Adiabatic vs Isothermal

[MedPR]

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So I'm listening to the wikipremed lecture(s) on thermochem and I realized I've been thinking about this stuff all wrong.

Isothermal doesn't mean no change in temperature, it means no change in internal energy. So if W is negative (expansion) then Q must be positive (absorbed) in order to keep the U (internal energy) from changing.

Adiabatic is no heat flow. So in an adiabatic expansion, W is negative and Q is 0, so dU=W, and since W is negative, U will be decreasing.

Not sure if anyone else has been having a problem with these two terms, but I wanted to type it out for my own benefit as well as anyone else's.

 

[chiddler]

Is it possible to change temperature without changing internal energy? I think the answer is no. If i'm right, then it is synonymous; isothermal means no change in temperature and no change in internal energy must be true as well.

 

[milski]

Yes, I'm not sure what they mean by isothermal not being the same temp. By definition isothermal is T=const. Since T is proportional to U, T=const -> ΔU=0 or Q+W=0 or Q=-W.

Adiabatic is Q=0 which means ΔU=W.

 

[MedPR]

I should've worded the OP more carefully. I meant that I was initially thinking that isothermal meant no change in temp in terms of no heat added or taken out of the system. But as you guys are saying, isothermal means constant temperature, which is the same thing as constant heat and the only way to keep it constant is to add heat as the system expands or remove heat as it compresses.

 

[milski]

I should've worded the OP more carefully. I meant that I was initially thinking that isothermal meant no change in temp in terms of no heat added or taken out of the system. But as you guys are saying, isothermal means constant temperature, which is the same thing as constant heat and the only way to keep it constant is to add heat as the system expands or remove heat as it compresses.

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