AP Chemistry

2 Questions (Multi-choice and T/F, though not sure if they're right, and these 2 questions have no correspondence to each other)

1.) a.) Consider the following system at equilibrium where Kc = 1.20E-2 and H° = 87.9 kJ/mol at 500 K. PCl5 (g) <=> PCl3 (g) + Cl2 (g)
The production of PCl3 (g) is favored by:
Indicate True (T) or False (F) for each of the following:
T 1. decreasing the temperature.
T 2. decreasing the pressure (by changing the volume).
T 3. decreasing the volume.
F 4. removing PCl5 .
F 5. adding Cl2 .
(I have other screenshots of different problems, but I keep only getting 1 wrong all the time and have to start all over)

2.) Consider the reaction, which is exothermic as written, PCl5(g) PCl3(g) + Cl2(g). Which of the following changes would result in the production of MORE Cl2(g)? I) adding PCl3(g)
II) removing PCl3(g)
III) reducing the volume of the container
IV) removing PCl5(g)
V) increasing the temperature
VI) increasing the volume of the container
VII) adding PCl5(g)
VIII) reducing the temperature
IX) adding a suitable catalyst
a) II, V, VI, VII
c) I, IV, V, VI
I wrote down the answers of which I was wrong in, and I don't think it's (a),(b) or (c). The only difference is whether to increase or decrease the volume in the container indicated by III or VI. I think it's (d) because reducing the volume of the container would produce Cl2 more wouldn't it?

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  1. In somewhat unorthodox language Le Chatelier's Principle says that a system in equilibrium tries to undo what we do to it.
    For #2 write it has follows:
    PCl5(g) ==> PCl3(g) + Cl2(g) + heat

    1. If we add heat the reaction shifts so as to use up heat. Since moving to the right would produce more heat the reaction will shift to the left so as to use up the added heat.
    2. If we increase the pressure the reaction shifts to occupy the smaller volume; therefore, count the mols on each side. In this case 2 mols on right and 1 on left so increase P shifts the reaction to the side with fewer mols (to the left for this reaction).
    3. You can answer volume changes by what changing volume does to the pressure.
    4. The only other change that affect the shift will be changing in concentration. The system works the same way as the T change above.
    I think the answer is e.
    d isn't right because of III. Decreasing volume means increasing P and that shifts the rxn to the left which will not produce more PCl3.

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  2. Thank you for #2!
    For #1, the same principles would apply right?

    Here are my reasons for the T/F.

    1.) Decreasing the temperature at 500K would be beneficial for the reaction, and therefore would shift the reaction forward which would favor PCl3. Also, the equation is endothermic because of the delta H. Therefore it is true.

    2.) Decreasing the pressure would therefore increase the volume, making it a favorable reaction to occur for PCl3 with a lower concentration of the gas. Therefore it is true.

    3.) If the volume decreases, pressure and the concentration increases for the gas, therefore promoting the PCl3 (similar to #2). Therefore it is true.

    4.) You'd have to add PCl5 for the equilibrium to balance out. Therefore it is false

    5.) You'd have to remove Cl2 for the same reason as #4. Therefore it is false.

    Am I correct with my assumptions?

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  3. I didn't look at #1 because I'm not sure what it says, particularly the Ho part. Ho to me means standard conditions for thermodynamics heat formation tables and those tables are for 25C or 298K. This reaction is at 500 C which doesn't agree with my understanding of Ho. In addition to that I noted that both problems have the same reaction with one being exothermic and the other endothermic. That doesn't compute either.

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  4. #1 is its own completely separate problem if it clears up any confusion. One of my friends says the Ho, being positive is endothermic, and she says it's more theoretically based too, and I don't think it uses any value of the heat formation table at all.

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  5. ok, on the assumption that the + H really means endothermic then
    PCl5(g) + heat ==> PCl3(g) + Cl2(g)
    All of your answers are correct IF the system is endothermic.

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  6. the answers for #1 is

    Ive just got done working in this problem

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  7. 1.F

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