Chapter 6 — Equilibrium

Chapter 6 of the Class 11 Chemistry NCERT textbook, "Equilibrium", explains that chemical equilibrium is a dynamic state in a closed system where forward and reverse reaction rates are equal, giving constant concentrations of reactants and products; it covers both physical processes and chemical reactions, characterised by an equilibrium constant (Kc or Kp) that depends on temperature.

• Equilibrium as a dynamic balance — The core idea is that at equilibrium opposing reactions have not stopped but proceed at equal rates in a closed system, so concentrations stay constant even though change continues at the molecular level.
• Physical versus chemical equilibria — The chapter shows the same principle governs phase changes and dissolution as well as chemical reactions, unifying melting, boiling, vapour pressure, and solubility with reacting systems.
• The equilibrium constant — It introduces the Law of Chemical Equilibrium and the constants Kc and Kp, relating them via Kp = Kc(RT)^Δn and showing how modifying a reaction changes its constant.
• Reaching equilibrium from either side — Using examples like the Haber process, the chapter demonstrates that identical equilibrium mixtures result whether you start from reactants or products at a given temperature.

1. 01Equilibrium is dynamic—both forward and reverse reactions continue simultaneously, though concentrations remain constant
2. 02Equilibrium constant expression Kc = [products]^coefficients / [reactants]^coefficients relates concentration of species at equilibrium for a given reaction
3. 03For gaseous reactions, Kp = Kc(RT)^Δn where Δn = (moles of gaseous products) – (moles of gaseous reactants)
4. 04The equilibrium constant for a reverse reaction K'c = 1/Kc; multiplying stoichiometric coefficients by n gives equilibrium constant K^n
5. 05Physical equilibria (melting point, boiling point, vapour pressure, solubility) are characterized by constant values at given temperature and pressure
6. 06Equilibrium can be established from either reactants or products; identical equilibrium mixtures result regardless of starting direction