Reactions of Group 2 Oxides, Hydroxides & Carbonates (CIE AS Chemistry)

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 with water to form alkaline solutions which generally get more alkaline going down the group
  • This happens because the hydroxides that form become more soluble as you move down the group
  • This means that more hydroxide ions, OH^–, dissociate into the solution causing the pH to increase

Group 2 oxide reactions with the water table

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

• • And that calcium hydroxide is also called limewater

Reactions of Group 2 metals with acid

• The Group 2 metals will react with dilute acids to form colourless solutions of metal salts 
  • For example, they will form colourless solutions of metal chlorides if reacted with hydrochloric acid
• When metals react with an acid, the by-product of this reaction is hydrogen gas

Group 2 reactions with dilute acids table

Group 2 element	Reaction with dilute HCl	Reaction with dilute H2SO4
Mg	Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)	Mg (s) + H2SO4 (aq) → MgSO4 (aq) + H2 (g)
Ca	Ca (s) + 2HCl (aq) → CaCl2 (aq) + H2 (g)	Ca (s) + H2SO4 (aq) → CaSO4 (aq) + H2 (g)
Sr	Sr (s) + 2HCl (aq) → SrCl2 (aq) + H2 (g)	Sr (s) + H2SO4 (aq) → SrSO4 (s) + H2 (g)
Ba	Ba (s) + 2HCl (aq) → BaCl2 (aq) + H2 (g)	Ba (s) + H2SO4 (aq) → BaSO4 (s) + H2 (g)

• When some Group 2 metals react with sulfuric acid rather than hydrochloric, an insoluble sulfate forms
• Going down the group, the Group 2 sulfates become less and less soluble
  • Calcium sulfate is sparingly soluble, but strontium sulfate and barium sulfate are insoluble

Reactions of Group 2 oxides with acid

• Group 2 sulfates also form when a Group 2 oxide is reacted with an 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 → salt + water

oxide + dilute sulfuric acid → sulfate + water

Reactions of group 2 hydroxides

• The Group 2 metal hydroxides form colourless solutions of metal chlorides 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 table

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 acid → salt + water

hydroxide + dilute sulfuric acid → sulfate + water

Reactions of group 2 carbonates

• All Group 2 carbonates (except for BeCO₃) are insoluble in water
• All Group 2 carbonates will form soluble chloride salts, water and carbon dioxide gas when reacted with dilute hydrochloric acid
• When reacted with sulfuric acid, the carbonates of Ca, Sr and Ba form an insoluble sulfate layer on their surface which stops any further reaction after the initial bubbling (effervescence) of carbon dioxide gas is seen

Group 2 carbonate reactions with dilute acids

Group 2 carbonate	Reaction with dilute HCl	Reaction with dilute H2SO4
MgCO3	MgCO3 (s) + 2HCl (aq) → MgCl2 (aq) + H2O (l) + CO2 (g)	MgCO3 (s) + H2SO4 (aq) → MgSO4 (aq) + H2O (l) + CO2 (g)
CaCO3	CaCO3 (s) + 2HCl (aq) → CaCl2 (aq) + H2O (l) + CO2 (g)	CaCO3 (s) + H2SO4 (aq) → CaSO4 (aq) + H2O (l) + CO2 (g)
SrCO3	SrCO3 (s) + 2HCl (aq) → SrCl2 (aq) + H2O (l) + CO2 (g)	SrCO3 (s) + H2SO4 (aq) → SrSO4 (s) + H2O (l) + CO2 (g)
BaCO3	BaCO3 (s) + 2HCl (aq) → BaCl2 (aq) + H2O (l) + CO2 (g)	BaCO3 (s) + H2SO4 (aq) → BaSO4 (s) + H2O (l) + CO2 (g)

 

• Remember that:

carbonate + dilute hydrochloric acid → salt + water + carbon dioxide

carbonate + dilute sulfuric acid → sulfate + water + carbon dioxide