Chemistry • Year 12 • Module 6 • Lesson 4
Neutralisation in Everyday Life & Industry
Apply antacid chemistry, agricultural liming, and FGD to real data, case scenarios, and comparative analysis.
1. Interpret antacid comparison data
A pharmacy student compared four common antacid formulations available at Chemist Warehouse, recording the active ingredient, active ingredient mass per tablet, moles of H⁺ neutralised per tablet, and approximate cost per tablet. 8 marks
| Brand (active ingredient) | Formula | Mass per tablet (mg) | Mol H⁺ neutralised per tablet | Cost per tablet (AUD) | CO₂ produced? |
|---|---|---|---|---|---|
| Rennie (calcium carbonate) | CaCO₃ | 680 | 0.0136 | $0.18 | Yes |
| Mylanta (magnesium hydroxide) | Mg(OH)₂ | 400 | 0.0137 | $0.24 | No |
| Gaviscon (aluminium hydroxide) | Al(OH)₃ | 500 | 0.0192 | $0.31 | No |
| Quick-Eze (sodium bicarbonate) | NaHCO₃ | 500 | 0.0060 | $0.09 | Yes |
Data adapted from product labels. Mol H⁺ calculated from stoichiometry of reaction with HCl.
1.1 Identify which antacid neutralises the most moles of H⁺ per tablet and calculate, using the data, the cost per mmol of H⁺ neutralised for that antacid. Show your working. 3 marks
1.2 The cheapest antacid (NaHCO₃, Quick-Eze) neutralises the fewest moles of H⁺ per tablet. Write the balanced equation for its reaction with HCl and explain, using the equation, why it neutralises fewer moles of H⁺ per 500 mg than Al(OH)₃ at the same mass. (Molar masses: NaHCO₃ = 84 g/mol; Al(OH)₃ = 78 g/mol.) 3 marks
1.3 A patient with kidney disease cannot efficiently excrete Ca²⁺ or Mg²⁺ ions. Using the data table and lesson content, recommend one antacid from the table and justify your choice against two chemical criteria. 2 marks
2. Interpret graph — NSW agricultural soil pH and lime application
The graph below shows soil pH measurements taken across a 5-year period in two NSW cropping paddocks. Paddock A received annual lime applications (CaCO₃, 2.5 t/ha) starting at Year 1. Paddock B received no lime treatment. 7 marks
Figure 2.1. Soil pH over five years in two NSW cropping paddocks. Adapted after NSW DPI (2022) Soil Acidification and Liming.
2.1 Describe the trend in soil pH for each paddock from Year 0 to Year 5. 2 marks
2.2 In which year did Paddock A first exceed the minimum optimal pH of 6.0? Estimate the pH at Year 5 for each paddock using the graph. 2 marks
2.3 Write the balanced ionic equation for the neutralisation reaction between CaCO₃ and soil H⁺ ions, and use it to explain the mechanism by which liming raises soil pH. 3 marks
3. Case study — Great Barrier Reef ocean acidification
Read the following passage, then answer the questions below. 6 marks
Ocean pH has declined from approximately 8.2 to 8.1 since pre-industrial times as atmospheric CO₂ concentrations have risen from 280 ppm to over 420 ppm. Dissolved CO₂ reacts with seawater to form carbonic acid (H₂CO₃), which dissociates to release H⁺, reducing pH and driving the equilibrium CO₃²⁻ + H⁺ ⇌ HCO₃⁻ to the right — consuming carbonate ions that corals need to build their CaCO₃ skeletons. AIMS (Australian Institute of Marine Science) monitoring at the Great Barrier Reef reports that average coral calcification rates have declined by approximately 16% since 1990. At pH 7.8, predicted under high-emissions scenarios by 2100, many coral species can no longer sustain net calcification.
3.1 Write the balanced equation for the reaction of dissolved CO₂ with seawater (as H₂CO₃) reacting with CaCO₃ in coral skeletons. Identify the role of CO₂ as an acidic oxide in this process. 2 marks
3.2 A student proposes adding Ca(OH)₂ directly to reef water to neutralise the excess H⁺ and restore carbonate ion concentration. Write the relevant equation and identify one limitation of this approach at the scale of the GBR. 2 marks
3.3 Explain why flue gas desulfurisation at coal-fired power stations is not a solution to ocean acidification, even though it removes an acid-forming gas. 2 marks
4. Predict and justify — Murray–Darling acid sulfate soils
Parts of the Murray–Darling Basin contain acid sulfate soils (ASS). When these soils are exposed by drought or water extraction, pyrite (FeS₂) oxidises: 4FeS₂ + 15O₂ + 14H₂O → 4Fe(OH)₃ + 8H₂SO₄. The H₂SO₄ produced lowers waterway pH to as low as 3.5. Land managers are considering applying agricultural lime (CaCO₃) to affected areas. 4 marks
4.1 Predict what will happen to the Fe³⁺ ions in the acidic runoff if the waterway pH is raised above 5.5 through liming, and write the relevant precipitation equation. 2 marks
4.2 H₂SO₄ is diprotic. Write the balanced equation for CaCO₃ neutralising H₂SO₄ in the acidic soil runoff, and justify why this reaction is more effective at raising pH than using CaCO₃ alone as a slow-dissolving solid. 2 marks
Q1.1 — Most H⁺ neutralised per tablet + cost calculation
Al(OH)₃ (Gaviscon) neutralises the most moles of H⁺ per tablet: 0.0192 mol per tablet [1]. Cost per mmol of H⁺: cost = $0.31; mol H⁺ = 0.0192 mol = 19.2 mmol. Cost per mmol = $0.31 ÷ 19.2 = $0.016/mmol (approximately 1.6 cents per mmol H⁺ neutralised) [1]. Correct working shown [1].
