Hess's Law

Also called the law of constant heat summation: the total enthalpy change for a chemical process between defined initial and final states is independent of the pathway taken and equals the sum of enthalpy changes of constituent steps.

The enthalpy change (ΔH) for a chemical transformation between fixed initial and final states is independent of the reaction pathway; therefore the enthalpy change for an overall reaction equals the algebraic sum of the enthalpy changes of any sequence of intermediate steps connecting the same states, provided pressure and reference states are specified.