In the synthesis of ammonia, what is the effect of a decrease in the reaction temperature?
The reaction is exothermic as written, and
Le Chatelier's principle states that a system at equilibrium that is stressed will move so as to counteract the external perturbation.
What does this mean?
It means that when the conditions at equilibrium are changed, the equilibrium will move (left or right) so as to oppose the effect. It will not wholly resist it.
So what does this mean for the Haber process? I can write it this way:
Industrially dinitrogen fixation is conducted at high pressures (favouring the forward reaction!) but also at high temperatures (which tends to favour the reactant side). Unfortunately, at low temperatures the rate of reaction is unacceptably low. The 1 thing that helps this reaction is that the product is condensable. Removal of the ammonia (by condensation on a cold finger for instance) can drive the equilibrium to the right by Le Chatelier's principle.