Reactions of Period III Oxides with Water

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Reactions of Period III Oxides with Water

The ionic oxides of Na2O and MgO, are soluble in water, although MgO is only slightly. In water, the small highly charged oxide ions strongly attract water molecules and hydration is followed by hydrolysis.

O2-(aq) + H2O(l)2OH-(aq)

Sodium ions, with only +1 charge and a relatively large radius, hence have a low charge density and hydrolysis does not occur.

Slight hydrolysis does occur with Mg2+ ions (as seen earlier with the chlorides) but this effect is small in comparison with the hydrolysis of the oxide ions.

The balance of these hydrolysis reactions means that these aqueous solutions of these oxides have high pH values with Na2O(aq)>MgO(aq).

Na2O(s) + H2O(l) → 2NaOH(aq) - pH 14

MgO(s) + H2O(l) → Mg(OH)2(aq) - pH 12

The oxide Al2O3 is insoluble in water due to it's high lattice energies.

It reacts with acids and alkalis showing properties that are intermediate between those of a metallic and a non-metallic oxide. Al2O3 is amphoteric.

In these oxides, the period 3 element is covalently bonded to oxygen.

In most covalent oxides the very electronegative oxygen induces a small positive charge on the atom to which it is attached.

The size of the positive charge on the atom increases with the number of doubly bonded oxygen atoms joined to it.

When these oxides are added to water, lone pairs of electrons on the oxygen atom of the water molecule are attracted to the positively charged central atom and hydrolysis occurs.

P2O5(S) + 3H2O(l) → 2H3PO4(aq)

SO3(g) + H20(l) → H2SO4(aq)

Cl2O(g) + H2O(l) → 2HClO(aq)