Hess's Law

The Reaction Enthalpy , ΔHrxn, is the sum of the enthalpies of any sequence of reactions (at the same temperature and pressure)
into which the overall reaction can be divided.

Using Hess's Law:
Reverse reaction
 <–––> Change sign of ΔH
Multiply reaction by a number
 <–––> Multiply ΔH by same number.
Add reactions
 <–––> Add ΔH's
Can calculate ΔHrxn from any convenient series of steps even if intermediate steps cannot be carried out !
Why does this work ?