The enthalpy change for a chemical reaction leading from reactants to products is the same no matter which route is chosen for the reaction. That is the enthalpy change going from reactants to intermediates1 then to products is the same as for going from reactants to intermediates2 then to products.

Addition of chemical equations can lead to a net equation. If energy is included for each equation and added, the result will be the energy for the net equation.

Hess's Law says that heat of enthalpy are additive.
ΔH(net) = ΣΔH(r), equals the sum of enthalpies of reaction.
Two rules:
* If the chemical equation is reversed, the sign of ΔH changes.
* If coefficients are multiplied, multiply ΔH by the same amount.