Solubility of Group 2 compounds
| English | Chinese | Pinyin |
|---|---|---|
| enthalpy of solution | 溶解焓 | róng jiě hán |
| lattice energy | 晶格能 | jīng gé néng |
| hydration | 水合 | shuǐ hé |
| insoluble | 不溶 | bù róng |
| antacid | 抗酸药 | kàng suān yào |
Solubility (Group 2)
- When an ionic solid dissolves, two energy changes compete.
- Their balance is the enthalpy of solution 溶解焓.
- This explains why hydroxides and sulfates trend oppositely.
Group 2 solubility lab
Classify Group 2 compounds by solubility trend.
Group 2 hydroxides become more soluble down the group.
While the sulfates become less soluble.
The energy balance
- first pull the lattice apart (costs the lattice energy 晶格能), then hydrate the ions (releases hydration 水合 energy).
- If hydration roughly matches or beats the lattice energy, the solid dissolves easily.
Why the Group 2 solubility trends are opposite
The enthalpy of solution is given by:
You reverse the lattice energy (to break the lattice) and add the hydration enthalpy.
A solid dissolves easily when the hydration energy:
If hydration releases about as much (or more) energy than is needed to break the lattice, dissolving is favourable.
Match each solubility trend.
Each item links the term to its correct meaning.
Opposite trends
Down the group both energies get smaller, but at different rates:
- hydroxides: the small $\text{OH}^-$ means the lattice energy falls a lot → solubility increases.
- sulfates: the large $\text{SO}_4^{2-}$ means the lattice energy barely changes, so the fall in hydration energy dominates → solubility decreases.
Down Group 2 the hydroxides get more soluble but the sulfates less
Group 2 hydroxides become MORE soluble down the group because:
With the small OH⁻ ion, the lattice energy drops sharply down the group, outweighing the hydration fall.
Group 2 sulfates become LESS soluble down the group because:
The big sulfate ion makes the lattice energy nearly constant, so the decreasing hydration energy controls the trend.
What you see in practice
- Hydroxides (more soluble down the group): Mg(OH)₂ is nearly insoluble 不溶 — it's milk of magnesia, a mild antacid 抗酸药; but Ba(OH)₂ dissolves well enough to use as an alkali.
- Sulfates (less soluble down the group): MgSO₄ is soluble — Epsom salts; but BaSO₄ is insoluble, which is why a barium meal is safe to swallow for an X-ray.
So the same two trends explain an antacid, a bath salt and a medical X-ray.
You've got it
- $\Delta H^{\ominus}_{\text{sol}} = -\Delta H_{\text{latt}} + \Delta H_{\text{hyd}}$ (break the lattice, then hydrate)
- hydroxides: lattice energy falls a lot down the group → more soluble (Mg(OH)₂ insoluble → Ba(OH)₂ soluble)
- sulfates: lattice energy ≈ constant, hydration fall dominates → less soluble (MgSO₄ soluble → BaSO₄ insoluble)