Chemistry • Year 11 • Module 3 • Lesson 5
Acid-Base & Acid-Carbonate Reactions
Build HSC Band 5–6 extended-response technique: evaluate data, justify recommendations, and synthesise reaction chemistry with real-world Australian contexts.
1. Stimulus-based extended response — acid rain and Sydney heritage limestone (Band 5–6)
8 marks Band 5–6
Stimulus. Atmospheric SO2 and NOx produced by burning fossil fuels dissolve in rainwater to form sulfuric acid (H2SO4) and nitric acid (HNO3), producing acid rain with pH as low as 4.2. Several sandstone and limestone structures in Sydney and the Hunter Valley — including the Paddington terraces and historic Hunter Valley church buildings — show accelerated surface erosion since the twentieth century. Chemical analysis confirms the surfaces contain high concentrations of CaCO3. Conservation engineers are assessing two protective strategies: (A) apply a silicone-resin surface sealant that blocks contact between acid rain and stone; (B) apply a thin Ca(OH)2 consolidant wash that reacts with the acid before it reaches the CaCO3 substrate.
Data table. Laboratory accelerated-weathering test: pH 4.2 acid solution applied for 72 h to three stone samples.
| Sample | Surface treatment | Mass loss (mg) | Surface pH after test |
|---|---|---|---|
| Control | None (bare CaCO3) | 48 | 4.4 |
| Strategy A | Silicone sealant | 3 | 4.3 |
| Strategy B | Ca(OH)2 wash | 11 | 6.8 |
Q1. Analyse and evaluate why Sydney's historic limestone structures are being eroded by acid rain, and assess which protective strategy (A or B) better conserves the buildings over the long term. In your response you must:
- Write the balanced molecular equations (with state symbols) for the reaction between H2SO4 and CaCO3, and between Ca(OH)2 and H2SO4.
- Identify the reaction type for each equation and name the salt produced in each.
- Interpret the data table: compare mass loss and surface pH across the three samples.
- Evaluate the chemical basis of each strategy (A and B), identifying at least one advantage and one limitation for each.
- Reach a justified recommendation, referencing both the data and the reaction chemistry.
2. Data + scenario — ocean acidification and the Great Barrier Reef (Band 5–6)
7 marks Band 5–6
Stimulus. The ocean absorbs approximately 25 % of anthropogenic CO2 emissions annually. When CO2 dissolves in seawater it forms carbonic acid (H2CO3), which partially dissociates to release H+ ions — a process known as ocean acidification. Since 1750, average ocean surface pH has dropped from ~8.2 to ~8.1. Coral skeletons are composed of calcium carbonate (CaCO3), and bleaching events in the Great Barrier Reef have increased in frequency. Marine chemists have observed that at pH below ~7.8, the rate of carbonate dissolution exceeds the rate of calcification (shell/skeleton building) in many coral species.
Data. Projected ocean pH and coral calcification rate (% relative to pre-industrial baseline).
| Year (projected) | Ocean surface pH | Coral calcification rate (%) |
|---|---|---|
| 1750 (baseline) | 8.20 | 100 |
| 2023 | 8.10 | 89 |
| 2050 (moderate emissions) | 7.95 | 72 |
| 2100 (high emissions) | 7.75 | 51 |
Q2. Evaluate, using the stimulus data and your knowledge of acid-carbonate reactions, the chemical threat that ocean acidification poses to coral reefs. In your response you must:
- Write the word equation and then the balanced molecular equation for the reaction between carbonic acid (H2CO3) and calcium carbonate (CaCO3).
- Describe the trend in calcification rate from the data table and link it to increasing [H+] in seawater.
- Explain, using the reaction type and products, why lower pH specifically threatens CaCO3-based structures such as coral skeletons.
- Assess the severity of the projected 2100 scenario relative to the marine chemists’ threshold of pH 7.8.
Q1 — Sample Band 6 response (8 marks), annotated
Equations (2 marks).
H2SO4(aq) + CaCO3(s) → CaSO4(s) + H2O(l) + CO2(g) — acid-carbonate reaction; salt: calcium sulfate [1 balanced + labelled].
H2SO4(aq) + Ca(OH)2(aq) → CaSO4(aq) + 2H2O(l) — neutralisation; salt: calcium sulfate [1 balanced + labelled].
Data interpretation (2 marks). The control sample suffered the greatest mass loss (48 mg) and remained highly acidic (pH 4.4 post-test), confirming ongoing CaCO3 dissolution [1]. Strategy A (sealant) almost eliminated mass loss (3 mg), but the surface pH remained 4.3 — the sealant physically blocked contact. Strategy B (Ca(OH)2) reduced mass loss to 11 mg and raised surface pH to 6.8, indicating the acid was chemically neutralised before reaching the CaCO3 [1].
