This lesson explains how reversible reactions reach equilibrium, how equilibrium responds to changes, and how equilibrium constants quantify reaction positions.

1. Core Concepts (Short Notes)

16.1 Reversible Reactions

Many reactions proceed in both forward and backward directions: A + B ⇌ C + D

16.2 Dynamic Equilibrium

A state where:

• Forward and backward reaction rates are equal.

• Concentrations remain constant, not equal.

• Occurs in a closed system.

16.3 Le Chatelier’s Principle

If an external change is applied to a system at equilibrium, the system shifts to oppose the change.

Changes and Their Effects:

• Increase concentration (reactant): shifts right.

• Decrease concentration (product): shifts right.

• Increase pressure (gases): shifts to side with fewer moles.

• Increase temperature: favors endothermic direction.

• Catalyst: no effect on equilibrium position (only speeds up attainment).

16.4 Equilibrium Constant (Kc)

For reaction: aA + bB ⇌ cC + dD Kc = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ

• Large Kc (>1): products favored.

• Small Kc (<1): reactants favored.

16.5 Reaction Quotient (Qc)

Used to determine direction of shift:

• Q < K → shifts right.

• Q > K → shifts left.

16.6 Effect of Temperature on Kc

• Endothermic reaction: ↑T → ↑Kc

• Exothermic reaction: ↑T → ↓Kc

2. Key Formulas to Memorize

Equilibrium Constant

Kc = [products]ᵖ / [reactants]ʳ

Reaction Quotient

Compare Qc with Kc.

ICE Table Method

Initial → Change → Equilibrium Used for calculating equilibrium concentrations.

3. Tips & Tricks for Exams

• Equilibrium never means reactants = products.

• Kc involves only aqueous and gaseous species (not solids/liquids).

• Changing temperature is the only factor that changes Kc.

• Always balance equations before writing Kc expressions.

• Use ICE tables to avoid algebra mistakes.

• For pressure changes, count moles of gas only.

4. Important Points to Remember

• At equilibrium, rate forward = rate backward.

• Le Chatelier’s principle predicts direction, not quantity.

• Catalysts do not alter equilibrium composition.

• Kc indicates the extent of reaction.

• Temperature changes alter Kc; concentration and pressure do not.

Equilibrium shifts aim to minimize applied changes.