Reactions of Group 2 (Edexcel A Level Chemistry)

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) 

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