Chemistry • Year 12 • Module 7 • Lesson 18
Organic Acids & Bases: pKa, Strength & Reactions
Build HSC Band 5–6 technique for evaluating quantitative pKa scenarios, source claims, and multi-step organic acid–base problems in Australian industrial and pharmaceutical contexts.
1. Data + scenario — AWRI wine acidity and amine reaction prediction (Band 5–6)
8 marks Band 5–6
Context. The Australian Wine Research Institute (AWRI) is investigating a process in which tartaric acid (pKa₁ 2.98, pKa₂ 4.34) is neutralised with an amine-based additive — specifically triethylamine (pKb ≈ 3.25, conjugate acid pKa ≈ 10.75) — to reduce acidity and produce a flavour-neutral ammonium carboxylate salt. A second additive being considered is aniline (Kb ≈ 4 × 10⁻¹⁰, conjugate acid pKa ≈ 4.6).
| Additive | Kb | Conjugate acid pKa | Compound class |
|---|---|---|---|
| Triethylamine | ~5.6 × 10⁻⁴ | ~10.75 | Tertiary alkylamine |
| Aniline | ~4 × 10⁻¹⁰ | ~4.6 | Primary arylamine |
Q1. Evaluate which additive is more effective at neutralising tartaric acid in wine. In your response you must:
- Use pKa values to predict whether each additive reacts with tartaric acid (pKa₁ 2.98); state the thermodynamic reasoning for each prediction.
- Write the net ionic equation for the reaction of triethylamine with tartaric acid (first ionisation only) and name the product salt.
- Compare the two additives on three criteria: reaction completeness, conjugate acid strength, and practical suitability for a food application.
- Reach a justified conclusion identifying which additive the AWRI should use, citing at least one specific pKa value in your reasoning.
2. Source critique — TGA pharmaceutical claim (Band 5–6)
7 marks Band 5–6
"Aspirin (pKa 3.5) is a strong acid because it completely ionises in the stomach. This is why it causes gastric irritation. A weak acid like ibuprofen (pKa 4.91) is much safer because it barely ionises in acidic conditions and therefore does not irritate the stomach lining. Adding a base like sodium hydrogen carbonate (NaHCO₃) neutralises aspirin by converting it to its conjugate base, which is less acidic. This confirms that aspirin and NaHCO₃ react because aspirin is a stronger acid than any carbonate species."
— Excerpt adapted from a fictional consumer health website.
Q2. This extract contains three distinct scientific errors. For each error: (i) identify what the claim states, (ii) explain the correct chemistry using lesson content, and (iii) state what evidence or reasoning confirms the correction. 7 marks
Error 1 — Identify:
Correct chemistry:
Evidence/reasoning:
Error 2 — Identify:
Correct chemistry:
Evidence/reasoning:
Error 3 — Identify:
Correct chemistry:
Evidence/reasoning:
Q1 — Sample Band 6 response (8 marks), marking criteria
Triethylamine reaction prediction: Triethylamine's conjugate acid has pKa ≈ 10.75. The rule is: a reaction proceeds when pKa(acid on left) < pKa(conjugate acid of base on left). Tartaric acid pKa₁ = 2.98 < 10.75 → the reaction proceeds strongly to the right. [1 mark — correct thermodynamic comparison with values]
Net ionic equation: (C₄H₅O₆H) + N(C₂H₅)₃ → (C₄H₅O₆)⁻ + [N(C₂H₅)₃H]⁺ (or written as: tartaric acid + triethylamine → triethylammonium hydrogen tartrate / triethylammonium tartrate salt). Product = triethylammonium hydrogen tartrate. [1 mark — equation correct, product named]
Aniline reaction prediction: Aniline conjugate acid pKa ≈ 4.6. Tartaric acid pKa₁ = 2.98 < 4.6 → tartaric acid is slightly stronger than aniline's conjugate acid, so the reaction could proceed, but with an equilibrium ratio Ka/Ka(conj) = 10^(4.6−2.98) = 10^1.62 ≈ 42:1 in favour of products. However, at typical wine concentrations and the need for full neutralisation, aniline would likely not go to completion as effectively, and the conjugate acid of aniline (pKa 4.6) is itself acidic enough to partially re-donate the proton. [1 mark — marginal/borderline prediction with reasoning]
Three-criteria comparison:
- Reaction completeness: Triethylamine (conjugate acid pKa 10.75) — reaction is essentially complete; the conjugate acid is very weak (barely re-ionises). Aniline (conjugate acid pKa 4.6) — reaction is less complete; the conjugate acid is itself moderately acidic and will partly re-donate H⁺ back to tartrate. [1 mark]
- Conjugate acid strength: Triethylammonium ion (pKa 10.75) is a very weak acid — produces minimal pH drop in wine. Anilinium ion (pKa 4.6) is a moderately strong organic acid that would re-acidify the wine significantly. [1 mark]
- Practical/food suitability: Aniline is toxic (FSANZ food additive restriction), carcinogenic, and imparts off-flavours. Triethylamine is also not an approved food additive in standard practice, but the comparison demonstrates that aniline is additionally problematic due to toxicity. Accept any reasonable practical criterion. [1 mark]
Justified conclusion: Triethylamine is far more effective for acid neutralisation in wine. Its conjugate acid (pKa 10.75) is much weaker than tartaric acid (pKa 2.98), so the neutralisation reaction proceeds essentially to completion; the resulting salt does not re-acidify the wine. By contrast, aniline's conjugate acid (pKa 4.6) is close in strength to tartaric acid, limiting neutralisation completion and causing re-acidification. The AWRI should use triethylamine (or an approved equivalent alkylamine) over aniline. [2 marks — cited pKa value, three-criteria evidence, justified conclusion that does not simply say "stronger base wins"]
Marking criteria (8 marks):
- 1 mark — Correctly predicts triethylamine reaction using pKa comparison (2.98 < 10.75).
