A LeChatelier's question

[meowkat444]

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So, I'm actually a chemistry teacher, and I thought I knew this....

If you've got a reaction going in a closed container, and you inject an inert gas, then the pressure increases and the reaction should shift accordingly, yes?

So how come my Kaplan practice question just told me equilibrium does not shift? Am I missing something?

Thanks!

 

[MystA RavE]

TPR states the same thing...no equilibrium shift. This is because it does not take part in the reaction. As for the change that should occur as you say due to pressure, they state that this does not happen for a container of a fixed volume. They're thinking about this along the lines of it being an ideal gas that take up no volume.

 

[meowkat444]

but but pressure is directly proportional to n 🙁 ah, well, if two test prep companies agree then at least it's probably not a mistake by one. thanks!

 

[baby methuselah]

adding an inert gas would not change the partial pressures of the other gasses so why would you expect the equilibrium to shift? am i missing something?

 

[BerkReviewTeach]

So, I'm actually a chemistry teacher, and I thought I knew this....

If you've got a reaction going in a closed container, and you inject an inert gas, then the pressure increases and the reaction should shift accordingly, yes?

Thanks!

The issue here involves the components of the reaction. The volume is fixed, the temperature is constant, and the moles of both the reactants and products do not change upon the addition of an inert gas. This means that the partial pressures of the reactants and products remain constant. As such, Q is still the exact same value, so if the system was originally at equilibrium (Q = K), it reamins at equilibrium (Q remains = to K).

If the container expands upon the addition of the inert gas AND the number of reactants is not equal to the number of products, then the reaction is displaced from equilibrium and the reaction will need to shift to re-establish equilibrium.

Hopefully this makes sense, because I'm typing away incoherently at this time of night.

 

[Creightonite]

well if you keep the volume constant, the addition of inert gas will increase total pressure --> the eq. fill shift toward the side where there are fewer gas molecules... so the proportions on either sides of the equation will change --> The equilibrium changes.... Maybe i am just confusing myself.

 

[BrokenGlass]

well if you keep the volume constant, the addition of inert gas will increase total pressure --> the eq. fill shift toward the side where there are fewer gas molecules... so the proportions on either sides of the equation will change --> The equilibrium changes.... Maybe i am just confusing myself.

Maybe this could be of some use...

http://forums.studentdoctor.net/showpost.php?p=4992107&postcount=808