Group 2 Compounds (OCR AS 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 iportant uses in agriculture and medicine.

Group 2 compounds in agriculture

• Over time, soil can become too acidic for optimal crop growth

• The pH of the soil can be raised by adding a Group 2 base to neutralise the excess acid

• However, 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

Calcium hydroxide (slaked lime), Ca(OH)₂:

• This is a common compound used to treat acidic soils

• It is reasonably soluble in water and acts relatively quickly

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

Calcium carbonate (limestone), CaCO₃:

• This is another widely used compound for the same purpose

• It is often preferred as it is cheaper and safer to handle than calcium hydroxide

• However, calcium carbonate is largely insoluble in water, so it acts more slowly

CaCO₃ (s) + 2H⁺ (aq) → Ca²⁺ (aq) + H₂O (l) + CO₂ (g)

Group 2 compounds in medicine

Antacids

Magnesium hydroxide, Mg(OH)₂:

• This compound is used in suspension (known as 'milk of magnesia') to neutralise excess hydrochloric acid in the stomach, relieving indigestion

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

• It is considered safe to use because magnesium hydroxide has a very low solubility

  • This means that the concentration of OH^- ions is low and the solution is only weakly alkaline (pH ≈ 10)

Medical imaging (barium meals)

Barium sulfate, BaSO₄:

• This compound is used in medicine as a 'barium meal'

• A patient drinks a suspension of barium sulfate before having an X-ray of their digestive system

• Barium is a large atom that absorbs X-rays well, so the BaSO₄ coats the tissues and allows the soft gut to be seen clearly on the X-ray image

• Safety:

  • While soluble barium compounds are extremely toxic, barium sulfate is safe to ingest because it is insoluble

  • Due to its very low solubility, it is not absorbed into the bloodstream from the gut