# Calculating heat changes

## Presentation on theme: "Calculating heat changes"— Presentation transcript:

Calculating heat changes
Section 11.4

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

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

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

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

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

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