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 |
|
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.
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The group 2 oxides and hydroxides have general formula XO and X(OH)2 respectively where X is the group 2 element.
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Therefore, radium oxide is RaO and radium hydroxide is Ra(OH)2
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- Its first ionisation energy.
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Radium is below barium so its atomic radius is larger than the atomic radius of barium.
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This means that radium’s outermost electrons are even further away and are therefore even more easily removed than barium’s outermost electron pair.
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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.
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The Group 2 sulfates become less soluble going down the group.
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Radium sulfate will therefore be less soluble than strontium sulfate.
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- 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 Tip
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
