Group 2 Hydroxides & Sulfates (Edexcel A Level Chemistry): Revision Note
Exam code: 9CHO
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 |
Ca(OH)2 | 1.5 x 10–3 |
Sr(OH)2 | 3.4 x 10–3 |
Ba(OH)2 | 1.5 x 10–2 |
The hydroxides dissolve in water as follows:
X(OH)2 (aq) → X(aq) + 2OH- (aq)
Where X is the Group 2 element
When the metal oxides react with water, a Group 2 hydroxide is formed
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
Going down the group, the solubility of the hydroxides increases which means that the solutions formed from the reactions of the Group 2 metal oxides and water become more alkaline going down the group
Group 2 sulfates
The solubility of the Group 2 sulfates decreasing going down the group
Going down the group, the solubility of the sulfates decreases
Worked Example
Predicting properties of radium
Radium (Ra) is a radioactive element found below barium at the bottom of group 2.
Applying your knowledge of the group 2 elements, predict:
The formula of the ion formed by Ra.
The formulae of its oxide and hydroxide
Its first ionisation energy
Its reactivity compared with barium
The relative pH of its saturated hydroxide solution compared with a saturated solution of calcium hydroxide
The solubility of its sulfate compared to strontium sulfate
The equation for the reaction of its solid oxide with dilute hydrochloric acid
What would you expect to see if you mixed radium hydroxide solution with dilute sulfuric acid?
Answers:
The formula of the ion formed by Ra.
Since, Ra is in group 2, it will form an ion with +2 charge to give Ra2+
The formulae of its oxide and hydroxide.
The group 2 oxides and hydroxides have general formula XO and X(OH)2 respectively where X is the group 2 element.
Therefore, radium oxide is RaO and radium hydroxide is Ra(OH)2
Its first ionisation energy.
Radium is below barium so its atomic radius is larger than the atomic radius of barium.
This means that radium’s outermost electrons are even further away and are therefore even more easily removed than barium’s outermost electron pair.
The first ionization energy is between 450-480 kJ mol-1
Its reactivity compared with barium.
Radium’s outermost electrons are even further away than in barium and are therefore more easily removed making radium more reactive than barium.
The relative pH of its saturated hydroxide solution compared with a saturated solution of calcium hydroxide.
The group 2 hydroxides become more soluble going down the group.
Radium hydroxide will therefore be more soluble than calcium hydroxide. Since more hydroxide ions will be present in solution the pH should be higher than the pH of calcium hydroxide
The solubility of its sulfate compared to strontium sulfate.
The Group 2 sulfates become less soluble going down the group.
Radium sulfate will therefore be less soluble than strontium sulfate.
The equation for the reaction of its solid oxide with dilute hydrochloric acid.
The general equation for the reaction of group 2 oxides with dilute hydrochloric acid is:
XO (s) + 2HCl (aq) → XCl2 (aq) + H2O (l)
The reaction of radium oxide with dilute hydrochloric acid is therefore:
RaO (s) + 2HCl (aq) → RaCl2 (aq) + H2O (l)
What would you expect to see if you mixed radium hydroxide solution with dilute sulfuric acid?
Radium sulfate will be formed in this reaction, however the solubility of group 2 sulfates decreases going down the group, therefore a white precipitate of radium sulfate will be formed in this reaction
Examiner Tips and Tricks
You may be wondering why there are no trends here for the solubility of Group 1 hydroxides and sulfates. You should recall from GCSE that Group 1 compounds are all soluble in water. They will therefore not produce any precipitates when testing for cations, so to identify them you need to use flame tests.
Group 1 hydroxides will be more soluble than Group 2 hydroxides. Even though we say the solubility of the Group 2 hydroxides increases down the group barium hydroxide is less soluble than a Group 1 hydroxide such as potassium hydroxide.
At 25 °C the solubility of Ba(OH)2 is 4.68 g / 100 cm3
At 25 °C the solubility of KOH is 121 g / 100 cm3
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