute trioxonitrate (V) acid on trilead tetraoxide.
Compare the equation with:
or, in the action of heat:
Other example of mixed oxides is Fe3O4.