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Standard entropy

  1. Oct 28, 2012 #1
    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?
  2. jcsd
  3. Oct 28, 2012 #2
    This, I think. Remember what entropy is about (things not wanting to be "pure", basically).
    what's the problem statement, anyway?
  4. Oct 29, 2012 #3


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    Do you mean the standard entropy of hydration?
    Maybe you could refer to the source of the values you are considering?
  5. Oct 29, 2012 #4
    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?
  6. Oct 30, 2012 #5


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    I am not familiar to the concept of standard entropies for ions. Maybe you can give a reference?
  7. Oct 30, 2012 #6
  8. Oct 30, 2012 #7
    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
    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.
    Last edited: Oct 30, 2012
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