Group 2 Compounds (OCR A Level Chemistry A): Revision Note

Group 2 Oxides & Water

• Group 2 oxides react with water, releasing hydroxide ions, to form alkaline solutions:

CaO (s) + H₂O (l) → Ca²⁺ (aq) + 2OH^- (aq)

• These solutions get more alkaline going down the group

  • This is due to increasing solubility meaning the resulting solutions formed contain more OH^- ions

• When the solution becomes saturated any further metal and hydroxide ions will form a precipitate:

 Ca²⁺ (aq) + 2OH^- (aq) → Ca(OH)₂ (s)

• Solid magnesium hydroxide, Mg(OH)₂ (s), is only slightly soluble in water

  • The solution has a low OH^-(aq) concentration and a pH ≅ 10

• Barium hydroxide, Ba(OH)₂ (s), is much more soluble in water

  • The solution has a greater OH^-(aq) concentration and a pH ≅ 13

Uses of Group 2 Compounds

• Due to their basic properties and ability to neutralise acids, Group 2 oxides, hydroxides and carbonates have many uses

Group 2 compounds in agriculture

• Calcium hydroxide, Ca(OH)₂, is reasonably soluble in water and is used in agriculture to neutralise acidic soils

Ca(OH)₂ (s) + 2H⁺ (aq) → Ca²⁺ (aq) + 2H₂O (l)

• It is important to regulate how much is added as If too much is added then this will result in soil becoming too alkaline to sustain crop growth

Group 2 compounds in medicine

• Magnesium hydroxide, Mg(OH)₂, is partially soluble in water and is used in suspension (known as 'milk of magnesia') to neutralise excess acid in the stomach and treat constipation

Mg(OH)₂ (s) + 2HCl (aq) → MgCl₂ (aq) + 2H₂O (l)

• It is safe to use as the magnesium hydroxide is only partially soluble making the solution only slightly alkaline (pH ≅ 10) due to the low OH^- concentration