Solubility of Group 2 compounds
Introduced| English | Meaning |
|---|---|
| enthalpy of solution/enˈθælpi ɒv səˈluːʃn/ | enthalpy of solution |
| lattice energy/ˈlætɪs ˈenədʒi/ | The enthalpy change associated with forming an ionic solid from gaseous ions, or the reverse process under a stated sign convention. |
| hydration/haɪˈdreɪʃn/ | Association or reaction of a substance with water, including water molecules surrounding dissolved ions. |
| insoluble/ɪnˈsɒljuːbl/ | Having very low solubility in a specified solvent under the stated conditions. |
| antacid/ænˈtæsɪd/ | antacid |
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.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
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:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
You reverse the lattice energy (to break the lattice) and add the hydration enthalpy.
A solid dissolves easily when the hydration energy:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
If hydration releases about as much (or more) energy than is needed to break the lattice, dissolving is favourable.
Match each solubility trend.
Difficulty 2/5 · 140 coins + a small bonus on first completion today
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:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
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:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
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)