HSC Exam Practice

Sample response. Enthalpy of dissolution (ΔH_soln) is the heat change when one mole of solute dissolves completely in excess solvent under standard conditions. Exothermic dissolution: ΔH_soln < 0 (solution warms); endothermic dissolution: ΔH_soln > 0 (solution cools).

Marking notes. 1 mark for defining ΔH_soln as heat change per mole of solute dissolved. 1 mark for correct sign convention (negative = exothermic / positive = endothermic).

Sample response. Dissolution of NaOH involves two competing energy processes. (1) Breaking the NaOH ionic lattice into Na⁺(g) and OH⁻(g) ions requires energy input (endothermic, lattice energy). (2) When Na⁺ and OH⁻ ions are surrounded by water molecules, energy is released via ion–dipole interactions (hydration energy, exothermic). For NaOH, the hydration energy released (particularly the strong hydration of OH⁻, which forms hydrogen bonds with water) is greater in magnitude than the lattice energy required. Net ΔH_soln < 0 (exothermic); energy is transferred to the surroundings, raising the solution temperature.

Marking notes. 1 mark — identifies both lattice energy (endothermic, energy input) and hydration energy (exothermic, energy released). 1 mark — states that |HE| > |LE| for NaOH; net ΔH < 0. 1 mark — concludes that energy released to surroundings causes temperature rise.

Sample response. A saturated NaCl solution with excess solid is at dynamic equilibrium: NaCl(s) ⇌ Na⁺(aq) + Cl⁻(aq). Na⁺ and Cl⁻ ions continuously leave the crystal surface (dissolution) and return to it (recrystallisation) at equal rates. Because both processes occur simultaneously at the same rate, the concentration of ions and the amount of solid remain constant macroscopically — the system is dynamic, not static.

Marking notes. 1 mark — correctly uses ‘dynamic equilibrium’ and states both dissolution and recrystallisation occur simultaneously at equal rates. 1 mark — states concentrations/amount of solid remain constant as a result.

Sample response. Although NH₄NO₃ dissolution is endothermic (ΔH > 0), it is spontaneous because the Gibbs free energy change ΔG = ΔH − TΔS is negative. Dissolution disperses NH₄⁺ and NO₃⁻ ions throughout the solution, greatly increasing the disorder of the system (ΔS > 0, large positive). At room temperature, the entropy term TΔS is sufficiently large that ΔH − TΔS < 0, giving ΔG < 0: the process is spontaneous. Dissolution is driven by entropy, not enthalpy.

Marking notes. 1 mark — states ΔG = ΔH − TΔS and identifies that ΔG must be negative for spontaneity. 1 mark — explains that dissolution increases entropy (ΔS > 0) due to dispersal of ions. 1 mark — concludes TΔS > ΔH at room temperature, so ΔG < 0 and the process is spontaneous.

Sample response. Cycasin is water-soluble and exits the seed tissue through dissolution equilibrium: cycasin(seed) ⇌ cycasin(aq). Running water continuously removes dissolved cycasin from the surrounding water, maintaining a near-zero [cycasin(aq)]. By Le Chatelier's Principle, the removal of product continuously shifts the dissolution equilibrium to the right, sustaining maximal extraction rates throughout the soaking period. Crushing the seeds increases the surface area of seed tissue in contact with water, providing more simultaneous dissolution sites and increasing the rate of cycasin entry into solution.

Marking notes. 1 mark — correctly identifies cycasin as water-soluble and describes the dissolution equilibrium. 1 mark — applies LCP: running water removes product → equilibrium shifts right → continuous maximum extraction. 1 mark — explains crushing increases surface area → greater rate of dissolution.

Sample response. In a static equilibrium, all molecular processes have ceased and there is no net movement or change at any level (e.g. a solid in a sealed container with no solvent — no dissolving or recrystallising). In a dynamic equilibrium, two opposing processes occur simultaneously at equal rates, so macroscopic properties are constant but molecular exchange is ongoing (e.g. a saturated NaCl solution: dissolution and recrystallisation occur at the same rate).

