How can entropy be negative?

Trowa

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!

Mapes

What reference are you looking at?

Trowa

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

Do you know more about negative entropy?

Mapes

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.

Trowa

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!