Strong and weak acids (Higher tier)
The strength of an acid is not the same as its concentration. A dilute acid has few moles of acid per dm³; a weak acid only partially ionises in water.
Strong vs weak acids
A strong acid ionises (dissociates) completely in water:
HCl(aq) → H⁺(aq) + Cl⁻(aq) (one-way arrow)
Examples: HCl (hydrochloric), H₂SO₄ (sulfuric), HNO₃ (nitric).
A weak acid only partially ionises — there is an equilibrium between the un-ionised acid and its ions:
CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq) (reversible arrows)
Examples: ethanoic acid (vinegar), citric acid, carbonic acid.
Why pH differs
For a given concentration, a strong acid releases more H⁺ ions than a weak acid, so the strong acid has the lower pH.
Example: 0.1 mol/dm³ HCl has pH ≈ 1; 0.1 mol/dm³ ethanoic acid has pH ≈ 3 — about 100× fewer H⁺ ions.
pH and H⁺ concentration — the factor of 10 rule
Every step on the pH scale corresponds to a ×10 change in [H⁺]:
- pH 1 has 10× more H⁺ than pH 2.
- pH 1 has 100× more H⁺ than pH 3.
- pH 1 has 1000× more H⁺ than pH 4.
In general: as pH decreases by 1, [H⁺] increases by a factor of 10.
Concentration vs strength — don't confuse them
- Concentration = how many moles of acid per dm³ (depends how much you put in).
- Strength = how much of those acid molecules ionise (a property of the acid).
A "concentrated weak acid" is possible (lots of vinegar). A "dilute strong acid" is also possible (a tiny amount of HCl in lots of water).
✦Worked example
If you reduce the pH of a solution from pH 4 to pH 2, how does [H⁺] change?
The pH decreased by 2 units, so [H⁺] increased by 10² = 100 times.
Reactions of weak acids
Weak acids react with metals, carbonates etc. like strong acids — but more slowly because there are fewer H⁺ ions present.
Ethanoic acid + magnesium: still gives MgEthanoate + H₂, but bubbles slowly.
⚠Common mistakes
- Confusing strong with concentrated. They're independent.
- Saying weak acids don't ionise. They partially do — equilibrium lies to the left.
- Using one-way arrow for weak acid ionisation. Use ⇌.
- Thinking pH 4 → 2 means [H⁺] doubled. It increased by 100× (10 per unit, twice).
Links
Extends C4.7 (pH and neutralisation). Connects to C6 (rate of reaction differences) and C6.5 (equilibrium HT).
AI-generated · claude-opus-4-7 · v3-deep-chemistry