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Change in Entropy and Entropy as a state function
Kyriee:
Hi!
I'm studying PChem1 and I'm doing some exercises about change in Entropy.
I cannot understand one basic and fundamental point about entropy: it is a state function and, like all state functions, what matter are the initial and the final states. If this is correct, how is it possible that the variation in entropy is different if I'm talking about, for example, a reversible or an irreversible (against an external pressure different from zero) gas expansion? The fact that the trasformation is reversible or not doesn't change my initial or final states, it only changes the way I arrive to the final state.
Sorry for my bad english and sorry if this is a stupid question, but I'm desperate.
Enthalpy:
Hi Kyriee!
Are the final states really identical? For instance a gas could attain the same final pressure, but with a different temperature and volume, if an expansion is lossy.
Corribus:
--- Quote from: Enthalpy on December 16, 2019, 01:34:13 PM ---Are the final states really identical?
--- End quote ---
No, which resolves the apparent contradiction.
Kyriee:
Why aren't they?
Let's suppose an isothermic expansion of an ideal gas. The states (Pi,Vi), (Pf, Vf) are the same if the expansion is reversible or irreversible; what does change is the work done. Does the variation in entropy have something to do with this work?
I repeat, sorry if these are basic questions, but I really can't understand.
Corribus:
For example. In an irreversible process there may be heat loss from the system. Therefore the final state of the system that undergoes an irreversible process is different from the final state of the system that undergoes a reversible process, in which no heat is lost. The environment is different in the two processes. Heat, in other words, is not a state function, and the "system" is everything, including the environment. The entropy change during the two processes is not the same, because the final states (which includes the environment) is not the same.
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