Marking notes. 1 mark — distinguishes static (no molecular processes occurring) from dynamic (opposing processes occurring at equal rates). 1 mark — gives one dissolution-context example of each (any valid examples accepted).

Part (a) — Sample response. From the graph, the solubility of KNO₃ at 50 °C is approximately 85 g per 100 g water. Accept values 80–90 g per 100 g water. [1]

Part (b) — Sample response. KNO₃ dissolution is endothermic (ΔH_soln = +35.4 kJ/mol) [1]. The dissolution equilibrium KNO₃(s) ⇌ K⁺(aq) + NO₃⁻(aq) can be treated as endothermic in the forward direction. By LCP, increasing temperature supplies energy that the system absorbs by favouring the forward (endothermic) reaction, increasing K_sp and solubility — hence the steep positive gradient for KNO₃ [1]. NaCl dissolution is only slightly endothermic (+3.9 kJ/mol); the very small ΔH means temperature has almost no effect on the position of equilibrium, so solubility changes very little with temperature — hence the near-flat line for NaCl [1].

Marking notes (b). 1 mark — identifies KNO₃ dissolution as endothermic. 1 mark — applies LCP: increasing T shifts endothermic equilibrium right, increasing KNO₃ solubility steeply. 1 mark — explains NaCl's near-zero ΔH_soln means temperature has negligible effect on solubility.

Part (a) — q = mcΔT. Total mass of solution = 100 mL × 1.00 g/mL + 8.00 g = 108.0 g. q = mcΔT = 108.0 × 4.18 × 5.4 = 2437 J (endothermic: solution cools, so dissolution absorbs heat). [1 mark for substitution; 1 for correct answer ± rounding]

Part (b) — ΔH_soln. Moles NH₄NO₃ = 8.00/80.0 = 0.100 mol. Heat absorbed per mole = 2437 J / 0.100 mol = 24 370 J/mol = 24.4 kJ/mol (endothermic, so ΔH_soln = +24.4 kJ/mol). [1 mark for calculating moles correctly; 1 for ΔH_soln with correct sign and units. Accept +24–26 kJ/mol given rounding.]

Part (c) — Assumption. Acceptable assumptions include: (i) the specific heat capacity and density of the solution equal those of pure water (in reality, dissolved ions alter both slightly, introducing a small systematic error); (ii) no heat loss to the container or surroundings (the container is not a perfect insulator, so some heat is lost to the environment, causing the recorded temperature drop to underestimate the true endothermic effect, making the calculated ΔH_soln slightly too small in magnitude); (iii) complete dissolution of NH₄NO₃. [1 mark for naming a valid assumption; 1 mark for explaining how it introduces uncertainty/error into the result.]

Error 1 — “No chemical bonds are broken or formed.” Incorrect. Dissolution of NaCl involves breaking the ionic lattice (electrostatic forces between Na⁺ and Cl⁻) — this requires energy input (lattice energy). New ion–dipole interactions form between Na⁺/Cl⁻ and water molecules (hydration) — this releases energy. Both lattice breaking and hydration involve changes to intermolecular and ionic interactions, not merely physical mixing. [1 identify + 1 correct]

Error 2 — “No energy is exchanged with the surroundings.” Incorrect. Dissolution has a measurable enthalpy (ΔH_soln). For NaCl, ΔH_soln = +3.9 kJ/mol (endothermic): the solution absorbs energy from the surroundings, causing a slight temperature drop. Exothermic dissolutions (e.g. NaOH: ΔH = −44.5 kJ/mol) release energy, warming the solution. Energy is always exchanged during dissolution. [1 identify + 1 correct]

Error 3 — “At saturation the process stops entirely.” Incorrect. At saturation, the system is at dynamic equilibrium: dissolution and recrystallisation continue simultaneously at equal rates (NaCl(s) ⇌ Na⁺(aq) + Cl⁻(aq)). The process does not stop; macroscopic properties are constant because opposing rates are equal, not because molecular processes have ceased. [1 identify + 1 correct]

Sample Band 6 response. The claim is well-supported by the chemistry: Aboriginal cycad detoxification is a scientifically valid application of dissolution equilibrium principles, and its inclusion in the Chemistry curriculum serves both scientific and epistemic purposes.

