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

1 Answer
Fatima
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 .

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