Why is the standard entropy of aqueous ions negative?

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The discussion centers on the negative standard entropy of aqueous ions, which some participants initially find counterintuitive, as they expect entropy to be non-negative based on the definition related to perfect crystals at absolute zero. It is clarified that the negative values for ions, such as PO32- (aq) with an entropy of -220 J/K·mol, arise because these ions are compared to the standard entropy of hydrogen ions, which is conventionally set to zero. This means that the standard entropies for ions are defined relative to the hydrogen ion, not to a perfect crystal state. The conversation also touches on the concept that ions in solution have restricted microstates, contributing to their lower entropy compared to neutral substances. Additionally, it is noted that the sign of standard entropy does not necessarily correlate with the signs of standard Gibbs free energy or enthalpy of formation.
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Why is the standard entropy of aqueous ions negative? I thought it could be no less than 0, which represents a perfect crystal at 0 K?

Is it negative so that calculations can be performed properly? Or is it because it because ions solutes actually have less entropy than a perfect crystal?
 
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Nikitin said:
Or is it because it because ions solutes actually have less entropy than a perfect crystal?

This, I think. Remember what entropy is about (things not wanting to be "pure", basically).
what's the problem statement, anyway?
 
Nikitin said:
Why is the standard entropy of aqueous ions negative?

Do you mean the standard entropy of hydration?
Maybe you could refer to the source of the values you are considering?
 
No, the standard entropy.

For example, PO32- (aq) has an entropy of -220J/K*mol. it's the same with many other ions in aqueous solutions. Why? Is it because ions floating in water are extremely restricted in their number of micro-states?
 
I am not familiar to the concept of standard entropies for ions. Maybe you can give a reference?
 
Standard entropies for ions are defined by convention as relative the the hydrogen as taken to be zero.

Standard entropies for electrically neutral substances are defined to be relative to the crystal state at 0°K

so S* for common hydrogen related species is

H2 130.6
H 114.6
H2O 188.7
OH 183.6
H+ 0
OH- -10.8

all in J/degreee K moles
Hf
Note also that the sign of S* does not follow that of ΔGf or ΔHf, the standard energies of formation, which may also be positive or negative.
 
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