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Temperature and the equilibrium constant

  1. Jul 12, 2012 #1
    The equilibrium constant changes with a change in temperature, I am confused as to the directions and why Qc is behaving oppositely of what I think it does

    For example, my textbook says a temperature increase favors an endothermic reaction (left to right) and a temperature decrease favors the exothermic direction (from right to left). This is ambiguous to me, I am not sure what they mean by favor, if somone can clarify.

    Anyway, here is an endothermic reaction

    heat + A ⇔ B + C

    If you increase the temperature, the heat is treated as a reactant and it shifts from left to right, that is fine

    Exothermic:

    A + B ⇔ C + heat
    decrease the temperature, shouldnt this shift from left to right, since the heat is treated like a concentration and it is taken away, so to make up for it more is going from left to right, yet the book says right to left.


    My other concern is how the equilibrium constant changes with temperature. We had a worksheet and its says heating an endothermic equilibrium system increases K. The answer given is true, and I have 2 thoughts on this.

    The first is that if you have an endothermic reaction, heating it up increases the number of the reactant side, so the denominator is bigger and K is lower. However, if you increase the temperature, equilibrium would shift to the right so now the numerator is bigger, and therefore K is bigger. I think the second statement is the correct one, but just want to confirm.

    It says cooling an exothermic equilibrium increases K. That mens equilibrium shifts to the right (which contradicts my textbook from above) and the numerator is now bigger, so K is bigger.
     
  2. jcsd
  3. Jul 13, 2012 #2
    No, it should be favoring the products (shifting right to left). Heat is not exactly the same thing as temperature. If temperature is decreased, heat is leaving the system. The system will "maintain" its temperature by reacting to the product side to release heat.

    Right ----> Left

    Le Chatelier's principle.
     
  4. Jul 13, 2012 #3
    Maybe we're reading this different, but if its favoring the products, you are saying some of the reactants are going to go to the product side. The reactants are on the left, and the products are on the right. Therefore, reactants (left) to products (right)?

    Right -----> Left is backwards, my right hand is on the side where you have written left

    Left -----> Right should be how you wrote it, no?
     
  5. Jul 13, 2012 #4
    Hahah I just realized that I did write it incorrectly. I apologize.

    Products are definitely favored in an exothermic reaction when temperature is decreased.
     
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