AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
A weak acid is only partially ionised in aqueous solution. Examples of…
Full specification wording
Strong and weak acids (HT only): A weak acid is only partially ionised in aqueous solution. Examples of weak acids are ethanoic, citric and carbonic acids.
In aqueous solution, a weak acid releases hydrogen ions only partially: most of its molecules remain intact and only a small fraction ionise.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 2 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In aqueous solution, a weak acid releases hydrogen ions only partially: most of its molecules remain intact and only a small fraction ionise. This is shown by an equilibrium, for example CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq), where the position lies well to the left. Because few H⁺ ions form, a weak acid of a given concentration has a higher pH and reacts more slowly than a strong acid of the same concentration. Ethanoic acid (in vinegar), citric acid (in citrus fruit) and carbonic acid (formed when carbon dioxide dissolves in water) are common examples. The double arrow is essential: it shows that ionisation is reversible and incomplete, unlike the single arrow used for a strong acid such as hydrochloric acid.
What earns the marks
- A weak acid is only partially ionised in aqueous solution, so only a small proportion of its molecules release hydrogen ions.
- The ionisation of a weak acid is represented as an equilibrium using a reversible arrow, for example CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq).
- Because ionisation is incomplete, the concentration of H⁺ ions is lower than for a strong acid of the same concentration, giving a higher pH.
- Ethanoic acid, citric acid and carbonic acid are named examples of weak acids.
- The contrast with strong acids is the degree of ionisation: strong acids ionise completely, weak acids do not.
- A weak acid can still be concentrated; weak refers to degree of ionisation, not to amount of substance dissolved.
Common mistakes and how to put them right
Confusing weak with dilute.
Put it right: A weak acid is partially ionised, whereas a dilute acid simply has a small amount of acid per unit volume; a weak acid can be concentrated.
Writing a single arrow for the ionisation of a weak acid.
Put it right: The correct notation uses a reversible arrow to show incomplete ionisation.
Believing that a weak acid produces no ions at all.
Put it right: It does produce H⁺ ions, but only a small fraction of its molecules ionise.
Exam tips
- When asked to explain why a weak acid has a higher pH than a strong acid of the same concentration, link partial ionisation to a lower H⁺ concentration.
- Quote at least one named weak acid from the specification, such as ethanoic acid, to support your answer.
- If you write an equation for a weak acid, use the reversible arrow and include state symbols to show it is in aqueous solution.
Try the lesson
The start of the Strong and weak acids (HT only): A weak acid is only partially ionised in aqueous solution. Examples of weak acids are ethanoic, citric and carbonic acids. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement describes how ethanoic acid behaves when it is dissolved in water?
Listen
In aqueous solution, a weak acid releases hydrogen ions only partially: most of its molecules remain intact and only a small fraction ionise. This is shown by an equilibrium, for example CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq), where the position lies well to the left. Because few H⁺ ions form, a weak acid of a given concentration has a higher pH and reacts more slowly than a strong acid of the same concentration. Ethanoic acid (in vinegar), citric acid (in citrus fruit) and carbonic acid (formed when carbon dioxide dissolves in water) are common examples. The double arrow is essential: it shows that ionisation is reversible and incomplete, unlike the single arrow used for a strong acid such as hydrochloric acid. A strong answer earns marks for these points. A weak acid is only partially ionised in aqueous solution, so only a small proportion of its molecules release hydrogen ions. The ionisation of a weak acid is represented as an equilibrium using a reversible arrow, for example CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq). Because ionisation is incomplete, the concentration of H⁺ ions is lower than for a strong acid of the same concentration, giving a higher pH. Ethanoic acid, citric acid and carbonic acid are named examples of weak acids. Exam tip: When asked to explain why a weak acid has a higher pH than a strong acid of the same concentration, link partial ionisation to a lower H⁺ concentration.
See it
Key words
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weak acid
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reversible arrow
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degree of ionisation
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dilute
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See every definition
- weak acid
- An acid that is only partially ionised in aqueous solution, so only a small proportion of its molecules release hydrogen ions.
- reversible arrow
- The double arrow used to show that the ionisation of a weak acid is reversible and incomplete, unlike the single arrow used for a strong acid.
- degree of ionisation
- The extent to which an acid ionises, which is complete for strong acids and partial for weak acids.
- dilute
- Having a small amount of acid per unit volume, which is not the same as being weak.
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