Ionic Equilibrium in Solution

Full Notes

Introduction

In chemical systems involving ions in solution, equilibrium can exist between the undissociated molecules and dissociated ions. This is known as ionic equilibrium.

It is fundamental to understanding reactions in aqueous environments, particularly those involving acids, bases, and salts.

Types of Solutes in Water

Electrolytes

Electrolytes are substances that produce ions in aqueous solution and therefore conduct electricity.

• Strong Electrolytes
  These dissociate completely into ions in solution.
  Examples:
  • Sodium chloride:
    NaCl (aq) → Na⁺(aq) + Cl⁻(aq)
  • Hydrochloric acid:
    HCl (aq) → H⁺(aq) + Cl⁻(aq)
  • Nitric acid, Sulfuric acid, etc.
• Weak Electrolytes
  These dissociate only partially, establishing a dynamic equilibrium between ions and undissociated molecules.
  Example: Acetic acid
  CH₃COOH (aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)
  In these cases, the double arrow (⇌) reflects the reversible nature of dissociation.

Non-Electrolytes

Non-electrolytes are substances that do not dissociate into ions in solution. As a result, they do not conduct electricity.

Examples:
• Glucose (C₆H₁₂O₆)
• Urea
• Ethanol

These substances dissolve in water as intact molecules, not ions.

Nature of Ionic Equilibrium

1. Dynamic: Forward (ion formation) and reverse (recombination) reactions occur simultaneously.
2. Sensitive to external conditions: Changes in temperature, concentration, or pressure can shift the equilibrium.
3. State-specific: The ionic equilibrium only exists in aqueous solution or molten state, where ions are free to move.

Summary

• Ionic equilibrium describes reversible dissociation of electrolytes in aqueous solution.
• Strong electrolytes dissociate completely while weak electrolytes establish equilibrium.
• K_a and K_b quantify acid and base dissociation following the Law of Mass Action.
• Non-electrolytes dissolve as molecules and do not conduct electricity.