2.4.3 Oxides & Hydroxides

Overall equations 

• We have seen that Group 1 and Group 2 metals can react with oxygen to form oxides 

metal + oxygen → metal oxide 

• Oxides can react with water to form hydroxides 

oxide + water → hydroxide

• If calcium oxide is added to water, you should know that calcium hydroxide is formed
  • Calcium hydroxide solution is also called limewater

• When metal oxides react with dilute hydrochloric acid and dilute sulfuric acid, chloride and sulfate salts are formed

metal oxide + dilute hydrochloric acid → metal chloride + water

metal oxide + dilute sulfuric acid → metal sulfate + water

• Similar reactions occur with hydroxides

metal hydroxide + dilute hydrochloric acid → metal chloride + water

metal hydroxide + dilute sulfuric acid → metal sulfate + water

Reactions of Group 1 oxides

With water

• The Group 1 metal oxides will react with water to give a colourless alkaline solution 
  • For example

Na₂O (s) + H₂O (l) → 2NaOH (aq) 

• The solution is alkaline as hydroxide ions are released which also occurs for the reaction of Group 2 oxides with water

O^2- (s) +  H₂O (l) → 2OH^- (aq) 

Reactions of Group 1 hydroxides with dilute acid

• The Group 1 metal hydroxide is acting as an alkali when it is added to dilute acid  
• When an alkali reacts with an acid, a neutralisation reaction occurs forming salt and water
  • For example

NaOH (aq) + HCl (aq) → NaCl (aq) + H₂O (l) 

NaOH (aq) + H₂SO₄ (aq) → Na₂SO₄ (aq) + H₂O (l) 

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

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

Reactions of Group 2 hydroxides with dilute acid 

• 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