1. Bond Energy and Reactions
AP Gr 11 Chem

2. Introduction
Bond Breaking – when two entities move apart. This requires energy
Bond Making – when two entities come together. Energy is released
Bond energy is the energy required to break a chemical bond; it is also the energy released when a bond is formed

3. Recall Endothermic Reactions Exothermic Reactions
If the energy required to break the bonds is greater than the energy released when bonds are formed, rH is positive
Exothermic Reactions
If the energy required to break the bonds is less than the energy released when bonds are formed, rH is negative

4. Bond Energy
is the amount of energy that would be required to break the bond.
Similarly, it is also the amount of energy that is released when the bond is formed
Every chem rxn involves breaking and forming bonds
Bond energies are considered to be “average” energies because the actual amount varies depending on the bonding environment.

5. Single bond < Double bond < Triple bond
The enthalpy change required to break a particular bond in one mole of gaseous molecules is the bond energy.
Bond Energy
H2 (g)
H (g) +
DH0 = kJ
Cl2 (g)
Cl (g) +
DH0 = kJ
HCl (g)
H (g) +
Cl (g)
DH0 = kJ
O2 (g)
O (g) +
DH0 = kJ O
N2 (g)
N (g) +
DH0 = kJ N
Bond Energies
Single bond < Double bond < Triple bond
9.10

6. Average bond energy in polyatomic molecules
H2O (g)
H (g) +
OH (g)
DH0 = 502 kJ
OH (g)
H (g) +
O (g)
DH0 = 427 kJ
Average OH bond energy = 2
= 464 kJ
9.10

7. Consider the reaction: H2 + F2  2HF
Bond energies can be a useful way of predicting the enthalpy of a reaction.
Consider the reaction: H2 + F2  2HF
H-H and F-F bonds must be broken (energy must be added) and 2 H-F bonds will form (energy will be released).

8. Bond Energies (BE) and Enthalpy changes in reactions
Imagine reaction proceeding by breaking all bonds in the reactants and then using the gaseous atoms to form all the bonds in the products.
DH0 = total energy input – total energy released
= SBE(reactants) – SBE(products)
9.10

9. H2 (g) + Cl2 (g) HCl (g)
2H2 (g) + O2 (g) H2O (g)
9.10

10. Bond breakage requires energy (sign positive)
So from the reactant total subtract the product total
For the reaction exo is still (-) and endo (+)

11. Steps:
Draw Lewis diagrams to determine the number and type of bonds in the molecules.
Look up the bond energies for each bond.
Add the reactants (don’t forget to multiple by molar coefficients).
Add the products (don’t forget to multiple by molar coefficients).
Subtract products from reactants.

12. Example:
Use bond energies to estimate the enthalpy change for the combustion of methane.
CH4 + 2O2  CO2 + 2H2O

13. Use bond energies to calculate the enthalpy change for:
H2 (g) + F2 (g) HF (g)
DH0 = SBE(reactants) – SBE(products)
Type of bonds broken
Number of bonds broken
Bond energy (kJ/mol)
Energy change (kJ) H 1
436.4 F 1
156.9
Type of bonds formed
Number of bonds formed
Bond energy (kJ/mol)
Energy change (kJ) H F 2
568.2
1136.4
DH0 = – 2 x = kJ
9.10