Chemistry • Year 12 • Module 5 • Lesson 15
Dissolution & ATSI Knowledge
Lock in the vocabulary of dissolution thermodynamics, dynamic equilibrium in saturated solutions, and the chemistry behind Aboriginal cycad detoxification.
1. Term–definition match
Match each term to its correct definition by writing the term in the right-hand column. Terms (some may be used once): dissolution, lattice energy, hydration energy, enthalpy of dissolution (ΔHsoln), dynamic equilibrium, saturated solution, cycasin, solubility, traditional ecological knowledge, Le Chatelier's Principle. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | The process by which a solute disperses uniformly into a solvent to form a homogeneous solution. | |
| 1.2 | The energy required to break one mole of an ionic lattice into its constituent gaseous ions; always endothermic. | |
| 1.3 | The energy released when one mole of gaseous ions is surrounded by water molecules via ion–dipole interactions; always exothermic. | |
| 1.4 | The net heat change when one mole of solute dissolves in excess solvent; can be positive (endo) or negative (exo). | |
| 1.5 | A solution in which the rate of dissolution of the solute equals the rate of recrystallisation, with excess solid present. | |
| 1.6 | A state in which two opposing processes occur simultaneously at equal rates, so that macroscopic properties remain constant. | |
| 1.7 | The maximum mass of solute that dissolves in a given volume of solvent at a specified temperature. | |
| 1.8 | The water-soluble neurotoxin and carcinogen found in cycad seeds, detoxified by Aboriginal communities through water soaking. | |
| 1.9 | When a system at equilibrium is disturbed, it shifts to partially counteract the disturbance and re-establish equilibrium. | |
| 1.10 | Accumulated knowledge of ecological and chemical phenomena developed by indigenous communities over generations through systematic observation and transmission. |
2. Fill-the-blank — dissolution thermodynamics
Complete the paragraph by writing a term from the word bank into each blank. Use each term once. 8 marks
Word bank: endothermic • exothermic • lattice energy • hydration energy • negative • positive • Le Chatelier's Principle • running water
When an ionic compound dissolves, two competing processes occur. Breaking the ionic lattice requires energy input: this step is always (2.1) _______________________ and has a (2.2) _______________________ value. When the released ions are surrounded by water molecules, energy is released: this hydration step is always (2.3) _______________________. The net enthalpy of dissolution is determined by comparing the magnitude of (2.4) _______________________ with that of (2.5) _______________________. If the hydration energy is larger in magnitude, the overall dissolution is (2.6) _______________________ and the solution warms. Traditional Aboriginal cycad detoxification uses (2.7) _______________________ to continuously remove dissolved cycasin, which shifts the dissolution equilibrium to the right in accordance with (2.8) _______________________, maximising the removal of toxin from the seeds.
3. True or false — with correction
Circle T or F. If the statement is false, write the correction. 10 marks (1 T/F + 1 correction each)
3.1 Dissolution of an ionic compound always produces a warmer solution because hydration energy is always released. T / F
3.2 Lattice energy is always an endothermic quantity because breaking the ionic lattice requires energy input. T / F
3.3 A saturated solution containing excess solid is at static equilibrium because the concentrations of ions do not change. T / F
3.4 Cycasin is water-soluble, which is the key chemical property exploited by Aboriginal communities when detoxifying cycad seeds. T / F
3.5 Crushing cycad seeds before soaking is chemically significant because it increases the surface area in contact with water, raising the rate of dissolution. T / F
4. Function recall
Answer each question in 1–2 sentences using precise lesson terms. 8 marks (2 each)
4.1 What is the role of hydration energy in determining whether dissolution is endothermic or exothermic?
4.2 What does it mean for a solution to be at dynamic equilibrium?
4.3 Why does NaOH dissolving in water cause the solution temperature to rise?
4.4 What is the role of water replacement (water changes) in the Aboriginal still-water cycad detoxification method, in terms of Le Chatelier's Principle?
5. Build a concept map
Draw labelled arrows between the six terms below to show how they connect. Each arrow must carry a short linking phrase. Aim for at least 6 labelled arrows. 6 marks
Supplied terms: ionic lattice · lattice energy · hydration energy · ΔHsoln · dynamic equilibrium · cycasin dissolution.
