N2 (g) + 3H2 (g) ⇌ 2NH3 (g)

How will the reaction be affected if the pressure on the system is decreased?

Reaction equilibrium will shift to the left (higher molar volume side of process).

Explanation:

For gas phase reactions, changes in pressure-volume conditions cause a stress on the process equilibrium shifting away from the applied pressure-volume change.

LeChatlier's Principle states for a gas phase reaction at equilibrium ...

if the pressure is increased (volume decreased) the reaction will shift toward the lower molar volume side of the process.

if the pressure is decreased (volume increased) the reaction will shift toward the higher molar volume side of the process.

if the sum of molar volumes of reactants equals the sum of molar volumes of products, changes in pressure-volume conditions will NOT affect the equilibrium stability. Example = > H₂ (g) + Cl₂ (g) ⇄ 2HCl (g) will not be affected by P-V changes as ∑Vm (reactants) = ∑Vm (products).