- 1 mark — Net ionic equation with correct product named.
- 1 mark — Correct prediction/analysis for aniline (marginal, with reasoning about closeness of pKa values).
- 1 mark — Criterion 1: reaction completeness compared for both additives.
- 1 mark — Criterion 2: conjugate acid strength compared for both.
- 1 mark — Criterion 3: practical/food application suitability (toxicity, regulatory, flavour).
- 1 mark — Justified conclusion citing a specific pKa value.
- 1 mark — Uses lesson framework accurately throughout (Ka × Kb = Kw or pKa comparison rule applied at least twice).
Q2 — Source critique: three errors (7 marks)
Error 1: "Aspirin is a strong acid because it completely ionises in the stomach."
What is wrong: Aspirin (pKa 3.5) is a weak acid, not a strong acid [1]. Strong acids have pKa << 0 and essentially fully dissociate (e.g. HCl, H₂SO₄). Aspirin partially ionises — at pH 2 (stomach), using the Henderson–Hasselbalch approximation: the ratio [A⁻]/[HA] = 10^(pH−pKa) = 10^(2−3.5) = 10⁻¹·⁵ ≈ 0.032; so only about 3% is ionised in the stomach [1]. Evidence: Ka(aspirin) = 10⁻³·⁵ = 3.16 × 10⁻⁴ << 1, confirming incomplete ionisation [1].
Error 2: "Ibuprofen (pKa 4.91) barely ionises in acidic conditions, making it safe."
What is wrong: It is true that ibuprofen barely ionises at stomach pH (~2), but the reasoning that reduced ionisation equals stomach safety is an oversimplification or incorrect causal claim [1]. Gastric irritation from NSAIDs like ibuprofen is primarily caused by inhibition of cyclo-oxygenase enzymes (COX-1/COX-2) that protect the stomach lining — not by acid ionisation. Furthermore, both aspirin and ibuprofen are weak acids with similar degrees of ionisation at stomach pH [1]. Evidence: At pH 2, ibuprofen ionisation ≈ 10^(2−4.91) ≈ 0.12% and aspirin ≈ 3.2% — both low, yet ibuprofen can still cause stomach irritation through its pharmacological mechanism [1]. Accept partial credit if student identifies that the safety claim is an oversimplification regarding the degree-of-ionisation–irritation link.
Error 3: "Adding NaHCO₃ confirms aspirin is stronger than any carbonate species."
What is wrong: The statement is imprecise — aspirin (pKa 3.5) is stronger than H₂CO₃ (pKa 6.4), which is why the NaHCO₃ reaction proceeds [1]. However, aspirin is NOT stronger than all carbonate species — the carbonate ion CO₃²⁻ (conjugate base of HCO₃⁻, pKa₂ of H₂CO₃ = 10.3) is a different species from HCO₃⁻. The claim should be: aspirin is a stronger acid than H₂CO₃ (the acid produced when NaHCO₃ accepts a proton), not stronger than "any carbonate species" [1]. Evidence: pKa(aspirin) 3.5 < pKa(H₂CO₃) 6.4 → forward reaction spontaneous; ionic equation: C₉H₇O₄COOH + HCO₃⁻ → C₉H₇O₄COO⁻ + H₂O + CO₂(g) [1].
Marking criteria (7 marks):
- Error 1 — 1 mark for identifying that aspirin is a weak acid; 1 mark for using Ka / ionisation % to confirm incomplete ionisation.
- Error 2 — 1 mark for identifying the incorrect causal link (ionisation ≠ safety); 1 mark for explaining that NSAID irritation is pharmacological (COX inhibition) or that both acids ionise equally little at stomach pH.
- Error 3 — 1 mark for correctly identifying the specific pKa comparison needed (aspirin 3.5 vs H₂CO₃ 6.4, not "any carbonate"); 1 mark for the correct ionic equation; 1 mark overall for reaching a precise, evidence-based reformulation of all three corrections.