Why is the standard entropy of aqueous ions negative?

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Discussion Overview

The discussion revolves around the standard entropy values of aqueous ions, particularly why these values can be negative. Participants explore the implications of these values in relation to thermodynamic principles and the nature of ions in solution.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question whether the negative standard entropy values indicate that ions in solution have less entropy than a perfect crystal.
  • There is a suggestion that the concept of entropy relates to the tendency of systems to avoid being "pure".
  • One participant asks for clarification on whether the discussion pertains to the standard entropy of hydration specifically.
  • Another participant notes that the negative entropy values for ions may be due to restrictions in their number of micro-states when in aqueous solution.
  • A participant mentions that standard entropies for ions are defined relative to the hydrogen ion, which is conventionally set to zero.
  • Reference to specific standard entropy values for various hydrogen-related species is provided, highlighting the differences in entropy values.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of negative standard entropy values, and the discussion remains unresolved regarding the underlying reasons for these values.

Contextual Notes

Some participants express unfamiliarity with the concept of standard entropies for ions, indicating a potential gap in understanding or definitions. The discussion also references specific entropy values and their conventions, which may not be universally accepted or understood.

Who May Find This Useful

This discussion may be of interest to students and professionals in chemistry and thermodynamics, particularly those exploring the properties of ions in solution and the concept of entropy.

Nikitin
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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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