Class 11 Chemistry – Equilibrium
CBSE 2026–27 | NCERT-aligned free study resource
Dynamic Equilibrium
Chemical equilibrium is dynamic: forward and reverse reactions continue at equal rates. Macroscopic concentrations remain constant under fixed conditions.
Equilibrium Constant
For a general reaction, equilibrium expressions are written using appropriate concentrations or partial pressures of participating species. Pure solids and pure liquids are omitted from concentration-based equilibrium expressions.
Le Chatelier’s Principle
When an equilibrium system is disturbed, it shifts in the direction that tends to counter the disturbance. Changes in concentration, pressure and temperature can influence equilibrium position.
Ionic Equilibrium
Acids and bases can be treated using concepts of ionisation and equilibrium. pH is related to hydrogen-ion concentration. Weak electrolytes establish equilibria in solution.
Buffers and Solubility
Buffer solutions resist large changes in pH upon addition of small amounts of acid or base. Solubility equilibria involve sparingly soluble salts and their ions.
MCQs
- At equilibrium, forward and reverse reaction rates are: (A) zero (B) equal (C) unrelated (D) infinite. Answer: B.
- A catalyst changes the equilibrium constant. Answer: False. It changes rates of both directions similarly.
- pH is related to: (A) [H+] (B) atomic mass only (C) density only (D) neutron number. Answer: A.
Practice
- Write equilibrium expressions for given reactions.
- Predict shifts using Le Chatelier’s principle.
- Calculate pH in simple strong-acid/strong-base situations.
HOTS
Explain why increasing pressure affects gaseous equilibria only when the total number of gaseous moles differs between the two sides.
Revision
Dynamic equilibrium → K → Le Chatelier → acids/bases → pH → buffers → solubility equilibrium.