It depends on a reaction. Commonly, this is not a trivial job. Plz, give an example of your reaction.

Optimize all of your reactants and products in the solvent medium you want to report. Then, you just calculate the frequency of the optimized structures. From frequency output, you will get the data that you have to put in trivial thermodynamic equations.

Suvendu Paul It would be interesting to see how you solve this "trivial" problem in the "real life"

Geletii Sir,

I described the method to obtain Gibbs Free Energy theoretically especially using Gaussian 09 software package.

Any book on chemical or process thermodynamics. Such information can also be found in books on chemical reactor and reaction engineering. Regards,

See section Thermodynamics in the attached file. Gibbs energy p.916

Dear Suvendu Paul,

You mention that it is possible to obtain Gibbs Free Energy theoretically using Gaussian 09 software package. This method is well known to calculate Enthalpy but the authors do not calculate the more important Gibbs energy. Could you give me more information.

Thank you

Guy

The so-called "entropy " doesn't exist at all.

During the process of deriving the so-called entropy, in fact, ΔQ/T can not be turned into dQ/T. That is, the so-called "entropy " doesn't exist at all.

The so-called entropy was such a concept that was derived by mistake in history.

It is well known that calculus has a definition,

any theory should follow the same principle of calculus; thermodynamics, of course, is no exception, for there's no other calculus at all, this is common sense.

Based on the definition of calculus, we know:

to the definite integral ∫T f(T)dQ, only when Q=F(T), ∫T f(T)dQ=∫T f(T)dF(T) is meaningful.

As long as Q is not a single-valued function of T, namely, Q=F( T, X, …), then,

∫T f(T)dQ=∫T f(T)dF(T, X, …) is meaningless.

1) Now, on the one hand, we all know that Q is not a single-valued function of T, this alone is enough to determine that the definite integral ∫T f(T)dQ=∫T 1/TdQ is meaningless.

2) On the other hand, In fact, Q=f(P, V, T), then

∫T 1/TdQ = ∫T 1/Tdf(T, V, P)= ∫T dF(T, V, P) is certainly meaningless. ( in ∫T , T is subscript ).

We know that dQ/T is used for the definite integral ∫T 1/TdQ, while ∫T 1/TdQ is meaningless, so, ΔQ/T can not be turned into dQ/T at all.

that is, the so-called "entropy " doesn't exist at all.

.

.

.

Why did the wrong "entropy" appear ?

In summary , this was due to the following two reasons:

1) Physically, people didn't know Q=f(P, V, T).

2) Mathematically, people didn't know AΔB couldn‘t become AdB directely .

If people knew any one of them, the mistake of entropy would not happen in history.

Please read my paper and those answers of the questions related to my paper in my Projects.

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

To reiterate other responses, it depends on the reaction. Using equilibrium constants is one means, using statistical thermodynamics and spectrospcopy data is another method. Enthalpies and entropies can be determine through van't Hoff plots to then determine free energies.

There are many good texts and websites. For example, the Khan academy has great tutorials. https://www.khanacademy.org/science/chemistry/thermodynamics-chemistry/gibbs-free-energy/v/gibbs-free-energy-example.

Thermodynamics by Lewis and Randell is a great text

Dear Matthew F. Ryan

I would like to thank you for your kind attention and nice answer.

Regards

It is always better to experimentally measure Gibbs formation energies as indicated by Matthew F. Ryan but this is not always possible. For unstable compounds in the gas phase, theoretical calculations using Gaussian 09 software package give the formation enthalpies with a good approximation. However, the authors of this kind of calculations give enthalpy values but no entropy, which does not allow to calculate equilibrium constants. There are reactions driven by entropy. How can entropy be theoretically calculated and why do not these authors do it?

Several interesting 'case studies' dealing with the thermochemistry of chemical reactions have been presented for discussion at this forum, to which you may possibly find of some interest to look at (e.g.): https://www.researchgate.net/post/What_is_the_heat_reaction_for_Ammonia_gas_scrubbing_in_60_nitric_acid_solution https://www.researchgate.net/post/How_do_I_determine_what_the_degree_of_dissociation_is_for_hydrogen_molecule_in_561K_and_88_atm_in_water https://www.researchgate.net/post/how_can_we_calculate_gamma_specific_heat_ratio_for_real_gas_chemical_non-equili https://www.researchgate.net/post/How_would_you_calculate_the_heat_capacity_of_a_1_wt_KOH_and_13_Ligin_solution_I_have_the_cps_of_dry_KOH_and_Ligin_can_I_sum_up_by_relative_weight

https://www.researchgate.net/post/Can_I_simply_dilute_25_NH3_solution_to_10_solution_by_pouring_water_in_tank_and_circulating_with_pump

If you can measure an equilibrium constant anyngen book will take you through the process Gibbs free energy a powerful tool in chemistry. Also check out vant off plots.

The easiest is in the case of chemical reactions in a gas system, where the starting point is the knowledge of the specific heats of individual reactants as a function of temperature. The correct method is also to use the equilibrium constants, however, in many processes, determining a reliable value of the equilibrium constant is not an easy task.