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Arrhenius, Brønsted–Lowry, Lewis Definitions

Learn and apply Arrhenius, Brønsted–Lowry, Lewis Definitions in the published Chemistry course sequence.

  • GCE A-Level H2 Chemistry 9476-2027
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Arrhenius, Brønsted–Lowry and Lewis Definitions: Orientation

The same reaction may fit more than one acid–base theory. This lesson shows what each model describes, when it applies, and how to justify a classification using the vocabulary requested. Continue with conjugate pairs and amphoteric species to apply the Brønsted–Lowry model.

Definitions (Must Know)

A. Arrhenius (aqueous only)

  • Arrhenius acid: produces H⁺ (or H₃O⁺) in aqueous solution.
  • Arrhenius base: produces OH⁻ in aqueous solution.

B. Brønsted–Lowry

  • Brønsted–Lowry acid: proton donor.
  • Brønsted–Lowry base: proton acceptor.

C. Lewis

  • Lewis acid: electron-pair acceptor.
  • Lewis base: electron-pair donor.

Detailed Explanations

A. Choosing the correct model (workflow)

Three acid-base theory lensesThree stacked panels compare Arrhenius aqueous ion production, Brønsted-Lowry proton transfer, and Lewis electron-pair transfer. A final note explains that one reaction may fit more than one theory.1ArrheniusLens: ion production in aqueous solutionAcid produces H+ / H3O+ in water.Base produces OH− in water.Evidence: name the ion produced.2Brønsted–LowryLens: proton transferAcid donates H+.Base accepts H+.Evidence: identify who loses and gains H+.3LewisLens: electron-pair transferAcid accepts an electron pair.Base donates an electron pair.Trace the electron pair: donor → acceptor.Theories can overlapNH3 accepting H+ is a Brønsted baseand a Lewis base: it accepts the protonwhile donating the new bonding pair.
Three theory lenses, not three mutually exclusive boxes: use aqueous ion production for Arrhenius, proton transfer for Brønsted–Lowry, and electron-pair transfer for Lewis.
  1. If the question is explicitly aqueous and talks about producing H⁺/OH⁻ → Arrhenius.
  2. If there is proton transfer in an equation → Brønsted–Lowry.
  3. If the question asks about electron-pair donation/acceptance or adduct formation → Lewis.

A single reaction can pass more than one check. For example, when NH₃ accepts H⁺, it is both a Brønsted–Lowry base (proton acceptor) and a Lewis base (electron-pair donor).

Phrasing templates (1 line each):

  • Arrhenius acid/base: “In aqueous solution, X produces H₃O⁺ / OH⁻, so it is an Arrhenius acid/base.”
  • Brønsted–Lowry acid/base: “X donates/accepts H⁺ (see equation), so it is a Brønsted–Lowry acid/base.”
  • Lewis acid/base: “X accepts/donates an electron pair, so it is a Lewis acid/base.”

Because Brønsted–Lowry is defined by proton transfer, the equation is your evidence; therefore a Brønsted–Lowry justification should point to the species that gains/loses H⁺ in the equation.

Mini example (spot the model): HCl + H₂O → H₃O⁺ + Cl⁻

  • In water it produces H₃O⁺ → Arrhenius acid.
  • HCl donates H⁺ to water → Brønsted–Lowry acid.

B. Brønsted–Lowry example: ammonia in water

NH₃ + H₂O ⇌ NH₄⁺ + OH⁻

  • NH₃ accepts H⁺ → Brønsted–Lowry base.
  • H₂O donates H⁺ → Brønsted–Lowry acid.

C. Lewis example: BF₃ reacting with NH₃

BF₃ + :NH₃ → F₃B < -NH₃

  • NH₃ donates a lone pair → Lewis base.
  • BF₃ accepts an electron pair (electron-deficient) → Lewis acid.
Lewis adduct diagramDiagram showing lone pair donor, electron pair acceptor, coordinate bond arrow direction, and final adduct charge check.Donor · Lewis baseNH₃N supplies the bonding pairelectron pairAcceptor · Lewis acidBF₃Electron-deficient B accepts the pairadduct formsProduct · Lewis adductF₃B ← NH₃Bond arrow: donor → acceptoroverall charge: 0 → 0
Lewis adduct wording for marks: NH3 donates an electron pair (base) to electron-deficient BF3 (acid), so the coordinate arrow runs donor to acceptor.

Worked Examples

Modelled example 1

Apply the Arrhenius definition

Core

Problem

In aqueous solution, explain why HCl is an Arrhenius acid.
Study the worked solution

  1. Use the aqueous equation

    Method

    Show hydrogen chloride reacting with water.

    Reason

    Arrhenius classification depends on the ions produced in aqueous solution.

    Working

    HCl + H₂O → H₃O⁺ + Cl⁻

  2. Match the definition

    Method

    Identify production of H₃O⁺, equivalently H⁺(aq).

    Reason

    An Arrhenius acid increases the aqueous hydrogen-ion concentration.