Evaluation of strategies (2 marks). Strategy A: advantage — near-complete protection from erosion (only 3 mg lost vs 48 mg). Limitation — the acid is not neutralised, it is merely deflected; sealant can crack, peel or degrade, and does not address acid that enters through gaps [1]. Strategy B: advantage — the Ca(OH)2 sacrificially neutralises the acid (neutralisation reaction) before it contacts the CaCO3 substrate, and the high post-test pH (6.8) confirms chemical buffering. Limitation — the Ca(OH)2 layer is consumed in the reaction and must be periodically reapplied; 11 mg of mass was still lost as the wash itself reacted [1].
Recommendation (2 marks). Strategy A provides greater short-term erosion protection based on the mass-loss data (3 mg vs 11 mg). However, Strategy B is chemically superior for long-term heritage conservation because it neutralises the acid and prevents [H+] from ever reaching the irreplaceable CaCO3; the eroded material is the sacrificial Ca(OH)2 consolidant, not the historic stone [1]. A combined approach — Ca(OH)2 consolidant wash beneath a sealant topcoat — would exploit both the physical barrier (A) and chemical neutralisation (B) [1 — for a qualified, evidence-based judgement that does not simply pick one winner].
Marking criteria (8 marks total).
- 1 mark — Balanced equation for H2SO4 + CaCO3 with state symbols, reaction type named, salt identified.
- 1 mark — Balanced equation for H2SO4 + Ca(OH)2 with state symbols, reaction type named, salt identified.
- 1 mark — Correctly describes control mass loss and pH data vs treated samples.
- 1 mark — Correctly interprets the difference in pH outcome between Strategy A (physical barrier, acid not consumed) and Strategy B (acid neutralised, pH rises).
- 1 mark — Evaluates one advantage of Strategy A with evidence from the data.
- 1 mark — Evaluates one limitation of Strategy A and one advantage and one limitation of Strategy B using chemistry.
- 1 mark — Reaches a justified recommendation that references specific data values.
- 1 mark — Recommendation considers the long-term / heritage conservation context, not only short-term data, and links to reaction chemistry (what is consumed vs what is preserved).
Q2 — Sample Band 6 response (7 marks), annotated
Equations (2 marks).
Word equation: carbonic acid + calcium carbonate → calcium hydrogen carbonate [1 for correct word equation].
Balanced molecular: H2CO3(aq) + CaCO3(s) → Ca(HCO3)2(aq) [1 balanced]. This is an acid-carbonate reaction; the product Ca(HCO3)2 is soluble, so the coral skeleton literally dissolves.
Data trend and H+ link (2 marks). From 1750 to the projected 2100 high-emissions scenario, ocean pH falls from 8.20 to 7.75 — a decrease of 0.45 pH units. Lower pH means a higher concentration of H+ ions in the seawater [1]. Calcification rate declines from 100% to 51% over the same period, and the trend accelerates as pH falls further — consistent with increasing acid-carbonate dissolution outpacing skeleton deposition [1].
Reaction-type explanation (2 marks). The acid-carbonate reaction between H2CO3 and CaCO3 produces the soluble salt Ca(HCO3)2, which diffuses away from the skeleton into seawater [1]. Unlike a physical acid attack that merely erodes the surface, the products of this reaction are aqueous — the solid CaCO3 skeleton is chemically converted to a dissolved ionic species, undermining structural integrity irreversibly [1].
2100 severity assessment (1 mark). The projected 2100 high-emissions pH (7.75) falls below the marine chemists’ threshold of ~7.8 at which dissolution exceeds calcification. With a calcification rate of only 51% of the pre-industrial baseline, coral frameworks would be net dissolving — the Great Barrier Reef would lose structural integrity faster than it can rebuild. This represents a qualitative shift from degraded reef to actively dissolving reef, substantially more severe than the 2050 moderate-emissions scenario (pH 7.95, 72% calcification, which is above the critical threshold) [1].
Marking criteria (7 marks total).
- 1 mark — Correct word equation for carbonic acid + calcium carbonate.
- 1 mark — Correct balanced molecular equation H2CO3(aq) + CaCO3(s) → Ca(HCO3)2(aq).
- 1 mark — Correctly describes calcification rate trend from data with specific figures.
- 1 mark — Correctly states that lower pH means higher [H+] and connects this to the acid-carbonate reaction with CaCO3.
- 1 mark — Explains why the acid-carbonate reaction specifically destroys CaCO3 skeletons (soluble product dissolves skeleton).
- 1 mark — Assesses the 2100 scenario relative to the pH 7.8 threshold with reference to data.
- 1 mark — Reaches an evaluative judgement about severity that uses precise chemical language (reaction type, dissolving product, net dissolution vs net calcification).