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Chemical Equilibrium Question

  1. Mar 23, 2013 #1
    Hi, I have simple question
    N2+3H2 = 2NH3 + heat

    How would the equilibrium position be affected if ammonia is removed from the vessel?, shouldnt it move to the left to produce more reactants and replace the ammonia? why does it move to the right?, because thats what my texbook says.
  2. jcsd
  3. Mar 23, 2013 #2
    Lets think about Le Chatelier's principal. This applies to a system at equilibrium. If the temperature, pressure or chemical concentration changes, the system will shift its equilibrium position to counteract the disturbance.

    You have a balanced equation
    N2+3H2 = 2NH3 + heat

    Alright, lets take the ammonia out, decreasing the chemical concentration of the right
    N2+3H2 = heat

    Le Chatelier's principal states that the system will shift its equilibrium position to counteract the disturbance. Ammonia was removed so the counteract is N2 reacting with 3H2 to form more ammonia.

    If we had added reactants N2 and H2, equilibrium would have shifted to the right to balanace the chemical concentration.

    higher concentration-------> lower concentration
    higher temperature--------> lower temperature
    higher pressure------------> lower pressure
  4. Mar 23, 2013 #3


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    Staff: Mentor

    Sorry, but what you wrote doesn't make any sense. Please elaborate on what you mean, chances are when you will try to explain what you mean you will find you are suggesting things that contradict themselves.
  5. Mar 23, 2013 #4


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    Staff: Mentor

    Never heard about the guy.
  6. Mar 23, 2013 #5
  7. Mar 23, 2013 #6


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    Staff: Mentor

    Ah, I thought you were talking about some headmaster that accidentally shared the name with Le Chatelier.
  8. Mar 23, 2013 #7


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    Or is it the quoted textbook that doesn't make sense?

    I wonder if it's only me, but from the first time I heard the expression I found the expression of equilibrium 'moving to the left' or right confusing and ambiguous and it never became part of my terminology or mental furniture, every time I heard it I translated into a vision what was really happening.
  9. Mar 25, 2013 #8


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  10. Mar 26, 2013 #9
    Developing Creative Problem Solving Approaches\

    Thanks for putting it that way. Since I'm still learning how to communicate these ideas, they can come out dry. Being technically correct is a good foundation from which the question can be answered. When answering questions on these forums I:
    1) identify the area of unknown information
    2) explain concepts and keywords
    3) explain how those work together
    4) pause to let the poster assimilate the information

    This method keeps me out of the spreading misinformation realm. Do you have any recommendations on how I can improve this technique? I really like the visualization element you are presenting. Big thumbs up for helping me start exploring the relationship with a clearer focus. I have so many trains rolling around up here it is hard to keep track of them.

    I don't know the things you don't know but I certainly know the things you tell me you don't know. This old (Oct '12) Wall Street Journal article provides a enlightening perspective on Internet Sarcasm. I would like for our correspondence be pleasant while we enrich our knowledge. Pleeease?:smile:
  11. Mar 26, 2013 #10


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    For the avoidance of doubt you are correct in substance - that removing ammonia causes the system to "produce more reactants and replace the ammonia".

    The difference is that the book calls that moving the eq. to the right and you call it moving to the left. And that seems to me an equally if not more valid manner of speaking (the idea being that if you think of a sort of representation as a continuum, maybe with shading or somthing with

    stuff on left ------------^----------- stuff on right

    stuff moving from left to right from corresponds to the ^ pointer moving to the left. From the above to

    stuff on left --^--------------------- stuff on right. )

    I always shared your perplexity and translated in my mind what anyone was sayng into what is happening and was OK. It is risky though to talk or write in a terminology of one's own so I always wrote in a what's happening terminology, products forming, concentrations increasing or decreasing etc. rather than quasi-metaphors of equilibria moving left or right.
    Last edited: Mar 26, 2013
  12. Mar 29, 2013 #11

    If in the system, nitrogen, hydrogen and ammonia, at equilibrium you "remove" ammonia, the system reacts according to Le Chateliers Principle, which states that a dynamic equilibrium at equilibrium opposes any change imposed. Thus, more ammonia will be produced by reaction between nitrogen and hydrogen, and will continue until equilibrium is established once more, ie moves to the right in the conventional sense. If the system is closed this second equilibrium will be at a lower pressure than the first because there are fewer molecules present. However, if we keep temperature constant the value of the equilibrium constant remains unchanged as does the position of equilibrium and hence the amount of ammonia produced will be the same.
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