1. Calculating heat changes
Section 11.4

2. After reading Section 11.4, you should know:
What Hess’s Law is
How to apply Hess’s Law to a chemical equation
How to calculate the heat of formation for a given chemical equation

3. Hess’s Law
The heat of 2 reactions added together will equal the heat of a 3rd reaction
Adding 2 equations together…you can cancel compounds out IF they are exactly the same on the reactants and products side
2Al + 11/2 O2 --> Al2O  H = kJ
Fe2O3 --> 2Fe /2 O  H = kJ
2Al + Fe2O3 --> 2Fe + Al2O  H = kJ

4. Hf = standard heat of formation
Hf = standard heat of formation
Change in enthalpy when 1 mol of new compounds form under standard conditions
Table of values on page 316
Ex: Calculate the H0 for NO(g) + O2 --> 2NO2
Hf NO = kJ / mol
Hf O2 = 0.0 kJ / mol
Hf NO2 = kJ / mol

5. You should have gotten: -113.04 kJ / mol
1. Find sum of Hf reactants
Remember, you have 2 moles of NO
2. Find sum of Hf products
3. Find the difference between Hf products and Hf reactants
You should have gotten: kJ / mol

6. After reading Section 11.4, you should know:
What Hess’s Law is
How to apply Hess’s Law to a chemical equation
How to calculate the heat of formation for a given chemical equation