Question

NH3 has a pka of 38, while NH4+ has a pka of 9. What accounts for thispka difference (why do they have different acidities?)

Answer 1

NH3 structure has --> 1 lone pair on the nitrogen, and 3 hydrogen atoms bonded to it

they are "strong" covalent bonds nad Nitrogen has already filld up his octect.

Theefore, NH3 --> NH2- + H+

is not likely to occu; since NH3 is the prefferred state for the moleucle

Now... for NH4+

recall the lone pair on nitrogen,

it cna be used to bon an extra proton, i.e. "proton acceptor" which is the definition of a base

when

NH3 + H+ --> NH4+ forms

then NH4+ is actualy the conjugate acid

This implies, NH4+ can donate H+ radily to form the stable NH3 sample

then, pKa is relativley lower ( i.e. more acidic ) than that of NH3 to NH2-