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Change in Entropy and Entropy as a state function

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sgojja:
The simplest answer is: entropy is a quantity that measures change in system as well as surrounding that certainly makes it different from other quantities like internal energy. In case of reversible or irreversible processes, the change in system is same as it is a state function. what makes these processes different is change in surroundings. we measure both add them up and say that quantity delta S (universe). we are interested in universe's entropy which always increases in an irreversible or spontaneous process. for more explanation on the topic, i would suggest you to read Horia Metiu's book on thermodynamics. for advanced explanations, see, Klotz and Rosenberg. They took a more numerical approach.
i understand, btw, the issue with entropy is that common sense doesn't come to rescue. the first law is intuitive to all but second isn't. it takes effort.
keep grinding! ;D

pm133:
My understanding of this (and I'm no expert) is that in an irreversible process you can't actually say anything about the path as regards entropy because it depends solely on heat.There are no known equations to solve. Irreversible processes can cause eddy currents, fluctuating pressures and all sorts of chaos which directly affect heat in unknowable ways.

For a reversible reaction you can say something about the heat and that is why you must find a reversible path to calculate entropy changes. Because entropy is a state function you are allowed to do this.

So in summary, irreversible processes mean you don't have a stable, known equation to solve in terms of heat to get entropy changes.
Entropy is a state variable so other paths can be used.
A reversible path is the path to do that.

You'll notice that delta S is described in terms of q_rev.

Hope this helps.
Have to be proven wrong if someone knows better.

Enthalpy:
What should be a state variable then? And how come is entropy tabulated?

Things like heat or work are not state variable. They depend on the path. That's the reason for the "reversible" dQ/T, by the way.

For H, S and some others, uniform conditions (same T everywhere, no vortices, and so on) suffice to define them, whatever the path.

pm133:
Enthalpy, I'm afraid I am struggling to understand your post.

What do you mean by "what should a state variable then" mean?
Why is entropy having tabulated values relevant?

I'm not disputing your second paragraph where you talk about the reason for using a "reversible" path. That's essentially what I said, or at least tried to say.

Your 3rd paragraph is confusing as well. If you have uniform conditions like constant temperature, are you saying you must also have reversible conditions? If not, then you will have an irreversible process and no way of calculating those state variables as far as I understand it. That, again to my understanding, is why we find and use reversible paths whenever possible. As an example, for S, we must find a reversible path. For an irreversible path, the most we can say is that S > 0.

Again, happy to be corrected if someone knows more about this than me.

Enthalpy:
Quite possibly I misunderstood your post.

My point was that, if we know the final state well enough, we can evaluate its entropy.

But if we know a transformation imperfectly, like initial P, T and final P, but some lossy transformation meanwhile, then we can't predict the final T nor S. Were you saying that?

In machines like turbines, we define an "isentropic efficiency" to improve the description over "some lossy transformation", and then computations become possible again.

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