CCL: SPAM CCL: Standard state conversions
- From: "Kathrin Helen Hopmann"
- Subject: CCL: SPAM CCL: Standard state conversions
- Date: Fri, 2 May 2014 11:08:31 -0400
Sent to CCL by: "Kathrin Helen Hopmann" [kathrin.hopmann(-)uit.no]
Chris and Torstein - Thank you very much for your answers!
I believe you are right, I can probably use any standard state, as long as I
specify it clearly. However, the convention for solution reactions seems to be
1M standard state.
I have seen at least one paper where the correction term for the conversion 1atm
-> 1M seems to have been applied to H2 as well. I was wondering if it is
correct to do so, but I assume it is, as long as one states it clearly.
Something else that puzzles me is what to do about activation free energies
(barriers). Would the correction also be applied to barriers? I have seen people
do this, but I am unsure about it. Assume I compute this reaction:
A(aq) + B(aq) -> [TS_A-B](aq) -> C(aq) + D(aq)
Clearly, for the reaction free energy, the correction cancels out. But it would
not for the TS (which is a complex of A and B). But should the correction at all
be applied to the TS? A colleague of mine suggested that it would not be
meaningful to do so ... but I am unsure.
Kathrin H. Hopmann
University of Troms, CTCC, Norway
> "torstein.fjermestad a unito.it" wrote:
> Sent to CCL by: torstein.fjermestad:+:unito.it
> Dear Kathrin,
> This is the first time I answer a question to CCL so I hope I get it
> As far as I have understood, the standard state of a species is
> completely arbitrary. You can choose whatever you want. You just have to
> make sure that you specify the concentrations of the corresponding
> species in units of the chosen standard state. In your example
> A(g) + B(aq) -> C(aq)
> it would probably be most practical to choose 1 atm for A(g) and 1 M
> for B(aq) and C(aq). This is because the concentration of gases is
> usually given in atm while the concentration of dissolved species is
> usually given in M. However, it is completely arbitrary. You could for
> instance choose the standard state 2.248583 atm for A(g), 3.45 atm for
> B(aq) and 4.2345 M for C(aq). In that case make sure you specify the
> concentration of A(g) in units of 2.248583 atm, the concentration of
> B(aq) in units of 3.45 atm, and the concentration of C(aq) in units of
> 4.2345 M.
> To Prof. Cramer: please correct me if I am wrong.
> Torstein Fjermestad
> On 01.05.2014 20:53, Kathrin Helen Hopmann kathrin.hopmann(a)uit.no
> > Sent to CCL by: "Kathrin Helen Hopmann"
> > 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
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