    Working

    HCl is an Arrhenius acid.

Guided practice 2

Track proton transfer to ammonia

About 6 min

Problem

Explain why NH₃ is a Brønsted–Lowry base in NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.

Try this before viewing the solution

Hints

Hint 1: compare formulas
Compare NH₃ with NH₄⁺.
Hint 2: name the role
The species that gains H⁺ is a Brønsted–Lowry proton acceptor.
View solution step by step

  1. Locate the transferred proton

    Method

    Recognise that ammonia becomes ammonium.

    Reason

    NH₄⁺ differs from NH₃ by one accepted proton.

    Working

    NH₃ + H⁺ → NH₄⁺.

  2. Assign the role

    Method

    Call ammonia the Brønsted–Lowry base.

    Reason

    A Brønsted–Lowry base accepts a proton.

    Working

    NH₃ accepts H⁺ from water.

Common misconception 3

Choose the theory before applying a definition

Find and correct the mistake

Learner claim

A learner says NH₃ cannot be a base because its formula contains no OH⁻. Diagnose the claim.

Choose the relevant evidence

Ammonia is a base because it

View solution step by step

  1. Identify the mixed definitions

    Method

    Separate aqueous hydroxide production from proton-transfer classification.

    Reason

    Different acid–base models answer different questions.

    Working

    Containing OH⁻ is not the Brønsted–Lowry base definition.

  2. Use the reaction evidence

    Method

    Show ammonia accepting H⁺ to form NH₄⁺.

    Reason

    Proton acceptance is sufficient for Brønsted–Lowry classification.

    Working

    NH₃ is a Brønsted–Lowry base.

Examiner practice 4

Classify a Lewis adduct

4 marks

Problem

In BF₃ + :NH₃ → F₃B < -NH₃, identify the Lewis acid and Lewis base and justify each choice. [4 marks]

Try this before viewing the solution

View solution step by step

  1. Identify the acceptor

    1 mark

    Method

    Name BF₃ as the Lewis acid.

    Reason

    Boron is electron-deficient in BF₃.

    Working

    BF₃ accepts an electron pair.

  2. Justify the acid role

    1 mark

    Method

    State the Lewis definition explicitly.

    Reason

    A Lewis acid is an electron-pair acceptor.

    Working

    Acceptor ⇒ Lewis acid.

  3. Identify the donor

    1 mark

    Method

    Name NH₃ as the Lewis base.

    Reason

    Nitrogen supplies its lone pair to form the coordinate bond.

    Working

    NH₃ donates an electron pair.

  4. Justify the base role

    1 mark

    Method

    State the complementary Lewis definition.

    Reason

    A Lewis base is an electron-pair donor.

    Working

    Donor ⇒ Lewis base.

Challenge 5

Transfer Lewis theory to complex formation

Minimal support

Problem

For Ag⁺ + 2NH₃ → [Ag(NH₃)₂]⁺, identify the Lewis acid and base, justify the electron-pair direction, and explain why Brønsted–Lowry theory is not the direct model.

Try this before viewing the solution

Hints

Hint 1: look for proton transfer
No species gains or loses H⁺ in the written equation.
Hint 2: follow lone pairs
Each ammonia ligand donates a nitrogen lone pair to the silver ion.
View solution step by step

  1. Choose the model

    Method

    Use Lewis electron-pair theory.

    Reason

    The equation forms coordinate bonds without proton transfer.

    Working

    Brønsted–Lowry is not the direct description here.

  2. Assign donor and acceptor

    Method

    Identify Ag⁺ as acid and NH₃ as base.

    Reason

    Ag⁺ accepts lone pairs; each NH₃ donates a lone pair.

    Working

    Lewis acid: Ag⁺; Lewis base: NH₃.

Mind Stretchers

Mind stretcher 1Extension

The reaction below occurs without any proton transfer:

AlCl₃ + Cl⁻ → AlCl₄⁻

(a) State which acid–base model is appropriate. (b) Identify the acid and base and justify.

Show Answer

Mark scheme:

  • (a) Lewis (electron-pair transfer; no proton transfer).
  • (b) Lewis acid: AlCl₃ (accepts an electron pair). Lewis base: Cl⁻ (donates an electron pair).
  • It is not a Brønsted–Lowry reaction because no proton is transferred, and Arrhenius does not describe this non-aqueous adduct formation.

Mind stretcher 2Extension

Consider the reaction:

NH₃ + HCl → NH₄⁺ + Cl⁻

(a) State why Arrhenius is not the best model here. (b) Identify the Brønsted–Lowry acid and base and justify using the equation.

Show Answer

Mark scheme:

  • (a) Arrhenius is an aqueous-only model (talks about producing H⁺ / OH⁻ in water). This reaction can be discussed without an aqueous context.
  • (b) Brønsted–Lowry acid: HCl (donates H⁺). Brønsted–Lowry base: NH₃ (accepts H⁺ to form NH₄⁺).