Why is the standard entropy of aqueous ions negative?

[Nikitin]

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?

 

[ohms law]

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?

 

[DrDu]

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?

 

[Nikitin]

No, the standard entropy.

For example, PO₃^2- (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?

 

[DrDu]

I am not familiar to the concept of standard entropies for ions. Maybe you can give a reference?

 

[Nikitin]

i used SI Chemical data

anyway,

http://answers.yahoo.com/question/index?qid=20100415135913AAn73mc

apparently it is given as a difference to the entropy of H+ ions. i think my question is answered by some googling.

 

[Studiot]

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
H_f
Note also that the sign of S^* does not follow that of ΔG_f or ΔH_f, the standard energies of formation, which may also be positive or negative.