Strong and Weak Acids (Higher)

This lesson provides a detailed treatment of strong and weak acids for Edexcel GCSE Chemistry (1CH0) Higher tier. You need to understand the difference between strong and weak acids in terms of ionisation, distinguish this from concentration, and explain how it affects pH and rate of reaction.

---

Ionisation of Acids in Water

When an acid dissolves in water, its molecules dissociate (split apart) to produce hydrogen ions (H⁺) and negative ions. The extent to which this happens determines whether the acid is classified as strong or weak.

---

Strong Acids

A strong acid is one that completely ionises (fully dissociates) in aqueous solution. Every single molecule of the acid breaks apart to form ions.

Examples of Strong Acids

Strong Acid	Formula	Ionisation Equation
Hydrochloric acid	HCl	HCl(aq) → H⁺(aq) + Cl⁻(aq)
Sulfuric acid	H₂SO₄	H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq)
Nitric acid	HNO₃	HNO₃(aq) → H⁺(aq) + NO₃⁻(aq)

Key Features

• The equation uses a single forward arrow (→) to show the reaction goes to completion.
• There are no undissociated acid molecules left in solution.
• The concentration of H⁺ ions equals the concentration of the acid (for a monoprotic acid like HCl).

---

Weak Acids

A weak acid is one that only partially ionises (partially dissociates) in aqueous solution. Only a small fraction of the acid molecules break apart at any given time. The majority remain as intact, undissociated molecules.

Examples of Weak Acids

Weak Acid	Formula	Ionisation Equation
Ethanoic acid (in vinegar)	CH₃COOH	CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)
Citric acid (in citrus fruits)	C₆H₈O₇	C₆H₈O₇(aq) ⇌ C₆H₇O₇⁻(aq) + H⁺(aq)
Carbonic acid (in fizzy drinks)	H₂CO₃	H₂CO₃(aq) ⇌ HCO₃⁻(aq) + H⁺(aq)

Key Features

• The equation uses a reversible arrow (⇌) to show that the reaction reaches a dynamic equilibrium.
• The equilibrium lies far to the left — most molecules remain undissociated.
• The concentration of H⁺ ions is much less than the concentration of the acid.

---

Visualising Strong vs Weak Acid Dissociation

graph LR
    subgraph Strong["Strong Acid (e.g. 1.0 mol/dm³ HCl)"]
        direction TB
        S1["100 HCl molecules<br/>dissolve in water"]
        S2["ALL 100 molecules<br/>dissociate"]
        S3["100 H⁺ ions<br/>+ 100 Cl⁻ ions"]
        S1 --> S2 --> S3
    end

    subgraph Weak["Weak Acid (e.g. 1.0 mol/dm³ CH₃COOH)"]
        direction TB
        W1["100 CH₃COOH molecules<br/>dissolve in water"]
        W2["Only ~1–2 molecules<br/>dissociate"]
        W3["~1–2 H⁺ ions<br/>+ ~98–99 undissociated<br/>CH₃COOH molecules"]
        W1 --> W2 --> W3
    end

    style S3 fill:#e74c3c,color:#fff
    style W3 fill:#f39c12,color:#fff

---

Comparing Strong and Weak Acids at the Same Concentration

If you have a strong acid and a weak acid at the same concentration (e.g. both 0.1 mol/dm³), they will differ in several measurable ways:

Property	Strong Acid (e.g. 0.1 mol/dm³ HCl)	Weak Acid (e.g. 0.1 mol/dm³ CH₃COOH)
Ionisation	Fully ionised	Partially ionised
[H⁺] in solution	High (0.1 mol/dm³)	Low (much less than 0.1 mol/dm³)
pH	Low (pH 1)	Higher (pH ≈ 3)
Rate of reaction with Mg ribbon	Fast — vigorous fizzing	Slow — gentle fizzing
Electrical conductivity	High (many ions)	Low (few ions)
Volume of H₂ produced with excess Mg	Same	Same

Why Is the Volume of Gas the Same?

Even though the weak acid reacts more slowly, it contains the same total number of moles of acid as the strong acid (same concentration and volume). As the H⁺ ions are used up by the reaction, the equilibrium for the weak acid shifts to the right, producing more H⁺ ions. Eventually, all the acid is used up and the same total amount of hydrogen gas is produced.

The difference is rate, not total yield.

Exam Tip: This is a very common exam question. Students often say that the weak acid produces less hydrogen. This is WRONG. At the same concentration and volume, both produce the same total volume of hydrogen. The weak acid just takes longer.

---

Concentration vs Strength: The Critical Distinction

Term	Definition	Example
Concentrated	A large amount of solute dissolved per unit volume	2.0 mol/dm³ HCl
Dilute	A small amount of solute dissolved per unit volume	0.01 mol/dm³ HCl
Strong	Fully ionised in solution	HCl (at any concentration)
Weak	Partially ionised in solution	CH₃COOH (at any concentration)

Key Point

• A concentrated weak acid (e.g. 2.0 mol/dm³ ethanoic acid) has a lot of acid dissolved, but most of it is not ionised.
• A dilute strong acid (e.g. 0.001 mol/dm³ HCl) has very little acid dissolved, but all of it is ionised.

You can have any combination of these:

	Dilute	Concentrated
Strong	Dilute strong acid (e.g. 0.01 mol/dm³ HCl)	Concentrated strong acid (e.g. 6 mol/dm³ HCl)
Weak	Dilute weak acid (e.g. 0.01 mol/dm³ CH₃COOH)	Concentrated weak acid (e.g. 6 mol/dm³ CH₃COOH)

Exam Tip: If an exam question says "explain the difference between a strong acid and a concentrated acid," you must make it clear that strong/weak refers to ionisation (how much the acid dissociates) while dilute/concentrated refers to the amount of acid dissolved per unit volume. These are two independent properties.

---

Effect on pH

For a given concentration:

• A strong acid has a lower pH (more H⁺ ions).
• A weak acid has a higher pH (fewer H⁺ ions).

For strong acids, if you increase the concentration by a factor of 10, the pH decreases by 1. For example:

• 0.1 mol/dm³ HCl has pH 1
• 0.01 mol/dm³ HCl has pH 2
• 0.001 mol/dm³ HCl has pH 3

This relationship does not hold as simply for weak acids because of the equilibrium.

---

Summary