IB Understanding
Classification by Ionisation Extent
Acids and bases are classified by the extent of their ionisation in water:
- Strong acids & bases: Fully ionise in aqueous solution (e.g. \(\text{HCl} \rightarrow \text{H}^+ + \text{Cl}^-\)). Single arrow.
- Weak acids & bases: Only partially ionise, establishing a dynamic equilibrium (e.g. \(\text{CH}_3\text{COOH} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}^+\)). Reversible arrow.
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) |
Examiner Trap
Strength vs Concentration Distinction
Strength ≠ Concentration:
- Strength: Extent of ionisation/dissociation in solution (fixed intrinsic property).
- Concentration: Amount of solute dissolved per unit volume in \(\text{mol dm}^{-3}\) (variable).
- You can have a dilute solution of a strong acid (\(0.001\text{ M HCl}\)) and a concentrated solution of a weak acid (\(10\text{ M CH}_3\text{COOH}\)).
Worked Example
Comparing Strong and Weak Acid pH
Question: Compare the \([\text{H}^+]\) and pH of \(0.10\text{ mol dm}^{-3}\text{ HCl}\) and \(0.10\text{ mol dm}^{-3}\text{ CH}_3\text{COOH}\).
- 0.10 M HCl (Strong): 100% ionised → \([\text{H}^+] = 0.10\text{ M} \implies \text{pH} = 1.00\).
- 0.10 M CH₃COOH (Weak, ~1.3% ionised): \([\text{H}^+] \approx 1.34 \times 10^{-3}\text{ M} \implies \text{pH} = 2.87\).
AQA GCSE & IB Chemistry
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