DF2- Hess cycles 

-       In some cases its not possible to measure the enthalpy change of a reaction directly so Hess cycles are used.

Hess’s law- the enthalpy change for any reaction is independent of the intermediate stages, so long as the initial and final conditions are the same for each route.

Hess cycle for enthalpy change using combustion data

Hess cycle for enthaply change using formation data