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
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5The Brønsted–Lowry Definitions
The definition focuses on what happens to the proton, not on the presence of water or hydroxide ions.
Acid
- Acid A Brønsted–Lowry acid is a proton (H⁺) donor, so it must contain a hydrogen atom that it can release.
Base
- Base A Brønsted–Lowry base is a proton acceptor and needs a lone pair of electrons to form a bond to H⁺.
Proton transfer
- 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
- 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
- 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
- Ammonia NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq): ammonia accepts a proton using its lone pair, so it is the base.
Ethanoic acid
- Ethanoic acid CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO⁻(aq) + H₃O⁺(aq): the acid donates one proton from its carboxyl group.
Hydroxide ion
- 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.
| Reaction | Acid (donor) | Base (acceptor) |
|---|---|---|
| HNO₃ + H₂O → H₃O⁺ + NO₃⁻ | HNO₃ | H₂O |
| NH₃ + HCl → NH₄⁺ + Cl⁻ | HCl | NH₃ |
| 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.
- Compare H₂PO₄⁻ with HPO₄²⁻one hydrogen ion has been lost.
- Compare H₂O with H₃O⁺one hydrogen ion has been gained.
- 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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