Lecture Notes: Chemistry Chapter 17 - Buffers and Related Concepts

17.1 The Common-Ion Effect

• Definition: Occurs when adding more of a product ion to an acid/base reaction.
• Example: Adding acetate to acetic acid.
  • Acetic acid is weak; sodium acetate dissociates fully.
  • Shifts equilibrium towards reactants (Le Chatelier's principle).
  • Can be applied to weak bases similarly.

Calculating pH for a Common Ion

• Example: Solution with 0.30 mol acetic acid and 0.30 mol sodium acetate.
  • Focus on acetic acid equilibrium.
  • Assumption: salt dissociates 100%.
  • Use equilibrium concentration to calculate pH.
  • Minimal apparent pH change due to buffer.

17.2 Importance of Buffers

• Purpose: Resist changes in pH by using a weak acid and its conjugate base.
• Buffer Capacity: Requires reasonable concentrations (≥10⁻³ M of each component).
• Buffer Creation:
  1. Mix weak acid with conjugate base salt (e.g., acetic acid and acetate).
  2. Mix weak base with conjugate acid salt (e.g., NH₃ and NH₄⁺).
  3. Add strong base to weak acid to form conjugate base.
  4. Add strong acid to weak base to form conjugate acid.

Buffer Mechanism

• Adding small amounts of acid/base neutralizes components slightly, stabilizing pH.
• Henderson-Hasselbalch Equation:
  • Used to calculate pH of buffers.
  • Considers ratio of concentrations of acid and conjugate base.

Buffer Performance

• Buffers work within certain pH ranges.
• Effectiveness based on concentration ratio rather than absolute concentration.

17.3 Acid-Base Titrations

• Process: Acid/base of known concentration is added to unknown until equivalence is reached.
• Equivalence Point: Seen by monitoring pH changes.
• Titration Types:
  • Strong Acid with Strong Base: Rapid pH rise near equivalence.
  • Strong Base with Strong Acid: Starts with high pH, levels off after equivalence.
  • Weak Acid with Strong Base: Distinct regions, use limiting reactant and Henderson-Hasselbalch.

Weak Acid Titration Differences

• Higher initial pH, smaller changes, pH at equivalence > 7.

17.4 Solubility Equilibria

• Solubility Product Constant (Ksp): Represents equilibrium of ionic compounds in solution.
• Factors Affecting Solubility:
  • Common-ion effect: Adding one ion decreases solubility.
  • pH impact: Anion solubility can increase in acidic conditions.
  • Formation of complex ions can increase solubility.

Precipitation and Ion Separation

• Precipitation: Determined by comparing reaction quotient (Q) with Ksp.
  • If Q > Ksp, a precipitate forms.
  • Used in selective precipitation and qualitative ion analysis.

17.6 and 17.7 Advanced Topics

• Amphoterism: Compounds acting as acids or bases based on context.
• Complex Ion Formation: Enhances solubility through complex ions.
• Qualitative Analysis: Utilizes differences in solubility for ion separation and recovery.