Thermodynamic potentials are state functions that describe the system's state as a function of thermodynamic variables. Internal energy is a function of entropy and volume. The temperature and pressure can be expressed as a partial differential of internal energy at constant volume and entropy, respectively. Enthalpy equals the sum of the system's internal energy and the product of its pressure and volume. Differentiating and substituting dU reduces enthalpy as a function of entropy and pressure. So, temperature and volume are obtained as the partial differentials of enthalpy. Helmholtz free energy is given as internal energy minus the product of the temperature and the system's entropy. For an infinitesimal change, the equation becomes a function of volume and temperature. So, entropy and pressure are determined using the partial differential of Helmholtz free energy. Replacing internal energy with enthalpy converts the Helmholtz free energy into the Gibbs free energy. Differentiating and simplifying further reduces the equation to a function of temperature and pressure. Similar calculations express entropy and volume as a partial derivative of Gibbs free energy.