[simpleton]

  Hi all, I'm quite confused here.

I understand that if it is expansion, there is considered to be no work done whereas for compression, it is considered to have work done. I tried hard to figure it out and could only come up with these reasonings (which I do not know if they are right):

(1) For compression, because external force is required to drive the compression, it is considered to have work done since force is required for the work to be done. And it involved change in just the system.

(2) In compression, because there can be conversion of state (gaseous to liquid).

(3) Expansion is not considered a work done because it does not involve change in the system but just the surrounding.

Could you guys please advise if I'm on the right track? Are my reasonings okay?

But then again... what if the expansion is result from a chemical reactions (with gas production) does it consider to have work done? I know based on the calculation, the work done will be negative. But why is it negative since it involved changes in the system?

Thank you very much! 

[Astrokel]

What is your system?

[Yggdrasil]

For systems at constant pressure, the work done on the system will be given by w = -pΔV.  When the system contracts in volume (ΔV < 0)--for example, when compressing a gas--the work is positive, meaning that the surroundings did work on the system.  When the system expands in volume (ΔV > 0)--for example, if a reaction creates more gas molecules--the work is negative, meaning that the system did work on the surroundings.

[renge ishyo]

"Expansion is not considered a work done"

The above statement is true only if the gas is expanding in a *vaccum*. If an atmosphere is present the expanding gas will do work on the surrounding atmosphere as it expands, and the system will lose energy as given in the equations above by Yggdrasil.

[simpleton]

yeah.  got it. thank you very much.

Astrokel, my system is just a general term used (not referring particularly to open, close or isolated).

this convention is really hard. got me quite sometime to accept this convention. Thanks all for your help.