Question

What is the direction of reaction when chlorine gas is added to an equilibrium mixture of `PCl_3+` `PCl_5+` and `Cl_2+`? The reaction is `PCl_3(g)` + `Cl_2(g)` `rightleftharpoons` `PCl_5(g)`

b) What is the direction of reaction when chlorine gas is removed from an equilibrium mixture of these gases?

Answer 1

A) To the right hand side.
i.e. towards the formation of phosphorus pentachloride (PCl5).
B) To the left hand side.
i.e. towards the formation of phosphorus trichloride (PCl3).

Explanation:

This can be illustrated by using Le Chatelier's Principle which states that:
"If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change".

This change may be decrease or increase in: concentration, volume, pressure or temperature.

For this case, we increase the concentration of chlorine gas which is one of the reactants. So, the equilibrium system will be driven to the side which decreases or counteracts this change to establish the equilibrium again.

To wrap up, increasing the amount of chlorine gas in this equilibrium will shift the reaction to the right hand side.

For the second part of the question, when chlorine gas concentration is decreased, the system will be shifted towards the left hand side. i.e. towards the formation of phosphorus trichloride gas (PCl3).

You have to read a little bit of Le Chatelier's Principle to solve such problems. Here's the link for your help.
https://www.chemguide.co.uk/physical/equilibria/lechatelier.html