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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: spirochete on March 01, 2009, 08:20:12 PM

Title: Why is complexation of Aluminum by EDTA thermodynamically favorable?
Post by: spirochete on March 01, 2009, 08:20:12 PM

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.

Title: Re: Why is complexation of Aluminum by EDTA thermodynamically favorable?
Post by: Borek on March 02, 2009, 02:51:26 AM

Quote from: spirochete on March 01, 2009, 08:20:12 PM

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-  ::equil:: AlEDTA^-

Title: Re: Why is complexation of Aluminum by EDTA thermodynamically favorable?
Post by: spirochete on March 02, 2009, 09:16:54 AM

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?

Title: Re: Why is complexation of Aluminum by EDTA thermodynamically favorable?
Post by: spirochete on March 02, 2009, 10:19:57 AM

there's a typo there but you get the point

Title: Re: Why is complexation of Aluminum by EDTA thermodynamically favorable?
Post by: Borek on March 02, 2009, 02:12:29 PM

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

Title: Re: Why is complexation of Aluminum by EDTA thermodynamically favorable?
Post by: spirochete on March 02, 2009, 02:34:19 PM

If that's the case then I'm stumped. 

Title: Re: Why is complexation of Aluminum by EDTA thermodynamically favorable?
Post by: Borek on March 02, 2009, 04:29:31 PM

Small, highly charged cation in water...

Title: Re: Why is complexation of Aluminum by EDTA thermodynamically favorable?
Post by: spirochete on March 02, 2009, 05:20:15 PM

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.

Title: Re: Why is complexation of Aluminum by EDTA thermodynamically favorable?
Post by: Borek on March 02, 2009, 05:30:07 PM

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

Title: Re: Why is complexation of Aluminum by EDTA thermodynamically favorable?
Post by: spirochete on March 02, 2009, 05:39:59 PM

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

edit:  Thank you!