6.1.1Additional lessons
Brønsted–Lowry Acids and Bases — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 116 of 172
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.
Section 1 of 4The Definitions
Proton transfer is the basis of all acid–base chemistry in this model.
Acid
- Acid A Brønsted–Lowry acid is a proton donor, meaning it gives away a hydrogen ion H⁺ to another species.
Base
- Base A Brønsted–Lowry base is a proton acceptor, meaning it uses a lone pair to take an H⁺ from another species.
Proton
- Proton In this model a proton is simply a hydrogen ion, H⁺, because a hydrogen atom that loses its electron is a bare proton.
Both needed
- Both needed An acid can only donate its proton if a base is present to accept it, so the two always react together.
Section 2 of 4In Water
In Water
Hydronium ion
- Hydronium ion In aqueous solution H⁺ is attached to a water molecule as H₃O⁺, but H⁺(aq) is accepted in equations.
Hydrogen chloride
- Hydrogen chloride HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq); water acts as the base by accepting the proton.
Ammonia
- Ammonia NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq); here water acts as the acid by donating a proton.
Alkalis
- Alkalis An alkali is a base that dissolves in water, so all alkalis are bases but not all bases are alkalis.
Section 3 of 4Recognising Acids and Bases
1
Look at the reactants and find which species loses an H⁺ in the equation; this species is the acid.
2
Find the species that gains an H⁺; this species is the base, and it must have a lone pair available.
3
Check that the charge is balanced: an acid that loses H⁺ loses one positive charge.
4
Remember that the same compound can behave as an acid in one reaction and as a base in another.
5
Use ⇌ for reactions that reach equilibrium and → for those that go essentially to completion.
Section 4 of 4Exam Checklist & Tips
6.1.1 – Key Points for the Exam
- Brønsted–Lowry acid A substance that donates a proton (H⁺) to another species.
- Brønsted–Lowry base A substance that accepts a proton (H⁺) from another species.
- Alkali A base that is soluble in water and produces OH⁻ ions in solution.
- Proton transfer The movement of an H⁺ from an acid to a base.
Common Mistakes
- Defining a base only as something that produces OH⁻; the Brønsted–Lowry definition is proton acceptance.
- Describing the acid as an electron acceptor, when this model is about proton transfer.
- Treating water as always neutral, when it can act as both acid and base.
- Forgetting that a proton transfer needs both an acid and a base.
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