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In everyday life, equilibrium describes the body’s sense of balance. In chemistry, chemical equilibrium refers to a state where the concentrations of reactants and products remain constant over time, even though the forward and reverse reactions continue to occur at equal rates.

Law of Chemical Equilibrium

The law of chemical equilibrium, often expressed via the equilibrium constant (K), states that for a reaction at constant temperature and pressure, the ratio of the product concentrations to the reactant concentrations (each raised to the power of their stoichiometric coefficients) remains unchanged.

Concept of Chemical Equilibrium

Consider a reversible reaction:

A + B <=> C + D

As the reaction proceeds, the concentrations of C and D rise while A and B fall. Eventually, the forward and reverse reaction rates equalize, and the concentrations plateau. This dynamic balance is the hallmark of chemical equilibrium.

Types of Chemical Equilibrium

Equilibria can be classified into:

• Homogeneous – reactants and products reside in the same phase.
• Heterogeneous – reactants and products occupy different phases.

Conditions for Equilibrium

For a system to reach equilibrium, the following must hold:

• All measurable properties (concentration, pressure, color, etc.) remain constant at a fixed temperature.
• The system can achieve equilibrium from either the forward or reverse direction.
• A catalyst may accelerate the approach to equilibrium but does not alter its position.
• The equilibrium is inherently dynamic.

Factors That Shift Equilibrium

Le Chatelier’s principle predicts how a system responds to disturbances:

• Concentration changes – Adding reactant drives the reaction forward; adding product drives it backward.
• Temperature – Raising temperature favors the endothermic direction; lowering temperature favors the exothermic direction.
• Pressure – Increasing pressure (decreasing volume) shifts equilibrium toward the side with fewer gas molecules.
• Catalysts – Speed up both directions equally, shortening the time to reach equilibrium.
• Inert gases – At constant volume, they have no effect; at constant pressure, they can influence the total pressure.