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Homework Help: Direction of equilibrium

  1. Apr 27, 2012 #1
    1. The problem statement, all variables and given/known data

    In what direction will the reaction go if the volume is increased.

    PCl5⇔ PCl3 + Cl2

    3. The attempt at a solution

    I would think using the ideal gas equation PV = nRT that since V goes up P must go down. In order for P to go down you need less molecules. There are less molecules on the left side of the equation, so the direction should be to the left but the book says to the right.
  2. jcsd
  3. Apr 27, 2012 #2
    The volume of what is increased?
  4. Apr 27, 2012 #3
    sorry, I don't know what I was saying in my last post (for some reason the editing screen wouldn't load)

    I like your logic with the whole PV =nRT, however the conclusion you've come to I disagree with (and so does your text book :P).

    You cannot simply "have less molecules". This is a closed system, so nothing gets in or gets out, the it's constant.

    Take opening a soft drink can as an analogy.

    Carbon dioxide(aq) <=> Carbon Dioxide(l)

    if you opened this a can of soft drink, which way do you think the position of the equilibrium will be in favour of?
  5. Apr 27, 2012 #4
    But on the right side of the equation there are twice as many molecules as on the left side. The number of atoms is the same on both sides, but no the same amount of molecules.
  6. Apr 27, 2012 #5
    yes that may be, but the volume and the pressure has been changed. It will tell you which one is will drive the equilibrium... in favour of the products, or the reactants...

    Have you learnt anything about entropy?
  7. Apr 27, 2012 #6
    I have learned about entropy but I don't see how it applies in this case.
  8. Apr 27, 2012 #7
    hmmm... i was going to try and make connection, but im having second thoughts.

    when pressure is increased, density is also increased right?

    which one is more dense out of the two?
  9. Apr 27, 2012 #8


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    Staff: Mentor

    Think in terms of LeChatelier's principle.

    When the volume goes up, pressure goes down, system tries to increase the pressure.
  10. Apr 27, 2012 #9
    Yes, I know that pressure goes down. That's not helping me find out whether or not more products are being consumed or produced. I would think that because the pressure is going down the number of molecules would also go down which would shift the equation to the left since there are less molecules on the left.
  11. Apr 27, 2012 #10


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    Staff: Mentor

    You have read only a small part of my post and you are still not seeing the bigger picture.

    What does the LeChatelier's principle state?
  12. Apr 27, 2012 #11
    Here's L's principle

    If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established.

    I'm still in the dark. The volume increases so the pressure decreases to restore equilibrium. I would think if pressure decreases than more reactants would form for reasons I've already stated.
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