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6.1.1Additional lessons

Brønsted–Lowry Acids and Bases — IB Chemistry SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 77 of 98

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Curriculum Equation Key pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: pH + pOH = 14
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Part 1 of 5The Brønsted–Lowry Definitions

6.1.1 Brønsted–Lowry Acids and Bases

The definition focuses on what happens to the proton, not on the presence of water or hydroxide ions.

Acid
  • A Brønsted–Lowry acid is a proton (H⁺) donor, so it must contain a hydrogen atom that it can release.
Base
  • A Brønsted–Lowry base is a proton acceptor and needs a lone pair of electrons to form a bond to H⁺.
Proton transfer
  • Every acid–base reaction involves a proton moving from the acid to the base, so an acid only acts when a base is present.
Hydronium ion
  • In water the proton is carried by a water molecule as H₃O⁺, often abbreviated as H⁺(aq).

Part 2 of 5Examples in Water

Examples in Water
Hydrogen chloride
  • HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq): HCl donates a proton, so it is the acid and water is the base.
Ammonia
  • NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq): ammonia accepts a proton using its lone pair, so it is the base.
Ethanoic acid
  • CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO⁻(aq) + H₃O⁺(aq): the acid donates one proton from its carboxyl group.
Hydroxide ion
  • OH⁻ accepts a proton from an acid to form H₂O, so it is a Brønsted–Lowry base.

Part 3 of 5Identifying the Acid and the Base

1
Write the equation and look for the species that loses a hydrogen ion; that reactant is the acid.
2
Find the reactant that gains the hydrogen ion; that reactant is the base.
3
Check that the total charge is balanced on both sides of the equation.
4
Remember that the same substance may act as an acid in one reaction and a base in another.
ReactionAcid (donor)Base (acceptor)
HNO₃ + H₂O → H₃O⁺ + NO₃⁻HNO₃H₂O
NH₃ + HCl → NH₄⁺ + Cl⁻HClNH₃
H₂PO₄⁻ + H₂O ⇌ HPO₄²⁻ + H₃O⁺H₂PO₄⁻H₂O

Part 4 of 5Worked Example

Question In the reaction H₂PO₄⁻(aq) + H₂O(l) ⇌ HPO₄²⁻(aq) + H₃O⁺(aq), identify the Brønsted–Lowry acid and base on the left-hand side.

  1. Compare H₂PO₄⁻ with HPO₄²⁻one hydrogen ion has been lost.
  2. Compare H₂O with H₃O⁺one hydrogen ion has been gained.
  3. The species that loses the proton is the acid; the species that gains it is the base.

Answer H₂PO₄⁻ is the acid (proton donor) and H₂O is the base (proton acceptor).

Part 5 of 5Exam Checklist & Tips

6.1.1 – Key Points for the Exam

  • Brønsted–Lowry acid A species that donates a proton (H⁺) to another species.
  • Brønsted–Lowry base A species that accepts a proton, using a lone pair of electrons.
  • Proton A hydrogen ion, H⁺, which is transferred in acid–base reactions.
  • Hydronium ion H₃O⁺, formed when a proton attaches to a water molecule.
  • Alkali A base that dissolves in water to release hydroxide ions.

Common Mistakes

  • Thinking that only substances containing OH⁻ are bases; ammonia is a base without containing hydroxide.
  • Describing the acid as a proton acceptor, which reverses the definition.
  • Writing the proton as an electron transfer; acid–base reactions are not redox reactions.
  • Forgetting that water takes part as a base when an acid is dissolved in it.

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