Understanding Equilibrium: Weak Acids and Bases

Introduction

• Discussed concepts relevant to equilibrium in the context of weak acids and bases
• Focus on understanding weak acids/bases and their dissociation behaviors

Weak Acids and Bases

• Weak Acids/Bases: Do not fully dissociate in solution
• Equilibrium Formation: Weak acids/bases form an equilibrium with their conjugate ions

KA: Equilibrium Constant for Acids

• KA Definition: Equilibrium constant for acids (Acid dissociation constant)
• Example - Acetic Acid:
  • Organic compound with a carboxylic acid group (COOH)
  • Partially dissociates into hydronium ions and conjugate base in water
• KA Expression:
  • Products (conjugate base and hydronium ions) over reactants (acetic acid, not water)
  • Water is a solvent, not included in the expression

Importance of KA

• Strength of Weak Acids: Larger KA = stronger weak acid; Smaller KA = weaker weak acid
• Comparison with Strong Acids: Strong acids do not form an equilibrium (no KA)

Percent Ionization

• Definition: Another measure of acid strength
• Formula: (Concentration of H+ at equilibrium / Initial concentration) * 100%
• Usefulness: Provides a more intuitive measure, easier to understand than small KA values*

Example of Weak Acid: Propanoic Acid

• Details: Organic, weak acid with Carboxylic group
• Challenge: Calculate percent ionization given initial concentration and pH
• Method:
  • Use ICE table to track changes in concentrations
  • Substitute values into KA expression using equilibrium concentrations derived from pH
  • Calculate percent ionization

KB: Equilibrium Constant for Bases

• KB Definition: Base ionization constant
• Relation to KA: Calculated similarly to KA but focuses on OH- ions
• Strength of Weak Bases: Larger KB = stronger weak base; Smaller KB = weaker weak base

Example of Weak Base: Aniline

• Properties: Contains NH2, acts as a base when dissolved in water
• Process:
  • Use initial concentration and pH to determine KB
  • Convert pH to POH to find OH- concentration
  • Apply ICE table to determine equilibrium concentrations
  • Use KB expression to solve for equilibrium concentrations

Conclusion

• Similarity in Approach: KA and KB follow similar steps; difference lies in ion focus (H+ for acids, OH- for bases)
• Homework Suggestions: Practice problems involving KA, KB, and percent ionization for deeper understanding

Next Lesson Preview

• Focus on strong acids/bases and redox reactions in upcoming lessons.

Additional Tips

• Pay attention to the relationships between pH, POH, and concentration changes in equilibrium scenarios
• Utilize ICE tables effectively to transition between initial and equilibrium conditions

These notes are designed to aid in understanding the key concepts discussed in the lecture on equilibrium, focusing specifically on weak acids and bases. Use them to prepare for exams and additional assignments.