What are the state functions telling me or how are they related to total energy?

Libby Owens

Libby Owens

Answered

2022-07-17

What are the state functions telling me or how are they related to total energy?

Answer & Explanation

Jeroronryca

Jeroronryca

Expert

2022-07-18Added 13 answers

"Total energy" is in fact, the internal energy U (unless you give a more precise definition).
We may consider also enthalpy H = U + p V, the Helmholtz free energy F = U T S, the Gibbs free energy G = U + p V T S
For instance, take Helmholtz free energy, we have (assuming the chemical potentials being zero) :
d U = p d V + T d S, and d F = p d V S d T
The natural variables associated to F are V and T, so if they are unchanged in some process, that means that F is constant in this process ( Δ F = 0). But in this same process, you have d U = T d S, so unless the process is isentropic, you will have a variation in Δ U 0),so you have, in the general case:
Δ F Δ U
A analog demonstration can be done for the enthalpy and the Gibbs free energy.

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