Why do some aqueous ions have negative standard entropy values?

[Conservation]

Certain aqueous ions have negative values for S^O, such as Ca²⁺ with -55.2 J/(K*mol); how is this possible when dissolution is usually an increase in entropy?

Also, all pure elements seem to have positive standard entropy value; why is this the case?

Thank you.

 

[Aceix]

Enthalpy or entropy?

Aceix..

 

[Conservation]

Oh jeez, I'm so sorry. I meant to say entropy. My bad.

 

[Bystander]

how is this possible when dissolution is usually an increase in entropy?

Start dropping solvent dipoles into an electrical potential well. There's more going on than just entropy of mixing.

 

[Conservation]

So is the dipole interaction between the solute and the solvent great enough that dissolution will result in an overall decrease in entropy despite the entropy of mixing?

 

[Bystander]

Can the charge-dipole interaction be that large? Yes. Is it always? No. That's what keeps measurement labs busy.

 

[Conservation]

Makes sense. Thank you.
Also, could you answer my other question about elements always having a positive standard enthalpy?

 

[Bystander]

elements always having a positive standard enthalpy?

Enthalpy? Again, you mean "entropy?" It's the integral of C_p/T from absolute zero to 298 K (or whatever T for the table).

 

[Conservation]

I really have to stop using the wrong term. I'm so sorry-the suggested threads about enthalpy at the bottom were throwing me off. :(
Okay, I think I see the integral idea now. Your explanation and this site (http://www.science.uwaterloo.ca/~cchieh/cact/applychem/entropy.html) was really helpful.

Thank you.