Problem: what direction will the reaction shift towards if PCl₃ is added;
Solution:
According to Le Chatelier's principle "if any conditions of a system in equilibrium is changed, the system will adjust itself in order to annul the effect of the change".
If the concentration of PCl₃ is increase, the equilibrium will shift towards the left because it is used up in that direction. More of the PCl₅ will be produced in order for the system to adjust back to equilibrium.
