Will NH3 decompose into N2 and H2?
Under standard conditions (100kPa, 0°C), nitrogen and hydrogen do not react with each other in any measurable way. It follows logically that also ammonia cannot decompose (as it would to a small extent in an equillibrium reaction), because otherwise it would decompose completely with time, although it is stable.

In equillibrium, the forward and backward reaction rates must be identical and finite.