The chemistry underpinning the detoxification is unambiguous. Cycasin, the principal toxin in Macrozamia and related cycad species, is water-soluble. This water solubility means cycasin participates in a dissolution equilibrium: cycasin(seed) ⇌ cycasin(aq). When seeds are submerged in water, cycasin moves from the solid seed matrix into the aqueous phase, driven by the concentration gradient. Running water continuously removes dissolved cycasin (product), maintaining [cycasin(aq)] near zero. By Le Chatelier's Principle, this continuous product removal shifts the equilibrium right, sustaining the maximum possible extraction rate. Crushing seeds increases surface area, providing more simultaneous dissolution sites and raising the rate of extraction. These are quantitative, reproducible chemical phenomena that can be modelled mathematically using solubility equilibrium constants and kinetic equations — they are emphatically not ‘just washing.’

The development of this knowledge also meets the criteria of scientific inquiry: Aboriginal communities observed that raw seeds cause neurological damage (observation); developed and tested various treatment methods (hypothesis testing); selected those that were most effective across different environmental conditions (comparative evaluation); verified safety by controlled community testing before declaring seeds fit for consumption (verification); and transmitted the knowledge systematically through language, ceremony and practice across 65,000+ years (transmission). Different language groups across Australia independently developed chemically equivalent solutions — using water soaking, roasting, burial or combinations — tailored to local water availability, temperature and fuel. This convergent development across geographically separated communities, producing equivalent chemical outcomes, is itself strong evidence of systematic empirical inquiry.

The significance for the curriculum is substantive, not symbolic. First, it demonstrates that chemistry describes natural phenomena that humans have understood and applied across all cultures and timescales — not only in Western laboratory traditions. Second, it provides an Australian-context application of dissolution equilibrium (LCP, concentration gradient, rate factors) that NESA requires students to understand and explain using the same chemical vocabulary as any other equilibrium application. Third, it requires students to use language that reflects the sophistication and validity of the knowledge system, resisting the misconception that science is culturally bounded. Students who dismiss this content as ‘cultural tokenism’ risk missing both HSC marks and a genuine scientific insight: that rigorous, evidence-based knowledge about the natural world develops wherever humans observe it systematically.

Marking criteria.

• 1 mark — States an overall evaluative position (claim is well-supported / Aboriginal cycad detoxification is a valid dissolution equilibrium application).
• 2 marks — Correctly describes the chemical mechanism: cycasin water-solubility + dissolution equilibrium [1]; LCP applied to running water maintaining concentration gradient → equilibrium shifts right + crushing increases surface area [1].
• 2 marks — Applies the criteria of scientific knowledge to cycad detoxification: names at least three criteria (observation, hypothesis, testing, verification, transmission) [1]; gives specific examples from the cycad knowledge system for each named criterion [1].
• 1 mark — Notes convergent development across language groups as evidence of systematic inquiry, or equivalent observation about the scope and consistency of the knowledge.
• 1 mark — Discusses curriculum significance: substantive chemical application (not symbolic); or importance of recognising that scientific knowledge develops across all cultures; or implication for how students should write about ATSI knowledge in HSC responses.
• 1 mark — Uses precise lesson terminology throughout (cycasin, dissolution equilibrium, Le Chatelier's Principle, concentration gradient, surface area, traditional ecological knowledge / ATSI knowledge system). Do not award if response is vague or uses non-chemical language only.