Effect of Concentration, Pressure and Temperature — Core Principles

Core Principles

Le Chatelier's Principle is a fundamental concept in chemical equilibrium, stating that a system at equilibrium will counteract any applied stress to re-establish a new equilibrium. Three primary stresses are concentration, pressure, and temperature.

Increasing reactant concentration or decreasing product concentration shifts equilibrium towards products. Conversely, decreasing reactant concentration or increasing product concentration shifts it towards reactants.

For gaseous reactions, increasing pressure shifts equilibrium towards the side with fewer moles of gas, while decreasing pressure shifts it towards the side with more moles of gas. Temperature changes affect the equilibrium constant (K) itself.

For exothermic reactions, increasing temperature shifts equilibrium to reactants (decreasing K), while for endothermic reactions, increasing temperature shifts it to products (increasing K). Catalysts accelerate the attainment of equilibrium but do not alter its position.

Important Differences

vs Effect of Catalyst

Aspect	This Topic	Effect of Catalyst
Equilibrium Position	Concentration/Pressure/Temperature	Catalyst
Effect on K	Only Temperature changes K; Concentration/Pressure do not	No effect on K
Mechanism of Action	Shifts reaction to consume/produce species or heat, or to reduce/increase gas moles	Lowers activation energy for both forward and reverse reactions equally
Time to reach Equilibrium	May change, but not directly its primary effect	Decreases time to reach equilibrium
Le Chatelier's Principle	Directly explained by Le Chatelier's Principle	Not directly explained by Le Chatelier's Principle as it doesn't relieve a 'stress' by shifting equilibrium

While concentration, pressure, and temperature changes directly influence the equilibrium position and, in the case of temperature, the equilibrium constant (K), a catalyst only affects the rate at which equilibrium is attained. A catalyst speeds up both the forward and reverse reactions equally by lowering the activation energy, thereby reducing the time required to reach equilibrium, but it does not alter the final equilibrium concentrations or the value of K. This distinction is crucial for NEET aspirants.