AQA A Level Chemistry复习笔记2.2.3 Reactions of Group 2

• The group 2 elements react with oxygen, water and dilute acids

Group 2 Reactions - Observations

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 of Group 2 metals with acid

• The Group 2 metals will react with dilute acids to form colourless solutions of metal salts
  • For example, they will form colourless solutions of metal chlorides if reacted with hydrochloric acid

   

• When metals react with an acid, the by-product of this reaction is hydrogen gas

Group 2 Reactions with Dilute Acids - Equations

• When some of Group 2 metals react with sulfuric acid rather than hydrochloric, an insoluble sulfate forms
• Going down the group, the Group 2 sulfates become less and less soluble
  • Calcium sulfate is sparingly soluble, but strontium sulfate and barium sulfate are insoluble

   

• The reaction of the metals with dilute HCl follows the following general equation:

M (s) + 2HCl (aq) → MCl2 (aq) + H2 (g)

• The reaction of the metals with dilute H2SO4 follows the following general equation:

M (s) + H2SO4 (aq) → MSO4 (aq) + H2 (g)

Remember that SrSO4 and BaSO4 are insoluble

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

Reactions of group 2 carbonates

• All group 2 carbonates (except for BeCO3) are insoluble in water
• All group 2 carbonates will form soluble chloride salts, water and carbon dioxide gas when reacted with dilute hydrochloric acid
• The carbonates of Ca, Sr and Ba form as an insoluble sulfate layer on their solid carbonates which stops any further reaction after the initial bubbling (effervescence) of carbon dioxide gas is seen

Group 2 Carbonates reacting with Dilute Acids

• Remember that:

carbonate + dilute hydrochloric acid → chloride + water + carbon dioxide

carbonate + dilute sulfuric acid → sulfate + water + carbon dioxide