Introduction

Equilibrium is actually a state, when forces from both the side become equal. According to chemistry: It is a point in a chemical reaction, when rate of forward reaction becomes equal to rate of backward reaction.  Or we can say, it is the state when concentration of reactants becomes equal to concentration of products.

Types of equilibrium

There are two types of equilibrium:

• Chemical equilibrium
• Ionic equilibrium

 Let us study both the types of equilibrium in detail:

Physical equilibrium: It is achieved in all physical processes, when all state variables like pressure, temperature etc becomes constant. These are basically of three types:

Let us take examples referring to above type of equilibrium:

• Solid to  liquid  or liquid to solid

For example: Conversion of ice to water.

In this forward reaction is: Ice to water (Melting).

In this backward reaction is: Water to Ice (Freezing).

So, when equilibrium is achieved: Rate of melting = Rate of freezing.

The temperature at which both solid and liquid states co exist is called Melting point. To attain melting point, we need to have certain state variable constant that is pressure.

• Liquid to  gas or gas to liquid

For example: Conversion of water to water vapour.

In this, forward reaction is: Evaporation

In this, backward reaction is: Condensation

At equilibrium: Rate of evaporation = Rate of condensation

Boiling point: Is that constant temperature, at which vapour pressure of liquid becomes equal to atmospheric pressure. The state variable that becomes constant is temperature.

• Solid to gas or gas to liquid

For example: Sublimation of Naphthalene, Camphor etc.

In this forward reaction is: Evaporation.

In this backward reaction is: Solidification. 

At equilibrium: Rate of evaporation = Rate of solidification. The state variable that becomes constant is temperature.

Equilibrium involving dissolution of solids

When any solid dissolves in liquid it is called as dissolution.

For example: When we add salt to water it dissolves .In this, the forward reaction is dissolution. When the solvent can’t dissolve more solute the process of crystallization occur in backward reaction.

At equilibrium: Rate of dissolution = Rate of crystallization .The state variable that becomes constant is concentration.

General characteristics of physical equilibrium

• The measurable properties become constant.
• It can be established only in closed vessel.
• At equilibrium, the opposing forces become equal.
• The equilibrium is dynamic in nature .That is the reaction keeps on going only the rate becomes constant.
• At equilibrium, the concentration becomes constant.
• The magnitude of equilibrium value, gives indication about the extent of reaction.