AQA · GCSE · Combined Science

    Course version: 8464 · Version from Sept 2018

    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.

    A weak acid dissolves in water but only some of its molecules donate a hydrogen ion, so ionisation is partial and reversible.

    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    A weak acid dissolves in water but only some of its molecules donate a hydrogen ion, so ionisation is partial and reversible. Ethanoic acid establishes an equilibrium: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq). At any moment, many intact CH₃COOH molecules remain alongside a relatively low concentration of H⁺(aq) ions. Citric acid and carbonic acid behave in the same partial way. Because the H⁺(aq) concentration is lower than in a strong acid of the same concentration, the pH is higher and reactions with metals or bases are slower and less vigorous. The equilibrium arrow (⇌) is essential: it shows that both forward ionisation and reverse recombination occur simultaneously.

    What earns the marks

    • A weak acid is only partially ionised in aqueous solution, meaning a significant proportion of intact acid molecules remains.
    • Ethanoic acid ionises reversibly: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq).
    • Citric acid and carbonic acid are additional examples of weak acids that ionise only partially in water.
    • Partial ionisation results in a lower concentration of H⁺(aq) ions than a strong acid of the same concentration, leading to a higher pH.
    • The equilibrium symbol (⇌) must be used because ionisation is reversible, with ions recombining to reform the acid molecules.

    Common mistakes and how to put them right

    • Writing a single arrow for ethanoic acid, such as CH₃COOH(aq) → H⁺(aq) + CH₃COO⁻(aq).

      Put it right: Use the equilibrium arrow (⇌) to show partial, reversible ionisation.

    • Thinking weak acids do not ionise at all.

      Put it right: They do ionise, but only partially, so some H⁺(aq) ions are present and the solution is acidic.

    • Assuming a weak acid must be dilute or harmless.

      Put it right: Strength depends on the extent of ionisation, while concentration and hazard are separate concepts.

    Exam tips

    • Use the word 'partially' and include the equilibrium arrow (⇌) whenever you describe or write the ionisation of a weak acid.
    • Link partial ionisation to a lower concentration of H⁺(aq) ions, a higher pH and slower reactions compared with a strong acid of the same concentration.

    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

    Citric acid is classified as a weak acid. Which statement correctly describes its behaviour in an aqueous solution?

    Listen

    A weak acid dissolves in water but only some of its molecules donate a hydrogen ion, so ionisation is partial and reversible. Ethanoic acid establishes an equilibrium: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq). At any moment, many intact CH₃COOH molecules remain alongside a relatively low concentration of H⁺(aq) ions. Citric acid and carbonic acid behave in the same partial way. Because the H⁺(aq) concentration is lower than in a strong acid of the same concentration, the pH is higher and reactions with metals or bases are slower and less vigorous. The equilibrium arrow (⇌) is essential: it shows that both forward ionisation and reverse recombination occur simultaneously. A strong answer earns marks for these points. A weak acid is only partially ionised in aqueous solution, meaning a significant proportion of intact acid molecules remains. Ethanoic acid ionises reversibly: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq). Citric acid and carbonic acid are additional examples of weak acids that ionise only partially in water. Partial ionisation results in a lower concentration of H⁺(aq) ions than a strong acid of the same concentration, leading to a higher pH. A common slip: Writing a single arrow for ethanoic acid, such as CH₃COOH(aq) → H⁺(aq) + CH₃COO⁻(aq). Instead, use the equilibrium arrow (⇌) to show partial, reversible ionisation. Exam tip: Use the word 'partially' and include the equilibrium arrow (⇌) whenever you describe or write the ionisation of a weak acid.

    See it

    A top-down flowchart. The top node reads 'Weak acid in aqueous solution' and points down to 'Only partially ionised'. From that node, five branches lead out. One goes to 'Many intact acid molecules remain'. One goes to 'Relatively low concentration of H⁺(aq) ions', which then splits into two further nodes: 'Higher pH than a strong acid of the same concentration' and 'Slower, less vigorous reactions with metals or bases'. One goes to 'Ethanoic acid: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq)', which leads to 'Equilibrium arrow shows forward ionisation and reverse recombination occur simultaneously'. The last goes to 'Citric acid and carbonic acid ionise only partially in water'.
    It shows how partial ionisation of a weak acid leads to intact molecules remaining, a low H⁺(aq) concentration, a higher pH and slower reactions, with the reversible ethanoic acid equilibrium and other weak acid examples.

    Key words

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    Weak acid

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    Partial and reversible ionisation

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    Equilibrium between intact acid molecules and ions

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    Lower H⁺(aq) concentration and less acidic behaviour

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    See every definition
    Weak acid
    An acid that is only partially ionised in aqueous solution, so a significant proportion of intact acid molecules remains.
    Partial and reversible ionisation
    Only some molecules donate a hydrogen ion, and both forward ionisation and reverse recombination occur simultaneously.
    Equilibrium between intact acid molecules and ions
    Many intact acid molecules remain alongside a relatively low concentration of H⁺(aq) ions, shown using the equilibrium arrow (⇌).
    Lower H⁺(aq) concentration and less acidic behaviour
    The H⁺(aq) concentration is lower than in a strong acid of the same concentration, so the pH is higher and reactions are slower and less vigorous.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The top node reads 'Weak acid in aqueous solution' and points down to 'Only partially ionised'. From that node, five branches lead out. One goes to 'Many intact acid molecules remain'. One goes to 'Relatively low concentration of H⁺(aq) ions', which then splits into two further nodes: 'Higher pH than a strong acid of the same concentration' and 'Slower, less vigorous reactions with metals or bases'. One goes to 'Ethanoic acid: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq)', which leads to 'Equilibrium arrow shows forward ionisation and reverse recombination occur simultaneously'. The last goes to 'Citric acid and carbonic acid ionise only partially in water'.

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