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

increasing the temperature

Explanation:

Now look carefully at the reaction equation, notice the inclusion on an energy term on the left hand side;

PCl5(g) + energy ⇌ PCl3(g) + Cl2(g)

The inclusion of an energy term means that the reaction is endothermic. Energy is absorbed as the reaction goes from left to right.

Since energy is absorbed, increasing the temperature (supplying energy in the form of heat) will favour the forward reaction over the reverse reaction in accordance with Le Chatelier's principle. Hence the answer.

Option (4) increasing the temperature

Explanation:

PCl5(g) + energy ⇌ PCl3(g) + Cl2(g)

The reaction above clearly indicates endothermic reaction since heat is required for the reaction to proceed to product.

As the heat is supplied, the temperature of reaction increases thereby making the reaction to proceed forward at a much faster rate and hence the equilibrium position will shift to the right. This is in accordance with Le Chatelier's principle will explained that for an endothermic reaction, an increase in temperature will cause the equilibrium position to shift to the right.