Measuring Enthalpy Changes
We can measure an enthalpy change by determining the amount of heat involved in a reaction when the only work done is PV work.
Calculating Enthalpy Changes
Enthalpy changes are calculated using Hess's law: If a process can be written as the sum of several steps, the enthalpy change of the process equals the sum of the enthalpy changes of the individual steps.
Using the enthalpy change for the reaction of Fe with Cl2 to give FeCl2 and the enthalpy change for the reaction of FeCl2 with Cl2 to give FeCl3, we can determine the enthalpy change for the reaction of Fe with Cl2 to give FeCl3.
Many handbooks and textbooks have tables of standard enthalpies of formation.
Note: As an approximation, introductory courses assume that enthalpy changes do not change with temperature.