📘 IB Understanding
Acids and bases are classified as strong or weak based on the extent of their ionisation in aqueous solution. This is distinct from concentration.
Strong Acids & Bases SL
Strong acids (e.g. HCl, HNO3, H2SO4) and strong bases (e.g. NaOH, KOH) dissociate almost 100% into ions in solution. The reaction goes to completion.
HCl(aq) + H2O(l) → H3O+(aq) + Cl-(aq)
Weak Acids & Bases SL
Weak acids (e.g. CH3COOH, H2CO3) and weak bases (e.g. NH3) only partially ionise. They set up a dynamic equilibrium heavily favouring the undissociated form.
CH3COOH(aq) + H2O(l) ⇌ CH3COO-(aq) + H3O+(aq)
Comparing Strong vs Weak (Same Concentration)
| Property | Strong Acid | Weak Acid |
|---|---|---|
| Dissociation | Complete (100%) | Partial (< 5%) |
| pH (0.1 M) | ~1 | ~3 |
| Electrical conductivity | High | Low |
| Reaction rate with metals | Fast | Slow |
| Equilibrium arrow | → (one way) | ⇌ (reversible) |
❗ Common Misconception
Strength ≠ concentration. Strength describes the proportion of molecules that ionise. Concentration describes the amount of solute per unit volume. You can have a concentrated weak acid or a dilute strong acid.
Worked Example
Q: Explain why 0.1 M HCl has a lower pH than 0.1 M CH3COOH.
A: HCl is a strong acid that fully dissociates, so [H+] = 0.1 M and pH = 1.0. CH3COOH is a weak acid that only partially ionises, so [H+] << 0.1 M and pH is higher (around 2.9).