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

Jun 8, 2018

This applies for Bronsted-Lowry acids and bases.

#### Explanation:

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

E.g.
${H}_{2} S {O}_{4}$ + ${H}_{2} O$ -----> $H S {O}_{4}^{-} 1$ + ${H}_{3} {O}^{+}$

Here, it's clear that sulfuric acid (${H}_{2} S {O}_{4}$) lost a proton and donated it to water (${H}_{2} O$), thus forming a hydroxonium ion (${H}_{3} {O}^{+}$). 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:
$N {H}_{3}$ + ${H}_{2} O$ -----> $N {H}_{4}^{+}$ + $O {H}^{-}$

Here, it is clear that ammonia ($N {H}_{3}$) accepted a proton from water to form an ammonium ion ($N {H}_{4}^{+}$) and a hydroxide ion ($O {H}^{-}$).

• Now if you look closely at the first equation, involving sulfuric acid, you'll find that water is acting as a base to form ${H}_{3} {O}^{+}$ from ${H}_{2} O$. It accepted a proton.

And if you look at the second equation, involving ammonia, water is acting as an acid and forming $O {H}^{-}$ from ${H}_{2} O$. It has donated a proton!

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