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Group 2: Question 8

Syllabus 10.1

Structured AS 5 marks

Solid magnesium oxide and solid barium oxide are added, separately, to distilled water at room temperature.

Magnesium oxide reacts only very slowly, giving a mixture that is only faintly alkaline even after standing for some time. Barium oxide, in contrast, reacts rapidly and vigorously as soon as it is added, giving a strongly alkaline solution.

(a) Write balanced symbol equations, including state symbols, for the reaction of magnesium oxide with water and for the reaction of barium oxide with water. [2]

(b) Describe and explain, in terms of the ionic radius of the M2+\text{M}^{2+} cation and the lattice energy of the oxide, why barium oxide reacts with water so much more readily than magnesium oxide does. [3]

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Worked solution

Part (a): Equations for the oxides reacting with water

Both magnesium oxide and barium oxide contain the O2−\text{O}^{2-} ion, and both react with water in a simple 1:1:11:1:1 combination to form the corresponding metal hydroxide:

MgO(s)+H2O(l)→Mg(OH)2(s)\text{MgO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Mg(OH)}_2\text{(s)}

BaO(s)+H2O(l)→Ba(OH)2(aq)\text{BaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ba(OH)}_2\text{(aq)}

Each equation is already balanced: one metal, one water molecule (contributing one extra oxygen and the two hydrogens needed for the two hydroxide groups) and the single oxide oxygen combine to give one formula unit of the hydroxide. Mg(OH)2\text{Mg(OH)}_2 is only sparingly soluble, so it is shown as a solid/suspension, (s)\text{(s)}; Ba(OH)2\text{Ba(OH)}_2 is considerably more soluble, so it is shown as (aq)\text{(aq)}.

Part (b): Why barium oxide reacts so much more readily than magnesium oxide

Down Group 2, the ionic radius of the M2+\text{M}^{2+} cation increases steadily from Mg2+\text{Mg}^{2+} to Ba2+\text{Ba}^{2+}, so its charge density (charge divided by radius) decreases.

Lattice energy becomes more exothermic (more negative) as the ions forming the lattice get smaller and more highly charged, because the electrostatic attraction between oppositely charged ions is stronger at shorter distances. Since the oxide ion O2−\text{O}^{2-} is the same in both compounds, it is the size of the M2+\text{M}^{2+} cation that controls how exothermic the lattice energy is:

  • MgO\text{MgO} is built from the small, high-charge-density Mg2+\text{Mg}^{2+} ion, so it has a very exothermic (strongly negative) lattice energy. The ionic lattice is held together very strongly, and a large amount of energy would be needed to pull it apart. Water molecules can only disrupt this strong lattice slowly, so MgO\text{MgO} reacts only very slowly with water.
  • BaO\text{BaO} is built from the much larger, lower-charge-density Ba2+\text{Ba}^{2+} ion, so its lattice energy is considerably less exothermic. The ionic lattice is held together much less strongly, so it is far more easily broken apart by attacking water molecules, BaO\text{BaO} therefore reacts rapidly and vigorously with water.

So the trend, reactivity of the Group 2 oxides with water increasing down the group, is a direct consequence of the decreasing lattice energy of the oxide, which in turn follows from the increasing ionic radius (decreasing charge density) of the M2+\text{M}^{2+} cation down the group.

Final answers

  • (a) MgO(s)+H2O(l)→Mg(OH)2(s)\text{MgO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Mg(OH)}_2\text{(s)}; BaO(s)+H2O(l)→Ba(OH)2(aq)\text{BaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ba(OH)}_2\text{(aq)}
  • (b) Down the group, ionic radius of M2+\text{M}^{2+} increases and charge density decreases, so the oxide’s lattice energy becomes less exothermic; BaO\text{BaO}‘s much weaker lattice (compared with MgO\text{MgO}‘s) is disrupted far more easily by water, so barium oxide reacts much more readily with water than magnesium oxide does.