[maakii]

Hi everyone, I was thinking about this problem:

1 mole of water was confined in a fixed container at 373K and 1 atm. By opening a valve, the water evaporates into an evacuated space, and ends up as all water vapour at 373K (temperature of the apparatus) and 0.1 atm.

How do we find out the heat change?

Since no work is done, w = 0, ?E = w + q = q

So what is ?E equal to? That's the part i'm stuck at. Can we split up the two processes, evaporation (water 1atm 373K -> water vapour 1atm 373K), then expansion (water vapour 1atm 373K -> water vapour 0.1atm 373K), and find individual ?Es and add them up?

Thanks!

[allanf]

Yes you would split up the evaporation part and the expansion part and just add them up.  You can rationalize this by recalling that energy is a state function, that is the difference in energy between two states depends only on the two states, not the path between them.

[maakii]

OK, I am more comfortable with doing this now..thanks Allan!