1. Lesson 1 - Enthalpy Changes in Chemistry

2. Starter
When a chemical reaction takes place heat may be given out or taken in.
Exothermic: when heat is given out
Endothermic: when heat is taken in

3. Energy level diagrams
Which diagram do you think is endothermic and which is exothermic?
Heat given out
Heat taken in

4. Change in energy
Chemical reactants have a certain amount of chemical energy stored within them.
When the reaction has taken place they have either more or less energy stored within them than before.

5. Reactants
Description of reaction
Initial temp. / °C
Final temp. /°C
Change in temp. ΔT / °C
Exo or endo?
Sodium and chlorine gas
N/A (demo)
exo
Sugar and sulfuric acid
Dissolving ammonium nitrate
endo
Barium hydroxide and ammonium nitrate

6. Definitions
In your notes write down a definition of an exothermic reaction;
AND
Describe what happened in the two experiments you have just seen.

7. Exothermic Reactions
Correct definition: An exothermic chemical reaction where the products have less energy than the reactants. Energy is given out to the surroundings (temperature of the surroundings goes up, because the reaction is releasing heat into the surroundings).

8. