Question

Can you explain in terms of Le Chatelier's principle why the concentration of NH3 decreases when the temperature of the equilibrium system increases?

Answer 1

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); `ΔH_"f"°` = -46.1 kJ/mol

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.

See What is Le Chatelier's principle?.

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.