📖 In-Depth Theory
Strong vs Weak Acids — Degree of Ionisation
All acids produce H⁺ ions when dissolved in water — but they differ in how COMPLETELY they ionise.
STRONG ACIDS — FULLY ionise in water (→ one-way arrow):
All of the acid molecules dissociate into H⁺ and the conjugate base.
HCl → H⁺ + Cl⁻ (complete dissociation)
H₂SO₄ → 2H⁺ + SO₄²⁻
HNO₃ → H⁺ + NO₃⁻
At a given concentration, a strong acid has the MAXIMUM possible H⁺ concentration.
WEAK ACIDS — PARTIALLY ionise (⇌ reversible arrow):
Only a small fraction of acid molecules dissociate — the rest remain as molecules.
CH₃COOH ⇌ CH₃COO⁻ + H⁺ (ethanoic acid — only ~1% ionised)
H₂CO₃ ⇌ H⁺ + HCO₃⁻ (carbonic acid)
HF ⇌ H⁺ + F⁻ (hydrofluoric acid)
Citric acid, lactic acid — weak acids found in foods.
pH Difference Between Strong and Weak Acids
At the SAME CONCENTRATION:
A strong acid has a LOWER pH than a weak acid.
Because: the strong acid provides far more H⁺ ions per dm³.
EXAMPLE:
0.1 mol/dm³ HCl (strong): nearly all molecules ionise → [H⁺] ≈ 0.1 mol/dm³ → pH ≈ 1.
0.1 mol/dm³ CH₃COOH (weak): ~1% ionised → [H⁺] ≈ 0.001 mol/dm³ → pH ≈ 3.
Both have the SAME CONCENTRATION but very different pH values.
EFFECT OF DILUTION on weak acids:
Adding water → equilibrium shifts RIGHT (more ionisation) → slightly more H⁺.
Weak acids become relatively more ionised on dilution.
COMPARING SAME pH:
A weak acid at pH 3 contains MORE acid molecules than a strong acid at pH 3.
The weak acid solution is at HIGHER CONCENTRATION to achieve the same pH.
Comparing Reactions of Strong and Weak Acids
At the SAME CONCENTRATION — a strong acid reacts MORE VIGOROUSLY than a weak acid because it has a higher [H⁺].
With magnesium:
0.1 mol/dm³ HCl: vigorous fizzing (high [H⁺]).
0.1 mol/dm³ CH₃COOH: gentle fizzing (low [H⁺] — only partly ionised).
But: if EXCESS acid is used, both eventually produce the SAME amount of H₂.
Why: both have the same TOTAL number of acid molecules — weak acid eventually fully reacts.
With sodium carbonate:
Strong acid: effervescence faster.
Weak acid: effervescence slower — same total CO₂ ultimately produced.
Conductivity comparison:
Strong acid: better electrical conductor (more ions in solution).
Weak acid: poorer conductor (fewer ions in solution at same concentration).
⚠️ Common Mistake
Strong acid ≠ concentrated acid. A STRONG acid is fully ionised (complete dissociation). A CONCENTRATED acid has a high number of moles per dm³. You can have a dilute strong acid (e.g. 0.001 mol/dm³ HCl) or a concentrated weak acid (e.g. 5 mol/dm³ CH₃COOH). These are independent concepts.
🎯 Test Yourself
Question 1 of 2
1. Two acids, 0.1 mol/dm³ HCl and 0.1 mol/dm³ CH₃COOH, are compared. Which has a lower pH and why?
Both have the same pH — they are the same concentration
HCl — it fully ionises, producing [H⁺] ≈ 0.1 mol/dm³. CH₃COOH only partially ionises, producing far less H⁺.
CH₃COOH — weak acids are more reactive and produce more H⁺
HCl — HCl molecules are larger and release more H⁺ per molecule
2. Is it possible to have a dilute solution of a strong acid?
No — strong acids must always be concentrated
Yes — but only if a catalyst is added to maintain ionisation
Yes — strong means fully ionised, dilute means low concentration. A 0.001 mol/dm³ HCl solution is dilute but still a strong acid.
No — diluting an acid makes it weak