📘 IB Understanding
Strong and weak acids and bases differ in the extent of their ionisation in aqueous solution. Strength is not the same as concentration.
Key Definitions
| Strong | Weak | |
|---|---|---|
| Dissociation | Complete (100%) | Partial (equilibrium) |
| Arrow | → (one-way) | ⇌ (reversible) |
| [H⁺] relative to c | [H⁺] = c | [H⁺] << c |
| Acid examples | HCl, HNO₃, H₂SO₄ | CH₃COOH, H₂CO₃, HF |
| Base examples | NaOH, KOH, Ba(OH)₂ | NH₃, CH₃NH₂ |
⚠️ Strength ≠ Concentration
Strength = degree of ionisation (a property of the substance). Concentration = moles of solute per dm³ (how much you dissolve). A dilute strong acid can have a higher pH than a concentrated weak acid.
Worked Example
Q: Compare [H⁺] in 0.10 M HCl and 0.10 M CH₃COOH.
HCl (strong): Ionises 100%, so [H⁺] = 0.10 mol dm⁻³
CH₃COOH (weak): Only partially ionises, so [H⁺] << 0.10 mol dm⁻³
📋 Exam Tip
Use a single arrow (→) for strong acid/base dissociation and a reversible arrow (⇌) for weak acid/base dissociation. Using the wrong arrow will cost marks!
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