This episode explains Hess's Law, a method for calculating the enthalpy change of a reaction by treating it as a sum of other known reactions. It presents three key rules for manipulating these equations: reversing an equation changes the sign of its ΔH, multiplying an equation by a coefficient requires multiplying its ΔH by the same number, and adding equations together means adding their ΔH values. By applying these rules to a sample problem, it demonstrates how to combine a series of reactions to find the ΔH for a target reaction, emphasizing that this method is based on enthalpy being a state function—its final value is independent of the path taken.