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

Stewart Hird

Last updated

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 OHions

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

 Ca2+ (aq) + 2OH(aq) → Ca(OH)(s)

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

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

  • Barium hydroxide, Ba(OH)(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)(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)(s) + 2HCl (aq) → MgCl(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

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Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

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