1. Energy changes in reactions
Exothermic and Endothermic – what do they mean?

2. What’s the difference between these two reactions?

3. HEAT OUT
HEAT IN

4. Exothermic Reactions Exothermic reactions increase in temperature.
HEAT EXITS the reaction so it gets hotter on the outside

5. Endothermic Reactions
Endothermic reactions decrease in temperature.
HEAT Enters the reaction so it gets colder on the outside.

7. Endothermic or exothermic
Liquid
Temperature (°C)
Solid
Min or Max temp (°C)
Endothermic or exothermic
Hydrochloric acid
Magnesium
Water
Ammonium nitrate
Citric acid solution
Sodium hydrogen carbonate
Copper sulfate solution
Zinc

8. Summary Q’s Name an example of an exothermic reaction
Name an example of an endothermic reaction
What happens to the energy in an endothermic reaction? How do you know?
What happens to the energy in an exothermic reaction? How do you know?

9. 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

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

11. 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.

12. Exothermic Reactions
Almost immediately the hot reaction products start to lose heat to the surroundings and eventually they return to room temperature.
25o C
45o C
Chemical energy becomes heat energy.
The reaction mixture gets hotter.
Eventually this heat is lost to the surroundings.
It follows that reaction products have less chemical energy than the reactants had to start with.

13. Say whether these processes are exothermic.
Activity
Say whether these processes are exothermic.
Charcoal burning
A candle burning.
A kettle boiling
Ice melting
A firework exploding
yes
yes no no
yes
You have to put heat in for boiling and melting.
You get heat out from all the other processes

14. Endothermic Reactions
Endothermic reactions cause a decrease in temperature.
Endothermic chemical reactions are relatively rare.
A few reactions that give off gases are highly endothermic - get very cold.
Dissolving salts in water is another process that is often endothermic.

15. Endothermic Reactions
Endothermic reactions cause a decrease in temperature.
Ammonium
nitrate
Water
Heat
energy
taken
in as the mixture returns back to room temp.
Cools
Starts 25°C
Cools to 5°C
Returns to 25°C

16. 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.

17. Endothermic Reactions
The cold reaction products start to gain heat from the surroundings and eventually return to room temperature.
25o C
5o C
25o C
The reactants gain energy.
This comes from the substances used in the reaction and the reaction gets cold.
Eventually heat is absorbed from the surroundings and the mixture returns to room temperature.
Overall the chemicals have gained energy.

18. Summary
Energy is absorbed to break bonds. Bond- breaking is an endothermic process.
Energy is released when new bonds form. Bond- making is an exothermic process.
Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break bonds and the energy released when new bonds form. If more heat energy is released when making the bonds than was taken in when they broke, the reaction is exothermic.

19. Endothermic or exothermic
Liquid
Temperature (°C)
Solid
Min or Max temp (°C)
Endothermic or exothermic
Hydrochloric acid
Magnesium
Water
Ammonium nitrate
Citric acid solution
Sodium hydrogen carbonate
Copper sulfate solution
Zinc
What is happening in an exothermic reaction?
What is happening in an endothermic reaction?