Chemistry - How can aluminium oxide be called an acid?

Solution 1:

The most basic definition of "acid" is that it is a proton donor (or one which accepts a lone pair)

All of this stuff is done in an aqueous medium, so we can assume that all aqueous ions and molecules are present. With this assumption (in this case, we are assuming that $\ce{OH-}$ is available to react), we get the following equation:

$$\ce{Al2O3 + OH- -> 2AlO2- + H+}$$

Similarly, we get:

$$\ce{Al2O3 + 6H+ -> 2Al^3+ + H2O}$$

where it is acting like a proton acceptor (base).

Solution 2:

We could extend the acidic/basic nature of $\ce{Al2O3}$ to the Lewis acid/base theory. For more about this see this question which has been asked before: Is there such a thing as an acid without a hydrogen?

Recall the Lewis Theory of acids and bases which states that an acid is an electron pair acceptor and a base is an electron pair donor.

Applying the theory to $\ce{Al2O3}$:

$$\ce{Al3+(aq) + 6 H2O(l) <=> Al(H2O)6^3+(aq)\tag{1}\label{a}}$$

Thus, in $(\ref{a})$ the $\ce{Al(H2O)6^3+}$ ion is formed when an $\ce{Al^3+}$ ion acting as a Lewis acid picks up six pairs of electrons from neighbouring water molecules acting as Lewis bases to give an acid-base complex, or complex ion

Similarly:

$$\ce{2NaOH(aq) + Al2O3 (s) + 3H2O (l) -> 2Na[Al(OH)4](aq)\tag{2}\label{b}}$$

In this case it has donated its lone electron pairs to form sodium tetrahydroxoaluminate.

The Brønsted-Lowry theory is also observed in:

$$\ce{Al2O3 + 6H+ -> 2Al^3+ + H2O} \qquad{(3) - \text{a typical Brønsted-Lowry base}}$$

This compound somewhat behaves as both a Lewis acid/base and Brønsted-Lowry base (as noted in the question) as well.