Reactions of Group 2 (Edexcel AS Chemistry): Revision Note
Group 2 Elements
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
| Reaction with oxygen | Reaction with water |
|---|---|---|
Mg | 2Mg (s) + O2 (g) → 2MgO (s) | Mg (s) + H2O (g) → MgO (s) + H2 (g) |
Ca | 2 Ca (s) + O2 (g) → 2CaO (s) | Ca (s) + 2H2O (l) → Ca(OH)2 (s) + H2 (g) |
Sr | 2Sr (s) + O2 (g) → 2SrO (s) Sr (s) + O2 (g) → SrO2 (s) | Sr (s) + 2H2O (l) → Sr(OH)2 (aq) + H2 (g) |
Ba | 2Ba (s) + O2 (g) → 2BaO (s) Ba (s) + O2 (g) → BaO2 (s) | Ba (s) + 2H2O (l) → Ba(OH)2 (aq) + H2 (g) |
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 with chlorine
Group 2 metals react with chlorine gas to give the metal chloride
For example
Mg (s) + Cl2 (g) → MgCl2 (s)
Group 2 Oxides
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) |
|
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
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) |
Remember that:
hydroxide + dilute hydrochloric acid → chloride + water
hydroxide + dilute sulfuric acid → sulfate + water
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