Effect of Concentration in Equilibrium for NEET: Key Points, Tricks & MCQs

What NEET Asks

• Questions on predicting the shift of equilibrium based on concentration changes are frequent (1-2 questions, 4-8 marks).
• Expect direct application of Le Chatelier's Principle and calculation-based problems involving reaction quotient.
• Conceptual understanding of K_c vs Q_c after concentration changes is crucial for higher-order problems.

Key Points

• Le Chatelier's Principle: If a system at equilibrium is subjected to a change in concentration, it will adjust itself to counteract the change.
• Adding Reactant: Increases reactant concentration, shifting equilibrium forward (towards products) to consume the added reactant.
• Removing Reactant: Decreases reactant concentration, shifting equilibrium backward (towards reactants) to replenish the removed reactant.
• Adding Product: Increases product concentration, shifting equilibrium backward (towards reactants) to consume the added product.
• Removing Product: Decreases product concentration, shifting equilibrium forward (towards products) to replenish the removed product.
• Pure Solids/Liquids: Changes in the amount of pure solids or liquids do not affect the equilibrium position as their concentrations are constant and not included in K_c or K_p expressions.
• Reaction Quotient (Q_c): Calculated at any non-equilibrium state, $Q_c = [Products]^p / [Reactants]^r$. Comparing Q_c with K_c predicts the direction of shift: if , reaction shifts forward; if , reaction shifts backward.

Must-Know Formula / Reaction

Q_c = [Products]^p / [Reactants]^r

• Q_c: Reaction Quotient, calculated using current (not necessarily equilibrium) molar concentrations.
• [Products], [Reactants]: Molar concentrations of species involved in the reaction.
• p, r: Stoichiometric coefficients from the balanced chemical equation.

Common Mistakes

• Students often forget that adding or removing pure solids/liquids does not shift the equilibrium position.
• Don't confuse the effect of changing concentration with changing pressure/volume, especially for gaseous reactions.
• Misinterpreting the $Q_c$ vs $K_c$ comparison; remember the system shifts to make $Q_c$ equal to .

Rapid Revision

Le Chatelier's Principle governs concentration changes. Add reactants/remove products: equilibrium shifts right. Add products/remove reactants: equilibrium shifts left. Pure solids/liquids have no effect. Use Q_c to predict shift: Q_c < K_c (forward), Q_c > K_c (backward).