Answer:
As temperature is decreased the reaction will shift towards the reactants i.e. it will favour the formation of PCl5
Step-by-step explanation:
According to Le Chatelier's principle, when a reaction at equilibrium is subject to a change in temperature, pressure, concentration etc then the equilibrium will shift in a direction so as to undo the effect of the induced change.
The given reaction is:
PCl5(g) + heat ↔ PCl3(g) + Cl2(g)
This reaction is endothermic i.e. heat is absorbed by the reactants. When the temperature is decreased, this would imply that heat is being removed from the system. Therefore, as per Le Chatelier's principle the equilibrium will shift in a direction that would favor more input of heat i.e. it will shift towards the reactants.