1. Exothermic and Endothermic
Reactions

2. Exothermic Reactions Examples include:
Burning reactions including the combustion of fuels.
Detonation of explosives.
Reaction of acids with metals.
Exothermic reactions increase in temperature.
Magnesium reacting with acid
Thermit reaction

3. Exothermic Reactions Magnesium + Hydrochloric acid 25o C 45o C
Heat
energy
given
out
Gets hot
Hydrochloric
acid

4. Exothermic Reactions
If heat is given out this energy must have come from chemical energy in the starting materials (reactants).
45o C
25o C
Reactants convert chemical energy to heat energy.
The temperature rises.

5. Explaining Exothermic Reactions
Energy / kJ)
Progress of reaction (time)
reactants
Reactants have more chemical energy.
Some of this is lost as heat which spreads out into the room.
products
Products now have less chemical energy than reactants.

6. Energy of reactants and products in
Exothermic Reactions
H is how
much energy
is given out
Energy / kJ
Progress of reaction
reactants
products
H=negative
H is given a negative sign for exothermic reactions

7. Definition of an Exothermic Reaction
Exothermic reactions give out energy. There is a temperature rise and H is negative.
products
Energy / kJ)
Progress of reaction
reactants
H is negative

8. Endothermic Reactions
Endothermic reactions cause a decrease in temperature.
Heat
energy
Is used up during the reaction so the contents in tube feel cold
Ammonium
nitrate
Water
Cools
Starts 25°C
Cools to 5°C

9. Explaining Endothermic Reactions
Extra energy is needed in order for endothermic reactions to occur.
This comes from the thermal energy of the reaction mixture which consequently gets colder.
5o C
25o C
Reactants convert heat energy into chemical energy as they change into products. The temperature drops.

10. Energy of Reactants and Products in an
Endothermic Process
This is how
much energy
is taken in
products
Energy / kJ)
Progress of reaction
reactants
H=+
H is given a positive sign for endothermic reactions

11. Definition of Endothermic Reactions
Endothermic reactions take in energy. There is a temperature drop and H is positive.
products
Energy / kJ
Progress of reaction
reactants
H=+

13. Are these endothermic or exothermic?
Activity
Are these endothermic or exothermic?
A red glow spread throughout the mixture and the temperature rose.
The mixture bubbled vigorously but the temperature dropped 150C.
Hydrazine and hydrogen peroxide react so explosively and powerfully that they are used to power rockets into space.
The decaying grass in the compost maker was considerably above the outside temperature.
exo
endo
exo
exo

14. Activation energy
Energy is given out as new bonds form
Chemical reactions involve the formation of bonds between atoms but often before new bonds can be formed old ones have to be broken.
This means that there has to be enough energy (activation energy)to start breaking the old bonds before a reaction can occur.
Activation energy needed to break existing bonds
New bonds form
Reactants
Old bonds start to break

15. Explaining Activation Energy
Most reactions are exothermic (give out heat) overall but there is still a need for energy to get the reaction started.
Energy in chemicals
Reaction
Activation energy
Break old bonds
Energy taken in
Form new bonds
Energy given out

16. Activation Energy and Exothermic Reactions
reactants
H= -
Energy / kJ)
products
Progress of reaction

17. Activation Energy and Endothermic Reactions
products
H=+
Energy / kJ)
reactants
Progress of reaction

18. Activity
Use this diagram to explain why garages can store petrol safely but always have notices about not smoking near the petrol pumps.
ActivationEnergy
Petrol + oxygen
Energy / kJ)
Carbon dioxide + water
Progress of reaction

19. Answer
The reaction is exothermic but requires the Activation energy to be provided before the reaction can get underway.
This is necessary to break some of the bonds in the oxygen or petrol before new bonds can start forming.
ActivationEnergy
Petrol + oxygen
Energy / kJ)
Carbon dioxide + water
Progress of reaction

21. Catalysts For chemical reactions to occur:
Existing bonds have to begin breaking so that new ones can be formed.
The molecules have to collide in such a way that the reacting parts of the molecules are brought together.
Catalysts can help with either or both of these processes.
A catalyst is a substance that speeds up a reaction without getting used up in the process.

22. Effect of a Catalyst on Activation Energy

23. Inside car engines some of the nitrogen and oxygen from the air combine to form poisonous nitrogen oxide. Inside the exhaust system a catalyst encourages decomposition back into nitrogen and oxygen.
Copy the energy profile for the uncatalysed reaction and draw in new lines showing how the presence of a catalyst will alter the profile.
2NO2
N2 + 2O2