5.8 Strong and Weak Acids

A free GCSE Chemistry lesson for AQA (8462) from OBA School: (Higher tier topic) The difference between strong and weak acids, concentrated and dilute acids, and how pH is linked to hydrogen ion concentration. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain the difference between strong and weak acids in terms of ionisation, with examples
• Explain the difference between the strength and the concentration of an acid
• Explain why a weak acid has a higher pH and reacts more slowly than a strong acid of the same concentration
• Use the rule that a decrease of 1 pH unit means the H⁺ concentration increases by a factor of 10

Key words

• Ionise: Split up into ions. Acids ionise in water to release H⁺(aq) ions.
• Strong acid: An acid that is completely ionised in aqueous solution, e.g. hydrochloric, nitric and sulfuric acids.
• Weak acid: An acid that is only partially ionised in aqueous solution, e.g. ethanoic, citric and carbonic acids.
• Concentration: The amount of acid (or of a substance) in a given volume of solution, e.g. in mol/dm³ or g/dm³.
• Dilute: A small amount of acid in a given volume of water (low concentration).
• Concentrated: A large amount of acid in a given volume of water (high concentration).

Strong and weak acids

Higher tier: This whole topic is Higher tier only.

A strong acid ionises completely in water: every acid molecule releases its hydrogen ions. A weak acid ionises only partially: most molecules stay whole, and only a small fraction release H⁺ ions. The ionisation of a weak acid is reversible, so we use ⇌.

strong: HCl(aq) → H⁺(aq) + Cl−(aq) HNO₃(aq) → H⁺(aq) + NO₃−(aq)

strong: H₂SO₄(aq) → 2H⁺(aq) + SO₄²−(aq)

weak: CH₃COOH(aq) ⇌ CH₃COO−(aq) + H⁺(aq) (ethanoic acid, in vinegar)

For a weak acid, the molecules ionise and the ions join back together at the same time, so a dynamic equilibrium is set up. The position of equilibrium lies well to the left: at any moment most of the acid is still whole molecules, and only a few are ions.

Strength is not the same as concentration

Strong and weak describe how much an acid ionises. Concentrated and dilute describe how much acid is dissolved in a volume of water. So you can have a dilute strong acid (a little hydrochloric acid in lots of water) or a concentrated weak acid (lots of ethanoic acid in a little water).

• hydrochloric acid; Strong or weak? strong; Typical pH 1.0
• nitric acid; Strong or weak? strong; Typical pH 1.0
• ethanoic acid; Strong or weak? weak; Typical pH about 2.9
• citric acid; Strong or weak? weak; Typical pH about 2.1

Concentration matters for safety too. A concentrated solution of a strong acid (for example 5 mol/dm³ hydrochloric acid) is corrosive; diluted to 0.1 mol/dm³ it is only an irritant. Adding water does not change the acid’s strength – it is still fully ionised – but it lowers the concentration of H⁺ ions.

Exam tip: At the same concentration, a strong acid has a lower pH than a weak acid, because it releases more H⁺ ions into the solution.

Strong and weak acids in reactions

With magnesium ribbon or marble chips, 0.10 mol/dm³ hydrochloric acid fizzes much faster than 0.10 mol/dm³ ethanoic acid. There are far more H⁺ ions in the strong acid at any moment, so there are more frequent successful collisions with the metal or carbonate.

If the metal is in excess, both acids eventually give the same total volume of gas (for the same volume and concentration of these acids). As the weak acid uses up its H⁺ ions, more acid molecules ionise to replace them.

pH and hydrogen ion concentration (Higher tier)

pH is a measure of the concentration of H⁺ ions. As the pH decreases by one unit, the H⁺ concentration increases by a factor of 10. So the scale works in powers of ten.

• 0.01; In standard form 1 × 10⁻²; pH 2
• 0.000 01; In standard form 1 × 10⁻⁵; pH 5
• 0.000 000 1 (pure water); In standard form 1 × 10⁻⁷; pH 7 (neutral)
• 0.000 000 000 1; In standard form 1 × 10⁻¹⁰; pH 10

Exam tip: When the H⁺ concentration is written as 1 × 10⁻ⁿ mol/dm³, the pH is n – the power of ten without its minus sign. (Even alkaline solutions contain a tiny concentration of H⁺ ions.)

Worked example: comparing pH values

How many times greater is the H⁺ concentration at pH 2 than at pH 5?
difference in pH = 5 − 2 = 3
factor = 10³ = 1000

Answer: 1000 times greater

Worked example: diluting a strong acid

A strong acid has pH 1.0. It is diluted so its concentration is 100 times smaller.
100 = 10², so the H⁺ concentration falls by a factor of 10² and the pH rises by 2.

Answer: pH 3.0

Common mistakes

• Using “strong” to mean “concentrated”. A strong acid is fully ionised; it can still be dilute.
• Saying a weak acid has fewer acid molecules. At the same concentration it has the same number of acid molecules – fewer of them are ionised.
• Thinking pH 2 is twice as acidic as pH 4. It has 10² = 100 times the H⁺ concentration.
• Using → instead of ⇌ for a weak acid. Partial ionisation is reversible.

Revision questions

Try these, then check your answers below each one.

Question 1. (Higher) Which of these is a weak acid? (Choose from: ethanoic acid, hydrochloric acid, nitric acid, sulfuric acid.)

Answer: ethanoic acid

Working: Ethanoic acid is weak. Hydrochloric, nitric and sulfuric acids are strong (completely ionised).

Question 2. (Higher) True or false? A dilute acid is always a weak acid.

Answer: False

Working: False. Dilute is about concentration (how much acid in the water). Weak is about how much the acid ionises. A dilute acid can be strong.

Question 3. (Higher) A strong acid is one that… (Choose from: is completely ionised in aqueous solution, has a high concentration, is only partially ionised in aqueous solution, has a pH above 7.)

Answer: is completely ionised in aqueous solution

Working: A strong acid ionises completely in water, e.g. HCl → H⁺ + Cl−.

Related lessons

• Previous lesson: 5.7 Neutralisation and the pH Scale
• Next lesson: 5.9 Required Practicals: Chemical Changes
• In this chapter: 5.1 The Reactivity Series
• In this chapter: 5.2 Displacement Reactions
• In this chapter: 5.3 Extracting Metals

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