Introduction to Chemical Equilibrium

Chemical equilibrium occurs in a reversible reaction when the rate of the forward reaction equals the rate of the backward reaction, and the concentrations of reactants and products remain constant over time.

• It is a dynamic equilibrium, meaning reactions continue at the molecular level, but macroscopic properties appear unchanged.

• Important for NEET as questions often involve equilibrium concepts, Le Chatelier’s principle, and factors affecting equilibrium.

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Characteristics of Chemical Equilibrium

• Dynamic Nature: Forward and backward reactions occur simultaneously.

• Constant Concentration: Concentrations of reactants and products remain constant, not necessarily equal.

• Reversibility: Only occurs in reversible reactions.

• Macroscopic Stability: Pressure, concentration, and color appear unchanged.

NEET Tip: Always remember dynamic vs static equilibrium – dynamic is continuous at molecular level; static is not.

Equilibrium Constant (K)

• The equilibrium constant expresses the ratio of product concentrations to reactant concentrations at equilibrium.

• For a reaction:
  aA+bB⇌cC+dDaA + bB ⇌ cC + dDaA+bB⇌cC+dD
  Conceptually, Kc=[C]c[D]d[A]a[B]bK_c = \frac{[C]^c [D]^d}{[A]^a [B]^b}Kc​=[A]a[B]b[C]c[D]d​

• Key points:

  • K is dimensionless when expressed correctly.

  • Large K (>1) → products favored

  • Small K (<1) → reactants favored

• NEET Focus: Compare relative amounts of products and reactants without calculations.

Types of Equilibrium

1. Homogeneous Equilibrium:

• All reactants and products are in same phase (gas or liquid).

• Example: N2(g)+3H2(g)⇌2NH3(g)N_2(g) + 3H_2(g) ⇌ 2NH_3(g)N2​(g)+3H2​(g)⇌2NH3​(g)

2. Heterogeneous Equilibrium:

• Reactants and products are in different phases.

• Example: CaCO3(s)⇌CaO(s)+CO2(g)CaCO_3(s) ⇌ CaO(s) + CO_2(g)CaCO3​(s)⇌CaO(s)+CO2​(g)

• NEET Tip: Concentrations of pure solids and liquids are ignored in equilibrium expressions.

Factors Affecting Chemical Equilibrium (Le Chatelier’s Principle)

Le Chatelier’s principle states:

If a system at equilibrium is disturbed, it shifts in a direction that opposes the disturbance.

1. Effect of Concentration:

• Increasing reactants → equilibrium shifts toward products

• Increasing products → equilibrium shifts toward reactants

2. Effect of Pressure (for gases):

• Increase in pressure → equilibrium shifts toward side with fewer moles of gas

• Decrease in pressure → shifts toward side with more moles of gas

3. Effect of Temperature:

• Exothermic reactions: Heat acts as a product

  • Increase temperature → shifts toward reactants

  • Decrease temperature → shifts toward products

• Endothermic reactions: Heat acts as a reactant

  • Increase temperature → shifts toward products

  • Decrease temperature → shifts toward reactants

4. Effect of Catalyst:

• Catalyst does not change position of equilibrium

• Increases rate of both forward and backward reactions equally

NEET Focus: Concentration, pressure, and temperature changes are commonly tested in reasoning-based questions.

Relationship Between Equilibrium and Gibbs Free Energy

• At equilibrium, Gibbs free energy change (ΔG) = 0.

• Conceptually, equilibrium is the point of maximum stability for a system.

• Reaction proceeds spontaneously until ΔG = 0.

NEET Tip: Understanding the thermodynamic perspective helps explain why reactions reach equilibrium.

Applications of Chemical Equilibrium

1. Industrial Processes:

• Haber Process: Production of ammonia, equilibrium manipulated by pressure, temperature, and catalyst.

• Contact Process: Production of sulfuric acid, equilibrium shifts influenced by temperature and pressure.

2. Biological Systems:

• Oxygen transport in hemoglobin involves equilibrium between oxygen-bound and free hemoglobin.

• Enzyme-catalyzed reversible reactions maintain equilibrium in metabolic pathways.

3. Environmental Chemistry:

• Equilibrium concepts explain CO₂ in oceans and acid-base buffering in natural waters.

Common NEET Conceptual Questions

• Predicting direction of shift when concentration, pressure, or temperature changes.

• Comparing K values to determine product vs reactant dominance.

• Differentiating homogeneous vs heterogeneous equilibrium.

• Understanding why catalysts do not affect equilibrium position.

• Linking thermodynamics (ΔG) with chemical equilibrium.

Quick Concept Summary Table

Conclusion

Chemical Equilibrium is a critical chapter for NEET Chemistry, emphasizing dynamic nature, equilibrium constant, Le Chatelier’s principle, and factors affecting equilibrium. Conceptual understanding is key for reasoning-based MCQs. Linking equilibrium concepts to thermodynamics and real-life applications ensures students can answer both theoretical and application-oriented NEET questions confidently. StudentBro.in provides structured, conceptual notes to help aspirants master Chemical Equilibrium efficiently.