Neutralisation in Everyday Life & Industry

Q1 — Sample Band 6 response (8 marks), annotated

Evaluation of each option:

Option 1 (CaCO₃): Reaction: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Fails criterion (b) — introduces Ca²⁺ ions which cannot be excreted by stage 3 CKD kidneys. Fails criterion (c) — constipating effect worsens existing constipation. Fails criterion (d) — produces CO₂ gas, exacerbating IBS. Not suitable (fails 3 criteria).

Option 2 (Mg(OH)₂): Reaction: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O. Fails criterion (b) — introduces Mg²⁺ ions which cannot be excreted. Satisfies criterion (c) — laxative effect could actually relieve constipation. Satisfies criterion (d) — no CO₂. Partially suitable but kidney criterion is critical.

Option 3 (Al(OH)₃): Reaction: Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O. Satisfies criterion (b) — introduces Al³⁺ not Ca²⁺/Mg²⁺. Fails criterion (c) — constipating effect worsens existing constipation. Satisfies criterion (d) — no CO₂. Partially suitable; 2 of 4 criteria met.

Option 4 (NaHCO₃): Reaction: NaHCO₃ + HCl → NaCl + H₂O + CO₂. Satisfies criterion (b) — introduces Na⁺ which is easily managed renally. Neutral bowel effect satisfies criterion (c). Fails criterion (d) — CO₂ produced exacerbates IBS. Disadvantage: lower efficacy per tablet. Partially suitable; 2 of 4 criteria met but CO₂ concern.

Recommendation: Option 3 (Al(OH)₃) is the best choice [1]. It is the only antacid that avoids Ca²⁺ and Mg²⁺ ion accumulation in a patient with reduced renal excretion (satisfies criterion (b), the most clinically critical) [1]. It produces no CO₂ (satisfies criterion (d)) [1]. The constipating effect (fails criterion (c)) can be managed by combining with a small dose of Mg(OH)₂ at physician discretion (a common clinical combination) [1].

Balanced equation: Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O [1]. Reaction type: acid + base → salt + water. No CO₂ because Al(OH)₃ is a hydroxide base, not a carbonate [1].

Residual limitation: Long-term Al³⁺ accumulation in renal patients can itself cause toxicity (Al³⁺ deposits in bone and brain tissue — dialysis encephalopathy is a documented risk). Regular monitoring of serum aluminium levels would be required for chronic use [1]. Accept any biologically valid limitation specific to Al(OH)₃.

Marking criteria:

• 1 — Balanced equation for recommended antacid with HCl, correctly balanced.
• 1 — Identifies critical kidney criterion (no Ca²⁺/Mg²⁺) and eliminates Options 1 and 2 with chemical reasoning.
• 1 — Explains why CO₂ is/is not produced (carbonate vs hydroxide mechanism) for at least two options.
• 1 — Evaluates all four options against at least 3 criteria with chemistry-based reasoning.
• 1 — Clear recommendation with explicit justification citing the most clinically critical criterion.
• 1 — Acknowledges the bowel effect limitation and suggests how it could be managed.
• 1 — Acknowledges residual limitation of recommended antacid with specific mechanism.
• 1 — Overall response is structured, uses precise chemical terminology, and reaches a defensible recommendation that correctly weighs criteria hierarchically.

Q2 — Source critique: three errors (7 marks)

Error 1 — “Complete solution to acid rain.” This is incorrect. FGD removes 90–99% of SO₂ from flue gas, but it does not remove CO₂. CO₂ is the primary driver of ocean acidification and a major contributor to climate change (greenhouse effect). Additionally, FGD does not remediate lakes, soils, or ecosystems already acidified by past acid rain — it only prevents further SO₂ emissions at the treated source. It is a highly effective preventive measure, not a complete environmental solution [2 marks: 1 for identifying, 1 for correct explanation].

