Thermodynamic Functions and Variables

In thermodynamics, each state of a system is characterized by the value given by a certain characteristic function, which depends on a certain number of variables the state variables. The most commonly cited variables include the internal energy U, enthalpy H, Helmholtz energy F and Gibbs energy G. Therefore, the complete properties of a system are known if one of the characteristic functions is known according to the variables chosen to define the problem. 

These characteristic functions also possess two properties, according to the two first principles of thermodynamics  

If the system is composed of several phases, the value of the characteristic function of the overall system is the sum of the values of this function for each phase. 

Phase Modeling Tools. Applications to Gases, First Edition. Michel Soustelle. ISTE Ltd 2015. Published by ISTE Ltd and John Wiley Sons, Inc. 

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A fundamental aspect of our MD treatment is its acceptance or assumption of the existence, in each surface parallel to the electrode, of local thermodynamic functions and variables and of thermodynamic properties for all species whose individual molecules or ions are centered in that surface. This assumption is equivalent to the statement that we consider only systems for which the Gibbs entropy-balance equation holds locally. In some regions of the interphase between the bulk of the metal and the bulk of... 

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