amphoteric substances

[Kailyncanez3G]

how can amphoteric substances act as Bronsted acids and bronzed base? especially by just looking at the equations

[Patrick Blankenberg]

Lets look at this example

Al2O3 (s) + 6 HCl (aq) --> 2 AlCl3 (aq) + 3H20 (l)

Here, the HCl is a strong acid and thus will want to donate the H+ heavily to the oxygen anions to form hydroxide. So, each oxygen anion (2-) gets two hydrogens from 6 hcl to make 3h2o. Then, each aluminum (3+) gets 3 cl (-1) to make the salt. So, we see Al2O3 as a base here as it accepts the protons.

2 NaOH (aq) + Al2O3 (s) + 3 H2O (l) -> 2Na[Al(oh)4] (aq)

Here, the NaOH acts as a strong base which reacts with the water to make 4 oh- hydroxide ions. These hydroxide ions with a -4 charge then rip the +3 aluminum to make a coordination compound, which is stabilized by the 2NA