1. Bond energies

2. Bond breaking - endothermic
Energy is always required to be inputted to break a bond. Bond breaking is always endothermic.

3. Bond making - exothermic
Energy is always released when a bond is formed. Bond making is always exothermic.

4. Bond energies
The energy released when a bond is formed or absorbed when it is broken is called the bond energy.
e.g. the C-H bond in methane has a bond energy of 413 KJ/mol

5. Examples of bond energies
Bond energy KJ/mol
H-H
436
Cl-Cl
242
H-Cl
431
C-H
413
C-C
347
C-O
335
O=O
498

6. Energy level diagrams

7. Exothermic reaction
The energy need to break the bonds is less than the energy released when new bonds are made
C + 4H + 4O
energy
Energy needed to break bonds
Energy released by forming bonds
CH4(g) + 2O2(g)
Energy released
CO2(g) + 2H2O(l)
“reaction path”

8. Endothermic reaction
The energy need to break the bonds is more than the energy released when new bonds are made
energy
Energy released by forming bonds
Energy needed to break bonds
NH4NO3(l)
Energy absorbed
NH4NO3(s) + H2O (l)
“reaction path”

9. ΔH – Energy change in a complete reaction
If heat is given out, the reaction has lost energy so ΔH is negative

10. ΔH – Energy change in a complete reaction
If heat is absorbed (reaction gets colder), the reaction has gained energy so ΔH is positive

11. Calculating ΔH
CH4(g) + 2O2(g) 2H2O(l) + CO2(g)

12. Calculating ΔH CH4(g) + 2O2(g) 2H2O(l) + CO2(g)
Bonds broken = 4 x (C-H) + 2 x (O=O)
= 4 x x 498
= = 2658 KJ/mol

13. Calculating ΔH CH4(g) + 2O2(g) 2H2O(l) + CO2(g)
Bonds broken = 4 x (C-H) + 2 x (O=O)
= 4 x x 498
= = 2658 KJ/mol
Bonds made = 4 x (O-H) + 2 x (C=O)
= 4 x x -805
= = KJ/mol

14. Calculating ΔH CH4(g) + 2O2(g) 2H2O(l) + CO2(g)
Bonds broken = 4 x (C-H) + 2 x (O=O)
= 4 x x 498
= = 2658 KJ/mol
Bonds made = 4 x (O-H) + 2 x (C=O)
= 4 x x -805
= = KJ/mol
Overall Energy change = = -808 KJ/mol
(Exothermic)

15. How well have you understood?

16. Bond energies?

17. Energy level diagrams?

18. Using bond energies in calculations?