Relationships Between Gibbs Free Energy and Other Thermodynamic Magnitudes

4 Relationships Between Gibbs Free Energy and Other Thermodynamic Magnitudes 

The internal energy of a system is a state function —i.e., the variation of the internal energy of the system when passing from state A to state B does not depend on the reaction path but only on initial and final state conditions (cf appendix 2). 

For a crystalline phase, we have already seen that internal energy corresponds to lattice energy at the zero point. At higher T, the internal energy of the phase increases as a result of the increase in vibrational motion of all the atoms in the lattice. 

We have also seen (in chapter 1) that enthalpy and lattice energy are related through the Born-Haber-Fayans thermochemical cycle, on the basis of the energy additivity principle of Hess. The enthalpy or heat content of a phase H) is composed of the internal energy U at the T of interest and the PV product  

The Gibbs free energy of a phase (or a system) is composed of its enthalpy minus the TS product  

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