PV work question

[rskhan29]

This is a question I've been thinking about, and I was wondering if anyone could confirm or correct me on this:

Say you have a gas placed in a container. Once you release this gas by opening the container, the gas willl expand. Will the gas be doing PV work?

I think the answer is yes. For the gas to expand, it has to exert a force at least equal to the force provided by the local atmopsheric pressure. Is this correct?

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[Phlame217]

Yes it will do PV work which can be detected by temperature. Think what happens when you puncture an aerosol can or even a CO2 tank, the air comming out feels very cold. Hence there is a change in energy and PV work occuring.

 

[Lukkie]

Yes it will do PV work which can be detected by temperature. Think what happens when you puncture an aerosol can or even a CO2 tank, the air comming out feels very cold. Hence there is a change in energy and PV work occuring.

good analogy! also, if you are still not convinced, try this. put your finger in your mouth, dont touch your teeth, tongue, etc. what temperature is the air inside your mouth? now put your hand in front of your mouth, make your mouth a thin 'o' shape like you're trying to whistle, and blow air out. is it warm or cold?!?!?

 

[silverlion]

Wow!! I might be totally wrong about this and would love to hear someone with good authority on the subject to clarify this....

In my opinion there is no PV work. the gas did not expand it simply diffused from a region of higher concentration to a region of lower concentration...

An analogy would be that if the gas had the same concentration/ proportion of molecules as the environment/atmosphere then there would be no directed outward movement.

If the gas was pressurized and then the container was opened (as in the case of a CO2 cylinder) then I THINK work is being done by the gas BUT I am not sure how to explain it...

 

[marele86]

Wow!! I might be totally wrong about this and would love to hear someone with good authority on the subject to clarify this....

In my opinion there is no PV work. the gas did not expand it simply diffused from a region of higher concentration to a region of lower concentration...

An analogy would be that if the gas had the same concentration/ proportion of molecules as the environment/atmosphere then there would be no directed outward movement.

If the gas was pressurized and then the container was opened (as in the case of a CO2 cylinder) then I THINK work is being done by the gas BUT I am not sure how to explain it...

Here's the key: You're right, it did diffuse from a region of higher concentration to a region of lower concentration!
HOWEVER, pressure is simply a measure of molecular collisions, and the number of collisions is directly related to the concentration of molecules. (Think about it for a second - it does make sense, and then if you still have doubts, check a physics textbook).

So when a gas is diffusing from a region of higher concentration to lower concentration, it's really moving from a region of higher pressure to a region of lower pressure (all else being equal, of course.)

PV work! Voila.