Equilibrium – JEE Mains Chemistry

• by Zenith LMS
• Updated April 2025

1. Concept of Equilibrium

Equilibrium is a state in which the rate of the forward reaction equals the rate of the reverse reaction, indicating dynamic equilibrium. This applies to both physical and chemical processes.

2. Physical Equilibria

• Solid-Liquid Equilibrium: Melting and freezing processes.
• Liquid-Gas Equilibrium: Evaporation and condensation.
• Gas-Gas Equilibrium: Gases in closed containers establish uniform composition.
• Solid-Gas Equilibrium: Sublimation and deposition.
• Henry’s Law: The solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid.

3. General Characteristics of Physical Equilibrium

Physical equilibria are dynamic, reversible, and occur under constant temperature and pressure with fixed equilibrium characteristics at a given condition.

4. Chemical Equilibrium

• Law of Chemical Equilibrium: For a reaction aA + bB ⇌ cC + dD, the equilibrium constant K_c is [C]^c[D]^d / [A]^a[B]^b.
• K_p and K_c Relationship: K_p = K_c(RT)^Δn
• ΔG and ΔG°: ΔG = ΔG° + RT lnQ. At equilibrium, ΔG = 0 and Q = K, hence ΔG° = -RT lnK.

5. Factors Affecting Equilibrium

• Concentration: Changes shift equilibrium to oppose the change.
• Pressure: Affects reactions involving gases with different moles of reactants and products.
• Temperature: Increases or decreases the equilibrium constant based on exothermic/endothermic nature.
• Catalyst: Does not change equilibrium position; speeds up both forward and reverse reactions equally.

6. Le Chatelier’s Principle

When a system at equilibrium is disturbed, it shifts in the direction that minimizes the disturbance to re-establish equilibrium.

7. Ionic Equilibrium in Solutions

• Electrolytes: Strong electrolytes dissociate completely, weak ones partially.
• Ionization Concepts: Includes Arrhenius, Bronsted-Lowry, and Lewis definitions of acids and bases.
• Ionization Constants: K_a and K_b values indicate strength of acids and bases.
• pH and pOH: pH = -log[H⁺], pOH = -log[OH⁻], and pH + pOH = 14.
• Ionization of Water: K_w = [H⁺][OH⁻] = 1 × 10⁻¹⁴ at 25°C.
• Common Ion Effect: Suppresses ionization of weak electrolytes by adding a common ion.
• Hydrolysis of Salts: Determines whether solution is acidic, basic or neutral.
• Solubility Product (K_sp): Product of ion concentrations in saturated solution of a sparingly soluble salt.
• Buffer Solutions: Resist changes in pH upon addition of small amounts of acid or base.