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) |
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) |