How to label acid/base

[080692]

So if you were given only the reactants side of an equation, how do you go about knowing which is the acid and which is the base?

For example, if you were give NH3 + H20 =________, the products can be NH4+ + OH- or NH2- + H30+.

How would you know which is a "better" product?

[mississippichem]

Which species has a lower [imath] pK_a [/imath]? That one tends to be de-protonated at equilibrium, and is therefore the acid on the reactant side.

Edited September 11, 2011 by mississippichem

[080692]

Well I'm asking which would be the acid/base on the reactants side.

[mississippichem]

Look up [imath] pK_a [/imath] values for water and ammonia. I know the answer but want you to find it for yourself.

[080692]

Oh I know the answer too. It's NH4+ + OH-. I was just using that as an example.

Could you find out without the pKa?

Edited September 11, 2011 by 080692

[mississippichem]

Oh okay. Well equilibrium constants always work, but in this case you can evoke the "leveling effect". Essentially no species that is more acidic than the solvent's conjugate acid can exist in that solvent. Likewise no species more basic than the solvent's conjugate base can exist in that solvent (btw, when I say "exist" I mean to have an appreciable lifetime or to constitute a large part of the solution mole fraction at equilibrium).

 

[ce]NH_{2}^-[/ce] is more basic than the hydroxide anion so will not be a product.