Question

Hi;
I'm just wondering, I need to write the dissocation equation for NH3 so that I can get Kb, and I just want to make sure that I am on the write track. I know that when some solids dissociate they just go from say CaCO3 --> CaO + CO2, but for this I'm wondering because it's a base it would be NH3 +H20 --> NH4+ + OH- or if it would still be NH3 --> NH2 + H2 ...

Answer 1

There are compounds, such as NaNH2 (sodamide) that have the negative ion, NH2^-) but you are right on track.
NH3 + HOH ==> NH4^+ + OH^-
Do you want to know a little of the history of this? When I took my first chemistry course, we were taught that
NH3 + HOH ==> produced NH4OH, and the NH4OH dissociated as follows:
NH4OH ==> NH4^+ + OH^-. But research has shown now that there is no NH4OH produced and the reaction procedes directly from NH3 to NH4^+ + OH^-. That intermediate of NH4OH was necessary in order to explain by the Arrhenius theory why NH3 was a base; i.e., NH4OH contained an OH part and that was the base. With the Bronsted-Lowry theory, however, no such intermediate is required. I think the Arrhenius theory still is useful; however, most texts now emphasize the Bronsted-Lowry theory initially.