HSC Exam Practice

Sample response. An antacid is a base (such as CaCO₃, Mg(OH)₂, or Al(OH)₃) that neutralises excess hydrochloric acid in the stomach. The reaction type is either acid + base → salt + water (for hydroxide antacids) or acid + carbonate → salt + water + CO₂ (for carbonate antacids).

Marking notes. 1 mark for defining antacid as a base used to neutralise stomach HCl; 1 mark for correctly identifying the reaction type (acid + base or acid + carbonate; accept either).

Sample response (any two of three).
CaCO₃: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. CO₂ IS produced because CaCO₃ contains a carbonate ion (CO₃²⁻) that reacts with H⁺ to form H₂CO₃, which decomposes to CO₂ + H₂O.
Mg(OH)₂: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O. CO₂ is NOT produced because Mg(OH)₂ is a hydroxide base — it contains no carbonate ion; the reaction follows acid + base → salt + water only.
Al(OH)₃: Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O. CO₂ is NOT produced for the same reason as Mg(OH)₂.

Marking notes. 1 mark per correct balanced equation (max 2); 1 mark per correct CO₂ statement with explanation of why (max 2). Total 4 marks.

Sample response. H₂SO₄ reacts with NH₃ in a 1:2 mole ratio (2 mol NH₃ per 1 mol H₂SO₄) because H₂SO₄ is diprotic and donates 2 protons, each accepted by one NH₃; the product is ammonium sulfate (NH₄)₂SO₄. HNO₃ reacts with NH₃ in a 1:1 mole ratio (1 mol NH₃ per 1 mol HNO₃) because HNO₃ is monoprotic and donates only 1 proton; the product is ammonium nitrate NH₄NO₃.

Marking notes. 1 mark for correct mole ratio (1:2) for H₂SO₄ + NH₃ with reason (diprotic); 1 mark for correct mole ratio (1:1) for HNO₃ + NH₃ with reason (monoprotic); 1 mark for both product names correctly stated.

Sample response. Flue gas desulfurisation (FGD) prevents acid rain by reacting SO₂ (produced by combustion of sulfur-containing coal) with Ca(OH)₂ in a wet scrubber before the exhaust is discharged. This converts the gas-phase acidic pollutant into a collectable solid. Balanced equation: Ca(OH)₂(aq) + SO₂(g) → CaSO₃(s) + H₂O(l). Acidic component: SO₂ (an acidic oxide). Basic component: Ca(OH)₂ (calcium hydroxide).

Marking notes. 1 mark for the correct balanced equation; 1 mark for correctly identifying SO₂ as the acidic component and Ca(OH)₂ as the basic component; 1 mark for explaining that FGD prevents SO₂ reaching the atmosphere and forming acid rain.

Sample response. Acidic industrial wastewater must be neutralised before discharge because pH 3 is far below the safe range for aquatic ecosystems (6.5–8.5). Direct discharge would lower the pH of the receiving waterway, dissolve carbonate sediments, mobilise toxic dissolved heavy metal ions (e.g. Al³⁺, Fe³⁺), and make the water uninhabitable for fish and aquatic invertebrates. One named neutralising agent: Ca(OH)₂ (slaked lime), which reacts with acid by Ca(OH)₂ + 2H⁺ → Ca²⁺ + 2H₂O, raising pH toward the safe range.

Marking notes. 1 mark for naming Ca(OH)₂, NaOH, or CaCO₃ as neutralising agent; 1 mark for describing the environmental consequence of pH 3 discharge (aquatic harm, with at least one specific effect); 1 mark for explaining the legal/ecological pH requirement (6.5–8.5 or equivalent).

Sample response.
CaCO₃: CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g).
Ca(OH)₂: Ca(OH)₂(s/aq) + 2H⁺(aq) → Ca²⁺(aq) + 2H₂O(l).
CaCO₃ reacts more slowly because it has low solubility in water — less dissolves at a time, so fewer CO₃²⁻ ions are available to react with H⁺. Ca(OH)₂ is more soluble, providing a higher concentration of OH⁻ ions immediately available to neutralise H⁺, so it acts faster. Both ultimately achieve pH correction, but Ca(OH)₂ is faster and more caustic (handling risk), while CaCO₃ is cheaper and safer to apply at large scale.

Marking notes. 1 mark per correct balanced ionic equation (max 2); 1 mark for explaining that CaCO₃ reacts more slowly due to lower solubility; 1 mark for correctly identifying Ca(OH)₂ as faster due to higher solubility/higher [OH⁻] immediately available. Total 4 marks.

Sample response (a). Percentage reduction = (2000 − 80)/2000 × 100 = 1920/2000 × 100 = 96%. The two-pass FGD system reduces high-sulfur coal SO₂ from 2000 ppm to 80 ppm, a 96% reduction.

Marking notes (a). 1 mark for correct substitution; 1 mark for correct answer (96%).

Sample response (b). Low-S coal: Stage 4 SO₂ = 20 ppm — below the 100 ppm limit. Meets the regulatory limit. High-S coal: Stage 4 SO₂ = 80 ppm — also below the 100 ppm limit. Both meet the limit. Additional reduction required for high-S coal = 0% (80 ppm < 100 ppm limit). Accept: student may note both meet the limit and state no additional reduction is required. Full marks if student correctly reads both values from the graph and correctly concludes which meet the limit.

Marking notes (b). 1 mark for correctly reading both Stage 4 values from the graph and identifying that both are below 100 ppm; 1 mark for conclusion that both meet the regulatory limit / no additional reduction required.

