Consider the following equilibrium:

Question

PCl3 (g) + Cl2 (g) yields PCl5 (g)
delta H= -92KJ

The concentration of PCl3 at equilibrium may be increased by:

decreasing the temp (think this is it)
adding Cl2 to the system
the addition of neon
the additio of PCl4

Answer 1

I don't think so. Remember that Le Chstelier's Principle says that when a system in equilibrium is disturbed it will shift so as to undo what we've done to it.
When dH is involved I rewrite the equation to make more sense.
PCl3 + Cl2 ==> PCl5 + heat
Now, adding PCl3 makes the system want to use up the added PCl3. How can it do that? By reacting more PCl3 with Cl2 to produce PCl6; i.e., shifting the rxn to the right.
Adding more cl2 will shift to the right.
Adding PCl5 will shift to the left.
Adding heat will make it shift to the left.

Answer 2

Sir, thank you for answering. I thought it was the adding the Cl2, but it was wrong. That is why I went with decreasing the temp, but now knowing that it is wrong as well. I think my only option is the addition of PCl5 (which I meant to put at the top vice Pcl4) I hope I am on the right track.

Answer 3

Adding heat and adding PCl5 is th only way to shift it to the left (or course decreasing PCl3 and Cl2 will shift it to the left also). Decreasing T shifts the reaction to the right (because if we take temperature away the reaction will shift to the right in order to produce more heat).