Can you explain in terms of Le Chatelier's principle why the concentration of NH3 decreases when the temperature of the equilibrium system increases?
When you increase the temperature by adding heat, the system responds by trying to remove the heat.
The synthesis of ammonia is an exothermic process:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g);
Heat is a "product" of the reaction.
Le Châtelier's Principle states that when a stress is applied to a system at equilibrium, the system will respond in such a way that it relieves the stress.
So if you add heat to the system, it will respond in a way that removes the added heat.
The position of equilibrium will move to the left. So the equilibrium concentration of ammonia will decrease.