system undergoing change at constant temp(#7.15)

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HilaryPhan_3N
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Joined: Fri Sep 26, 2014 2:02 pm

system undergoing change at constant temp(#7.15)

Postby HilaryPhan_3N » Tue Jan 13, 2015 5:40 pm

(Number 15 for Ch 7 HW)
Each of the pictures below shows a molecular view of a system undergoing a change at constant temperature. In each case, indicate whether heat is absorbed or given off by the system, whether expansion work is done on or by the system, and predict signs of q and w for the process.

I'm confused by all parts of this problem. Can someone please explain how to do this? The pictures are attached.
Attachments
7.15.JPG

Niharika Reddy 1D
Posts: 127
Joined: Fri Sep 26, 2014 2:02 pm

Re: system undergoing change at constant temp(#7.15)

Postby Niharika Reddy 1D » Tue Jan 13, 2015 5:55 pm

The change in internal energy, ΔU = q+w. For an isothermal change, or change under constant temperature, ΔU = 0. In a), going from solid to liquid requires energy, as the solid state is more stable than the liquid state. This would be endothermic, so heat is required, making q positive. Since ΔU = 0 and ΔU = q+w, 0 = q+w and w = -q. Thus, work must be negative, indicating the expansion work is done by the system.

The same approach works for part b), where condensation is occurring. Heat is released because condensation is an exothermic process, making q negative. Since w = -q, work is positive in this case and work is being done on the system.


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