Weak Acids and Bases

Jul 3, 2024

Lecture on Weak Acids and Bases

Weak Acids

  • Definition: Weak acids partially dissociate in solution.
  • Strength: Depends on the bond strength within the acid molecule.
    • Weaker bond = stronger acid (dissociates faster)
    • Stronger bond = weaker acid (dissociates slower)
  • Concepts: Electron donors/withdrawers and delocalization.
    • Electron donors (e.g., CH3 in ethanoic acid) decrease acidity.
    • Electron withdrawers increase acidity.
  • Examples:
    • Methanoic Acid (Formic Acid) vs Ethanoic Acid (Acetic Acid)
      • Methanoic acid has a higher Ka (2.5 x 10^-5) than ethanoic acid (1.51 x 10^-5).
      • Explanation: CH3 group in ethanoic acid is an electron donor, reducing the polarity of the O-H bond, making it stronger and less likely to dissociate.

Reactant and Product Analysis

  • Reactant Side
    • CH3 is an electron donor, making the O-H bond in ethanoic acid less polar and stronger.
  • Product Side
    • Ethanoate ion is more negative due to CH3 electron donor, making it a stronger base but less stable than methanoate.
  • Conclusion: Stronger acids have more stable anions; weaker acids have less stable anions.

Homologous Series of Carboxylic Acids

  • Methanoic acid > Ethanoic acid > Propanoic acid in terms of acidity.
  • Larger electron donor groups reduce acidity.
  • Methanoic Acid is the strongest carboxylic acid (also known as formic acid; antiviral and antibacterial properties).

Comparison with Benzoic Acid

  • Benzene ring creates a withdrawing effect, making the O-H bond weaker compared to methanoic acid.
  • Benzoate ion is more stable due to electron delocalization, making benzoic acid stronger than methanoic acid.
  • Strong acids have more stable anions (Benzoate > Methanoate).

Additional Examples

  • Benzoic Acid vs Chlorobenzoic Acid vs 4-Methylbenzoic Acid
    • Chlorobenzoic acid (electron withdrawer) is the strongest acid.
    • 4-Methylbenzoic acid (electron donor) is the weakest acid.

Weak Bases

  • Definition: Weak bases partially dissociate in aqueous solution.
  • Examples: Ammonia, amines.
  • Ka and Kb values: Indicators of weak acid/base strength.

Ammonia Dissociation

  • NH3 + H2O ⇌ NH4+ + OH-
  • Kb = (NH4+)(OH-)/(NH3)
  • Calculation Example: pH calculation for methylamine given PKb.

Comparison of Weak Bases

  • Aliphatic Amines: Stronger bases due to electron-donating alkyl groups.
    • Primary, secondary, tertiary amines (tertiary may be slightly weaker due to solubility issues).
  • Aromatic Amines: Weaker bases than ammonia due to electron delocalization with the benzene ring.
  • Ranking: Aliphatic amines > Ammonia > Aromatic amines.

Electron Donors and Withdrawers

  • Electron Donors: Increase basicity.
  • Electron Withdrawers: Decrease basicity.
  • Examples:
    • 4-Methylphenylamine (strongest base due to electron donor).
    • Phenylamine (moderate base).
    • 4-Chlorophenylamine (weakest base due to electron withdrawer).

Acidity and Basicity Trends

  • Weak Acid/Base Categories:
    • Stronger acids have higher Ka, weaker acids have lower Ka.
    • Stronger bases have higher Kb, weaker bases have lower Kb.

Monobasic, Dibasic, and Tribasic Acids

  • Monobasic Acids: One H+ (e.g., HCl, HF).
  • Dibasic Acids: Two H+ (e.g., H2SO4, H2CO3), each subsequent dissociation has a lower Ka value.
  • Tribasic Acids: Three H+ (e.g., H3PO4), each subsequent dissociation has a lower Ka.
  • Ka and pKa Relation: As Ka decreases, pKa increases.

Practical Example for Weak Bases Calculation

  • pH Calculation Steps:
    1. Write dissociation equation.
    2. Use given PKb to find Kb.
    3. Use Kb to solve for OH- concentration.
    4. Use KW to find H+ concentration.
    5. Calculate pH using -log[H+].

Summary

  • Understanding weak acids/bases involves knowing the effect of bond strengths, electron donors/withdrawers, and the stability of ions formed.
  • Comparative examples help highlight how different factors influence acidity and basicity.