where the reference species are graphite and H 2 . Accordingly, the ∆ f H of the standard reference species are 0 since it is a null reaction. Experimental ∆ f H can only be obtained directly for a limited set of species such as H 2 O (oxidation of H 2 ) and CO 2 (oxidation of graphitic C). In nearly all other cases, ∆ f H are derived from other measured reaction enthalpies, using the fact that the enthalpy is a state variable. Hess's law states that ∆ f H of a species is invariant to the reaction through which it is formed. This law can be exploited since it is relatively easy to measure the combustion enthalpy of C 2 H 6 in a bomb calorimeter.