This lecture covers the definition of equilibrium, reaction quotient, and equilibrium constants. It explains the thermodynamic aspects of equilibrium, such as free enthalpy and entropy variations. The lecture also delves into the concept of dynamic equilibrium, activity of chemical species, and the equilibrium constant. It discusses the relationship between reaction quotient and equilibrium constant, as well as the evolution of reactions towards product formation. The lecture concludes with the calculation of equilibrium constants in terms of concentrations and partial pressures, emphasizing the thermodynamic equilibrium constant.