Oxides & Hydroxides (Edexcel International AS Chemistry)

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

Group 2 oxide	Reaction with water	Observations
MgO	MgO (s) + H2O (l) → Mg(OH)2 (s)	MgO is only slightly soluble in water, therefore a weakly alkaline solution (pH 10.0) is formed
CaO	CaO (s) + H2O (l) → Ca(OH)2 (s)	A vigorous reaction which releases a lot of energy, causing some of the water to boil off as the solid lump seems to expand and open (pH 11.0)
SrO	SrO (s) + H2O (l) → Sr(OH)2 (aq)
BaO	BaO (s) + H2O (l) → Ba(OH)2 (aq)

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

Group 2 hydroxide	Reaction with dilute HCl	Reaction with dilute H2SO4
Mg(OH)2	Mg(OH)2 (s) + 2HCl (aq) → MgCl2 (aq) + 2H2O (l)	Mg(OH)2 (s) + H2SO4 (aq) → MgSO4 (aq) + 2H2O (l)
Ca(OH)2	Ca(OH)2 (s) + 2HCl (aq) → CaCl2 (aq) + 2H2O (l)	Ca(OH)2 (s) + H2SO4 (aq) → CaSO4 (aq) + 2H2O (l)
Sr(OH)2	Sr(OH)2 (s) + 2HCl (aq) → SrCl2 (aq) + 2H2O (l)	Sr(OH)2 (s) + H2SO4 (aq) → SrSO4 (s) + 2H2O (l)
Ba(OH)2	Ba(OH)2 (s) + 2HCl (aq) → BaCl2 (aq) + 2H2O (l)	Ba(OH)2 (s) + H2SO4 (aq) → BaSO4 (s) + 2H2O (l)