This lecture covers the second law of thermodynamics, the impossibility of converting heat entirely into work, the spontaneous flow of heat from hot to cold, the differences between quasi-static and out-of-equilibrium evolution, the concept of irreversibility, and the prediction of irreversible transformations. It also discusses the writing of elementary heat exchanges in the form of Tdx, the associated extensive quantity, and the relationship between entropy, heat exchanges, and work. The lecture includes examples such as the Carnot cycle and Clapeyron diagram, emphasizing the importance of understanding irreversible transformations and predicting their outcomes.