It is generally observed that many of the chemical reactions do not proceed to completion when they are carried out in a closed container. This implies that the reactants are not completely converted into the products. Instead, after some time concentration of reactants do not undergo further decrease and the reaction appears to have stopped. This state of the system in which no further net change occurs is called a state of equilibrium.
Equilibrium can be attained for both the physical and chemical processes. The equilibrium achieved in physical processes such as dissolution of salt, evaporation of water, etc., is called physical equilibrium, whereas the equilibrium achieved in chemical processes such as decomposition of calcium carbonate or reaction between hydrogen and iodine is called chemical equilibrium.
The equilibrium situation raises many interesting questions such as:
- What is the equilibrium state and how is it recognised?
- What is the molecular behaviour when state of equilibrium is reached?
- What are the factors which alter the state of equilibrium?
- In this unit, we shall seek answers to the above questions.