I carried out the DFT calculations using Gaussian. By varying the pressures (.8GPa, 5GPa,8GPa...) , I coud observe that there is a change in value the following parameters.
- Thermal correction to Gibbs Free Energy
- Thermal energy. specific heat capacity and entropy caused by translation .
- also. no change in the Thermal energy. specific heat capacity and entropy caused by rotational and vibrational motion
can anyone give a clear explanation
Dear Jenepha, Arrhenius equation explains your results, as the gas constant (R) directly influences the solid-gas interface and the sample's vapor pressure, which impacts the thermodynamic parameters.
This question can be answered on a lot of different levels. I'll give a few ideas and you can decide where to go with the discussion.
Phenomenologically, in classical (chemical) thermodynamics the total differential of G is dG=Vdp-SdT. Therefore, at constant temperature, an increase of the pressure should lead to an increase of G. If that is the case for you, everything is stil normal.
Also, the heat capacity is directly connected to G, so if G changes at the same T, the heat capacity should also change.
Now let's go to the microscopic level. For a meaningful discussion, it would be good to know if you perform your statistical thermodynamics on single-molecule data or whether you are using an ensemble, maybe in some sort of a supercell approach. If you have a scenario in which intermolecular interactions play a role, it is clear that the translation will have to change since additional binding energies start playing a role. The vibrations, on the other hand, will barely be affected, the major effect in reality would probably be that at a higher pressure the equilibrium state might be reached faster due to more collisions. Discussing entropic changes without knowing the actual scenario is a bit difficult.
This last paragraph was, of course, quite a "random facts compilation" and every one of these sentences could be expanded into a full textbook chapter, so please let us know what topic you want to discuss in more detail.
i) For RG discussions 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
https://www.researchgate.net/post/Could-anyone-please-tell-me-how-to-determine-the-enthalpy-of-solution-from-henrys-constant-at-different-temperatures
ii) Elsewhere at this forum, I have 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
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