Strong and Weak Acids

Section 1 of 7Ionisation in water (HT only)

Acids release H⁺

• Acids release H⁺ When an acid dissolves in water its molecules split into hydrogen ions and a negative ion.
• In generalHX → H⁺ + X⁻.

Strength is about the extent of ionisation

• Strength is about the extent of ionisation A strong acid ionises completely, so every molecule splits into ions.
• A weak acid ionises only partially, so most molecules remain intact.

Effect on pH

• Effect on pH More H⁺ ions at the same concentration means a lower pH.
• Strong acids typically have pH 0–3 while weak acids are around pH 4–6.

Section 2 of 7Strong versus weak (HT only)

Feature	Strong acid	Weak acid
Ionisation	Complete	Partial
Arrow in equation	→ (goes to completion)	⇌ (reversible)
Concentration of H+	High	Low
Typical pH at same concentration	Low	Higher, nearer the middle of the scale
Examples	Hydrochloric, nitric, sulfuric	Ethanoic, citric, carbonic

Section 3 of 7Equations to know (HT only)

Ionisation equations

Hydrochloric acid

• Hydrochloric acid HCl(aq) → H⁺(aq) + Cl⁻(aq)

Nitric acid

• Nitric acid HNO₃(aq) → H⁺(aq) + NO₃⁻(aq)

Sulfuric acid

• Sulfuric acid H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq)

Ethanoic acid

• Ethanoic acid CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq)

Section 4 of 7Equilibrium in a weak acid (HT only)

1Dissolve

• Dissolve Weak acid molecules enter the water.

2Some ionise

• Some ionise A few molecules split into H⁺ and anions.

3Reverse reaction

• Reverse reaction Ions recombine to give molecules again, so the change is reversible.

4Equilibrium lies left

• Equilibrium lies left Mostly molecules, few ions, so the H⁺ concentration is low.

Section 5 of 7Strength versus concentration

Strength

• Strength A property of the acid itself: how much it ionises in water.

Concentration

• Concentration The amount of acid dissolved per dm³;
  • you can have a dilute strong acid or a concentrated weak acid.

Reactions with alkali

• Reactions with alkali Weak acids still neutralise alkalis completely, for example CH₃COOH + NaOH → CH₃COONa + H₂O.

Section 6 of 7Key terms

Term	Meaning
Strong acid	An acid that ionises completely in water.
Weak acid	An acid that ionises only partially in water.
Ionisation	The splitting of a molecule into ions when it dissolves.
Reversible reaction	A reaction in which products can react to re-form the reactants, shown by ⇌.
Concentration	The amount of a substance dissolved in a given volume of solution.

Section 7 of 7Exam technique

1

Confusing strong with concentrated; strength is about ionisation, concentration is about quantity per volume.

2

Writing a single arrow for weak acids when the reaction is reversible.

3

Saying weak acids do not neutralise alkalis fully — they do, only more slowly to ionise.

4

Assuming a weak acid has no H⁺ ions; it has fewer, not none.