Oxides & Hydroxides (Edexcel International AS Chemistry): Revision Note
Reactions with Acid
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
O2- (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) |
|
Group 2 Oxides reacting 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) |
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