Chemistry · Lesson

Arrhenius, Brønsted-Lowry and Lewis acid-base models

Use an acid–base model suited to the interaction. Identify proton-transfer conjugate pairs and electron-pair donor/acceptor roles without assuming every acid or base has the same formula.

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By the end, you can

Distinguish aqueous Arrhenius, proton-transfer and Lewis descriptions.
Identify conjugate pairs in a balanced reaction.
Assign acid/base roles relative to a specified partner.

The Arrhenius description has an aqueous scope

An Arrhenius acid increases hydrogen-ion concentration in water, and an Arrhenius base increases hydroxide-ion concentration. Aqueous H⁺ is shorthand for solvated hydrogen ions; H₃O⁺ makes the simplest hydration representation explicit. It does not mean bare isolated protons move through the solution.

This operational aqueous description is useful but has a defined setting. The ammonia–water reaction can increase hydroxide even though NH₃ contains no OH group. A formula checklist is less informative than the actual interaction and medium.

Assign the donor and acceptor in one reaction

A Brønsted–Lowry acid donates a proton and a base accepts one. For NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq), ammonia accepts and water donates in the forward direction. Each side has one N, one O and five H atoms, with net charge zero.

NH₄⁺/NH₃ is one conjugate acid–base pair; H₂O/OH⁻ is the other. Each pair differs by one proton and one charge unit. The reverse direction exchanges the donor/acceptor roles of the reacting conjugate species.

Identify roles and conjugate partners in the stated interaction

Interaction	Acid role	Base role	Bookkeeping check
NH₃ + H₂O → NH₄⁺ + OH⁻	H₂O donates H⁺	NH₃ accepts H⁺	N₁O₁H₅ and total charge 0 on each side
Conjugate partners	NH₄⁺ / NH₃	H₂O / OH⁻	Each pair differs by H₁ and one charge unit
BF₃ + NH₃ → F₃B←NH₃	BF₃ accepts a pair	NH₃ supplies a pair	B₁F₃N₁H₃ and total charge 0; no proton transferred

Balanced NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ proton-transfer model, plus a separate simple BF₃/NH₃ Lewis-adduct representation. Roles refer to the forward interactions shown; no geometry or equilibrium constant is implied.

A species’ role depends on its partner

Water is amphiprotic: it can donate a proton to one partner or accept a proton from another. A species that acts as a base in one interaction is not forbidden from donating a proton to a sufficiently strong base under other appropriate conditions.

Specify the reaction and solvent when assigning roles or comparing equilibria. Calling a substance an acid in this model is a statement about proton donation; it is not by itself a pH measurement, a concentration label or a claim that proton transfer is complete.

An electron-pair interaction need not transfer a proton

A Lewis acid accepts an electron pair and a Lewis base supplies one in forming an adduct. In the simple representation BF₃ + NH₃ ⇌ F₃B←NH₃, ammonia supplies its lone pair to an accepting orbital on boron. The atom counts and total neutral charge remain the same, and no proton is transferred.

The pair-origin arrow describes formation of the bond; it does not label physically distinguishable electrons after bonding. Electron-pair donation into a bond is also different from assigning a one-electron redox transfer. A Lewis acid need not contain hydrogen or be a positive ion.

Worked examples

Find the conjugate pairs

For NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, identify the forward acid and base and both conjugate pairs.

NH₃ gains H⁺ to form NH₄⁺, so NH₃ is the forward base.
H₂O loses H⁺ to form OH⁻, so H₂O is the forward acid.
Pair NH₄⁺ with NH₃ and H₂O with OH⁻.
Check that each pair changes by one H and one charge unit.

Forward acid H₂O; base NH₃. Conjugate pairs NH₄⁺/NH₃ and H₂O/OH⁻.

Try it yourself

What is water’s forward role in the stated ammonia–water reaction?
Show the answer and reasoning
Proton donor, or Brønsted–Lowry acid.
It becomes OH⁻ after donating H⁺.
How do members of a Brønsted conjugate pair differ?
Show the answer and reasoning
By one proton and one charge unit.
Adding H⁺ changes both hydrogen count and charge by +1.
Must every Lewis acid contain hydrogen?
Show the answer and reasoning
No.
BF₃ can accept an electron pair without transferring a proton.
Does aqueous H⁺ mean an isolated bare proton moving freely in water?
Show the answer and reasoning
No.
It is a convention for solvated hydrogen ions; H₃O⁺ is a simple explicit representation.

What to carry forward

Match the acid–base description to the interaction and medium.
Check atoms and charge in proton-transfer equations.
Assign conjugate partners by one-proton differences.
Keep Lewis pair donation distinct from a proton-transfer or redox claim.

References

Teaching resources and reference material supporting this lesson’s explanations and examples.