Reactions of Group II Elements: MDCAT Chemistry notes

General features

Group II (Be, Mg, Ca, Sr, Ba) are the alkaline earth metals, with valence configuration $ns^2$. Each atom supplies two electrons to the metallic bond, so their melting points are higher than those of group I. Reactivity increases down the group. Beryllium is the least reactive: it is the smallest atom, has a high ionization energy, and its compounds are largely covalent.

Reaction with oxygen

$$\mathrm{2M + O_2 \to 2MO}$$

• Be and Mg form a thin oxide layer that protects the metal from further oxidation. Heated Mg burns with a brilliant white flame.
• BeO is amphoteric: it reacts with both acids and alkalis. MgO, CaO, SrO and BaO are basic. Basic character increases down the group.
• BeO is covalent and inert and does not react with water even at red heat. CaO reacts vigorously to give $\mathrm{Ca(OH)_2}$ (slaked lime).

Reaction with nitrogen

When heated in nitrogen, the metals form nitrides of formula $\mathrm{M_3N_2}$:

$$\mathrm{3Mg + N_2 \to Mg_3N_2}$$

With water, the nitride hydrolyses to the hydroxide and ammonia, so the mixture becomes basic:

$$\mathrm{Mg_3N_2 + 6H_2O \to 3Mg(OH)_2 + 2NH_3}$$

Reaction with water

Be does not react. Mg reacts very slowly with cold water but readily with steam, forming MgO and $\mathrm{H_2}$. Ca, Sr and Ba react with cold water with increasing vigour: $\mathrm{M + 2H_2O \to M(OH)_2 + H_2}$.

Solubility trends: opposite for hydroxides and sulphates

Compound	Trend down the group	Extremes
Hydroxides $\mathrm{M(OH)_2}$	Solubility increases	$\mathrm{Be(OH)_2}$ and $\mathrm{Mg(OH)_2}$ are least soluble, $\mathrm{Ba(OH)_2}$ most soluble
Sulphates $\mathrm{MSO_4}$	Solubility decreases	$\mathrm{BeSO_4}$ and $\mathrm{MgSO_4}$ are soluble, $\mathrm{CaSO_4}$ sparingly soluble, $\mathrm{BaSO_4}$ insoluble

Solubility depends on the balance between lattice energy and hydration energy.

• For sulphates, the large $\mathrm{SO_4^{2-}}$ ion keeps the lattice energy nearly constant, while the hydration energy of the cation falls down the group. So solubility falls.
• For hydroxides, the lattice energy falls faster than the hydration energy, so solubility rises.

Thermal stability of carbonates

$$\mathrm{MCO_3 \xrightarrow{\Delta} MO + CO_2}$$

Stability increases down the group. A small cation such as $\mathrm{Be^{2+}}$ strongly polarizes the carbonate ion, so $\mathrm{BeCO_3}$ is very unstable and decomposes most readily. $\mathrm{BaCO_3}$ needs the highest temperature.

Common MDCAT traps

• Hydroxide solubility goes up while sulphate solubility goes down. Read which compound is asked.
• The nitride formula is $\mathrm{Mg_3N_2}$, not MgN or $\mathrm{Mg_2N_2}$.
• BeO is the amphoteric oxide. $\mathrm{Li_2O}$, CaO and $\mathrm{K_2O}$ are basic.
• Be's oxide layer protects the metal. It does not accelerate oxidation.
• The easiest carbonate to decompose is $\mathrm{BeCO_3}$, the top of the group.

Quick revision

• $\mathrm{BaSO_4}$ is insoluble.
• $\mathrm{Ba(OH)_2}$ is the most soluble group II hydroxide.
• Oxides become more basic down the group.
• Group II metals melt higher than group I because of two bonding electrons per atom.
• $\mathrm{Mg_3N_2}$ with water gives ammonia.