Given the equation representing a system at equilibrium:
PCl5(g) + energy ⇌ PCl3(g) + Cl2(g)

Which change will cause the equilibrium to shift to the right?
(1) adding a catalyst (2) adding more PCl3(g)
(3) increasing the pressure (4) increasing the temperature

(4) increasing the temperature  

Explanation:

PCl₅(g) + energy ⇌ PCl₃(g) + Cl₂(g)

According to Le Châtelier's Principle, when we apply a stress to a system at equilibrium, the system will respond in a way that tends to relieve the stress.

If you increase the temperature, the position of equilibrium will move to the right to get rid of the added heat.

(1) is wrong. Adding a catalyst does not change the position of equilibrium.

(2) is wrong. If you add more PCl₃, the position of equilibrium will move to the left to get rid of the added PCl₃.

(3) is wrong. The left-hand side has fewer moles of gas. If you increase the pressure, the position of equilibrium will move to the left to relieve the pressure.

c) should be the answer.