’s law allows you to find the net enthalpy of the magnesium combustion by adding the enthalpies of intermediate reactions.

Hess's law allows you to find the net enthalpy of the magnesium combustion by adding the enthalpies of intermediate reactions. The Hess's Law of Constant Heat Summation states that the total enthalpy change for the reaction is the sum of all changes (no matter how many stages the reaction has).

Answer is: Hess's law

Hess's Law of Constant Heat Summation, or just Hess's Law, states that the enthalpies of several intermediate reactions will be equal to the total net enthalpy of the reaction. This is due to the conservation of thermal energy across reactions.