Ch. 14

14.1 Brønsted-Lowry Acids and Bases

  • Acid: proton (H+) donor

  • Base: proton (H+) acceptor

  • Conjugate Acid-Base Pair: Acid becomes conjugate base after donating H+; base becomes conjugate acid after accepting H+

  • Amphiprotic: can act as both acid and base (e.g. H2O, HCO3−)

  • Acid ionization: HA + H2O → H3O+ + A−

  • Base ionization: B + H2O → BH+ + OH−

  • Autoionization of water: 2H2O → H3O+ + OH−

  • Kw = [H3O+][OH−] = 1.0 × 10⁻¹⁴ at 25 °C



14.2 pH and pOH

  • pH = -log[H3O+]

  • pOH = -log[OH−]

  • [H3O+][OH−] = 1.0 × 10⁻¹⁴

  • pH + pOH = 14 at 25 °C

  • Acidic: pH < 7, Neutral: pH = 7, Basic: pH > 7



14.3 Relative Strengths of Acids and Bases

  • Strong acid: complete ionization (Ka ≈ ∞)

  • Weak acid: partial ionization (Ka measurable)

  • Ka = [H3O+][A−]/[HA]

  • Kb = [BH+][OH−]/[B]

  • Ka × Kb = Kw

  • Stronger acid → weaker conjugate base

  • Binary acid strength: Increases down a group, across a period with increasing electronegativity

  • Oxyacid strength: Increases with electronegativity and oxidation number of central atom



14.4 Hydrolysis of Salts

  • Cation from weak base (e.g. NH4+): acidic solution

  • Anion from weak acid (e.g. CH3COO−): basic solution

  • Neutral ions: conjugates of strong acids/bases (e.g. Na+, Cl−)

  • Hydrated metal ions (e.g. Al3+): act as weak acids due to polarizing water



14.5 Polyprotic Acids

  • Monoprotic: 1 ionizable H+ (e.g. HCl)

  • Diprotic: 2 ionizable H+ (e.g. H2SO4, H2CO3)

  • Ionization happens stepwise: Ka1 > Ka2



14.6 Buffers

  • Buffer = weak acid + conjugate base or weak base + conjugate acid

  • Resists pH change upon small additions of acid/base

  • Henderson-Hasselbalch equation:

    • pH = pKa + log([A−]/[HA])

  • Best buffer: when [A−] ≈ [HA] (pH ≈ pKa)

  • Buffers are effective within ±1 pH unit of pKa



14.7 Acid-Base Titrations

  • Titration curve: pH vs. volume of titrant

  • Equivalence point: moles acid = moles base

  • Strong acid + strong base: pH = 7 at equivalence

  • Weak acid + strong base: pH > 7 at equivalence

  • Indicators: weak acids/bases that change color with pH

    • Change color over range of pKa ± 1