Rate of Effusion - TBR Gases #47

[Lifeman]

47. All of the following affect the effusion rate of a gas EXCEPT:
A. the size of the molecule relative to the pore
B. the temperature of the gas
C. the presence of isotopes in the compounds
D. the concentration of the species outside the container

The correct answer is D, and TBR explains it by saying the the outside concentration will only affect the backwards rate of effusion, not the forward rate. Earlier in the chapter, however, they say that concentration and molecular mass affect the rate of effusion. So my question is, if there is a backwards reaction, won't that increase the concentration inside of the container, thus increasing the forward rate as well?

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[cheetoh]

What did you think the answer was?

Using Graham's Law for effusion is a one way street per scenario you're looking at. Effusion is the rate that molecules can 'escape' through a hole.

If your molecules have escape, and you're wanting to look at a reverse process then you're now talking about 'infusion rate' and that's no where to be found in this question.

 

[Lifeman]

I know what effusion is, but I still think that if I have 100 molecules of gas in a container with a small opening, vs 5 molecules, the 100 molecule scenario will have a greater rate of effusing out of the hole because it is more likely that a molecule will find the opening.

Regarding the question, I think that there will more molecules inside the container if we started with some molecules outside too, because the infusion is adding molecules to the inside of the container.

 

[drechie]

so i chose D. the concentration of the species outside the container.
I was thinking that effusion somehow was similar to diffusion in that it traveled down a concentration gradient. I guess for effusion, ion concentration is not applicable? If you would, please quote me in your reply I would appreciate it. Thank you!

 

[DrknoSDN]

I know what effusion is, but I still think that if I have 100 molecules of gas in a container with a small opening, vs 5 molecules, the 100 molecule scenario will have a greater rate of effusing out of the hole because it is more likely that a molecule will find the opening.

Regarding the question, I think that there will more molecules inside the container if we started with some molecules outside too, because the infusion is adding molecules to the inside of the container.

I think that your assumptions are too small and you are thinking that the concentration inside the container is changing.

If you had 1/10 of a mole (2.24 L) inside the container that is 6.022x10^22 molecules. The rate of effusion into or out of the container is not going to change the concentration to any significant degree because the hole is so small only 1 molecule can pass at a time. If the hole was larger it wouldn't really be effusion anymore, it would be diffusion.

And even if external concentration is high, any level of effusion in would not change the concentration inside the container quickly so over a short time interval concentration inside should be assumed to be constant.

Your scenario of 100 molecules vs 5 molecules would be true with those numbers but when you approach zero pressure then rules for both effusion and diffusion break down.