Chemistry • Year 11 • Module 4 • Lesson 4
Calorimetry — Dissolution of Ionic Substances
Build core vocabulary, lock in the two-step dissolution energy model, and practise recognising exothermic vs endothermic dissolution from temperature data.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list:
enthalpy of dissolution (ΔHsoln), endothermic dissolution, exothermic dissolution, lattice energy, hydration enthalpy, Born–Haber cycle for dissolution.
6 marks (1 per row)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | The heat change when one mole of an ionic solid dissolves completely in a solvent under standard conditions; can be positive or negative. | |
| 1.2 | Dissolution in which energy is absorbed from the surroundings; the solution temperature falls; ΔHsoln > 0. | |
| 1.3 | Dissolution in which energy is released to the surroundings; the solution temperature rises; ΔHsoln < 0. | |
| 1.4 | Energy required to separate one mole of an ionic solid into its gaseous ions; always positive (endothermic step). | |
| 1.5 | Energy released when gaseous ions are surrounded by water molecules; always negative (exothermic step). | |
| 1.6 | Energy cycle for dissolution: ΔHsoln = lattice energy + hydration enthalpy; if hydration energy exceeds lattice energy, dissolution is exothermic. |
2. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version on the line below it. 10 marks (1 T/F + 1 correction where needed)
2.1 When ammonium nitrate (NH4NO3) dissolves in water, the solution temperature rises because this is an endothermic process. T / F
2.2 In a dissolution calorimetry calculation, the mass ‘m’ in q = mcΔT is the mass of water plus the mass of the dissolved solid. T / F
2.3 Lattice energy is an exothermic process because energy is released when the ionic lattice breaks apart. T / F
2.4 A negative ΔHsoln value indicates that a dissolution process is exothermic. T / F
2.5 NaOH dissolved in water is commonly used in AFL first aid cold packs because its dissolution warms the surrounding tissue. T / F
3. Function recall
Answer each in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
3.1 What is the function of lattice energy in the two-step dissolution model?
3.2 What is the function of hydration enthalpy in the two-step dissolution model?
3.3 Why is the ‘n’ in ΔHsoln = −q/n the moles of ionic solid dissolved, not the moles of water?
3.4 What function does a polystyrene cup serve in a dissolution calorimetry experiment?
4. Fill in the blanks
Complete the paragraph below by selecting the correct word from the word bank. Use each word once only. 8 marks (1 per blank)
Word bank: positive, negative, lattice energy, hydration enthalpy, exothermic, endothermic, rises, falls
When an ionic solid dissolves, two energy changes occur. First, energy is absorbed to overcome the _______________________ and separate the ions from the solid lattice. Second, energy is released as _______________________ when water molecules surround the separated ions. If the energy released in step two is greater than the energy absorbed in step one, the overall dissolution is _______________________ and the solution temperature _______________________. In this case ΔHsoln is _______________________. If step one requires more energy than step two releases, the dissolution is _______________________, the temperature _______________________, and ΔHsoln is _______________________. NH4NO3, which is used in AFL cold packs, is an example of this second type.
5. Build a concept map
Draw labelled arrows between the five terms below to show how they connect. Each arrow must carry a linking phrase. Aim for at least 5 labelled arrows. 5 marks
Supplied terms: ΔHsoln · lattice energy · hydration enthalpy · temperature change · sign of ΔHsoln
6. Classify exothermic or endothermic dissolution
For each ionic substance, write Exothermic or Endothermic in the second column, and a brief reason (one sentence) in the third column using lattice energy vs hydration enthalpy language. 6 marks (1 per substance)
| Ionic substance | Exo or endo? | Reason (lattice vs hydration) |
|---|---|---|
| NH4NO3 (used in AFL cold packs) | ||
| NaOH | ||
| CaCl2 (road de-icing in alpine NSW/VIC) | ||
| KNO3 | ||
| MgSO4 (Epsom salts in Australian physiotherapy) | ||
| NH4Cl |
Q1 — Term–definition match
1.1 enthalpy of dissolution (ΔHsoln) • 1.2 endothermic dissolution • 1.3 exothermic dissolution • 1.4 lattice energy • 1.5 hydration enthalpy • 1.6 Born–Haber cycle for dissolution.
Q2 — True / false with correction
2.1 False. Correction: when NH4NO3 dissolves in water the solution temperature falls — this is an endothermic process, not exothermic.
2.2 True.
2.3 False. Correction: lattice energy is an endothermic process — energy is absorbed (not released) to break apart the ionic lattice into separated gaseous ions.
2.4 True.
2.5 False. Correction: AFL cold packs use NH4NO3 (ammonium nitrate), whose endothermic dissolution cools the surrounding tissue. NaOH dissolves exothermically and would warm the solution, which is the opposite of what a cold pack does.
Q3 — Function recall
3.1 Lattice energy represents the energy that must be absorbed (endothermic) to overcome the electrostatic forces holding the ionic lattice together, separating the solid into individual gaseous ions before hydration can occur.
3.2 Hydration enthalpy is the energy released (exothermic) when water molecules form ion–dipole attractions around each separated gaseous ion, stabilising the ions in solution. It is the energy-releasing step that can offset the lattice energy cost.
3.3 ΔHsoln is defined as the enthalpy change per mole of substance dissolved. Dividing by moles of water would give a completely meaningless ratio because water is the solvent, not what is dissolving — the energy change arises from the ionic solid's lattice breaking and ions being hydrated.
3.4 Polystyrene is a thermal insulator. It reduces heat transfer between the solution and the external environment, minimising the systematic error from heat loss (or heat gain) during the experiment and making the measured temperature change more representative of the actual enthalpy change of dissolution.
Q4 — Cloze
In order: lattice energy • hydration enthalpy • exothermic • rises • negative • endothermic • falls • positive.
Q5 — Sample concept map
Correct arrows include:
- lattice energy — is absorbed in step 1 of → ΔHsoln
- hydration enthalpy — is released in step 2 of → ΔHsoln
- ΔHsoln — is calculated from → temperature change
- temperature change — determines the direction and thus → sign of ΔHsoln
- sign of ΔHsoln — reflects whether lattice energy exceeds hydration enthalpy in → ΔHsoln
Any chemically valid linking phrases are accepted. Award 1 mark per correct labelled arrow (minimum 5).
Q6 — Classify exo/endo
NH4NO3: Endothermic — lattice energy > hydration enthalpy; the lattice is hard to break and the hydration energy released is insufficient to compensate; used in AFL cold packs because the solution cools.
NaOH: Exothermic — hydration enthalpy > lattice energy; the strong hydration of Na+ and OH− ions releases more energy than is needed to break the NaOH lattice.
CaCl2: Exothermic — hydration enthalpy (especially for the highly charged Ca2+ ion) exceeds lattice energy; used in alpine NSW/VIC road de-icing where the exothermic dissolution helps melt ice.
KNO3: Endothermic — lattice energy > hydration enthalpy; temperature of solution falls on dissolution.
MgSO4 (Epsom salts): Endothermic overall (slightly) — lattice energy slightly exceeds hydration enthalpy; solution cools marginally; used in physiotherapy soaks in Australia.
NH4Cl: Endothermic — lattice energy > hydration enthalpy; temperature falls on dissolution (used in some cold pack formulations as an alternative to NH4NO3).