AP®ChemistryCollege BoardRevision NotesUnit 4: Chemical ReactionsBrønsted-Lowry Acids & BasesBrønsted-Lowry Acids & Bases

Brønsted-Lowry Acids & Bases (College Board AP® Chemistry): Revision Note

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Written by: Martín

Reviewed by: Stewart Hird

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Brønsted-Lowry Acids & Bases

  • The Brønsted-Lowry theory defines acids and bases in terms of proton transfer

    • A Brønsted-Lowry acid donates a proton (H+): it is a proton donor

    • A Brønsted-Lowry base accepts a proton (H+): it is a proton acceptor

  • The Brønsted-Lowry model is not limited to aqueous solutions: it also applies to reactions in the gas phase. AP Chemistry focuses on reactions in aqueous solution

Brønsted-Lowry acid reacting with a Brønsted-Lowry base

Diagram of a Brønsted–Lowry acid–base equilibrium: HA plus B with a lone pair is in equilibrium with A⁻ plus HB⁺. Labels: proton (on H), Brønsted–Lowry acid (HA), Brønsted–Lowry base (B), lone pair of electrons (on B).
The Brønsted-Lowry acid donates a proton to the Brønsted-Lowry base that accepts a proton

Examples

  • HCl (aq) + H2O (l) → H3O+ (aq) + Cl- (aq)

    • Brønsted-Lowry acid = HCl

    • Brønsted-Lowry base = H2O

  • NH3 (aq) + H2O (l) ⇌ NH4+ (aq) + OH- (aq)

    • Brønsted-Lowry acid = H2O

    • Brønsted-Lowry base = NH3

    • NH3 is a weak base, so only a small fraction of NH3 molecules accept a proton

  • CH3COOH (aq) + NH3 (aq) → CH3COO- (aq) + NH4+ (aq)

    • Brønsted-Lowry acid = CH3COOH

    • Brønsted-Lowry base = NH3

  • H2SO3 (aq) + H2O (l) ⇌ HSO3- (aq) + H3O+ (aq)

    • Brønsted-Lowry acid = H2SO3

    • Brønsted-Lowry base = H2O

  • H2PO4- (aq) + NH3 (aq) → HPO42- (aq) + NH4+ (aq)

    • Brønsted-Lowry acid = H2PO4-

    • Brønsted-Lowry base = NH3

Examiner Tips and Tricks

  • To justify that a species is a Brønsted–Lowry acid or base, name the species that donates the proton, the species that accepts it, and the H+ that moves between them

  • Weak acids and weak bases react only partly with water, so write their reactions with ⇌, e.g. NH3 (aq) + H2O (l) ⇌ NH4+ (aq) + OH– (aq)

  • Nitrogen bases such as hydrazine (H2NNH2) and ammonia are bases: they accept a proton

Worked Example

Identify the Brønsted-Lowry acid and the Brønsted-Lowry base in the following chemical reaction

H2SO4(aq) + H2O(l) → HSO4−(aq) + H3O+(aq)

 

Brønsted-Lowry acid

Brønsted-Lowry base

(A)

H2SO4

 H3O+

(B)

H2SO4

H2O

(C)

H2O

H3O+

(D)

H2O

H2SO4

Answer:

  • The correct option is B

    • H2SO4 donates a proton to H2O

    • So, H2SO4 must be the Brønsted-Lowry acid and H2O the Brønsted-Lowry base

Water as a Brønsted-Lowry Acid & Base

  • In aqueous solution, water takes part in many acid–base reactions

  • Water can act both as a Brønsted-Lowry acid and Brønsted-Lowry base

    • Species that can donate or receive protons are called amphiprotic

  • Water's structure allows this:

    • A lone pair on the oxygen atom can accept H+ (forming H3O+)

    • A polar O–H bond can release H+ (forming OH–)

Water as a Brønsted-Lowry acid

  • An equation showing water acting as Brønsted-Lowry acid is:

NH3 (aq) + H2O (l) ⇌ OH– (aq) + NH4+ (aq)

  • Water (H2O) is the Brønsted-Lowry acid: it donates a proton (H+) to ammonia

  • Ammonia (NH3) is the Brønsted-Lowry base: it accepts the proton

Lewis structure for the reaction between water and ammonia

Dot-and-cross diagrams of water reacting with ammonia. A lone pair on nitrogen forms a bond with a hydrogen ion from water, giving NH₄⁺. The electron from the broken O–H bond stays on oxygen, giving OH⁻ a negative charge.
Lewis structures that show the reaction between water and ammonia, and the charges they acquire when they become ions

Water as a Brønsted-Lowry base

  • An equation showing water acting as Brønsted-Lowry base is:

HCl (g) + H2O (l) → Cl– (aq) + H3O+ (aq)

  • Hydrogen chloride (HCl) is the Brønsted-Lowry acid: it donates a proton (H+)

  • Water (H2O) is the Brønsted-Lowry base: it accepts the proton

Lewis structure for the reaction between water and hydrochloric acid

Dot-and-cross diagrams of hydrogen chloride reacting with water. A lone pair on oxygen forms a bond with the hydrogen ion from HCl, giving H₃O⁺. The electron from the broken H–Cl bond stays on chlorine, giving Cl⁻.
Lewis structures that show the reaction between water and hydrochloric acid, and the charges they acquire when they become ions

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Martín

Author: Martín

Expertise: Chemistry Content Creator

Martín, a dedicated chemistry teacher and tutor, excels in guiding students through IB, AP, and IGCSE Chemistry. As an IB Chemistry student, he came from hands-on preparation, focusing on practical exam techniques and rigorous practice. While at Universidad San Francisco de Quito, his academic journey sparked a passion for computational and physical chemistry. Martín specializes in chemistry, and he knows that SaveMyExams is the right place if he wants to have a positive impact all around the world.

Stewart Hird

Reviewer: Stewart Hird

Expertise: Chemistry Content Creator

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.