Re: How can some aqueous ions be listed as having a negative entropy?

How can some aqueous ions be listed as having a negative entropy?

I thought that the smallest entropy assigned was zero (for a perfect crystal at zero Kelvin). How can some aqueous ions be assigned a negative value in thermodynamic tables?

Furthermore, the standard partial molar entropy of the aquated proton is defined as zero:

Relative to this, many ions have negative solution entropies.

Now solution is very different from the solid state; we are really comparing apples to oranges here. Even so, we can make some hand-waving statements on the order of "well, the apple is red and the orange is orange, and they have different detailed shapes."

A perfect crystal is in a highly ordered state, but its component ions are not imposing order on their surroundings, so to speak. However, in solution the ions are solvated. They are therefore imposing a great deal of order on their surroundings, compared to the random arrangements of water molecules in pure water. You have gone from pure water plus a perfect crystal, to a series of ordered aggregates of the form ion· nH₂O.

And remember that solvated H⁺ is assigned an entropy of zero. H⁺ is the smallest ion and so can't surround itself with as many water molecules as, say, Na⁺.