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entropy of an adiabatic expansion? Rate Topic: -----

#1 5 February 2012 - 03:45 AM Chap 


Quark
If an ideal gas undergoes a reversible adiabatic expansion, then the entropy change would be zero, since reversible heat exchange is zero.

What happens if the gas undergoes an irreversible adiabatic expansion? I know, since its irreversible, the entropy should increase, but my undergraduate chemistry course book argues that since entropy is a thermodynamic function, it doesn't depend on the path; so should be equal to the reversible one: zero?

Can anyone shed some light on this issue?

Thanks!
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#2 5 February 2012 - 04:58 AM DrRocket 


Primate

View PostChap, on 5 February 2012 - 03:45 AM, said:

If an ideal gas undergoes a reversible adiabatic expansion, then the entropy change would be zero, since reversible heat exchange is zero.

What happens if the gas undergoes an irreversible adiabatic expansion? I know, since its irreversible, the entropy should increase, but my undergraduate chemistry course book argues that since entropy is a thermodynamic function, it doesn't depend on the path; so should be equal to the reversible one: zero?

Can anyone shed some light on this issue?

Thanks!


While the change in entropy does not depend on the path it most certainly does depend on the end points of the path (independence of path is a statement about paths in state space with the same end points).

In an irreversible process energy is being lost somewhere or added to the system from an external source and the end points of the path will not both coincide with those of a reversible process.

You can know the name of a bird in all the languages of the world, but when you're finished, you'll know absolutely nothing whatever about the bird... -- Richard P. Feynman
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#3 5 February 2012 - 05:01 AM Chap 


Quark
Thanks for clearing it up! :)
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#4 5 February 2012 - 03:24 PM mississippichem 


Icon
fluorescent protein

View PostChap, on 5 February 2012 - 03:45 AM, said:

If an ideal gas undergoes a reversible adiabatic expansion, then the entropy change would be zero, since reversible heat exchange is zero.

What happens if the gas undergoes an irreversible adiabatic expansion? I know, since its irreversible, the entropy should increase, but my undergraduate chemistry course book argues that since entropy is a thermodynamic function, it doesn't depend on the path; so should be equal to the reversible one: zero?

Can anyone shed some light on this issue?

Thanks!


A thermodynamic function CAN depend on path. A thermodynamic state function does not depend on path by definition.
You've come a long way. Remember back when we defined what a velocity meant? Now we are talking about an antisymmetric tensor of second rank in four dimensions.

-Feynman Lectures on Physics II
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