Short on time? This page is the recap. For explanations, the resonance figure and worked examples, use the full study guide.
Recap
- Acid strength depends on how stable the conjugate base A⁻ is with its negative charge.
- Acidic hydrogens are bonded to electronegative atoms (usually O or a halogen). C–H hydrogens are not acidic in water.
- Strong acids: HCl, HBr, HI, HClO₄, H₂SO₄, HNO₃. Their conjugate bases are very weak.
- Strong bases: group 1 and heavier group 2 hydroxides (NaOH, KOH, Ca(OH)₂, Ba(OH)₂). Their conjugate acid, water, is very weak.
- Weak acids: carboxylic acids, R–COOH. Weak bases: nitrogen bases (NH₃, amines) and carboxylate ions, R–COO⁻.
Key relationships
| Structural feature | Effect on the conjugate base | Example (acid strength increases →) |
|---|---|---|
| More electronegative atom bearing the charge (same period) | holds charge better | CH₄ < NH₃ < H₂O < HF |
| More electronegative central atom in an oxoacid | pulls density from O–H | HOI < HOBr < HOCl |
| Electronegative atoms nearby (inductive effect) | charge spread through bonds | CH₃COOH < ClCH₂COOH < Cl₂CHCOOH < Cl₃CCOOH |
| More O atoms on the central atom | inductive pull plus resonance | HClO < HClO₂ < HClO₃ < HClO₄ |
| Resonance in A⁻ | charge shared by several atoms | CH₃CH₂OH ≪ CH₃COOH |
| Larger anion down a group | charge spread over bigger volume, weaker H–X bond | HF < HCl < HBr < HI |
| Conjugate pair | Ka × Kb = Kw (1.0 × 10⁻¹⁴ at 25 °C) | weaker acid ↔ stronger conjugate base |
Assumptions behind the comparisons
- Compare closely related acids, changing one feature at a time.
- Strength means extent of ionisation in water at the same temperature.
- Electronegativity arguments apply across a period or for atoms near the acidic site, not to the halide ion itself down a group.
Mistakes to avoid
- Counting every H as acidic. Check what each H is bonded to.
- Talking only about the acid. Explain using the stability of the conjugate base.
- Reversing the halogen acid trend. HF is weak; HI is strongest.
- Confusing strong with concentrated. Strength is fixed by structure, not by concentration.
- Saying a strong acid has a strong conjugate base. It is the reverse.
- Naming a feature without its effect. “Cl is electronegative” earns little; “Cl pulls electron density and stabilises the negative charge on A⁻” earns the point.
Quick self-check
- How many acidic hydrogens does HCOOH (methanoic acid) have? (One: the H on O)
- Which is stronger, HOCl or HOI? (HOCl: Cl is more electronegative)
- Ka of HF is 6.8 × 10⁻⁴. What is Kb of F⁻? (1.5 × 10⁻¹¹)
- Why is ethoxide a stronger base than acetate? (Ethoxide’s charge sits on one O; acetate shares its charge over two O atoms by resonance.)
Next: practice questions.