Gen Chem - Keq

[jefff]

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Which of the following will change the numerical value for the equilibrium constant?
A. increase in pressure
B. decrease in temperature
C. increase in concentration of a reactant
D. addition of a catalyst
E. an increase in the volume of the reaction vessel


It says the correct answer is a decrease in temp (B)... I was wondering if anyone could help explain this to me. I honestly thought A B C and E would SHIFT the equilibrium to produce either more products or reactants but not actually change the numerical value.

 

[dentrilla]

the VALUE of Keq is strictly dependent on temp. Just somethin ya gotta know

 

[nze82]

Which of the following will change the numerical value for the equilibrium constant?
A. increase in pressure
B. decrease in temperature
C. increase in concentration of a reactant
D. addition of a catalyst
E. an increase in the volume of the reaction vessel


It says the correct answer is a decrease in temp (B)... I was wondering if anyone could help explain this to me. I honestly thought A B C and E would SHIFT the equilibrium to produce either more products or reactants but not actually change the numerical value.

Keq is the equilibrium constant that shows where the equilibrium lays (in favor of reactants vs. in favor of products).
You're right saying that A and C could cause the reaction to shift to left or right. But why is that?
Changing any of these choices disrupts the equilibrium. The reaction shifts to left or right to compensate for this disturbance and restore the equilibrium, which will in turn prevent Keq from changing.

Temperature on the other hand, is the only factor that changes the equilibrium constants.

 

[jefff]

Changing any of these choices disrupts the equilibrium. The reaction shifts to left or right to compensate for this disturbance and restore the equilibrium, which will in turn prevent Keq from changing.

Temperature on the other hand, is the only factor that changes the equilibrium constants.

This makes sense, but if they said it was exothermic ( A + B -> C +heat) wouldn't a change in temperature shift to the right with a decrease in temp or to the left if the temp was increased? (to compensate) I mean I thought I understood Le Chatelier's principle so I am still kinda of confused. I mean...on an exam if they try and trick me in regards to this I don't feel like i still have a full understanding.

PS thanks for your time (again)

 

[dentrilla]

Changing the temperature favours one reaction over the other. Therefore a shift in the Keq value is caused, as well as a change in the [participants]. A new equilibrium will be reached with it's own Keq value for the reaction at that temperature. At constant temperature, changing the equilibrium concentration does not affect Keq because the rate constants are not affected by the concentration changes.
When the concentration of one of the participants is changed, the concentration of the others vary in such a way as to maintain a constant value for the Keq.

 

[nze82]

This makes sense, but if they said it was exothermic ( A + B -> C +heat) wouldn't a change in temperature shift to the right with a decrease in temp or to the left if the temp was increased? (to compensate) I mean I thought I understood Le Chatelier's principle so I am still kinda of confused. I mean...on an exam if they try and trick me in regards to this I don't feel like i still have a full understanding.

PS thanks for your time (again)

This is a great question! But I can't explain it in few paragraphs.
This is a short article that explains this whole thing clearly. If you have time, I suggest you read through it. It should clarify this for you:

http://www.chemguide.co.uk/physical/equilibria/change.html

P.S. Make sure you read all the way to end, because they actually answer your question toward the very end.

 

[r8 dds]

Did you get this question from Princeton?

I think if you look at the Arrhenius equation, you'll have a more clear understanding of it. 👍

 

[jefff]

Awesome, thanks for the link. It definitely cleared it up.

 

[jefff]

Did you get this question from Princeton?

Nope, Kaplan. And I will hopefully get it from the ADA too