Why is ammonia considered a weak base?

Answer 1

The lone pair of electrons on the Nitrogen atom of Ammonia, is available for donation. That is why ammonia is considered as a base.
But it not as readily available as it is From OH- radical or other basic radicals. So it is much weaker than them.
In aq. soln, NH3 combines with water to form a very weak electrolyte NH4OH, which gets dissociated to a very small extent furnishing only a few OH- ions. So the aq. soln. of NH3 is also mild basic.

Ammonia acts both as a weak base in the Brernsted-Lowry sense
NH3 + H2O = NH4(+) + OH(-)and
as a complexing agent (Lewis acid) with some metal ions
Zn(2+)+ 4NH 3 + Zn(NH3)4(2+)
Both weak and strong bases produce hydroxide ion in solution which precipitates some metal ions, e.g.
A13(+) + 30H- + Al(OH)3(s)
and Zn92+) + 20H- + Zn(OH)2(s).
These precipitates appear as flocculent white solids which settle slowly from solution. In higher
concentrations hydroxide ion acts as a complexing agent for amphoteric cations, e.g.
Zn(OH)2(s) + 2OH- -+ Zn(OH),2-
Al(OH)3(s) + OH- -+ A1(OH)4-
This experiment demonstrates the action of OH- and NH3 on Al”(aq) and Zn*‘(aq) ions.

Although ammonia is well-known as a base, it can also act as an extremely weak acid. It is a protic substance, and is capable of dissociation into the amide (NH2−) ion, for example when solid lithium nitride is added to liquid ammonia, forming a lithium amide solution:

Li3N(s)+ 2 NH3 (l) → 3 Li+(am) + 3 NH2−(am)

This is a Brønsted-Lowry acid-base reaction in which ammonia is acting as an acid.

I think you are interested about dry ammonia.

Answer 2

Ammonia Is A Weak Base

Answer 3

Any substance that produces ions in solution can be considered an electrolyte; dissociation isn't the only way a substance can produce ions. Ammonia is an electrolyte because it produces ions by the following reaction:

NH3(aq) + H2O = NH4+(aq) + OH-(aq)

Ammonia is a weak rather than strong electrolyte because this reaction runs both ways; ammonia reacts with water to produce ammonium and hydroxide ions, while hydroxide ions react with ammonium ions to produce ammonia and water. The reaction clearly shows that ammonia is a base* because it accepts a hydrogen ion from the water.

Answer 4

A strong acid or a strong base indicates a great extent or even complete dissociation of the ions when in the aqueous solution.

A weak acid or base means that the ions do not dissociate completely

So ammonia when dissove in water, only a small fraction of the ammonia molecules react with water to for OH- (aq) ions. Most of the ammonia molecules remain unchanged. hence it is a weak base.

Answer 5

when ammonia is dissolved in water most of it get hydrated due to hydrogen bonding. very few molecules dissociates to form NH4+ and OH- ions. so it is a weak base.

Answer 6

http://en.wikipedia.org/wiki/Ammonia#Acidity