Arrhenius, Brønsted–Lowry, and Lewis Acid–Base Models

Core Concept

Acids and bases are defined by three nested models of increasing generality. The Arrhenius model says an acid produces H⁺ in water and a base produces OH⁻. The Brønsted–Lowry model reframes this as proton transfer: an acid is a proton donor, a base a proton acceptor, and every acid–base reaction leaves behind a conjugate pair. The Lewis model is the most general of all: an acid is an electron-pair acceptor and a base an electron-pair donor, so it covers reactions that involve no proton at all. Each later model contains the earlier one, and the Brønsted–Lowry model is the workhorse of aqueous acid–base equilibria.

Key Ideas

• Arrhenius (1884): acid → H⁺(aq); base → OH⁻(aq). Limited to aqueous solutions; treats H⁺ as a bare proton and fails for bases like NH₃.
• Brønsted–Lowry (1923): acid = proton donor; base = proton acceptor. Works in any solvent; requires a conjugate pair.
• Conjugate pair: after an acid donates H⁺, the species that remains is its conjugate base (one fewer H, one more negative charge); after a base accepts H⁺, the product is its conjugate acid.
• Amphoteric / amphiprotic: a species (water, HCO₃⁻, H₂PO₄⁻) that can both donate and accept protons.
• Lewis (1923): acid = electron-pair acceptor; base = electron-pair donor. Covers BF₃, metal cations, and every Brønsted reaction.

Equations and Variables

• Brønsted acid ionization: HA + H₂O ⇌ H₃O⁺ + A⁻ (A⁻ = conjugate base)
• Brønsted base ionization: B + H₂O ⇌ BH⁺ + OH⁻ (BH⁺ = conjugate acid)
• Conjugate pair shorthand: HCl / Cl⁻, NH₄⁺ / NH₃, H₂O / OH⁻, H₃O⁺ / H₂O
• Lewis adduct formation: BF₃ + :NH₃ → F₃B–NH₃ (BF₃ = Lewis acid, NH₃ = Lewis base)

How It Works

1. In the Brønsted–Lowry picture, a proton (H⁺, just a bare nucleus) moves from the acid to the base, but only when the base has a lone pair to grab it.
2. The acid donates H⁺ and becomes its conjugate base (one charge unit more negative). Example: HCl → Cl⁻.
3. The base accepts H⁺ and becomes its conjugate acid (one charge unit more positive). Example: NH₃ → NH₄⁺.
4. The reaction is always written in both directions, so every Brønsted reaction has two conjugate pairs, one on each side.
5. The Lewis model looks one level deeper: it ignores the proton and asks "who is accepting a lone pair?" A Brønsted acid's H⁺ accepts the base's lone pair, so every Brønsted acid is automatically a Lewis acid.

Worked Example

Identify the conjugate pairs and the Lewis roles in each reaction.

(a) HF + H₂O ⇌ H₃O⁺ + F⁻

• HF donates H⁺ → its conjugate base is F⁻.
• H₂O accepts H⁺ → its conjugate acid is H₃O⁺.
• Pairs: (HF/F⁻) and (H₃O⁺/H₂O).

(b) BF₃ + :NH₃ → F₃B–NH₃

• No proton moves. NH₃ donates its lone pair (Lewis base); BF₃ accepts it (Lewis acid). This is a Lewis acid–base reaction that is not a Brønsted reaction, because BF₃ has no H to give.

How it works

1. In the Brønsted–Lowry picture, a proton (H⁺, just a bare nucleus) moves from the acid to the base, but only when the base has a lone pair to grab it.
2. The acid donates H⁺ and becomes its conjugate base (one charge unit more negative). Example: HCl → Cl⁻.
3. The base accepts H⁺ and becomes its conjugate acid (one charge unit more positive). Example: NH₃ → NH₄⁺.
4. The reaction is always written in both directions, so every Brønsted reaction has two conjugate pairs, one on each side.
5. The Lewis model looks one level deeper: it ignores the proton and asks "who is accepting a lone pair?" A Brønsted acid's H⁺ accepts the base's lone pair, so every Brønsted acid is automatically a Lewis acid.

Common confusions

• "H⁺ and H₃O⁺ are different substances." — In water, a bare H⁺ immediately attaches to H₂O to form H₃O⁺ (hydronium); the two notations are used interchangeably for aqueous work.
• "A conjugate base is a base in its own right." — It is a base in the reverse reaction, but "conjugate" just labels the partner; whether it is appreciably basic depends on strength.
• "Lewis acids must contain hydrogen." — Wrong; BF₃, AlCl₃, and Fe³⁺ are Lewis acids with no acidic proton.
• "Arrhenius and Brønsted are equally general." — Brønsted is broader; it explains NH₃ and works in non-aqueous solvents.

Quick review

• Arrhenius: H⁺ producer (acid) / OH⁻ producer (base), aqueous only.
• Brønsted–Lowry: proton donor / proton acceptor, conjugate pairs, amphoteric water.
• Lewis: electron-pair acceptor / donor, most general, no proton required.
• HF/F⁻, H₃O⁺/H₂O, NH₄⁺/NH₃ are classic conjugate pairs.
• BF₃ + NH₃ is Lewis-only; HF + H₂O is both Brønsted and Lewis.