Re: Checking species
Evgenii Rudnyi
> 3. Finally, I entered the same parameters in the Reference object
> (files: refNH3.mod and outNH3.mod) as well as the associated_solution
> object (file: NH3_ass1.mod and out.mod). None of them delivered
> satisfactory results when I let tdlib do the computation.
Let us first check information that you have entered for three species.
Along this way I will demonstrate some useful features. See
testspecies.mod.
1) Almost any objects can have an attribute id. Then later you can refer
to these objects by this id. I have inserted ids to species and now I was
able to check the data that you have entered directly.
2) When TDLIB looks for the phase id and there is no such phase, it tries
to find a species with such id and if successful it promotes this species
to the point phase. A point phase is actually a species.
This way I can print tables for all species without big changes.
When I have done it, I have seen one problem. All species reproduces the
tables from the NIST Web site. However, this is not enough. You have
forgotten about the enthalpy of formation.
What you compute is actually G – H298. However, H298(H2) != H298(N2) !=
H298(NH3): these are three different values. This means that with this
information you can compute correctly only the change in entropy, as one
can determine an absolute entropy. And you cannot compute the enthalpy of
reaction and the Gibbs energy of reaction correctly.
Well, with your data TDLIB will compute that DelH of reaction is zero at
298.15, as TDLIB just does it as DelH = 2 H(NH3) – H(N2) – 3H(H2). This
means that TDLIB treats enthalpies and Gibbs energy as “absolute”.
In order to solve the problem above one introduces the enthalpy of
formation and it should be added. One chooses some state of elements at
H298 as the reference and refers the enthalpy to this reference.
Pragmatically speaking one takes H298 for elements in the standard state
as 0, and then H(298) as the enthalpy of formation for other compounds.
This means that we have to add the enthalpy of formation to NH3 and this
is what I have done. Note that in TDLIB you can use expression, for
example -7408.61-45940. They are evaluated at input.
Best wishes,
Evgenii