Chemistry / Unit 4 Chem 0709 worked example, 60 to 90 seconds
Stepwise Hess's Law Addition
The idea
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.
In practice
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.
Where people slip
Leaving an intermediate species like FeCl2 uncanceled anywhere in the summed equation signals the reactions were not combined correctly for the intended target.
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
Builds on
Something wrong in this lesson? Flag it · Sources for Chemistry