Question

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

Answer 1

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 .