Question #1887e

1 Answer
Jan 29, 2015

An amphoteric compound is a compound that can act either as an acid, or as a base in a chemical reaction.

A first example of water's amphoteric character is the fact that it can self-ionize to a small extent. What that means is that a water molecule is capable of donating one of its protons (#"H"^(+)#) to a neighbouring water molecule, forming the hydroxyde and hydronium ions.

#H_2O + H_2O rightleftharpoons H_3O^(+) + OH^(-)#

One of the two water molecules acts as an Bronsted-Lowry acid and donates a proton to the other water molecule, which subsequently acts as a Bronsted-Lowry base.

Water can act as an acid or as a base in various chemical reactions, the most common two examples involving the ammonia and hydrochloric acid reactions.

#NH_(3(aq)) + H_2O_((l)) rightleftharpoons NH_(4(aq))^(+) + OH_((aq))^(-)#

Here water acts as a Bronsted-Lowry acid because it donates a proton to ammonia.

#HCl_((g)) + H_2O((l)) rightleftharpoons H_3O_((aq))^(+) + Cl_((aq))^(-)#

This time water acts a Bronsted-Lowry base because it accepts a proton from hydrochloric acid.

To be exact, in reactions that involve the transfer of protons, the term is actually amphiprotic.

Water can act as a Lewis acid or as a Lewis base as well; in such reactions, water can either accept a pair of electrons, which makes it a Lewis acid, or donate a pair of electrons, which makes it a Lewis base.

Here's another example of water's amphiprotic (amphoteric) nature: