Hello Azd,

In the table you have shown, enthalpy and Gibbs energy are represented as changes due to the formation of the molecules from their elemental atoms. The entropy is the absolute entropy, and not the entropy due to the formation of the molecules from elemental atoms. In the first example you show (the red slide) the entropy that is given is the change in entropy upon creating the species from elements (i.e. it is the entropy of formation, not the absolute entropy). So, you are given an "S" in the bottom slide (green slide) but you need a "Delta_S" if you want to use Delta_G = Delta_H-T*Delta_S.

Here I used Delta to refer to the change in energy in forming species from elements.

There is probably a better way to getting the Gibbs energy at a different temperature than I am about to show you. But, it should still be okay.

Keq =- RTln(delta_G), so you can calculate the equilibrium constant at the current reference temperature with the information you have already. Then, you can use the van't Hoff equation to calculate the Keq at a different  temperature, and then convert that Keq to the Delta_G at that temperature (i.e. delta_G = exp[-Keq/RT]).

The van't Hoff equation essentially has a hard way, and a very common shortcut way. The shortcut way is this: ln(Keq2/Keq1) = -Delta_H/R *( 1/T2 - 1/T1)

This can be rearranged to get Keq2 = Keq1*exp[-Delta_H/R *( 1/T2 - 1/T1)]. The shortcut assumes that the Delta_H is a constant value(i.e. doesn't depend on temperature, so use the given Delta_H at the reference conditions), Thus, as your temperature difference gets larger, the assumption gets worse. That said, the shortcut way is quite common.

So in short. Calculate Keq1, then calculate Keq2 using the van't Hoff equation, then calculate Delta_G @T2 using Keq2.

Here is a wikipedia link to the van't hoff if you want to try the longer version of the van't hoff.

https://en.wikipedia.org/wiki/Van_'t_Hoff_equation

Cheers

Dear  @Braden Kelly,

Thanks for your informative sufficient answer.

Sincerely,

Azd.

Hey Azd,

I have had a chance to look over my thermo textbook, and while the way I have shown using the Keq (equilibrium constants) is correct, it is faster to just use: Delta_G1/(R*T1) - Delta_G2/(R*T2) = -Delta_H1/R*(1/T2-1/T1) or rearranging to be more direct

• Delta_G2/(R*T2) = Delta_G1/(R*T1) + Delta_H1/R*(1/T2-1/T1)

Again this form assumes that Delta_H does not depend on temperature, which is probably a reasonable assumption for all but very high temperatures of your components of interest.

All I have done here is realize that 

• ln(Keq2/Keq1)= Delta_G1/(R*T1) - Delta_G2/(R*T2)

and replaced the left hand side of the equation I gave you earlier with the equivalent but more direct right hand side of the equation I have just given.

Cheers,

Braden

Entropy: A concept that is not a physical quantity

https://www.researchgate.net/publication/230554936_Entropy_A_concept_that_is_not_a_physical_quantity

The changes in H and S can be determined here based on the specific heat of the reactants as a function of temperature. As a result, the determined change in G will be a function of temperature.

i) For a RG discussion concerning to correlations used to express the temperature dependency of the equilibrium constant:  https://www.researchgate.net/post/How_to_determine_coefficients_for_equilibrium_constants

ii) Elsewhere at this forum, I have discussed the apparent possiblity of high temperature formation of hydrogen cyanide (HCN) after CHi radicals (i = 1, 2, 3) reacting with (gaseous) nitrogen species, possibly involving heterogeneous catalysis:

https://www.researchgate.net/post/formation-of-cyanide-in-vacuum-sintering-in-high-temps-like-2000-C-with-nitrogen-atmosphere

iii) I have also discussed the case of high temperature dissociation of H2 to atomic H for a (previous) gas misture of H2 and He ― while specifically adressing its heat capacity ratio (γ = Cp/Cv):  https://www.researchgate.net/post/how_can_we_calculate_gamma_specific_heat_ratio_for_real_gas_chemical_non-equili/1

iv) About the reaction enthalpy for CH3F → CH3+ + F-:

https://www.researchgate.net/post/What_basis_set_and_method_to_calculate_reaction_mechanism_of_CH3F_CH3_F