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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: angiems on January 07, 2005, 03:21:13 PM

Title: Equilibrium Constants
Post by: angiems on January 07, 2005, 03:21:13 PM

Does anyone know the difference between thermodynamic and practical equilibrium constants?  I can't seem to find much anywhere :(

Title: Re:Equilibrium Constants
Post by: Mitch on January 07, 2005, 06:20:42 PM

 ???

Title: Re:Equilibrium Constants
Post by: dexangeles on January 07, 2005, 07:19:04 PM

huh???  isn't equilibrium constant = k = products/reactants (on any reaction)

Title: Re:Equilibrium Constants
Post by: Demotivator on January 07, 2005, 11:43:50 PM

The question is a bit confused. The so called "practical" equil const and thermodynamic equil const are the same constant. It's all thermodynamics.

Perhaps by "practical" she means, by direct measurement of the concentrations of species to solve for K.

Perhaps thermodynamic means:
One can obtain K indirectly by measuring the thermodynamic properties of the system.
DG = DG(0) + RT ln K = -nfe(0) + RT lnK
at equilibrium DG = 0, so
nfe(0) = RT lnK  meaning K can be meaured through the standard emf.
Or by thermochemistry:
DG(0) = DH(0) - TdS(0) = RTlnK

Title: Re:Equilibrium Constants
Post by: Tiger on January 08, 2005, 12:08:52 AM

The practical one is probably one that ignores minor effects like species
with a small conc.
Thermodynamic is exact.
I have never heard of a practical one before.

Title: Re:Equilibrium Constants
Post by: angiems on January 08, 2005, 10:19:38 PM

Rereading the text (for some reason, the first fifteen times I read this part, I did NOT catch this), it states that a thermodynamic equilibrium constant is unitless, while a practical equilibrium constant has units.

So... I don't think any of you is right, per se, at least not in what they were getting at, but thanks a lot for the help :D