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Hello,

 

I'm looking for some help reconciling (or understanding) log K values for aqueous species with values I'm calculating from various references (Solutions, Minerals, and Equilibria - Garrels and Christ; Chemistry of Glasses - A. Paul). (As a disclaimer - my formal training is in nuclear science and I have little academic training in geochemistry, environmental chemistry or physical chemistry - and I'm hoping I'm just missing something about how these values are calculated.) For instance, the value for Zr++++ is set to log K (25°C) = -0.2385 in the thermo database, and there are no other log K's listed at other temperatures for this species (I need to simulate everything at 90°C). However, the delG0f is given as -125.350 kcal/mol (524.46 kJ/mol) in the header. Elsewhere (e.g., in Paul and in Garrels & Christ) the value is 142.0 kcal. That discrepancy aside, if I use the log K value given at 25°C in the database to back calculate the delG0f from the equation log K = -delG0f/2.303*R*T, I get a value of 1.3615 kJ/mol, which is significantly different from the other values listed. I have noticed this with a few other aqueous species; also, when I'm trying to add new species not found in the database using values I calculate, I think I'm getting skewed results in terms of activity diagrams and species not showing up or having extremely low activities when we are thinking they should be playing a larger role in the systems I'm investigating. This is especially true in terms of species that I'm adding to the database for which I'm using reference values for delG0f which yield log Ks that are significantly higher than those for other preexisting species in the database.

 

What am I missing about how these values are determined in the file and the discrepancies between the delG0f values from the literature? Is is "ok" to back calculate the delG0f from the log K given in the database and then use that same equation to calculate values for log K at different temperatures?

 

Please let me know when you have a chance...like I said, I feel like I'm missing a fundamental piece of knowledge here. Any help is very much appreciated.

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Ok...so after some more reading, I think I've realized where I've gone wrong. Are the log k's given for the aqueous species in the database calculated from the delG0's for the reactions (involving the basis species) and NOT DIRECTLY from the delG0f's? So, delG0-reaction = SUM (delG0f-products) - SUM (delG0f-reactants)?? If this is the case, then I understand where my large discrepancies were coming from and I believe that I can calculate the values at different temperatures by the simple method I described in the first post. Sorry for the confusion on my end....if what I wrote here is correct, things make much more sense now. If not, well....I better keep reading...

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Hi,

 

I think you're on the right track now. The delG0f's are properties of individual species, while the delG0-reaction depends on the stoichiometry of the reaction written between those species. The log K also depends on the reaction, not the individual species, so you need to figure out the delG0-reaction (as described in your second post) before you can calculate the log K for the reaction.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions LLC

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Hello again,

 

I'm not sure whether you're trying to go with part of the approach described in your first post. You cannot, with knowledge only of delG0f at 25 C, simply plug in a different temperature into the equation log K = -delG0-reaction/2.303*R*T (you had delG0-f in your first post) to get the log K at that temperature. You would need to know delGo-reaction for every temperature at which you want the log K. For more information, please see a reference like section 9.6, Change of K with temperature I, in Greg Anderson's Thermodynamics of Natural Systems.

 

Regards,

Brian

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Thanks, Brian, for the responses. Between your guidance here and the help of a colleague, I think I have this pinned down now. I'm calculating the enthalpies of the reactions and using the van't Hoff equation to find the log k's. For the purpose of what I'm trying to do, I'm assuming a constant delH0f over the temperature range I'm interested in. I was able to find access to the book you suggested, but found similar texts and they were very helpful. Thanks, again.

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