Kb: NH3+H2O<=>NH4++OH-: Why Omit H2O?

[alingy1]

Why is it that for the constant Kb, we always have to write H2O in the equation and we can omit H2O in acid reactions:

My textbook always writes:
NH3+H2O<=>NH4++OH-
NH4+<=>NH3+H+

Why can we not write NH3+H+<=>NH4+?

 

[Borek]

Why can we not write NH3+H+<=>NH4+?

We can. NH₄⁺ is a weak acid, with a dissociation constant of 5.6×10^-10, and above dissociation reaction is perfectly valid.

Your book my be sticking to one convention to make things easier to follow.

 

[alingy1]

Would the equilibrium constant be the same of that reaction? Or should we substitute OH- with Kw/H+?
Kw=10^-14

 

[Borek]

Dissociation constant is an equilibrium constant.

Compare

http://www.chembuddy.com/?left=pH-calculation&right=introduction-acid-base-equilibrium

and

http://www.chembuddy.com/?left=pH-calculation&right=bronsted-lowry-theory

 

[alingy1]

I guess this means yes. :)