The relative magnitudes of the reaction quotient Q and the equilibrium constant K indicate how the reaction mixture changes as it moves toward equilibrium.  If Q is smaller than K, the reaction proceeds from left to right until Q = K.  When Q = K, the reaction is at equilibrium and has no tendency to change.  If Q is larger than K, the reaction proceeds from right to left until Q = K.

When H2 is added to an equilibrium mixture of N2, H2, and NH3, a portion of the H2 reacts with N2 to form NH3, thereby establishing a new equilibrium position that has the same equilibrium constant.  The results are shown in accordance with Le Châtelier’s principle.