This lecture focuses on the study of thermodynamic potentials and their applications in understanding systems in contact with reservoirs. The instructor begins by discussing the theoretical aspects of thermodynamic potentials, emphasizing their significance when a system interacts with a reservoir, which maintains constant intensive properties like temperature and pressure. The lecture illustrates how to analyze a system composed of subsystems that may not be in equilibrium but are in partial equilibrium with the reservoir. The instructor explains the irreversible evolution of the system when subsystems have different pressures, leading to a decrease in a thermodynamic potential towards a minimum value at equilibrium. The concepts of energy free and enthalpy are introduced, detailing how these potentials guide the system towards equilibrium. The lecture also covers the implications of maintaining constant temperature and pressure during processes, highlighting the importance of Gibbs free energy in characterizing the system's behavior. The instructor concludes with practical examples and experiments to demonstrate these principles in action.