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Arrhenius Bronsted Theories

Learn and apply Arrhenius Bronsted Theories in the published Chemistry course sequence.

  • GCE A-Level H1 Chemistry 8873-2027
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Arrhenius and Brønsted–Lowry Theories: Orientation

Choose the acid–base model that fits the evidence, then show the transferred proton explicitly when using Brønsted–Lowry theory.

H1 8873 scope
  • Lewis acid-base theory and Lewis adducts are excluded from H1.

Definitions (Must Know)

  • An Arrhenius acid produces H+(aq) in water; an Arrhenius base produces OH−(aq) in water.
  • A Brønsted–Lowry acid donates a proton, H+; a Brønsted–Lowry base accepts a proton.

Detailed Explanations

In HCl + H2O → H3O+ + Cl−, HCl donates H+ and is the Brønsted–Lowry acid; H2O accepts H+ and is the base.

In NH3 + H2O ⇌ NH4+ + OH−, NH3 accepts H+ and is the base; H2O donates H+ and is the acid.

Arrhenius theory is restricted to aqueous formation of H⁺ or OH⁻. Brønsted–Lowry theory follows proton donation and acceptance and therefore handles a wider range of reactions.

Water may act as either an acid or a base; its role depends on the reaction partner.

Worked Examples

Modelled example 1

Core application

Core

Problem

Classify HCO₃⁻ in HCO₃⁻ + H₂O ⇌ H₂CO₃ + OH⁻ and justify.
Study the worked solution

  1. Compare the species

    Method

    Track HCO₃⁻ becoming H₂CO₃.

    Reason

    The product has gained one proton.

    Working

    HCO₃⁻ + H⁺ → H₂CO₃.

  2. Apply the definition

    Method

    Classify hydrogencarbonate as the Brønsted–Lowry base.

    Reason

    A Brønsted–Lowry base accepts H⁺.

    Working

    HCO₃⁻ accepts a proton from water.

Guided practice 2

Assigning roles from an equation

About 6 min

Problem

For NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, identify the Brønsted–Lowry acid and base on the left and justify each choice.

Try this before viewing the solution

Hints

Hint 1: follow ammonia
NH₃ gains one H⁺ to become NH₄⁺.
Hint 2: follow water
H₂O loses one H⁺ to become OH⁻.
View solution step by step

  1. Identify the donor

    Method

    Name water as the acid.

    Reason

    Water donates a proton and becomes hydroxide.

    Working

    H₂O is the Brønsted–Lowry acid.

  2. Identify the acceptor

    Method

    Name ammonia as the base.

    Reason

    Ammonia accepts the proton and becomes ammonium.

    Working

    NH₃ is the Brønsted–Lowry base.

Common misconception 3

Select the model demanded by the evidence

Find and correct the mistake

Learner claim

A learner says every acid–base reaction involving water must be explained only with Arrhenius theory. Correct the claim.

Choose the direct evidence

When the equation shows proton transfer, use

View solution step by step

  1. State the Arrhenius boundary

    Method

    Reserve Arrhenius reasoning for aqueous production of H⁺ or OH⁻.

    Reason

    Its definition is tied to aqueous ions.

    Working

    Water being present does not exclude another applicable model.

  2. Follow proton transfer

    Method

    Use Brønsted–Lowry theory when one species donates and another accepts H⁺.

    Reason

    The equation directly reveals those roles.

    Working

    A reaction may be discussed through more than one applicable theory.

Examiner practice 4

Explain one reaction through two H1 theories

4 marks

Problem

For HCl + H₂O → H₃O⁺ + Cl⁻, explain why HCl is an Arrhenius acid and a Brønsted–Lowry acid, and identify the Brønsted–Lowry base. [4 marks]

Try this before viewing the solution

View solution step by step

  1. Apply Arrhenius theory

    1 mark

    Method

    State that hydrogen chloride produces aqueous hydronium ions.

    Reason

    This increases H⁺(aq) in water.

    Working

    HCl is an Arrhenius acid.

  2. Identify proton donation

    1 mark

    Method

    State that HCl donates H⁺.

    Reason

    Proton donation is the Brønsted–Lowry acid definition.

    Working

    HCl is also the Brønsted–Lowry acid.

  3. Identify the acceptor

    1 mark

    Method

    Name water as the Brønsted–Lowry base.

    Reason

    Water accepts the proton.

    Working

    H₂O + H⁺ → H₃O⁺.

  4. Use product evidence

    1 mark

    Method

    Connect acceptance to hydronium formation.

    Reason

    The changed formula verifies the direction of proton transfer.

    Working

    H₂O becomes H₃O⁺.

Challenge 5

Transfer proton roles beyond water

Minimal support

Problem

For HSO₄⁻ + NH₃ ⇌ SO₄²⁻ + NH₄⁺, identify the Brønsted–Lowry acid and base and justify both roles.

Try this before viewing the solution

Hints

Hint 1: compare hydrogensulfate
It becomes sulfate by losing one proton.
Hint 2: compare ammonia
It becomes ammonium by gaining one proton.
View solution step by step

  1. Identify the acid

    Method

    Name HSO₄⁻ as the proton donor.

    Reason

    It loses H⁺ to form SO₄²⁻.

    Working

    HSO₄⁻ is the Brønsted–Lowry acid.

  2. Identify the base

    Method

    Name NH₃ as the proton acceptor.

    Reason

    It gains H⁺ to form NH₄⁺.

    Working

    NH₃ is the Brønsted–Lowry base.

Mind Stretchers

Attempt the independent prompts before opening a hint or solution.

  • Identify the acid and base in NH4+ + H2O ⇌ NH3 + H3O+ and justify each choice.
  • Explain why NH3 is not an Arrhenius base by direct dissociation but behaves as a base in water.

Mind stretcher 1: Model selectionExtension

Question. Hydrogen chloride reacts with ammonia gas to form ammonium chloride. Explain why Brønsted–Lowry theory describes this reaction more directly than Arrhenius theory.

Show Hint

Ask whether aqueous H⁺ or OH⁻ is needed.

Show Answer

No aqueous solution is required. HCl donates a proton and NH₃ accepts it, so Brønsted–Lowry theory identifies the acid and base directly.

Mind stretcher 2: Amphoteric evidenceExtension

Question. Use two equations to show that water can act as both a Brønsted–Lowry acid and a base.

Show Hint

Pair water once with NH₃ and once with HCl.

Show Answer

With NH₃, H₂O donates H⁺: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. With HCl, H₂O accepts H⁺: HCl + H₂O → H₃O⁺ + Cl⁻.