6. Sequence the steps — running-water cycad detoxification
The six events below describe the running-water cycad detoxification process. Number them 1–6 in the correct order in the “Order” column. 6 marks (1 per correct placement)
| Event | Order (1–6) |
|---|---|
| Dissolved cycasin is carried away by the current, keeping [cycasin] in surrounding water near zero. | |
| Seeds are crushed or sliced to maximise the surface area exposed to water. | |
| The concentration gradient between seed tissue (high cycasin) and water (low cycasin) is continuously maintained. | |
| Cycasin molecules enter the aqueous phase through dissolution equilibrium: cycasin(s) ⇌ cycasin(aq). | |
| LCP shifts the dissolution equilibrium right, maintaining maximum extraction rate. | |
| Processed seeds are tested and declared safe for consumption by community members. |
Q1 — Term–definition matches
1.1 dissolution • 1.2 lattice energy • 1.3 hydration energy • 1.4 enthalpy of dissolution (ΔHsoln) • 1.5 saturated solution • 1.6 dynamic equilibrium • 1.7 solubility • 1.8 cycasin • 1.9 Le Chatelier's Principle • 1.10 traditional ecological knowledge.
Q2 — Fill-the-blank
2.1 endothermic • 2.2 positive • 2.3 exothermic • 2.4 lattice energy • 2.5 hydration energy • 2.6 exothermic • 2.7 running water • 2.8 Le Chatelier's Principle.
Q3 — True / False
3.1 False. Dissolution is not always exothermic. When lattice energy exceeds hydration energy in magnitude (ΔH > 0), dissolution is endothermic and the solution cools (e.g. NH4NO3).
3.2 True. Breaking an ionic lattice always requires energy input, so lattice energy is always positive (endothermic).
3.3 False. A saturated solution with excess solid is at dynamic equilibrium, not static. Dissolution and recrystallisation occur simultaneously at equal rates; concentrations remain constant because the rates are equal, not because no molecular processes occur.
3.4 True. Cycasin's water solubility allows it to move from solid seed tissue into the surrounding aqueous phase through dissolution equilibrium, making water-based extraction possible.
3.5 True. Crushing increases the surface area of seed tissue in contact with water, providing more sites for cycasin to enter solution simultaneously, increasing the rate of dissolution.
Q4 — Function recall
4.1 Hydration energy is the energy released when gaseous ions are surrounded by water molecules. If |HE| > |LE|, the net ΔHsoln is negative (exothermic); if |HE| < |LE|, the net ΔHsoln is positive (endothermic). It is one of the two competing energetic terms that determine the sign of dissolution enthalpy.
4.2 A solution is at dynamic equilibrium when two opposing processes (dissolution and recrystallisation) occur simultaneously at equal rates, so macroscopic properties such as ion concentration and the amount of solid remain constant, but molecular-level exchange is ongoing.
4.3 NaOH dissolves exothermically because the hydration energy released when Na+ and OH− ions are surrounded by water molecules (especially OH−, which forms strong ion–dipole interactions) is greater in magnitude than the lattice energy required to break the NaOH ionic lattice. The net ΔHsoln is negative, so energy is released to the surroundings and the solution warms.
4.4 Each water change removes dissolved cycasin (the product) from the surrounding water, decreasing [cycasin(aq)]. By LCP, the system responds by shifting the dissolution equilibrium to the right to partially replace the removed product, causing more cycasin to leave the seed tissue. This cycle of removal and re-equilibration maximises extraction with each water change.
Q5 — Sample concept map
Valid labelled arrows include:
- ionic lattice —requires energy to break →— lattice energy (endothermic, +)
- ionic lattice —breaking releases ions that are— hydrated → hydration energy released (exothermic, −)
- lattice energy + hydration energy —determines sign of— ΔHsoln
- ΔHsoln —positive when |LE|>|HE| → endothermic dissolution; negative when |HE|>|LE| → exothermic
- dynamic equilibrium —exists in saturated cycasin solution; disturbed by removal of product— cycasin dissolution
- cycasin dissolution —shifts right by LCP when product removed by— running water
Q6 — Sequence
Correct order: Seeds crushed (1) → Cycasin enters aqueous phase via dissolution equilibrium (2) → Dissolved cycasin carried away by current (3) → [cycasin] in surrounding water kept near zero (4) → Concentration gradient continuously maintained (5) → LCP shifts equilibrium right (6). Note: steps 3–5 are a near-simultaneous cycle; award marks for the logical chain. Final step is community verification and safe-use declaration (6).
Intended sequence numbers: Event 1 (cycasin enters aqueous phase) = 2; Event 2 (seeds crushed) = 1; Event 3 (concentration gradient maintained) = 4; Event 4 (cycasin enters equilibrium) = 2; Event 5 (LCP shifts right) = 5; Event 6 (community safe-use test) = 6. Award 1 mark per correctly placed step.