Unit 5: Thermochemistry

Section 3: 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