Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- Brønsted-Lowry acid: proton (H⁺) donor. Brønsted-Lowry base: proton acceptor, using a lone pair.
- A base does not need OH⁻: NH₃, CO₃²⁻ and H₂O can all accept protons.
- Each proton transfer has two conjugate pairs: acid / conjugate base and base / conjugate acid.
- Water can accept a proton (becoming H₃O⁺) or donate one (becoming OH⁻). Species that can do both are amphiprotic.
- H⁺(aq) and H₃O⁺(aq) mean the same thing in equations.
- The course uses the Brønsted-Lowry model in aqueous solution; the Lewis model is not assessed.
Key relationships
| Relationship | Rule | Example |
|---|---|---|
| Conjugate base | acid − H⁺; charge down 1 | HSO₃⁻ → SO₃²⁻ |
| Conjugate acid | base + H⁺; charge up 1 | NH₃ → NH₄⁺ |
| Water as base | H₂O + H⁺ → H₃O⁺ | with HNO₂ |
| Water as acid | H₂O − H⁺ → OH⁻ | with CH₃NH₂ |
| Strength link | stronger acid ↔ weaker conjugate base | HCl / Cl⁻ (negligible base) |
Strong and weak acids
| Strong acid | Weak acid | |
|---|---|---|
| Ionisation in water | essentially complete | partial; mostly molecules |
| Particles in a model | H₃O⁺ and anions only | mostly HA, a few H₃O⁺ and A⁻ |
| Conjugate base | negligible base | weak base that takes protons back |
| Examples | HCl, HBr, HI, HNO₃, HClO₄, H₂SO₄ (first H⁺) | HF, HNO₂, ethanoic acid |
Assumptions
- Reactions are in aqueous solution, where water takes part.
- “Strong” and “weak” compare how fully acids ionise, not how concentrated they are.
Mistakes to avoid
- Charge not changed when writing a conjugate (SO₃⁻ instead of SO₃²⁻).
- Pairs differing by two H⁺ (H₂SO₃ and SO₃²⁻ are not a pair).
- Labelling only one pair in an equation; there are always two.
- “Conjugate base of a strong acid is strong.” It is negligibly weak.
- Confusing strong with concentrated.
- Ignoring water as a reactant.
Quick self-check
- Conjugate base of NH₄⁺? (NH₃)
- Conjugate acid of OH⁻? (H₂O)
- Conjugate base of HCO₃⁻? (CO₃²⁻)
- In HF + H₂O ⇌ H₃O⁺ + F⁻, what role does water play? (Base: it accepts the proton.)
- HNO₂ is weak and HNO₃ is strong. Which is the stronger base, NO₂⁻ or NO₃⁻? (NO₂⁻)
Next: practice questions.