Acids and Bases: Arrhenius to Brønsted–Lowry — Conjugate Pairs, Polyprotic Acids, Ionization vs Dissociation, and Ampholytes

Arrhenius definition of acids and bases

  • An acid forms H3O+\mathrm{H_3O^+} in water; a base forms OH−\mathrm{OH^-} in water.

  • Limited as it relies on water and not all bases contain OH−\mathrm{OH^-}.

Brønsted–Lowry definition (proton transfer)

  • Brønsted–Lowry acid = proton donor; Brønsted–Lowry base = proton acceptor.

  • This definition is broader and accounts for proton transfer without requiring OH−\mathrm{OH^-} formation.

Acid-base conjugate pairs (concept and examples)

  • A conjugate base is what remains after an acid donates a proton; a conjugate acid forms after a base accepts a proton.

  • Example: HNO<em>3\mathrm{HNO<em>3} (acid) and NO</em>3−\mathrm{NO</em>3^-} (conjugate base); H<em>2O\mathrm{H<em>2O} (base) and H</em>3O+\mathrm{H</em>3O^+} (conjugate acid).

Diprotic, Triprotic, and Monoprotic acids

  • Monoprotic acids donate one proton (e.g., HCl\mathrm{HCl}).

  • Diprotic acids donate two protons (e.g., H<em>2SO</em>4\mathrm{H<em>2SO</em>4}).

  • Triprotic acids donate three protons (e.g., H<em>3PO</em>4\mathrm{H<em>3PO</em>4}).

  • Polyprotic acids donate more than one proton.

Ionization vs dissociation

  • Ionization: formation of ions from a previously non-ionic molecule (covalent) in water (e.g., HCl\mathrm{HCl}).

  • Dissociation: breaking apart of existing ions in an ionic compound in water (e.g., NaOH\mathrm{NaOH}).

Ampholytes and amphiprotic species

  • Ampholyte (amphiprotic) substances can act as both acids and bases (e.g., water, HSO4−\mathrm{HSO_4^-}).

Neutralization and spectator ions (practical example)

  • Neutralization: Acid + Base
    →\rightarrow
    Salt + Water (e.g., HCl+NaOH→NaCl+H2O\mathrm{HCl + NaOH \rightarrow NaCl + H_2O}).

  • Spectator ions (e.g., Na+,Cl−\mathrm{Na^+}, \mathrm{Cl^-}) do not participate in the chemical change.

Strength of acids: strong vs weak

  • Strong acids ionize completely in water (one-way arrow).

  • Weak acids ionize incompletely/partially in water (reversible arrow, equilibrium).

Summary remarks

  • The Brønsted–Lowry model is a broader, proton-transfer based definition than Arrhenius.

  • Identify conjugate acid–base pairs from proton transfer.

  • Distinguish ionization (new ion formation from covalent molecule) from dissociation (separation of pre-existing ions).

  • Ampholytes exhibit dual acid/base behavior.

  • Acid strength is determined by the extent of ionization in water: complete for strong, partial for weak.