Question

Question #3c6f1

Answer 1

The concentration of `Cl_2` would increase.

Explanation:

I published a note on this topic ~two weeks ago... Here is the link to my answer presentation. => https://socratic.org/questions/what-is-meant-by-a-stress-on-a-reaction-at-equilibrium?source=search

Quick Answer => Adding `PCl_5` overloads the product side of the reaction much like extra kids getting onto a balance beam (seesaw). Visualize the reaction tilting toward the product side because of the extra `PCl_5`. To reestablish the equilibrium, the reaction needs to shift away from the applied stress. This means some of the `PCl_5` must decompose into `PCl_3 and Cl_2` thereby increasing the concentration of `Cl_2` (and `PCl_3)` causing the balance to shift to the reactant side to establish a new equilibrium.

Go to the link mentioned above for more support on this. It is really very simple concept. Good luck.

Answer 2

Principle of Le Chatalier applies.....

Explanation:

The equilibrium position shifts so as to nullify the change you have made. The change you made was to add `PCl_5` so to nullify this the equilibrium shifts in the direction of reagents, thereby using up some of the `PCl_5` by decomposing it into `PCl_3` and `Cl_2`, so `Cl_2` concentration increases.