Final answer:
When the volume of the reaction vessel is decreased, the system will shift towards the side with fewer gas molecules to reduce pressure. The reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g) will shift to the right (product side), resulting in the formation of more ammonia.
Step-by-step explanation:
When the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g) is at equilibrium in a closed vessel and the volume is reduced from 4L to 3L, the reaction will respond according to Le Chatelier's Principle. This principle states that if a stress is applied to a system at equilibrium, the system will adjust to partially counteract the imposed stress.
In this case, decreasing the volume increases the pressure of the system. Since there are fewer gas molecules on the right side of the equilibrium (2 moles of NH₃) compared to the left side (1 mole of N₂ and 3 moles of H₂, hence 4 moles in total), the system will shift towards the right to produce more NH₃ and reduce the pressure. This shift to the product side (formation of ammonia) is the reaction's response to the increase in pressure caused by the volume reduction.