Entropy zero?

[m25]

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Given the fact that the enthalpy of formation deltaH and Gibbs free energy deltaG is zero for a pure substance in their natural phase under standard condition,
and the equation deltaG=deltaH - TdeltaS,
why isn't the deltaS (also known as change in standard molar entropy in this case) also zero for a pure substance in their natural phase under standard condition?
Or is it supposed to be zero?

 

[The Brown Knight]

Delta S of the substance when it's not undergoing a physical/chemical change and has no energy added to it WILL be 0 since its final state is EXACTLY the same as its initial state.
Entropy is unique from Gibbs energy and enthalpy in that every object above absolute zero has an "entropy" associated with it (remember, I am distinguishing between "change in entropy" delta S and "entropy" S). A gas at room temperature has higher "entropy" than a solid. This "entropy" is 0 only for a perfect crystal at absolute zero.

 

[Lighthill-FFT]

Lets look at the equation first.

If deltaS is 0, that would simply deltaH is 0 in order for your facts to work (assuming your facts are correct, of course). That doesn't sound right.

Upon second inspection, we notices deltaH is not 0. There exists a heat of fusion/heat of vaporization; we have to put in or take out energy to do a phase change. Merely bringing the substance to the transition temperature is not enough for it to evaporate; we have to keep putting in energy to complete it. This requires an entropy to counteract the enthralpy.

An alternative way of looking at it is through the extremely rough "how ordered is it" criterion. Lets say we are moving from a solid ice cube to liquid water. Would you say this change preserves its "randomness"? No, because the liquid water is obviously much more chaotic than the ice cube.

The harder theoretical question that you should be able to awnser if you understand whats going on is "What does Gibbs Free Energy tell me about the system I am studying?".

EDIT: Extra credit if you know all the other canonical metrics off the top of your head! Not sure what extra credit would be, but I would be impressed.

 

[m25]

Okay, I guess my question wasn't very clear.
What I meant to ask is, what will be the change in entropy when we are dealing with pure substance in their natural phase under standard condition?

 

[Lighthill-FFT]

Do the math. You are given Delta G, Delta H and T. All we have to calculate is Delta S.

If you want a foundational method, you integrate the incomplete differential of heat over the temperature of the change. At most, the change in entropy is this number because we probably aren't changing the phase reversibly.

 

[The Brown Knight]

Okay, I guess my question wasn't very clear.
What I meant to ask is, what will be the change in entropy when we are dealing with pure substance in their natural phase under standard condition?

I thought my reply answered this a bit.
As long as it's in an isolated state and no matter or energy is exchanged with it, delta S will be 0.
However, (as long as it's not absolute zero and it's not a perfect crystal) the object will have a nonzero absolute entropy associated with it - its INTRINSIC entropy is always nonzero.

 

[m25]

Okay, so the change in entropy for a pure substance in their natural phase under standard condition is zero, correct?