Why amonia is a base and not water?

[SciencePF]

Why amonia NH3 in water acts as a base and not an acid?
Thanks

 

[chroot]

Because the nitrogen atom has a lone electron pair, and thus is a proton acceptor. Proton acceptors are bases.

- Warren

 

[SciencePF]

but water has two lone electron pair!

 

[chroot]

Water is amphoteric -- it can indeed act as a base, but it can also act as an acid.

- Warren

 

[SciencePF]

Sorry, Warren, but water is amphoteric and why it can not act as a proton acceptor from NH3?

 

[Oaksinstructor]

Technically speaking, water is amphiprotic. Wiki has a good explanation of this: http://en.wikipedia.org/wiki/Amphiprotic

But as to the original question:
When you combine water with say, Hydrogen Chloride, HCl is the acid and Water is the base (according to Brønsted-Lowry theory because water is accepting the proton)
H2O + HCl → H3O+ + Cl−

In the case of combining water with Ammonia you get this reaction:
H2O + NH3 → NH4+ + OH−
Ammonia is clearly accepting the proton and thus (again according to Brønsted-Lowry theory) is the base. (and water is the acid).

Your explanation about electron pairs would fit in more with Lewis Theory of acids and bases, but it still doesn't change anything, as Ammonia is donating its electron pair in that reaction which still makes it a base.

So the reason it is a base is simply because the proton goes there (or because the electron pair leaves)

 

[Borek]

Ammonia is clearly accepting the proton and thus (again according to Brønsted-Lowry theory) is the base. (and water is the acid).

It still doesn't answer the question WHY ammonia is accepting th eproton, instead of donating one of its own to water molecule. And that's the question SciencePF asked.

 

[Weave]

If you look at the geometry(bent) at an H_{2}0 it is a polar molecule. The Oxygen is pretty electronegitive: So the oxygen has a partial charge of -2\delta. Then each Hydrogen has a +\delta. Then the lewis structure of the ammonia has the lone pair of e^- and will take one of the H^+. Hope that helps

 

[pixel01]

Both H2O and NH3 are the sp3 hybridization which have the tetrahedral molecule with O and N as the center. For H2O, there are 2 H and 2 lone pairs. The two lone pairs of electrons in H2O can accept 2 H+, but it is far more difficult than in the case of NH3 which has only one lone pair. If water takes two protons, it will be too positive. If it takes one, it is not symmetrical. NH3 can take one proton making it a complete tetrahedron.
Overmore oxygen has higher electronegativity compared to nitrogen, so oxygen 'hates' H+ more than nitrogen.

 

[Oaksinstructor]

Yeah, like pixel said, it's because of energy in the bonds. There is either less energy required or more energy released (I didn't look it up to find out which) in the process of breaking bonds with water's H+ ion and forming NH4 than there is for NH3 to break a bond with an H+ and form H3O. Since this is the 'easier' process (in terms of energy) this is what happens.