AQA A-Level Chemistry Paper 3, June 2022: Question 13

Identifying Brønsted-Lowry Acids & Bases

📌 What this question tests

This question evaluates your core understanding of fundamental acid-base definitions under the Brønsted-Lowry theory:

Definition of a Brønsted-Lowry acid (proton / H⁺ donor) vs. a Brønsted-Lowry base (proton / H⁺ acceptor).
Ability to distinguish between acidic hydrogens attached to electronegative atoms (e.g. O-H, N-H) and non-acidic hydrogens on alkyl groups (e.g. C-H).
Recognition of amphiprotic (amphoteric) species that can both donate and accept protons.

Question 13 • Multiple Choice • 1 Mark

Analysis of Question 13

"Which can not function as a Brønsted-Lowry acid?"

✅ Correct Answer

A: CH₃COO⁻ (ethanoate ion)

Mark Scheme: Option A [1 mark] (AO3)

The ethanoate ion lacks an available acidic proton. The hydrogens are bonded covalently to carbon ( C-H ) with very low polarity and cannot be donated as H⁺ in aqueous acid-base reactions. It only acts as a Brønsted-Lowry base by accepting H⁺ to form ethanoic acid ( CH₃COOH ).

💡 Key Knowledge

Brønsted-Lowry Acid: A proton donor (must be able to lose an H⁺ ).
Brønsted-Lowry Base: A proton acceptor (must have a lone pair of electrons to bind an H⁺ ).
Amphiprotic species: Can act as an acid or a base (e.g. HCO₃⁻ , H₂O , HSO₄⁻ ).

Evaluating Each Given Option

Species	Can it donate H⁺?	Reaction as an Acid	Conclusion
A: CH₃COO⁻	❌ No	Cannot form CH₂COO²⁻ ; C-H protons are not acidic.	Cannot act as an acid (Correct answer)
B: HCO₃⁻	✅ Yes	HCO₃⁻ ⇌ H⁺ + CO₃²⁻	Can act as an acid (amphiprotic)
C: H₃O⁺	✅ Yes	H₃O⁺ ⇌ H⁺ + H₂O	Can act as an acid (hydronium ion)
D: NH₄⁺	✅ Yes	NH₄⁺ ⇌ H⁺ + NH₃	Can act as an acid (ammonium ion)

🧠 Exam Technique

Watch the negative wording: The word not is bolded in the stem. Always read carefully so you don't accidentally pick an option that can act as an acid.
Test the hypothetical conjugate base: Remove an H⁺ from each formula and evaluate the stability/feasibility of the resulting product:
• HCO₃⁻ → CO₃²⁻ (well-known carbonate ion)
• H₃O⁺ → H₂O (stable neutral water)
• NH₄⁺ → NH₃ (stable ammonia molecule)
• CH₃COO⁻ → CH₂COO²⁻ (extremely unstable carbanion; does not occur in acid-base chemistry)

❌ Common Errors & Misconceptions

Seeing "H" and assuming acidity: Students often spot the three hydrogens in CH₃COO⁻ and assume it can donate one. Remember: C-H hydrogens in alkyl groups are non-acidic!
Confusing negative charge with inability to act as an acid: Students frequently eliminate HCO₃⁻ thinking negative ions cannot be acids. However, anions containing acidic hydrogens ( HCO₃⁻ , HSO₄⁻ , H₂PO₄⁻ ) are well-known Brønsted-Lowry acids.

AQA A-Level Chemistry Paper 3, June 2022: Question 13

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