Chemical Equilibrium

Based on the direction of their occurrence, chemical reactions are two types.

Irreversible reactions: In these reactions, reactants are converted into products and products cannot be converted into reactants. There are unidirectional as they occur in one direction.

Reactants → products

These reactions almost go for completion i.e. reactants are almost completely convert into products.

Reversible reactions: A reaction is said to be reversible, if both the forward and the backward reactions are taking place simultaneously under the given experimental conditions.

Reactants giving rise to products is known as forward reaction. Products giving rise to reactants is known as reverse reaction or backward reaction. A reverse reaction does not go to completion.

Equilibrium State

The stage or state at which the rate of forward reaction is equal to the rate of the reverse reaction in a reversible reaction is known as the equilibrium stage or state. The chemical equilibrium is considered as dynamic equilibrium because the forward and the reverse reactions continue to take place simultaneously at this stage also.

In the beginning in a reversible reaction the rate of forward reaction is more since the concentration of reactants is more. As time proceeds the rate of forward reaction decreases as the concentrations of reactants decreases.

In the beginning in a reversible reaction the rate of backward reaction is absolutely zero because the concentration of products is zero. As time proceeds the rate of backward reaction increases since the concentrations of products also increases.

At one stage the rate of forward reaction becomes equal to rate of backward reaction and no further change occurs in the concentration of reactions or products. At equilibrium, the concentration of reactants and products may not be equal but they remain constant.

Types of Chemical Equilibrium

Based on the physical states of substances equilibrium is of two types:

Homogeneous equilibrium: All the reactants and products are present in same physical state.

Heterogeneous equilibrium: Reactants and products are in different physical states or different phase.