Acid Base Definitions and Influence of Acid Structure

Acid Base Definitions

• Weak bases: Bases that do not completely ionize in H₂O.
  • Example: NH3(aq) + H2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)
    • K_b is the base dissociation constant.
• Monoprotic Acids: Acids that have only one acidic proton.
  • Example:
    HNO3(aq) \rightarrow H^+(aq) + NO3^-(aq)
• Polyprotic Acids: Acids with more than one acidic proton.
  • Example: H2SO4(aq) \rightleftharpoons H^+(aq) + HSO4^-(aq) HSO4^-(aq) \rightleftharpoons H^+(aq) + SO_4^{2-}(aq)
    • K_a is the acid dissociation constant.

Acid-Base Equilibrium and Constants

• General acid dissociation: HA(aq) + H2O(l) \rightleftharpoons H3O^+(aq) + A^-(aq) K_a = \frac{[H^+][A^-]}{[HA]}
  • pKa = -\log[Ka]: A lower pK_a indicates a stronger acid.
  • [H_3O^+] = [H^+]
• General base dissociation: B(aq) + H2O(l) \rightleftharpoons BH^+(aq) + OH^-(aq) Kb = \frac{[BH^+][OH^-]}{[B]}
  • pKb = -\log[Kb]
• Autoionization of water: H2O(l) + H2O(l) \rightleftharpoons H_3O^+(aq) + OH^-(aq)
  • Ion product of water: Kw = [H3O^+][OH^-]
    • At 25°C, K_w = 1.0 \times 10^{-14}
• pH and pOH:
  • pH = -\log[H_3O^+]
  • pOH = -\log[OH^-]
• Relationship between Ka, Kb, pKa, pKb, and K_w:
  • pKw = pKa + pK_b = 14 for an acid/base conjugate pair
  • pH + pOH = pK_w = 14

Influence of Acid Structure on Acidity and Other Aspects

• Solute Effects on pH
  • Salts can affect the pH of a solution depending on the acidity/basicity of their ions.
  • Example 1: NaCl(s) \rightarrow Na^+(aq) + Cl^-(aq)
    • Na^+: Cation from a strong base - No effect on pH.
    • Cl^-: Anion from a strong acid - No effect on pH.
    • The solution remains neutral.
  • Example 2: NH4Cl(aq) \rightarrow NH4^+(aq) + Cl^-(aq)
    • Adding NH_4Cl will lower the pH.
    • NH4^+(aq) \rightleftharpoons NH3(aq) + H^+(aq)
    • NH4^+ is the conjugate acid of a weak base (NH3).
    • Cl^-: Anion from a strong acid - No effect.
  • Example 3: NaOCl(aq) \rightarrow Na^+(aq) + OCl^-(aq)
    • Adding NaOCl will increase the pH.
    • Na^+: Cation of a strong base - No effect.
    • OCl^-(aq) + H_2O(l) \rightleftharpoons HOCl(aq) + OH^-(aq)
    • OCl^- is the conjugate base of a weak acid (HOCl).
• Oxyacids: Oxygen-containing acids.
  • Acidic hydrogen attached to an oxygen bonded to another element.
  • Acidity increases with the electronegativity of the central element and the number of oxygen atoms.
  • Example: HNO2 vs. HNO3
    • HNO2 (weak acid) vs. HNO3 (strong acid)
    • Resonance structures contribute to the stability of the conjugate base.
    • More resonance structures lead to greater stability.