AS Chemistry | Trends in Group II Compounds
In this topic we consider the compounds of (going down the group) calcium, magnesium, strontium and barium.
(Beryllium compounds are somewhat covalent so buck the trend a bit and radium is radioactive, so we leave them out.)
THERMAL STABILITY OF GROUP II CARBONATES/HYDROXIDES
The solid group II carbonates and hydroxides become more thermally stable going down the group.
(For example, calcium carbonate [limestone] can be decomposed to calcium oxide with a bunsen burner but we can't do
that with barium carbonate)
Explanation:
the cations (metal ions) get larger going down the group,
but they all have the same charge (2+);
this means they have lower charge densities,
so they have a lower polarising power;
this means they polarise the carbonate/hydroxide ion less,
so the anion is less easily decomposed.
SOLUBILITY OF GROUP II HYDROXIDES
The group II hydroxides become more soluble in water going down the group.
(Calcium hydroxide is sparingly soluble, magnesium hydroxide is slightly soluble, and strontium and barium hydroxides are
considered to be soluble.)
Before we give an explanation, two definitions are required.

The lattice enthalpy of an ionic compound is the energy needed to separate one mole of an ionic compound into its
gaseous anions and cations. This is an ENDOTHERMIC process. It is related to the sum of the radii of the anion and
cation.

The enthalpy of hydration of an ionic compound is the energy released when one mole of ions are hydrated (i.e.
when attractions form between the ions and water molecules). This is an EXOTHERMIC process.

If the enthalpy of hydration of an ionic compound is greater than its lattice enthalpy, the compond will dissolve.
Explanation:
the cations get larger going down the group,
so differ much more in size to the small hydroxide ion;
this means the lattice enthalpy decreases significantly;
the enthalpy of hydration also decreases with larger cations;
BUT the decrease in the lattice enthalpy outweighs the decrease in hydration enthalpy,
hence the larger group II hydroxides are more soluble.
SOLUBILITY OF GROUP II SULPHATES
The group II sulphates become less soluble in water going down the group.
(Calcium sulphate is slightly soluble, barium sulphate is insoluble [think sulphate ion test].)
Explanation:
the cations get larger going down the group,
so the enthalpy of hydration decreases,
but the lattice enthalpy only decreases slightly because the sulphate ion is so large compared to the cations;
the decrease in enthalpy of hydration outweighs the decrease in lattice enthalpy,
hence the larger group II sulphates are less soluble.
Samuel Millar, 29 May 2012