This document discusses the concept of Hess's law in thermodynamics. It explains that Hess's law states that the enthalpy change of a reaction is the same whether the reaction occurs in one step or multiple steps. It provides examples of calculating enthalpy changes of reactions using Hess's law. Important notes are provided on applying Hess's law, such as reversing the sign of enthalpy changes when reversing reactions or multiplying/dividing reactions.