Sample response (c). Balanced equation: Ca(OH)₂(aq) + SO₂(g) → CaSO₃(s) + H₂O(l). Role of SO₂: SO₂ acts as an acidic oxide — it behaves as the acid component in the acid + base reaction. When dissolved in water it forms H₂SO₃, but in the scrubber it reacts directly with the base Ca(OH)₂. In Brønsted–Lowry terms, SO₂ is the proton-accepting species (or the acid when considered via the H₂SO₃ it would form); Ca(OH)₂ supplies the OH⁻ to complete the neutralisation.

Marking notes (c). 1 mark for correct balanced equation; 1 mark for identifying SO₂ as the acidic component/acidic oxide and explaining its acid-equivalent role.

Sample response (a). From P1 to P5, soil pH increases from 3.1 to 6.8 (a rise of 3.7 pH units), while dissolved Fe³⁺ concentration decreases dramatically from 48 mg/L to less than 0.5 mg/L. The two trends are inversely related — as pH increases, dissolved Fe³⁺ decreases.

Marking notes (a). 1 mark for correctly describing the pH increase trend with data; 1 mark for correctly describing the Fe³⁺ decrease trend with data. Total 2 marks.

Sample response (b). As pH increases from P1 to P5, the hydroxide ion concentration [OH⁻] increases. Fe³⁺ ions react with OH⁻ to form insoluble Fe(OH)₃: Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s)↓. This precipitation removes Fe³⁺ from solution (reducing dissolved concentration) and produces solid Fe(OH)₃ (orange-red precipitate) that accumulates in the soil and is visible from P3 onward. At P1 (pH 3.1), [OH⁻] is too low to precipitate Fe(OH)₃, so dissolved Fe³⁺ remains high. As pH rises naturally from dilution and buffering by soil minerals, the precipitation threshold is crossed and increasing amounts of Fe(OH)₃ form, explaining the pattern of heavy orange staining at higher pH points (P4, P5) despite near-zero dissolved Fe³⁺.

Marking notes (b). 1 mark for correct balanced precipitation equation; 1 mark for explaining that increasing pH raises [OH⁻], driving precipitation and removing Fe³⁺ from solution; 1 mark for linking the visible orange precipitate to Fe(OH)₃ and explaining why it increases while dissolved Fe³⁺ decreases. Total 3 marks.

Sample response. Coal combustion generates SO₂ by oxidation of sulfur impurities: S + O₂ → SO₂. In the atmosphere, SO₂ dissolves in water droplets to form sulfurous acid (SO₂ + H₂O ⇌ H₂SO₃) and is further oxidised to sulfuric acid (2SO₂ + O₂ + 2H₂O → 2H₂SO₄). The resulting acid precipitation (pH 4–5) causes significant environmental damage: it acidifies lakes below pH 5, at which point fish cannot survive; it mobilises toxic Al³⁺ ions from soil minerals (Al(OH)₃ + 3H⁺ → Al³⁺ + 3H₂O), which are toxic to fish gills; and it corrodes limestone and marble structures (CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂).

Flue gas desulfurisation (FGD) addresses this at source by reacting SO₂ with Ca(OH)₂ in a wet scrubber: Ca(OH)₂(aq) + SO₂(g) → CaSO₃(s) + H₂O(l). Modern FGD systems achieve 90–99% SO₂ removal, preventing the majority of SO₂ from entering the atmosphere. The byproduct CaSO₃ can be oxidised to CaSO₄ (gypsum) with commercial value in construction. This is highly effective as a preventive measure: intercepting SO₂ before it becomes H₂SO₄ in the atmosphere.

Acid-affected freshwater systems can be remediated directly by adding CaCO₃ (lime) to the water body: CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂. This raises pH and can restore conditions for aquatic life. The technique has been used to rehabilitate Scandinavian lakes acidified by decades of acid rain. However, this is a remediation measure, not a preventive one — without FGD, acid deposition continues and the lake would re-acidify.

Key limitation: Neither FGD nor lake liming addresses CO₂ emissions from the same combustion process. CO₂ dissolves in ocean water to form H₂CO₃ (CO₂ + H₂O ⇌ H₂CO₃), reducing ocean pH and driving ocean acidification — a global problem affecting coral calcification (including Australia’s Great Barrier Reef) that cannot be mitigated by Ca(OH)₂ scrubbing. Comprehensive management of coal combustion environmental impact ultimately requires reduced fossil fuel combustion or carbon capture, not just acid-gas scrubbing.

Marking criteria.

• 1 — Describes acid rain formation with at least one correct balanced equation (SO₂ → H₂SO₃ or → H₂SO₄) and identifies it as atmospheric neutralisation chemistry.
• 1 — Identifies at least two specific environmental impacts of acid rain with chemical reasoning (lake acidification, Al³⁺ mobilisation, building corrosion — any two).
• 1 — States the correct FGD equation: Ca(OH)₂ + SO₂ → CaSO₃ + H₂O (balanced).
• 1 — States FGD removal efficiency (90–99%) and identifies CaSO₃/CaSO₄ byproduct and its commercial use.
• 1 — Describes lake liming with correct equation (CaCO₃ or Ca(OH)₂ + H⁺) and explains the mechanism by which it raises pH.
• 1 — Identifies and explains at least one named limitation of these approaches (CO₂/ocean acidification, or lake re-acidification without FGD, or remediation vs prevention distinction).
• 1 — Response demonstrates coherent analysis: links the chemistry of each process to its environmental management role, and reaches a justified overall assessment of effectiveness with explicit limitations. Uses precise chemical terminology throughout.