Why is ammonia considered a base?

1 Answer
Apr 22, 2018

It's a Bronsted base because it can accept a proton.


The Brondsted-Lowry definition defines bases as substances that accept protons, or #H^+# ions, in an aqueous solution.

When ammonia, #NH_3#, is placed into an aqueous solution, this reaction will occur:

#NH_3(aq) + H_2O(l) rightleftharpoons NH_4^+(aq) + OH^(-)(aq)#

We can see that #NH_3# is accepting an #H^+# ion from #H_2O#.

The forward reaction doesn't occur that readily, so, although ammonia is a base, it's a weak base.

If we were to be really specific, ammonia sometimes also behaves as an acid by donating an #H^+# ion.
This would make ammonia amphoteric, which means that it can behave as both an acid and a base.

#NH_3(aq) + H_2O(l) rightleftharpoons NH_2^(-)(aq) + H_3O^(+)(aq)#

BUT, the above reaction (when ammonia is behaving as an acid) happens at a way lower rate than when ammonia accepts a proton, so we call it a base.