Error 2 — Wrong gas / wrong equation. The passage states the reaction as Ca(OH)₂ + SO₃ → CaSO₄ + H₂O. Two sub-errors: (a) The gas is SO₂ (sulfur dioxide — the primary combustion product of sulfur in coal), not SO₃ (sulfur trioxide). (b) The product of SO₂ with Ca(OH)₂ is CaSO₃ (calcium sulfite), not CaSO₄ (calcium sulfate / gypsum). Correct primary FGD equation: Ca(OH)₂(aq) + SO₂(g) → CaSO₃(s) + H₂O(l). Gypsum (CaSO₄) is obtained only by the separate oxidation step: 2CaSO₃ + O₂ → 2CaSO₄ [2 marks: 1 for correct gas + correct equation; 1 for explaining CaSO₃ vs CaSO₄ distinction].

Error 3 — Self-funding claim. While CaSO₄ (gypsum) does have commercial value in wallboard and cement manufacture, the sale of synthetic gypsum does not typically cover the full capital and operating costs of a large-scale FGD system. The claim that the system is “self-funding” overstates the economic benefit. Furthermore, gypsum is only produced after the additional oxidation step — the primary product of the FGD reaction is CaSO₃, which has fewer commercial uses [1 mark for identifying the overclaim; partial credit if student identifies only the CaSO₃/CaSO₄ distinction without the economic overclaim].

Reformulated claim: Flue gas desulfurisation is a highly effective technology for reducing SO₂ emissions from coal-fired power stations. In the primary reaction, Ca(OH)₂ neutralises SO₂ to produce CaSO₃(s), which can be further oxidised to CaSO₄ (gypsum) and sold to the construction industry, partially offsetting operating costs. Modern FGD systems remove 90–99% of SO₂ from flue gas. However, FGD does not remove CO₂ and cannot remediate ecosystems already damaged by acid rain; it should be considered one component of a broader environmental management approach rather than a complete solution [2 marks: 1 for a scientifically accurate reformulation that corrects all three errors; 1 for retaining accurate information while removing overclaims].

Q3 — Sample Band 6 response (7 marks), annotated

Key equations:

Strategy A (FGD): Ca(OH)₂(aq) + SO₂(g) → CaSO₃(s) + H₂O(l). Removes SO₂ at source before it forms acid rain [1 — correct equation].

Strategy B (lake liming): CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g). Neutralises existing acid in the lake [1 — correct equation].

Comparison on three criteria:

Scope: Strategy A addresses the source of the problem (prevents ongoing SO₂ release). Strategy B addresses a consequence (the already-acidified lake) but does nothing to prevent further SO₂ emissions — without Strategy A, the lake would re-acidify and require repeated treatment [1].

Permanence: Strategy A provides ongoing protection while the scrubber operates, removing 90–99% of SO₂. Strategy B provides a temporary pH correction — without addressing SO₂ source, the lake re-acidifies over time as more acid rain falls [1].

By-products and scale: Strategy A produces CaSO₃ (collectable solid, can be oxidised to gypsum for commercial use). Strategy B produces CO₂ gas (which escapes) and Ca²⁺ (remains in lake water as a harmless nutrient). Both by-products are benign at normal application rates [1].

Recommendation: A combination of both strategies is required [1]. Strategy A alone solves the ongoing SO₂ emission problem but does nothing to restore the lake’s current pH 4.3 to a level that supports fish (pH > 6.5 required). Strategy B alone temporarily raises lake pH, but without scrubbers the lake will re-acidify. Together: Strategy A prevents further acidification; Strategy B rehabilitates the existing ecosystem damage.

Limitation neither strategy addresses: Neither strategy addresses CO₂ emissions from the plant, which contribute to global ocean acidification and climate change through a separate chemical pathway. Broader decarbonisation (switching to renewables or carbon capture) would be required to address this limitation [1].

Marking criteria:

• 1 — Correct balanced equation for Strategy A (FGD with Ca(OH)₂ + SO₂).
• 1 — Correct balanced equation for Strategy B (CaCO₃ or Ca(OH)₂ with H⁺ in lake).
• 1 — Compares strategies on scope (source vs consequence), explaining chemistry.
• 1 — Compares strategies on permanence / ongoing vs one-off, with reasoning.
• 1 — Compares strategies on one further criterion (by-products, cost, speed, scale).
• 1 — Justifies a combination approach linking it explicitly to both the lake and the ongoing SO₂ issue.
• 1 — Identifies one limitation that neither strategy addresses (CO₂ / climate / ocean acidification, or equivalent valid limitation).