This lecture introduces the field of chemical thermodynamics, focusing on state variables and their molecular interpretation. It covers the concept of internal energy, the first law of thermodynamics, and the relationship between state functions. The lecture delves into the definition of enthalpy, its calculation, and its significance in chemical reactions. Various types of reactions, such as exothermic and endothermic processes, are discussed, along with the enthalpy of state changes and the enthalpy of formation. The application of Hess's law in determining enthalpy changes in reactions is also explored.