Acid–Base Theories
Brønsted–Lowry Acids and Bases · The pH Scale and Acid Strength
IB Chemistry topic guide
Reactivity 3: Mechanisms of Change
Reactivity 3: Mechanisms of Change is a core part of IB Chemistry. This guide connects the syllabus ideas behind Acid–Base Theories, pH Calculations and Buffers, Oxidation States and Redox, Electrochemical Cells, Substitution Reactions and 2 more units, shows how they appear in worked problems, and points you to the formulas and full lessons needed for exam revision.
What you will learn
Reactivity 3: Mechanisms of Change syllabus outline
The units below follow the structure used in the full Study to Learn course. Use the outline to identify exactly which idea needs attention, then work through the public example before continuing to the complete lesson path.
pH Calculations and Buffers
Weak Acids, Kₐ, and Buffer Solutions · Titration Curves and Indicators
Oxidation States and Redox
Oxidation and Reduction · Balancing Redox Equations
Electrochemical Cells
Voltaic (Galvanic) Cells · Electrolytic Cells and Applications
Substitution Reactions
Free Radical Substitution · Nucleophilic Substitution: Sₙ1 and Sₙ2
Addition Reactions
Electrophilic Addition to Alkenes · Addition Polymerisation and Markovnikov’s Rule
Electron-Pair Sharing Reactions
Condensation Polymerisation · Polymer Properties and Environmental Impact
Free worked preview
Brønsted–Lowry Acids and Bases
This complete preview comes from the Acid–Base Theories unit. It introduces the core language, shows the method in context, and gives you a real example of the lesson quality before you create an account.
Defining Acids and Bases by Proton Transfer
Acids are not just “things that taste sour” — the Brønsted–Lowry theory defines them rigorously as proton donors and bases as proton acceptors, creating conjugate acid–base pairs. This lesson establishes the theoretical foundation for all acid–base chemistry, which extends into buffers, titrations, and biochemical systems. After this lesson, you will be able to identify the acid, base, conjugate acid, and conjugate base in any proton-transfer reaction, understand the critical distinction between strong and weak acids/bases, and explain why water is amphiprotic.
A Brønsted–Lowry acid is a proton (H⁺) donor. A Brønsted–Lowry base is a proton acceptor. When an acid donates a proton, it forms its conjugate base; when a base accepts a proton, it forms its conjugate acid. Together, they form a conjugate acid–base pair that differs by exactly one H⁺.
Conjugate Pairs
HCl + H₂O ⇌ H₃O⁺ + Cl⁻. HCl (acid, donates H⁺) / Cl⁻ (conjugate base). H₂O (base, accepts H⁺) / H₃O⁺ (conjugate acid). Water is amphiprotic (can act as either an acid or a base depending on the other reactant).
Strong vs Weak
- Strong acid: completely dissociates in water (essentially 100%). HCl, HNO₃, H₂SO₄. HCl → H⁺ + Cl⁻.
- Weak acid: partially dissociates; exists as an equilibrium. CH₃COOH ⇌ CH₃COO⁻ + H⁺. Most molecules remain undissociated.
- Strong base: completely dissociates (NaOH, KOH).
- Weak base: partially protonates (NH₃, amines). NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
Common Error“Strong” and “concentrated” mean different things. A strong acid is fully dissociated regardless of concentration. Dilute HCl is still a strong acid; concentrated ethanoic acid is still a weak acid. Strength = extent of dissociation; concentration = amount of solute per volume.
Reviewed by the Study to Learn editorial team · Updated 2026-08-04