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Why is aluminium oxide amphoteric?

At ‘O’ levels, we accept that aluminium oxide is amphoteric because it is able to behave as a base in the presence of an acid and behave as an acid in the presence of an alkali.

But, we know that metal oxides are basic. So, what gives rise to this special property?

In order to answer this question, we first need to understand why metal oxides are basic.

Key idea to bear in mind throughout:

  1. Charge density of the ion: Ions with high charge density are able to cause the water molecules to break apart (hydrolyse) into H+ and OH ions.

 

Why are metal oxides basic?

The simple answer is that they react with water to produce an alkali. For example:

Sodium oxide and water

When Na2O is dissolved in water, there are two ions formed, Na+ and O2-. Na+ has a relatively low charge density, hence only forms ion-dipole interactions with water but it does not hydrolyse the water molecule.

On the other hand, O2- ion has high negative charge density. Hence it is strongly attracted to the partially positive H atom of the water, such that it is able to break the H – O bond in water to form 2 OH-  ions!

Step 1

metal oxide basic1

Step 2

metal oxide basic2

Step 3

metal oxide basic3

Step 4

metal oxide basic4

Why is Al2O3 amphoteric?

As discussed above, the presence of the O2- ion makes Al2O3 basic (i.e. able to react with acid). However, the Al3+ ion has high positive charge density, hence is strongly attracted to and reacts with the electron rich OH- ions present in alkalis to form aluminate ions [Al(OH)4-]. See below.

Why is the OH – ion “electron-rich”? Recall the bonding in sodium hydroxide:

NaOH

There is a shared pair of electrons between the hydrogen and oxygen atoms in the hydroxide ion. Since oxygen is more electronegative than hydrogen, the shared pair of electrons within the ion are pulled closer to the oxygen atom, making the oxygen atom “electron rich”.

The aluminium ion having high positive charge density attracts and forms dative covalent bonds with hydroxide ions to form the aluminate ion! See diagram below

aluminate final

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