Reactions of group 2 Elements with Oxygen and Nitrogen

Reactions with Oxygen

Group 2 elements, also known as alkaline earth metals, readily react with oxygen to form oxides. Let’s explore the formation, properties, and trends within the group.

Formation of Simple Oxides (MO)

These oxides are prepared by:

• Direct Heating with Oxygen: Heating the Group 2 metal with oxygen gas.

  • Example: 2Ca (s) + O₂ (g) → 2CaO (s) (Calcium oxide formation)

• Thermal Decomposition of Carbonates: Heating carbonates of these metals at high temperatures.

  • Example: CaCO₃ (s) → CaO (s) + CO₂ (g) (Calcium carbonate decomposes to calcium oxide and carbon dioxide)

Properties of Simple Oxides (MO):

1. Stability and Structure: Due to the strong electrostatic attraction between doubly charged cations (M²⁺) and oxide anions (O²⁻), these oxides are highly stable, white, crystalline solids with a high melting point. They adopt a sodium chloride type crystal lattice for efficient packing of ions.

2. Solubility in Water: Solubility varies down the group:

   • BeO and MgO: Insoluble in water.

   • CaO, SrO, and BaO: React with water to form soluble hydroxides (M(OH)₂) which are strong bases.

   • Example: CaO (s) + H₂O (l) → Ca(OH)₂ (aq) (Calcium oxide reacts with water to form calcium hydroxide)

3. Amphoteric Nature:

   • BeO is amphoteric, meaning it can react with both acids and bases. It exhibits weak basic character.

   • Reactions:

     • BeO (s) + 2HCl (aq) → BeCl₂ (aq) + H₂O (l) (BeO with hydrochloric acid)
     • BeO (s) + 2NaOH (aq) → Na₂BeO₂ (aq) + H₂O (l) (BeO with sodium hydroxide)

4. Basic Character: Except for BeO, the basic character of these oxides increases down the group. This is because the larger cations down the group have a weaker attraction for the oxide anions, making the hydroxides formed upon reaction with water more readily soluble and basic.

5. Covalent nature of BeO: Due to the small size of Be²⁺ ion, BeO has a significant covalent character compared to the other ionic oxides. This contributes to its higher melting point and hardness compared to other Group 2 metal oxides. Interestingly, BeO has a polymeric structure where each Be atom is tetrahedrally coordinated by four oxygen atoms.

Peroxides (MO₂)

Heavier Group 2 elements (Ca, Sr, Ba) form peroxides upon heating their corresponding oxides with excess oxygen at high temperatures.

• Example: 2BaO (s) + O₂ (g) → 2BaO₂ (s) (Barium oxide reacts with oxygen to form barium peroxide)

Properties of Peroxides (MO₂):

• White, ionic solids containing the peroxide anion (O-O)²⁻.
• They react with acids to liberate hydrogen peroxide (H₂O₂).

Reactions with Nitrogen

When heated in air (a mixture of oxygen and nitrogen), all Group 2 elements react with nitrogen to form nitrides (M₃N₂).

• Example: 3Ca (s) + N₂ (g) → Ca₃N₂ (s) (Calcium reacts with nitrogen to form calcium nitride)

These nitrides react with water to release ammonia (NH₃) gas.

• Example: Ca₃N₂ (s) + 6H₂O (l) → 3Ca(OH)₂ (aq) + 2NH₃ (g) (Calcium nitride reacts with water to form calcium hydroxide and ammonia)

Note: Beryllium nitride (Be₃N₂) is volatile unlike other nitrides in the group.