Acid Base Fundamentals

Acid/Base Fundamentals

Acid/Base Fundamentals (Arrhenius, Brønsted–Lowry, Lewis)

Acids and bases can be framed three ways: Arrhenius (H⁺/OH⁻ in water), Brønsted–Lowry (proton donors/acceptors), and Lewis (electron-pair acceptors/donors). Picking the right definition lets you describe acid–base behavior in water, non-aqueous solvents, and even purely Lewis interactions.

Arrhenius: H⁺/OH⁻ in Water

  • Arrhenius acid: produces H⁺ (as H₃O⁺) in water; e.g., HCl → H₃O⁺ + Cl⁻.
  • Arrhenius base: produces OH⁻ in water; e.g., NaOH → Na⁺ + OH⁻.
  • Useful for aqueous systems only.

Arrhenius acids and bases generating H₃O⁺ or OH⁻ in water

Brønsted–Lowry: Proton Transfers

  • Acid = proton donor; base = proton acceptor (works in any medium).
  • Example: HCl donates H⁺ to NH₃ → NH₄⁺ (conjugate acid) + Cl⁻ (conjugate base).
  • Always two conjugate pairs; acid/base strength is inversely related to their conjugates.

Brønsted–Lowry proton transfer between HCl and NH₃ forming NH₄⁺ and Cl⁻

Lewis: Electron-Pair Interactions

  • Lewis acid = electron-pair acceptor; Lewis base = electron-pair donor.
  • Captures reactions without proton transfer (e.g., BF₃ accepting a lone pair from NH₃).
  • Mirrors electrophile (acid) vs nucleophile (base) language.

Lewis adduct formation between BF₃ and NH₃ showing electron-pair donation

Summary

  • Arrhenius covers aqueous H⁺/OH⁻ generation; Brønsted–Lowry generalizes proton transfers to any medium.
  • Lewis focuses on electron pairs, covering electrophile–nucleophile interactions with or without protons.
  • Conjugate acid/base strengths are inversely related; choose the broadest definition that fits the reaction.