Proton Transfer, Strong & Weak Acids
Section: 7. Acids, Bases & Salts | Syllabus: Cambridge IGCSE Chemistry (0620)
Acids and Bases as Proton Transfer
Using the Brønsted-Lowry model:
- An acid is a proton (H⁺) donor
- A base is a proton (H⁺) acceptor
Neutralisation, H⁺(aq) + OH⁻(aq) → H₂O(l), is a proton transfer: the acid donates H⁺, which the base (OH⁻) accepts.
Strong Acids vs. Weak Acids
- A strong acid is completely dissociated in aqueous solution
- A weak acid is only partially dissociated in aqueous solution
| Acid | Strength | Equation |
|---|---|---|
| Hydrochloric acid | strong | HCl(aq) → H⁺(aq) + Cl⁻(aq) |
| Ethanoic acid | weak | CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq) |
A single arrow (→) shows complete dissociation; a reversible arrow (⇌) shows that most molecules remain undissociated at equilibrium.
Worked Example: Identifying a Strong Acid
- Question: which statements about aqueous hydrochloric acid are correct? 1 - it is completely dissociated in aqueous solution. 2 - it turns thymolphthalein blue. 3 - it acts as a strong acid in aqueous solution. 4 - it produces OH⁻ ions.
- Answer: 1 and 3. HCl completely dissociates (HCl → H⁺ + Cl⁻), so it is a strong acid - these are two ways of saying the same thing. Statement 2 is wrong (thymolphthalein is colourless in acid, not blue). Statement 4 is wrong (acids produce H⁺, not OH⁻).
Concentration vs. Strength
Concentration and strength are different properties:
| What it means | Can change? | |
|---|---|---|
| Concentration | amount of acid dissolved per dm³ of solution | yes - dilute or concentrate the same acid |
| Strength | degree of dissociation into ions | no - a fixed property of that acid |
For the same concentration, a strong acid has a lower pH (more H⁺) than a weak acid, reacts faster with metals/carbonates, and conducts electricity better (more ions). A dilute strong acid and a concentrated weak acid can end up with a similar pH, since one factor can offset the other.
Worked Example: Sulfuric Acid's Two-Stage Dissociation
H₂SO₄ ⇌ H⁺ + HSO₄⁻ then HSO₄⁻ ⇌ H⁺ + SO₄²⁻
- Question: which statements are correct? 1 - HSO₄⁻ can donate and accept protons. 2 - H₂SO₄ is a proton donor. 3 - SO₄²⁻ is the only base.
- Answer: 1 and 2. HSO₄⁻ donates a proton in the second equation (forward) but accepts one in the first equation (reverse), so it is amphiprotic - statement 1 is correct. H₂SO₄ donates H⁺ in the first equation, so it is a proton donor (acid) - statement 2 is correct. Statement 3 is wrong: HSO₄⁻ also acts as a base (accepting H⁺ in the reverse of the first equation), so SO₄²⁻ is not the only base.
Common Mistakes
- ❌ Confusing "weak" with "dilute"
- ✓ weak/strong describes how much of the acid dissociates (a fixed property); dilute/concentrated describes how much acid is present per dm³ (something you control)
- ❌ Assuming an ion can only ever be an acid OR a base
- ✓ some species, like HSO₄⁻, can donate a proton in one reaction and accept one in another - this dual behaviour is called amphiprotic
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