Why Does OH- Have a Negative Standard Molar Entropy?

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The discussion centers on the negative standard molar entropy of the hydroxide ion (OH-), contrasting it with the positive values of H+ and H2O. Participants reference the textbook "Physical Chemistry" by Atkins, which suggests that the negative entropy arises from the increased order when OH- is added to water. This phenomenon may be due to the strong repulsion between anions in solution, leading to a lower entropy compared to a hypothetical isolated state. The conversation highlights the complexities of measuring entropy in different states and solutions. Understanding these concepts is crucial for grasping the behavior of ions in aqueous environments.
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Hi there!

The Standard molar entropy of H+ and H2O are positive.
But OH- has a negative standard molar entropy.
Can somebody explain why the standard molar entropy of OH- is negative?

I just can't imagine how entropy could be negative.

Thanx in advance!
 
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What reference are you looking at?
 
Mapes said:
What reference are you looking at?

Physical chemistry (Atkins) - 8th edition
And this question appeared on my exam.

Do you know more about negative entropy?
 
It looks like Atkins is talking about the partial molar entropy of the molecule in solution; the addition of the molecule to water increases order.
 
That makes sense; pure OH- could than still be positive but that is hard to isolate. Maybe the entropy is lower in water because the anions can be closer together than without solution (strong repulsion) (basically: the reference point must be higher in entropy).

Thanx!
 

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