What definition of acids and bases emphasizes the role of protons?

1 Answer
Jun 8, 2018

This applies for Bronsted-Lowry acids and bases.

Explanation:

  • A Bronsted-Lowry acid is defined as a proton donor.

E.g.
H_2SO_4 + H_2O -----> HSO_4^-1 + H_3O^+

Here, it's clear that sulfuric acid (H_2SO_4) lost a proton and donated it to water (H_2O), thus forming a hydroxonium ion (H_3O^+). So sulfuric acid is a strong Bronsted-Lowry acid with a pH of around 2, which will turn a blue litmus paper red.

  • A Bronsted-Lowry base however, is a proton acceptor .

E.g:
NH_3 + H_2O -----> NH_4^+ + OH^-

Here, it is clear that ammonia (NH_3) accepted a proton from water to form an ammonium ion (NH_4^+) and a hydroxide ion (OH^-).

  • Now if you look closely at the first equation, involving sulfuric acid, you'll find that water is acting as a base to form H_3O^+ from H_2O. It accepted a proton.

And if you look at the second equation, involving ammonia, water is acting as an acid and forming OH^- from H_2O. It has donated a proton!

Thus, water can act as both , an acid and a base. We say that water is amphoteric .