(78) Chemical Equilibrium FULL CHAPTER | Class 11th Physical Chemistry | Arjuna NEET

Chemical Reactions Overview

• Types of Reactions: There are two types of chemical reactions: reversible reactions and irreversible reactions. Reversible reactions allow both forward and backward reactions to occur, while irreversible reactions only progress in one direction.

• Equilibrium: When a reaction reaches equilibrium, the concentrations of reactants and products remain constant over time, but they may not be equal.

Key Concepts of Equilibrium

• Equilibrium Expression: The equilibrium constant (K) quantifies the relationship between the concentrations of the reactants and products at equilibrium. It is typically expressed in terms of the concentration of products raised to the power of their coefficients divided by the concentration of reactants raised to the power of their coefficients.

• Le Chatelier's Principle: If a change is made to a system at equilibrium, the system will adjust itself to counteract that change. For instance, if the pressure is increased by reducing the volume, the equilibrium will shift toward the side with fewer moles of gas.

• Effect of Temperature:

  • Endothermic Reactions: For reactions that absorb heat (endothermic), increasing the temperature shifts the equilibrium toward the products, while decreasing the temperature shifts it toward the reactants.

  • Exothermic Reactions: Conversely, for exothermic reactions that release heat, increasing temperature shifts the equilibrium toward the reactants, while decreasing temperature shifts it toward the products.

Equilibrium Constant Calculation

• Equilibrium Constant (K): For a general reaction aA + bB ⇌ cC + dD, the equilibrium constant expression is given by:

[ K = \frac{[C]^c [D]^d}{[A]^a [B]^b} ]

• Influence of Catalysts: Catalysts do not affect the position of equilibrium; rather, they increase the rate at which equilibrium is achieved.

Effect of Changing Concentration

• Adding Reactants or Products: Adding more reactant shifts the equilibrium to the right (towards products); adding product shifts it to the left (toward reactants).

Example of Equilibrium Shifts

• Consider the reaction: 2N2 + O2 ⇌ 2NO.

  • If you increase the amount of O2, the reaction will shift to the right to form more NO. Conversely, if NO is removed, the reaction will shift to the left to compensate.

Relationship Between Gibbs Free Energy and Equilibrium constant

• The relationship is given by: [ \Delta G = \Delta G^\circ + RT \ln K ]

• When K > 1, the reaction favors the products (spontaneous). When K < 1, it favors reactants (non-spontaneous).

Summary of Core Principles

• The reaction at equilibrium can be disturbed by changing concentration, pressure, or temperature, and shifts in the position of equilibrium can be predicted using Le Chatelier's principle.

• Catalysts speed up the rate of achieving equilibrium but do not alter the equilibrium concentrations of reactants and products.