Chemical Equilibrium

Concept overview

Chemical equilibrium describes the balance that reversible reactions reach when the forward and backward rates become equal. The equilibrium constant K quantifies the ratio of product to reactant concentrations at that balance point, and its magnitude tells you whether the reaction favours products (K ≫ 1), reactants (K ≪ 1), or sits somewhere in between.

For gaseous reactions, K comes in two flavours — K_c using molar concentrations and K_p using partial pressures — related by K_p = K_c × (RT)^Δn where Δn is the change in moles of gas. For solution reactions, only K_c makes sense. Solids and pure liquids do not appear in the K expression because their activities are conventionally taken as 1.

The workhorse tool of this chapter is the ICE table — Initial, Change, Equilibrium — that lets you solve any equilibrium problem symbolically. Given initial concentrations and one equilibrium concentration (or the value of K), you set up ICE, solve the resulting algebraic equation, and read off every other concentration.

Le Chatelier's principle predicts qualitative shifts when the system is perturbed. Adding a reactant shifts equilibrium toward products. Increasing pressure on a gas-phase reaction shifts toward the side with fewer moles of gas. Raising temperature shifts toward the endothermic direction. Adding an inert gas at constant volume changes nothing; adding it at constant pressure dilutes the mixture and shifts toward more moles of gas.

Key formulas & concepts

• K_c definition: K_c = [products]^ν / [reactants]^ν
• K_p vs K_c: K_p = K_c × (RT)^Δn_gas
• ΔG° and K: ΔG° = −RT ln K
• Reaction quotient: Q = same expression as K, at any instant
• Q vs K forecast: Q < K → forward; Q > K → backward; Q = K → at equilibrium
• Degree of dissociation: α = moles dissociated / initial moles
• For A → B + C, K: K = α² × C / (1 − α), where C is initial [A]

Common mistakes to avoid

• Including pure solids and pure liquids in the K expression — they don't appear (activity is 1).
• Using K_p when the reaction has no gases — K_p is defined only for gas-phase reactions.
• Applying the 'small α' approximation when α is not small — always verify at the end.
• Assuming adding a catalyst shifts equilibrium — a catalyst speeds both directions equally and doesn't shift the position of equilibrium.

JEE Main relevance

JEE Main asks 2–3 equilibrium questions every paper: one direct K_c or K_p calculation, one Le Chatelier prediction, and often one degree-of-dissociation numerical for a gas-phase dimerisation or dissociation.

JEE Advanced relevance

Advanced problems combine equilibrium with thermodynamics — using ΔG° = −RT ln K to compute K from tabulated ΔH°/ΔS° or predicting how K changes with temperature via the van't Hoff equation. Multi-equilibrium problems (three coupled equilibria in solution) also appear in Advanced.

Previous-year questions (PYQ)

Equilibrium is the largest chapter in the JEETutorAI Chemistry PYQ bank — 1000+ tagged problems. Split practice into K_c/K_p, ICE-tables and Le Chatelier — each archetype has its own trap set.