Adiabatic expansion

[m25]

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In a free adiabatic expansion, a real gas is allowed to spread to twice its original volume with no energy trasnfer from the surroundings. All of the following are true concerning this process EXCEPT:

A No work is done
B Increased potential energy between molecules results in decreased KE and the gas cools
C Entropy increases
D The gas loses heat

The answer given is D since there's no transfer of heat during adiabatic process, but that automatically makes other choices true. How is answer choice A and C a true statement during adiabatic expansion? how can no work be done??

 

[Cawolf]

This is a weird question if you don't know what "free adiabatic expansion" is.

This is when a gas expands into a vacuum. There is no external pressure (P = 0) for the gas to expand against so no work is done.

You are correct in your thinking, but I think you assumed there was some external pressure.

 

[m25]

This is a weird question if you don't know what "free adiabatic expansion" is.

This is when a gas expands into a vacuum. There is no external pressure (P = 0) for the gas to expand against so no work is done.

You are correct in your thinking, but I think you assumed there was some external pressure.

Ooh, I see. So is there also such thing as free compression with W=0?
Also, how do entropy increase in free adiabatic expansion??

 

[Cawolf]

No. Free adiabatic expansion is expansion only. Work would be required to compress a gas.

Entropy is increasing because there is more space between each molecule. The distance between the molecules theoretically goes to infinity - the lack of order in such an open system is much more entropically favorable than being enclosed in a container.

 

[m25]

No. Free adiabatic expansion is expansion only. Work would be required to compress a gas.

Entropy is increasing because there is more space between each molecule. The distance between the molecules theoretically goes to infinity - the lack of order in such an open system is much more entropically favorable than being enclosed in a container.

Oh okay thanks!
Does that mean whenever there's compression, entropy always decreases?

 

[Cawolf]

That is a good generalization to make.

It may not be pan-applicable, but I would say it is a great direction to lean on any introductory courses or MCAT problems.

 

[m25]

That is a good generalization to make.

It may not be pan-applicable, but I would say it is a great direction to lean on any introductory courses or MCAT problems.

Hmm, okay. But conceptually thinking, compression makes the system gain energy=temperature increases=KE increases, and it feels like higher temperature would actually be a an increase in entropy??

 

[Cawolf]

Compression does not always make a system gain energy - suppose it loses heat in an equal amount (isothermal reaction)?

That is why I said you can't just apply it to everything. Temperature also plays a role as your correctly described. If you want to compare the effects of compression it is best to look at one variable at a time - intermolecular distance and total energy (temperature).