1. Can you guess the topic for today?

2. Bond energies

3. Combustion of methane
One molecule of methane and two molecules of oxygen

4. Combustion of methane
CH4(g) + 2O2(g) 2H2O(l) + CO2(g)

5. Combustion of methane CH4(g) + 2O2(g) 2H2O(l) + CO2(g)
All reactions involve bond breaking and bond making as the atoms “swap partners”

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

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

8. Chemical Energy
When chemical reactions occur, bonds are first broken and then formed.
The amount of energy released / absorbed during a chemical reaction can be measured and calculated from the bond energies

9. Bond Energy Energy is absorbed by atoms when their bonds break.
(+∆H) - bond dissociation
Energy is released when bonds form between atoms.
(-∆H) - bond formation
When two atoms form a bond, energy is stored in the molecule as part of the process
When the bond is broken, energy must be injected into the molecule to disrupt the bond and separate the two atoms. This bond energy can be measured. It is equal and opposite ot the energy used to form the bond. Generically we refer to this energy as bond energy

10. Bond Energy
Bond
Energy (kJ/mol)
H - H
436
C - H
413
N - H
393
436 kJ is required to disrupt H-H
436 kJ is released when H-H is formed
The higher the bond energy, the more work is required to break the bond, the more stable it is

11. Bond Energy Bond Energy (kJ/mol) H - H 436 N - N 160 C - H 413 N = O
631
N - H
393
N triple N
941
P - H
297
N - O
201
C - C
347
N - P
C - O
358
O - H
464
C - N
305
O - S
265
C - Cl
397
O - Cl
269
C = C
607
O - O
204
C = O
805
C - F
552
O = O
498
C - S
259
Which of these bonds are most stable? – C=O, N≡N
Least stable? - N-N, N-O

12. Energy level diagrams

13. 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”

14. 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”

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

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

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

18. 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

19. 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

20. 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)

21. Calculating Energy of a Reaction
H2 + Cl2  2HCl
Breaking H bond = 436 kJ/mol
Breaking Cl bond = 242 kJ/mol
H-Cl bond forming = kJ/mol
When the net energy released is negative, the reaction is exothermic

22. Reactions and Energy
If heat is generated during a reaction ≡ exothermic
If heat is absorbed during a reaction ≡ endothermic

23. Calculating Energy of a Reaction
H2 + Cl2  2HCl
Moles of Bonds Broken
Energy absorbed (kJ)
Moles of Bonds Formed
Energy released (kJ) kJ
436
2 kJ
854 1
243
679
Heat of Reaction is 679 – 854 = -175kJ; energy is released during the reaction
When the net energy released is negative, the reaction is exothermic
Endothermic or exothermic?

24. Chemical Reactions and Energy
Energy released
This is an example on an exothermic reaction. In an exothermic reaction, the energy required to break the bonds is less than the energy required to make the new bonds. The excess energy is released as heat. Conversely, if more energy is needed to form the bonds in the product than is released with the reactants’ bonds are broken, the reaction absorbs heat.
H2 + Cl2
Bonds are broken
HCl

25. Calculating chemical energy
N2 + O2  2NO
Breaking one N bond = 946 kJ/mol
Breaking one O bond = 498 kJ/mol
Forming 2 N-O bonds = 2 x 631 kJ/mol = 1262 kJ/mol
Net energy released = ( ) + 2(-631) = +182 kJ/mol
When the net energy value is positive, the reaction is endothermic.
Endothermic or exothermic?

26. Chemical Reactions and Energy
Energy absorbed
This is an example on an endothermic reaction. In an endothermic reaction, the energy required to break the bonds is more than the energy required to make the new bonds. Energy is drawn in from the surroundings to complete the reaction.
N2 + O2
Bonds are broken NO

27. Calculating chemical energy
2H2 + O2  2H2O
Breaking H bond = 436 kJ/mol
Breaking O bond = 498 kJ/mol
O-H bond forming = kJ/mol
Net energy released = ((2*436) + 498) + 4(-464) = -442 kJ/mol
When the net energy value is positive, the reaction is endothermic.
Breaking H bond = 436 kJ/mol
Breaking O bond = 498 kJ/mol
O-H bond forming = kJ/mol
Net energy released = ((2*436) + 498) + 4(-464) = -442 kJ/mol
When the net energy value is positive, the reaction is endothermic.

29. FORMULA TO DETERMINE THE NUMBER OF RINGS AND/OR DOUBLE AND TRIPLE BONDS
2 + 2C + N – H – X 2
ρ =
where
C = number of carbon atoms
N = number of nitrogen atoms
H = number of hydrogen atoms
X = number of halogen (Group 17) atoms