1. Unit 5: Thermochemistry
Bond Energy

2. Bond Energy Breaking chemical bonds requires energy
Forming new chemical bonds releases energy

3. Calculating Enthalpy from Bond Energy
Recall that enthalpy (H) is the heat energy absorbed or lost in a reaction
To determine H from bond energies:
H = ΣHbonds broken – ΣHbonds formed
H = ΣHreactant bonds– ΣHproduct bonds
Used when H of the entire molecule is not known – this is just an approximation (although a good one)

4. Calculating Enthalpy from Bond Energy
How do you use the bond energies to calculate the enthalpy?
You must draw Lewis structures for each molecule!
Determine the total number of valence electrons
Make sure each atom has 8 valence electrons, either through lone pairs or bonds
Remember, hydrogen only has 2 valence electrons

5. Bond Energy Example #1 2 H2(g) + O2(g)  2 H2O(g)
Determine H for the following reaction:
2 H2(g) + O2(g)  2 H2O(g)

6. Bond Energy Example #1 2 H2(g) + O2(g)  2 H2O(g)
Determine H for the following reaction:
2 H2(g) + O2(g)  2 H2O(g)

7. CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)
Bond Energy Example #2
Calculate ΔH for the following reaction:
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)

8. Bond Energy Exit Slip 6H2 (g) + N2 (g) → 2NH3 (g)
Calculate ΔH for this reaction:
6H2 (g) + N2 (g) → 2NH3 (g)