HSC Exam Practice

Sample response. Neutralisation is the reaction between an acid and a base to produce a salt and water [1]. The universal net ionic equation for any strong acid reacting with any strong base is H⁺(aq) + OH⁻(aq) → H₂O(l) [1]. This equation is the same regardless of which specific acid and base are used, because the spectator ions (e.g. Na⁺, Cl⁻) do not participate in the reaction [1].

Marking notes. 1 mark: correct definition (acid + base → salt + water). 1 mark: correct net ionic equation. 1 mark: explanation of why it is universal (spectator ions, or equivalent).

Sample response. An acid-base neutralisation produces two products: salt and water [1]. An acid-carbonate reaction produces three products: salt, water, and CO₂ gas [1]. The fizzing in an acid-carbonate reaction is caused by CO₂(g) being evolved as the carbonate ion (CO₃²⁻) reacts with H⁺ ions from the acid [1].

Marking notes. 1 mark each for the three points listed above.

Sample response.

(a) H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l). Salt: potassium sulfate, K₂SO₄. [2 marks: 1 for balanced equation with state symbols, 1 for correct salt name and formula].

(b) 2HCl(aq) + Na₂CO₃(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g). Salt: sodium chloride, NaCl. [2 marks: 1 for balanced equation with state symbols, 1 for correct salt].

Sample response. The carbon in the carbonate ion (CO₃²⁻) is already at oxidation state +4, the maximum oxidation state for carbon [1]. It cannot be further oxidised, so CO₂ (carbon also at +4) is the only possible gaseous carbon product. CO (carbon at +2) is the product of incomplete combustion, which requires a reduction in oxidation state from higher values — this does not occur in an acid-carbonate reaction [1].

Marking notes. 1 mark for identifying C is at +4 in CO₃²⁻. 1 mark for explaining why CO₂ (not CO) must result.

Sample response. Arrhenius (1884): an acid is a substance that produces H⁺ ions in water; a base is a substance that produces OH⁻ ions in water [1 for both definitions]. One limitation: the Arrhenius model requires water as a solvent and requires bases to contain OH⁻. This excludes substances such as NH₃ and Na₂CO₃, which can accept protons and act as bases but do not contain hydroxide [1]. The Brønsted-Lowry model (1923) defines an acid as a proton donor and a base as a proton acceptor, which is broader and applies to a wider range of substances and non-aqueous contexts [1].

Marking notes. 1 mark for both Arrhenius definitions correct. 1 mark for a valid limitation. 1 mark for a correct comparison to Brønsted-Lowry.

Sample response. Acidic NSW soils contain excess H⁺ ions (low pH). Agricultural lime (CaCO₃) is spread across paddocks and reacts with these H⁺ ions via an acid-carbonate reaction [1]: CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g) [1 for balanced equation]. H⁺ ions are consumed in this reaction, reducing their concentration and raising the soil pH, making the soil less acidic and more suitable for crops such as wheat and canola [1].

Marking notes. 1 mark: identifies reaction type (acid-carbonate). 1 mark: balanced equation (state symbols). 1 mark: explains H⁺ consumed, pH rises.

Sample response (a) — 2 marks. NaHCO₃ neutralised the acid the fastest (25 s), followed by Mg(OH)₂ (40 s), and CaCO₃ took the longest (70 s) [1]. The trend shows that carbonate-based antacids are slower to reach pH 7 than hydroxide-based or hydrogen-carbonate-based antacids under these conditions [1].

Sample response (b) — 3 marks. CaCO₃ took the longest. Reaction type: acid-carbonate reaction [1]. Balanced equation: 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g) [1 for balanced with state symbols]. Products: calcium chloride (CaCl₂), water (H₂O), and carbon dioxide (CO₂) [1].

Sample response (c) — 3 marks. Mg(OH)₂ is a metal hydroxide base. It undergoes neutralisation: 2HCl(aq) + Mg(OH)₂(s) → MgCl₂(aq) + 2H₂O(l). The products are only a salt and water; there is no CO₃²⁻ ion to react with H⁺ and release CO₂ [1]. NaHCO₃ is a hydrogen carbonate: HCl(aq) + NaHCO₃(aq) → NaCl(aq) + H₂O(l) + CO₂(g). The HCO₃⁻ ion reacts with H⁺ to form CO₂ [1]. CaCO₃ undergoes an acid-carbonate reaction with the CO₃²⁻ ion reacting with 2H⁺ to form CO₂. Both carbonate/hydrogen carbonate reactions produce gas because the CO₃²⁻ or HCO₃⁻ ion combines with H⁺ to form H₂CO₃, which immediately decomposes to H₂O and CO₂(g) [1].

Sample response. Atmospheric CO₂ produced by human activities dissolves in ocean water to form carbonic acid: CO₂(g) + H₂O(l) → H₂CO₃(aq) [1]. H₂CO₃ then dissociates to produce H⁺ ions, lowering ocean pH — a process called ocean acidification. Since 1750, ocean surface pH has dropped from ~8.2 to ~8.1, meaning the H⁺ concentration in seawater has increased [1]. This elevated [H⁺] drives an acid-carbonate reaction with the CaCO₃ skeletons of coral: H₂CO₃(aq) + CaCO₃(s) → Ca(HCO₃)₂(aq) [1 for balanced equation, reaction type identified]. The product, calcium hydrogen carbonate Ca(HCO₃)₂, is fully soluble, meaning the solid CaCO₃ skeleton dissolves into the surrounding seawater [1]. At projected 2100 pH values of ~7.75, dissolution of coral exceeds reef growth, threatening the biodiversity, fisheries, and coastal protection the reef provides [1]. This is significant because unlike acid rain on buildings (which can be treated with sealants or base washes), ocean acidification operates at a planetary scale and cannot be simply reversed by surface treatment — the only long-term solution is reducing atmospheric CO₂ emissions [1].

Marking notes. 1 mark: CO₂ + H₂O → H₂CO₃ (mechanism of acidification). 1 mark: lower pH means increased [H⁺] in seawater (with reference to the pH drop from ~8.2 to ~8.1 since 1750). 1 mark: balanced equation H₂CO₃ + CaCO₃ → Ca(HCO₃)₂, reaction type named. 1 mark: soluble product means skeleton dissolves (dissolution mechanism). 1 mark: significance to Great Barrier Reef including projected 2100 pH ~7.75 or dissolution exceeding growth. 1 mark: evaluative judgement on the scale/reversibility of the threat distinguishing it from localised acid-base problems.