CCL: Standard state conversions

 Sent to CCL by: Christopher Cramer [cramer#,#umn.edu]
 Kathrin,
    It is conventional to tabulate thermodynamic quantities with gases taken to
 be at a 1 bar standard state pressure and solutes at a 1 M standard state
 concentration, but there are many, many exceptions to be found in the
 literature, so in general one should simply be extremely careful to be specific
 about what one is doing (and about what one is comparing to).
    You’ve summarized the 1 bar —> 1 M free energy change
 nicely. Note that another key wrinkle is when one of the species in solution is
 buffered to a concentration other than 1 M (in which case the standard-state
 tabulation typically employs the buffer concentration — this is often
 true for electrochemical reactions that generate or consume ions, for example).
 And, there is the case where one of the solutes is also the SOLVENT (e.g., water
 is 55.6 M in water, not 1 M).
    The standard-state symbol can sometimes seem ambiguous, and ALWAYS merits
 close scrutiny with respect to its meaning. With apologies to Lewis Carroll, one
 should approach it bearing in mind: "When I use a [standard-state
 symbol]," Humpty Dumpty said, in rather a scornful tone, "it means
 just what I choose it to mean—neither more nor less.”
 Best regards,
 Chris
 On May 1, 2014, at 13:53, Kathrin Helen Hopmann kathrin.hopmann(a)uit.no
 <owner-chemistry(-)ccl.net> wrote:
 >
 > Sent to CCL by: "Kathrin Helen  Hopmann"
 [kathrin.hopmann[*]uit.no]
 >
 > Dear all,
 > I have a question regarding standard state conversion when computing free
 energies.
 > As far as I understand, if I compute a free energy with e.g. DFT (in gas
 phase or also using PCM), the energy will be a standard state free energy at 1
 atm. If I wish to convert to a solution reaction, i.e. a 1 M standard state, a
 correction term has to be added to each species amounting to +1.89 kcal/mol at
 298 K. If the number of moles remains the same during the reaction, this
 correction cancels out, but if I have a reaction of the type:
 > A + B -> C
 > the correction term will not cancel out and the overall reaction energy
 will in this case be lowered by 1.89 kcal/mol (compared to the computed value).
 >
 > I have the following question:  What if the reacting species have different
 standard states? E.g. if A is a gas (such as H2) reacting with B, which is
 dissolved in a solvent, to form complex C (also dissolved):
 > A(g) + B(aq) -> C(aq)
 > If I compute the free energies for the species in this reaction with DFT
 (all in 1 atm standard state) and I wish to do a standard state conversion to
 obtain the free energy for this reaction in solution, would the correction term
 of 1.89 kcal/mol be applied to A, the gas?
 > I appreciate your thoughts on this.
 >
 > best regards
 > Kathrin H. Hopmann
 > CTCC, University of Troms, Norway
 > kathrin.hopmann^uit.no>
 >
 --
 Christopher J. Cramer
 Elmore H. Northey Professor and
   Associate Dean for Academic Affairs
 University of Minnesota
 Department of Chemistry and
   College of Science & Engineering
 Minneapolis, MN 55455-0431
 Phone:  (612) 624-0859 (Chemistry)
 Phone:  (612) 624-9371 (CSE)
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