Given the equation representing a reaction at equilibrium:

N2 (g) + 3H2 (g) = = > 2NH3 (g) + energy

Which change causes the equilibrium to shift to the right?

(1) decreasing the concentration of H2 (g)

(2) decreasing the pressure

(3) increasing the concentration of N2 (g)

(4) increasing the temperature

The answer is (3) increasing the concentration of N2 (g). This is an application of Le Chatelier's Principle, which states that when a system is changed in some way, the system will react to minimize the effects of that change. In this case, when you add N2 (g) to increase the concentration of N2 (g), the chemical system will shift to the right so as to consume some of the N2 (g) that was added and therefore minimize the disturbance associated with the N2 (g) addition.