Why Does OH- Have a Negative Standard Molar Entropy?

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Homework Help Overview

The discussion revolves around the concept of standard molar entropy, specifically focusing on the negative standard molar entropy of the hydroxide ion (OH-). Participants are exploring the implications of this property in the context of physical chemistry.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants are questioning the nature of negative entropy and its implications. There are inquiries about the references being used and discussions about the context of entropy in solution versus in isolation.

Discussion Status

Some participants have provided insights regarding the relationship between the hydroxide ion and its behavior in solution, suggesting that the addition of OH- to water may increase order, which could explain the observed negative entropy. However, there is no explicit consensus on the interpretation of these findings.

Contextual Notes

There is mention of a specific reference (Atkins' Physical Chemistry) and the context of an exam question, indicating that the discussion is grounded in academic study. Participants are also considering the implications of molecular interactions in solution.

Trowa
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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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