[spirochete]

The formation of the EDTA/Aluminum complex is endothermic, yet the complex is thermodynamically favored over free aluminum and EDTA.  Why is the formation constant for the EDTA/Aluminum complex 10^16?  In other words why is the complex thermodynamically favored.

EDTA is a hexadentate ligand if that helps.  I don't understand how something can be endothermic and thermodynamically favored if the entropy of the reaction is also so obviously disfavored.

[Borek]

don't understand how something can be endothermic and thermodynamically favored if the entropy of the reaction is also so obviously disfavored.

You must be missing something then. Perhaps the reaction is not as simple as

Al³⁺ + EDTA^4-  AlEDTA^-

[spirochete]

For charges to be neutral there would be a stoichiometry of 4 EDTA ^4-: 3Al(III)

Does each complex contain 4 aluminum and 3 EDTA?

[spirochete]

there's a typo there but you get the point

[Borek]

EDTA complexes are not neutral. EDTA is so called chelating ligand, it surrounds central ion. It always reacts 1:1 with metals.

[spirochete]

If that's the case then I'm stumped. 

[Borek]

Small, highly charged cation in water...

[spirochete]

So it's more favorable for water to solvate the complex than it is to solvate the aluminum ion and free edta?  I considered rationalizing with hard soft acid base theory but water is a hard base and Al3+ is a hard acid, meaning there should be a favorable interaction there.

In other words, even if the first thing I said is true it still doesn't make sense to me.

[Borek]

Think about entropy - several tightly bounded water molecules are replaced by one EDTA molecule.

[spirochete]

ooooh alright.  Well at least I thought of every wrong answer first.

edit:  Thank you!