Hess's Law

Enthalpy does not depend on the reaction method. In other words, when going from one set of reactants to the product, the total enthalpy change will be the same as if you had gone through an intermediate reaction before acheiving the final product. Here is a reaction that shows this:

N₂ + 2O₂ -->2NO₂        H = 68 kJ
The product (2NO₂), can also be achieved by taking these reactions:

N₂ + O₂ --> 2NO        H = 180 kJ
2NO + O₂ --> 2NO₂        H = -112 kJ

If you "added" these two reactions, you'd end up with:
N₂ + 2O₂ -->2NO₂, because the 2NO gets canceled out. Now if you add the H, you end up with a total H of 68 kJ. Hey, that's the same number we got from the one single reaction up top!! This principle is called Hess's Law.

In order to use Hess's Law, there are two important rules that must be used:

1. If a reaction is reversed (products become reactants and vice versa), the sign of H is reversed.
   
   N₂ + 2O₂ -->2NO₂        H = 180 kJ
   2NO₂ --> N₂ + 2O₂        H = -180 kJ
   
2. The magnitude of H is directly proportional to how much reactant and product you have. So, if an entire reaction is multiplied by two, the H is also doubled.
   
   N₂ + 2O₂ -->2NO₂        H = 180 kJ
   2N₂ + 4O₂ -->4NO₂        H = 360 kJ