Equilibrium in Chemical Processes – Dynamic Equilibrium

NCERT Reference: Chapter 6 – Equilibrium – Page 167

Quick Notes

Chemical equilibrium occurs in reversible reactions when the rates of forward and reverse reactions become equal.
At this point, concentrations of reactants and products remain constant (but not necessarily equal).
Equilibrium is dynamic: reactions continue at the molecular level.
Requires a closed system to prevent loss of reactants/products.
Can be reached whether you start from reactants or products.

Full Notes

Introduction to Chemical Equilibrium

In many reactions, particularly those involving gases or solutions, the process is reversible.

Products can reform the reactants. Equilibrium in such systems is a balance in the rate of the forward and reverse reactions.

For example:

If A + B are mixed and react together according to the reversible reaction below, an equilibrium system will form.

A + B ⇌ C + D

As time passes: Product (C and D) concentration increases . Reactant (A and B) concentration decreases.

NCERT 11 Chemistry time-course graph showing concentrations of A, B decreasing and C, D increasing until levels flatten at equilibrium.

This causes the forward reaction rate to decrease and the reverse reaction rate to increase. Eventually the rates become the same and the concentrations of everything remain constant.

NCERT 11 Chemistry plot of forward and reverse reaction rates versus time showing equal rates at dynamic equilibrium.

Rate of forward reaction = Rate of reverse reaction
This is known as dynamic equilibrium.

Key Features of Dynamic Equilibrium

Dynamic nature: Reactions continue in both directions even though the system appears static.
Constant properties: Concentrations, pressure, and colour remain unchanged once equilibrium is reached.
Reversibility: Only reversible reactions can reach equilibrium.
Closed system: No matter can enter or leave – required to maintain equilibrium.
Molecular activity persists: Bonds break and form continuously; it’s just that the rates cancel each other out.

Approaching Equilibrium from Either Direction

One of the most important aspects of chemical equilibrium is that it can be reached from either direction – starting with only reactants, only products, or a mix of both.

Let’s take the example:

H₂(g) + I₂(g) ⇌ 2HI(g)

Starting from reactants (H₂ and I₂):
Forward reaction predominates at first, forming HI. As HI accumulates, reverse reaction begins. Eventually both balance, and equilibrium is reached.
Starting from products (only HI):
Reverse reaction dominates at first, producing H₂ and I₂. As they accumulate, the forward reaction begins. Again, equilibrium is reached.

The important point is the same equilibrium concentrations are achieved regardless of starting point, provided the temperature, pressure, and volume are constant.

Summary

Dynamic equilibrium is when forward and reverse rates are equal.
Concentrations remain constant once equilibrium is reached.
A closed system is required to maintain equilibrium.
The same equilibrium is reached from reactants or products.