Edexcel A Level Chemistry:复习笔记2.2.2 Reactions of Group 2

Reactions with water and oxygen

• The reaction of group 2 metals with oxygen follows the following general equation:

2M (s) + O2 (g) → 2MO (s)

Where M is any metal in group 2

Remember than Sr and Ba also form a peroxide, MO2

• The reaction of all metals with water follows the following general equation:

M (s) + 2H2O (l) → M(OH)2 (s) + H2 (g)

Except for, Be which does not react with water

Group 2 Metals reacting with Water and with Oxygen - Equations

• Magnesium reacts extremely slowly with cold water:

Mg (s) + 2H2O (l) → Mg(OH)2 (aq) + H2 (g)

• The solution formed is weakly alkaline (pH 9-10) as magnesium hydroxide is only slightly soluble
• However, when magnesium is heated in steam, it reacts vigorously with steam to make magnesium oxide and hydrogen gas:

Mg (s) + H2O (g) → MgO (s) + H2 (g)

Reactions with chlorine

• Group 2 metals react with chlorine gas to give the metal chloride
  • For example

Mg (s) + Cl2 (g) → MgCl2 (s)

Reactions of Group 2 oxides with water

• All Group 2 oxides are basic, except for BeO which is amphoteric (it can act both as an acid and base)
• Group 2 oxides react water to form alkaline solutions which get more alkaline going down the group

Group 2 Oxides reacting with Water

• Remember that:

oxide + water → hydroxide

• You should know that calcium hydroxide, when in solution, is also called limewater

Reactions of Group 2 oxides with acid

• Group 2 sulfates also form when a Group 2 oxide is reacted with sulfuric acid
• The insoluble sulfates form at the surface of the oxide, which means that the solid oxide beneath it can’t react with the acid
• This can be prevented to an extent by using the oxide in powder form and stirring, in which case neutralisation can take place
• Remember that:

oxide + dilute hydrochloric acid → chloride + water

oxide + dilute sulfuric acid → sulfate + water

Reactions of Group 2 hydroxides

• The Group 2 metal hydroxides form colourless solutions of metal salts when they react with a dilute acid
• The sulfates decrease in solubility going down the group (barium sulfate is an insoluble white precipitate)

Group 2 Hydroxide Reactions with Dilute Acids

• Remember that:

hydroxide + dilute hydrochloric acid → chloride + water

hydroxide + dilute sulfuric acid → sulfate + water