Trends in Solubility of Group 2 Hydroxides & Sulfates (Cambridge (CIE) AS Chemistry): Revision Note
Exam code: 9701
Trends in Solubility of Group 2 Hydroxides & Sulfates
Group 2 hydroxides
Going down the group, the solutions formed from the reaction of Group 2 oxides with water become more alkaline
When the oxides are dissolved in water, the following ionic reaction takes place:
O2- (aq) + H2O(l) → 2OH- (aq)
The higher the concentration of OH- ions formed, the more alkaline the solution
The alkalinity of the formed solution can therefore be explained by the solubility of the Group 2 hydroxides
Solubility of the Group 2 hydroxides table
Group 2 hydroxide | Solubility at 298 K (mol / 100 g of water) |
|---|---|
Mg(OH)2 | 2.0 x 10–5 (sparingly soluble) |
Ca(OH)2 | 1.5 x 10–3 |
Sr(OH)2 | 3.4 x 10–3 |
Ba(OH)2 | 1.5 x 10–2 (soluble) |
The hydroxides dissolve in water as follows:
X(OH)2 (aq) → X2+ (aq) + 2OH– (aq)
Where X is the Group 2 element
When the metal oxides react with water, a Group 2 hydroxide is formed:
XO (s) + H2O (l) → X(OH)2 (aq)
Going down the group, the solubility of these hydroxides increases
This means that the concentration of OH- ions increases, increasing the pH of the solution
As a result, going down the group, the alkalinity of the solution formed increases when Group 2 oxides react with water
The solubility of the Group 2 hydroxides
Group 2 sulfates
Group 2 sulfate | Solubility at 298 K (mol / 100 g of water) |
|---|---|
MgSO4 | Highly soluble |
CaSO4 | 1.2 x 10–3 |
SrSO4 | 7.6 x 10–4 |
BaSO4 | 1.0 x 10–4 (insoluble) |
The sulfates dissolve in water as follows:
XSO4 (s) → X2+ (aq) + SO42- (aq)
Where X is the Group 2 element
When the metal oxides react with sulfuric acid, a Group 2 sulfate is formed:
XO (s) + H2SO4 (aq) → XSO4 (aq) + H2O (l)
Going down the group, the solubility of these sulfates decreases
The solubility of the Group 2 sulfates
Examiner Tips and Tricks
You only need to be able to state the variation in the solubilities of the hydroxides and sulfates for this topic.
Enthalpy of Hydration: Ionic Charge & Radius in the A level course gives more detail on the solubility of Group 2 sulfates.
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