In the last section, we have learnt how chemical reactions have been classified into various types on the basis of the nature of chemical change that occurs in them. In this section we shall learn about some other features of chemical reactions. These features have been discussed below.

Homogeneous-hetrogeneous reacctions

Chemical reactions can be classified on the basis of physical states of reactants and products as homogeneous and heterogeneous reactions.

a) Homogeneous reactions

The reactions in which all the reactants and products are present in the same phase are called homogeneous reactions. Such reactions can occur in gas phase or solution phase only.

A. Gas phase homogeneous reactions

These are the reactions in which all reactants and products are gases.

( i ) H2(g)	+	Cl2(g)	—>	2HCl(g)
hydrogen		chlorine		hydrogen chloride

( ii ) 2SO2(g)	+	O2(g)	—>	2SO3(g)
sulphur dioxide		oxygen		sulphur trioxide

(iii) N2(g)	+	3H2(g)	—>	2NH3(g)
nitrogen		hydrogen		ammonia

B.Solution phase homogeneous reactions

These are the reactions in which all reactants and products are present in a solution

( i )	HCl(aq)	+	NaOH(aq)	—>	NaCl(aq)	+	H2O(l)
	hydrochloric acid		sodium hydroxide		sodium chloride		water

( ii )

CH3COOC2H5(l)

+

H2O(l)

—>

CH3COOH(l)

+

C2H5OH(l)

b) Hetrogeneous reactions

The reactions in which reactants and products are present in more than one phase are called heterogeneous reactions. Such reactions involve at least one solid substance along with one or more substances in solid, solution or gaseous phase. The following are the examples of heterogeneous reactions.

(i) CaCO3(s)	—>	CaO(s)	+	CO2(g)
calcium carbonate		calcium oxide		carbon dioxide

(ii) 2Mg (s)	+	O2(g)	—>	2MgO(s)
magnesium		oxygen		magnesium oxide

(iii) BaCl2 (aq)	+	Na2SO4(aq)	—>	BaSO4(s)	+	2NaCl(aq)
barium chloride		sodium sulphate		barium sulphate		sodium chloride