HW 8.17 Work done in a phase change

[kshalbi]

For image b, how do you know that work is done on the system?

[Wendy_Xie]

Because of the phase change.

[Janae Yip 3G]

I am confused by this question as well. If the phase change goes from vapor to liquid, then heat should be released, signifying that the system is doing work. However, the answer says that work is done on the system. If that were the case, wouldn't the energy inside the system increase? Then there is no way vapor could change to liquid.

[kshalbi]

Yeah, I still don't get it.

[Chem_Mod]

Vapor to liquid is a reduction in volume which is doing work on the system (w<0). Also this phase change is exothermic (q>0). These signs being opposite allows U=q+w to be ZERO when the phase change occurs at a constant temp.