6.0 Introduction

An equilibrium represents a state in a process when the observable properties such as colour, temperature, pressure, concentration, etc do not show any change. The word equilibrium means ‘balance’ which indicates that a chemical reaction represents a balance between the reactants and products taking part in the reaction. The equilibrium state is also noticed in certain physical processes such as the melting point of ice at \(0^{\circ} \mathrm{C}\), both ice and water are present at equilibrium.

In the case of physical processes such as the melting of solid, dissolution of salt in water, etc., the equilibrium is called physical equilibrium while the equilibrium associated with chemical reaction is known as a chemical equilibrium.

For example, when a liquid evaporates in a closed container, molecules with relatively higher kinetic energy escape the liquid surface into the vapour phase and number of liquid molecules from the vapour phase strike the liquid surface and are retained in the liquid phase. It gives rise to a constant vapour pressure because of an equilibrium in which the number of molecules leaving the liquid equals the number returning to liquid from the vapour. We say that the system has reached an equilibrium state at this stage. However, this is not static equilibrium and there is a lot of activity at the boundary between the liquid and the vapour. Thus, at equilibrium, the rate of evaporation is equal to the rate of condensation. It may be represented by
\(
\mathrm{H}_2 \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{H}_2 \mathrm{O}(\text { vap) }
\)
The double half arrows indicate that the processes in both directions are going on simultaneously. The mixture of reactants and products in the equilibrium state is called an equilibrium mixture.

Dynamic Equilibrium

Equilibrium can be established for both physical processes and chemical reactions. The reaction may be fast or slow depending on the experimental conditions and the nature of the reactants. When the reactants in a closed vessel at a particular temperature react to give products, the concentrations of the reactants keep on decreasing, while those of products keep on increasing for some time after which there is no change in the concentrations of either of the reactants or products. This stage of the system is the dynamic equilibrium and the rates of the forward and reverse reactions become equal. It is due to this dynamic equilibrium stage that there is no change in the concentrations of various species in the reaction mixture. Based on the extent to which the reactions proceed to reach the state of chemical equilibrium, these may be classified in three groups.

• The reactions that proceed nearly to completion and only negligible concentrations of the reactants are left. In some cases, it may not be even possible to detect these experimentally.
• The reactions in which only small amounts of products are formed and most of the reactants remain unchanged at equilibrium stage.
• The reactions in which the concentrations of the reactants and products are comparable, when the system is in equilibrium.

Ionic equilibrium

Equilibrium involving ions in aqueous solutions which is called as ionic equilibrium.