HL Extension
Quantitative Dissociation Extents
At Higher Level, equilibrium quantitative treatments apply: strong acids dissociate completely (\(K_{\text{a}} \gg 1\)), while weak acids establish an equilibrium characterised by an equilibrium dissociation constant \(K_{\text{a}} < 10^{-2}\).
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 |
Examiner Trap
Strength vs Concentration in Quantitative Problems
Never confuse acid strength with concentration:
- Concentration: Total moles of acid molecules added per \(\text{dm}^3\) of solution.
- Strength: The percentage fraction of those acid molecules that actually ionise into \(\text{H}^+\) and \(\text{A}^-\).
Worked Example
Worked Example
Quantitative Comparison of [H⁺]
Problem: Compare \([\text{H}^+]\) in \(0.10\text{ M HCl}\) and \(0.10\text{ M CH}_3\text{COOH}\) (\(K_{\text{a}} = 1.8 \times 10^{-5}\)).
- \(\text{HCl}: [\text{H}^+] = 0.10\text{ M}\) (100% ionised).
- \(\text{CH}_3\text{COOH}: [\text{H}^+] = \sqrt{K_{\text{a}} \times c} = \sqrt{1.8 \times 10^{-5} \times 0.10} = 1.34 \times 10^{-3}\text{ M}\) (only ~1.3% ionised).
Exam Tip
Arrow Notation in Acid Equations
In all HL written mechanisms and equations, use a single forward arrow (\(\rightarrow\)) for strong acids and a reversible equilibrium arrow (\(\rightleftharpoons\)) for weak acids.
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