Stepwise Hess's Law Addition

A stepwise Hess's-law calculation adds two or more known reactions together, canceling any species that appears on both sides of the combined total, so the resulting sum equation exactly matches the target reaction, then sums the corresponding known ΔH values to reach the target's own enthalpy change.

Iron reacting with chlorine gas to form FeCl3 can be built from a known reaction forming FeCl2 directly, plus a second known reaction converting that FeCl2 further into FeCl3. Adding these two equations cancels the intermediate FeCl2 completely, leaving only the overall formation of FeCl3 from iron and chlorine. Adding their two known ΔH values in the same way gives a formation enthalpy for FeCl3 near −399.5 kJ per mole.