Conjugate Acid-Base Pairs

Acid Strength and Conjugate Base Strength

Main Idea: The strength of an acid is inversely related to the strength of its conjugate base. Strong acids have weak conjugate bases, and weak acids have strong conjugate bases.

• Acid Dissociation Constant ( $ K_a $ ): This equilibrium constant quantifies the strength of an acid in aqueous solution. A larger $ K_a $ value indicates a stronger acid.

  $ HA(aq) \rightleftharpoons H^+(aq) + A^-(aq) $

  $ K_a = \frac{[H^+]][A^-]]}{[HA]]} $

• pKa: The negative logarithm of $ K_a $ . A smaller $ pK_a $ value indicates a stronger acid. $ pK_a = -\log_{10}K_a $

• Factors Affecting Acid Strength:

  • Bond Strength: Weaker bonds are easier to break, leading to stronger acids. Consider the H-X bond strength in hydrohalic acids (HF, HCl, HBr, HI). Bond Strength and Acid Strength
  • Electronegativity: More electronegative atoms stabilize the conjugate base by attracting the negative charge more effectively. This makes the acid stronger. Electronegativity and Acid Strength
  • Resonance: If the conjugate base can delocalize the negative charge through resonance, the acid will be stronger. Resonance and Acid Strength
  • Size/Polarizability: Larger atoms can better stabilize the negative charge on the conjugate base through polarizability, leading to stronger acidity. Atomic Size and Acid Strength
  • Inductive Effect: Electron-withdrawing groups can stabilize the conjugate base and increase acid strength. Inductive Effect and Acid Strength

• Conjugate Base Strength: The strength of a conjugate base is directly related to the stability of the anion ( $ A^- $ ). Factors that stabilize the anion ( $ A^- $ ) make it a weaker conjugate base and, therefore, the corresponding acid stronger.

• **Relationship between $ K_a $ and $ K_b $ **: For a conjugate acid-base pair, the product of their dissociation constants equals the ion product of water ( $ K_w $ ).

  $ K_a \times K_b = K_w = 1.0 \times 10^{-14} $ at 25°C

• Examples:

  • HCl (strong acid) has a very weak conjugate base, $ Cl^- $ .
  • $ CH_3COOH $ (weak acid) has a relatively strong conjugate base, $ CH_3COO^- $ .

• Further Notes: Acid-Base Equilibria, Titration Curves