I am aware of amphoteric properties of aluminum oxide and that it can react with both strong acids and bases. I was recently asked a question whether amphoteric properties allow for reaction with oxides, which form strong acids when reacted with water, namely sulphur (VI) oxide. This question of course can be extended to oxides such as sodium oxide, which would form a strong base in water.

While I know that appropriate equations for such reactions can be easily written and balanced, I'd love to know whether such reactions can actually happen without intermediate reaction of oxides with water to form a respective strong acid/base.

  • $\begingroup$ If you consider the Lewis theory of acids and bases, oxides can display acidic or basic properties and react on their own, not necessarily in the presence of a solvent. $\endgroup$ – Thomas Jungers Apr 9 '18 at 11:11
  • $\begingroup$ Theory or no theory, they will react. $\endgroup$ – Ivan Neretin Apr 9 '18 at 11:15

The oxides which can dissolve in water to give a base, can react with the oxides which can dissolve in water to give acids, to form a salt, similar to an acid-base reaction. They can be thought as bases and acids respectively according to Lux-Flood concept of acid-base.

Here in this case, $\ce{Al_2O_3}$ is the anhydride of $\ce{Al(OH)_3}$, and $\ce{SO_3}$ is the anhydride of $\ce{H_2SO_4}$. So, they react to form the salt aluminium sulfate $(\ce{Al_2(SO_4)_3})$, as:

$$\ce{Al_2O_3 (base) + 3SO_3 (acid) -> Al_2(SO_4)_3 (salt)}$$

However, according to Lux-Flood definition of acid and base, acids are oxide acceptors, whereas, bases are oxide donors. Here, in this example, you can clearly see that $\ce{SO_3}$ accepts oxide $(\ce{O^2-})$ to form $\ce{SO_4^2-}$, and $\ce{Al_2O_3}$ donates oxide and forms $\ce{Al^3+}$, thus it is a base. As, it is simply an acid base reaction, this can easily happen.

Some other examples of Lux-Flood acid-base reaction (spontaneous) are,

$$\ce{CaO (base) + SiO_2 (acid) -> CaSiO_3 (salt)}$$ $$\ce{MgO (base) + CO_2 (acid) -> MgCO_3 (salt)}$$

Thus, it is seen that, those oxides which can dissolve in water to form acids and bases, can actually react spontaneously, without water to form salts.


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