The relationships between physical quantities determining thermodynamic state of an ideal gas are analysed in the paper. It will be proved that such relationships can be obtained based on convex analysis of the formula describing specific energy of the gas. This formula is expressed as a functional with specific volume and specific entropy being its constrains. The resulting constitutive equations of ideal gas, determining pressure and temperature as a function of entropy and specific volume, are given. It will be proved that the equation of the ideal gas formulated by Clapeyron, can be easy obtain from the constitutive equations, eliminating the variable describing entropy. Moreover, it will be shown that the functional of specific energy of ideal gas is convex. Because of this mathematical property of the functional, Legendre transform is used in order to determine three conjugated functionals, i.e. enthalpy, free enthalpy and free energy. The method of description of thermodynamic relationships to be introduced herein, differs from well-known classic handbook’s presentations. Moreover, it permits a better understanding of the structure of thermodynamics equations.
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