Q1.2 — NaHCO₃ equation + comparison
Balanced equation: NaHCO₃ + HCl → NaCl + H₂O + CO₂ [1]. Each mole of NaHCO₃ neutralises only 1 mole of H⁺ (monoprotic mechanism — one HCO₃⁻ accepts one H⁺ to form H₂CO₃) [1]. At 500 mg: NaHCO₃ gives 500/84000 mol = 0.00595 mol H⁺ neutralised; Al(OH)₃ gives 500/78000 × 3 = 0.01923 mol H⁺ (3 mol H⁺ per mol Al(OH)₃). Al(OH)₃ has a higher mole-to-mass ratio and neutralises 3 mol H⁺ per mole, making it much more effective per gram [1].
Q1.3 — Antacid recommendation for kidney patient
Recommended: Al(OH)₃ (Gaviscon) [1]. Justification: (i) It produces Al³⁺ ions (not Ca²⁺ or Mg²⁺), satisfying the criterion of avoiding ions the kidneys cannot excrete; (ii) it produces no CO₂ gas (acid + base reaction: Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O), so the patient is not at risk of gas-related discomfort [1]. (NaHCO₃ also avoids Ca/Mg but neutralises less H⁺ per tablet and produces CO₂ — Al(OH)₃ is superior on both criteria as stated.)
Q2.1 — Trend description
Paddock A (lime treated): soil pH increases steadily from approximately 4.8 at Year 0 to approximately 6.6 at Year 5 — a rise of approximately 1.8 pH units over five years [1]. Paddock B (untreated): soil pH decreases gradually from approximately 4.9 at Year 0 to approximately 4.2 at Year 5, a drop of 0.7 pH units, consistent with ongoing soil acidification from leaching and fertiliser nitrification [1].
Q2.2 — Year threshold crossed + Year 5 estimates
Paddock A first exceeds pH 6.0 between Year 3 and Year 4 (the line crosses the dashed pH 6.0 threshold at approximately Year 3.5) [1]. Year 5 estimates: Paddock A ≈ pH 6.6; Paddock B ≈ pH 4.2 (accept ±0.1) [1].
Q2.3 — Ionic equation + mechanism (3 marks)
Balanced ionic equation: CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g) [1]. Mechanism: CaCO₃ dissolves slowly and the carbonate ion (CO₃²⁻) reacts with two H⁺ ions in the soil water, consuming them [1]. This reduces the concentration of H⁺ ions in the soil solution — by definition, reducing [H⁺] raises pH. The CO₂ escapes as gas and Ca²⁺ remains as a nutrient; no OH⁻ is produced [1].
Q3.1 — CO₂ as acidic oxide + equation (2 marks)
Equation for CO₂ dissolving: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. Reaction with coral CaCO₃: CaCO₃ + H₂CO₃ → Ca²⁺ + 2HCO₃⁻ [1]. CO₂ acts as an acidic oxide: it dissolves in water to form H₂CO₃ (an acid), which donates H⁺, lowering pH and dissolving the carbonate structure of coral skeletons [1].
Q3.2 — Ca(OH)₂ proposal + limitation (2 marks)
Equation: Ca(OH)₂ + 2H⁺ → Ca²⁺ + 2H₂O [1]. Limitation: The GBR covers approximately 344,400 km²; the volume of seawater is immense. Adding sufficient Ca(OH)₂ to raise ocean pH globally or even reef-wide is technically and logistically impossible at scale — it would require astronomical quantities and could have unintended consequences on marine chemistry and organisms (e.g. localised pH spikes toxic to reef organisms, calcium sedimentation) [1]. Accept any one valid scale/feasibility/ecological limitation.
Q3.3 — Why FGD does not fix ocean acidification (2 marks)
FGD targets SO₂ — a sulfur-based acidic oxide from coal combustion. Ocean acidification is driven by CO₂ — a different acidic oxide, produced in far greater quantities, that dissolves in seawater [1]. FGD does not capture or remove CO₂ from flue gas; it only scrubs SO₂. Furthermore, even if SO₂ reached the ocean it would contribute to ocean acidification only marginally compared with CO₂; the root cause of ocean acidification requires CO₂ emission reductions, not SO₂ scrubbing [1].
Q4.1 — Fe³⁺ precipitation + equation (2 marks)
As pH rises above 5.5 (and toward neutral), the increasing [OH⁻] causes Fe³⁺ ions to precipitate as insoluble iron(III) hydroxide [1]. Equation: Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s) [1]. The orange-red Fe(OH)₃ precipitate can be seen at acid sulfate soil drainage sites and settles out of suspension once pH is raised.
Q4.2 — CaCO₃ + H₂SO₄ equation + justification (2 marks)
Balanced equation: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂ [1]. Note: CaSO₄ (gypsum) is sparingly soluble and may precipitate, potentially limiting further reaction if a crust forms. CaCO₃ applied as a powder or slurry provides more surface area and reacts faster, raising pH more quickly [1]. Accept also: Ca(OH)₂ is more soluble and acts faster than CaCO₃ in emergency situations, but CaCO₃ is cheaper for large-scale field application.