Group 2 Compounds: Reactions

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Group 2 oxides and hydroxides are bases. They are neutralised by acids to form a salt and water.

For example:

MgO + 2HCl ----> MgCl2 + H2O

Ca(OH)2 + 2HCl ----> CaCl2 + 2H2O

As these reactions with hydrochloric acid take place you will see the solid oxide or hydroxide dissolve.

Group 2 Oxides

The Group 2 oxides react with water to form a solution of the metal hydroxide.

MgO + H2O -----> Mg(OH)2

The typical pH of these solutions is 10-12.

Group 2 Hydroxides

Group 2 hydroxides dissolve in water to form alkaline solutions.

Ca(OH)2 + aq ------> Ca2+ + 2OH-

The solubility of the hydroxides in water increases down the group. The resulting solutions are also more alkaline.

Mg(OH)2 is only slightly soluble in water. The resulting solution is dilute with a comparatively low OH- concentration.

Ba(OH)2 is much more soluble in water than MG(OH)2, with a greater OH- concentration. The resulting solution is more alkaline than a solution of Mg(OH)2.

Group 2 Carbonates

The Group 2 carbonates are decomposed by heat, forming the metal oxide and carbon dioxide gas.

MgCO3 -----> MgO + CO2

This type of reaction is called thermal decomposition.

The carbonates become more difficult to decompose with heat as you move down the group.

Properties of Group 2 Elements and Their Compounds

Group 2 elements become more reactive down the group

Group 2 carbonates decompose at higher temperatures down the group

Hydroxides become more soluble in water and the resulting solutions become more alkaline.

Uses of Group 2 Hydroxides

The alkalinity of the Group 2 hydroxides is used to combat acidity.

Two common examples are:

Calcium hydroxide - used by farmers and gardeners as 'lime' to neutralise acid soils

Magnesium hydroxide - is used in 'milk of magnesia' to relieve indigestion. It works by neutralising any excess acid in the stomach.

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