1. Kaupapa / Learning Outcome
To be able to use bond energies to calculate ∆H of a reaction.

2. Bond energy
The bond energy is the amount of energy required to break the bonds of one mole of a particular bond when both reactants and products are in their gaseous states.

3. Why would bond energies be average values?

4. Bond energies are average values
This is because the energy required to break a bond depends on the environment the bond is in.

5. Bond energies
A chemical reaction is a combination of bond breaking and bond forming
Bond breaking has a +ve ∆H (endothermic)
Bond forming has a –ve ∆H (exothermic)

6. Calculating ΔrH using bond energies
Step 1: draw the balanced equation showing the structure of the molecules
Step 2: Show all the bonds you would have to break to get the individual atoms
Step 3: Show all the bonds you need to form to make the products
Step 4: Use data from a data table to write in the bond energies for each bond
Step 5: Calculate the total Energy using the equation:
ΔrH = Σ(energy of bonds broken) – Σ(energy of bonds formed)

7. Example:
Calculate the ΔrH for the reaction above given the following bond energies Step 1

8. Step 2
Bonds Broken
Bonds Formed
C-H *6
C-C *1
Cl-Cl *1

9. Step 3 Bonds Broken Bonds Formed C-H *6 C-H *5 C-C *1 C-Cl*1 Cl-Cl *1
H-Cl*1

10. Step 4 Bond Energy C - H 413 kJ mol-1 C – Cl 339 kJ mol-1 Cl – Cl
Bonds Broken
Bonds Formed
C-H *6
6*413
C-H *5
5*413
C-C *1
1*437
C-Cl*1
1*339
Cl-Cl *1
1*242
H-Cl*1
1*431
Total
3157
3272
Bond
Energy
C - H
413 kJ mol-1
C – Cl
339 kJ mol-1
Cl – Cl
242 kJ mol-1
H – Cl
431 kJ mol-1
C - C
437 kJ mol-1
These will always be given

11. ΔrH = 3157 kJ mol-1 - 3272 kJ mol-1 = - 115 kJ mol-1
Step 5
Bonds Broken
Bonds Formed
C-H *6
C-H *5
C-C *1
C-Cl*1
Cl-Cl *1
H-Cl*1
Bonds Broken: Total 3157 kJ mol-1
Bonds Formed:
Total kJ mol-1
ΔrH = Σ(energy of bonds broken) – Σ(energy of bonds formed)
ΔrH = 3157 kJ mol kJ mol-1 = kJ mol-1