2. ENERGY LEVEL DIAGRAM FOR EXOTHERMIC REACTIONS
These reactions give out heat energy therefore these reactions feel warm/hot.
ENERGY LEVEL DIAGRAM FOR EXOTHERMIC REACTIONS
The diagram below shows an energy profile diagram for an exothermic reaction.
In exothermic reactions the products are below the reactants therefore energy is released as heat.
Take note that the arrow show the change in energy and this arrow always goes downwards in an exothermic reaction.

3. ENDOTHERMIC REACTION
In these reactions heat energy is taken in therefore the reaction feel cold.
The diagram below shows an energy profile diagram for an endothermic reaction.
In endothermic reactions the products are above the reactants therefore energy is taken in as heat.
Take note that the arrow shows the change in energy and this arrow always goes upwards in an endothermic reaction.

4. ACTIVATION ENERGY AND ENERGY PROFILE DIAGRAMS
EXOTHERMIC REACTION
REMEMBER:
ACTIVATION ENERGY is the MINIMAL amount of energy needed to kick start a reaction
this shows the change in energy (DH) which always has a negative value for an exothermic reaction
∴ FOR EXOTHERMIC REACTION DH = -ve

5. Examples of exothermic reactions include:
Combustion of fuels
Neutralisation
Displacement Reactions
Respiration

6. ∴ FOR ENDOTHERMIC REACTION DH = +ve
this shows the change in energy (DH) which always has a positive value for an endothermic reaction
∴ FOR ENDOTHERMIC REACTION DH = +ve

7. Examples of endothermic reactions are:
Adding salt to water
Photosynthesis
Thermal decomposition
Cracking

8. CATALYSTS
REMEMBER – A catalyst is a substance which speeds up the rate of reaction without being used up.
It does this by lowering the activation energy therefore allowing more particles with less energy in their collisions to overcome the activation energy and form products.

9. BOND BREAKING AND BOND MAKING
All chemical reactions take the form:
REACTANTS  PRODUCTS
For a reaction to occur bonds in the reactant molecules MUST be broken.
This requires energy, so this is an ENDOTHERMIC process
When products form new bonds are made.
This process gives out energy, so this is an EXOTHERMIC process.
ENDOTHERMIC
EXOTHERMIC

10. So, if:
More bonds are broken than formed then the overall reaction will be ENDOTHERMIC
More bonds are formed than broken then the overall reaction will be EXOTHERMIC.

11. QUESTION 1
QUESTION 2

12. BOND ENERGY CALCULATIONS
Bond energy is the energy needed to break bonds between two atoms
Bond energy is measured in kJ/mol

14. +  DH = 2253 +-2346 = -93kJ/mol N2(g) + 3H2(g)  2NH3(g)
EXAMPLE – Ammonia is made in the following way:
N2(g) + 3H2(g)  2NH3(g)
Its structural equation is given below: + 
REACTANTS
PRODUCTS
1(N≡𝑁) = 1(+945) = 945
3(H-H) = 3(+436) =
+2253
6(N-H) = 6(-391) = -2346
DH = = -93kJ/mol
NOTE: The fact that we have a negative answer for the overall energy change for the reaction means that the reaction is exothermic

15. C2H5OH + 3O2  2CO2 + 3H2O + 2O=O 2 O=C=O + 3 -5764
ANOTHER EXAMPLE
C2H5OH + 3O2  2CO H2O +
2O=O
2 O=C=O + 3
PRODUCTS
4(C=O) = 4 x -745 = -2980
6(O-H) = 6 x -464 = -2784
REACTANTS
1(C-C) = 1 x +347
5(C-H) = 5 x +413
1(C-O) = 1 x (+358)
1(O-H) = 1 x (+464)
2(O=O) = 2 x (+498)
-5764
DH = = kJ/mol
∴ again since the final answer is negative it shows that this reaction is exothermic

16. DH = 3754 +- 4824 = -1070 kJ/mol ∴ EXOTHERMIC
QUESTION 1 – Given the following bond energies calculate the energy change in the following reactions and state whether the reaction is exo or endothermic:
REACTANTS
4(C-H) = 4 x 412 = +1648
1(C=C) = 1 x 612 = +612
3(O=O) = 3 x 498 = +1494
PRODUCTS
4(C=O) = 4 x -743 = -2972
4(O-H) = 4 x -463 = -1852
+3754
-4824
DH = = kJ/mol ∴ EXOTHERMIC

17. DH = 3868 +-3992 = -124kJ/mol ∴ again an EXOTHERMIC reaction
QUESTION 2
REACTANTS
1(C=C) = 1 x 612 = +612
1(C-C) = 1 x 348 = +348
6(C-H) = 6 x 412 = +2472
1(H-H) = 1 x 436 = 436
PRODUCTS
2(C-C) = 2(-348) = -696
8(C-H) = 8(-412) = -3296
-3992
+3868
DH = = -124kJ/mol ∴ again an EXOTHERMIC reaction