According to the Le
Chatelier's Principle, if a condition of system is changed, then the system try
to become equilibrium again by correcting that change.
N2(g) + 3H2(g) ⇄ 2NH3(g)
If H2(g) is added, then the concentration of H2(g) is high in the system. So
the system tries to reduce it and become to equilibrium
state. Hence to reduce that H2(g) the forward reaction is promoted. Then
Concentration of product, NH3 gas
is increased.