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Many physical and chemical changes which we encounter are reversible. For e.g. water changes into ice when temperature is lowered; increasing the temperature reverses this process. Conversely, on heating water, it converts into steam; lowering its temperature reverses the process.

When the conditions are such that forward ( represented by ) and backward (represented by)reactions can both occur to a noticeable extent, the process is described as a reversible reaction.

It has been found that after a certain time interval, reversible reactions attain a state of chemical equilibrium i.e. a state in which no further change in composition with time can be detected, provided the temperature and pressure are not altered.

11.1 Types of Reactions

a) Reversible reactions

Reactions in which products themselves react (or decompose) to produce the starting reactants, are termed as reversible reactions. Such reactions are denoted by the use of two half arrows.

For a reverse reaction to occur, the reaction should be carried out in a closed container when one of the products is gaseous.

b) Irreversible reactions

The reactions in which products do not recombine to form the original reactants is called an irreversible reaction.

c) Exothermic reactions

It is a chemical reaction which proceeds with the liberation of heat energy.

d) Endothermic reactions

It is a chemical reaction which proceeds with the absorption of heat energy.

Chemical Equilibrium

Consider a reversible homogenous reaction occurring in a closed container

Initially the molar concentration of 'A' & 'B' being maximum the rate of forward reaction is maximum. As the reaction proceeds 'C' & 'D' appear in the reaction and the backward reaction sets in but at much lower rate.

The changing rates of forward and backward reactions with time can be represented as follows :

Figure 22 Equilibrium graph


11.1 Types of Reactions
11.2 Equilibrium Law Expression
11.3 Factors Affecting Chemical Equilibrium
11.4 Le-Chatelier's Principle

Chapter 12

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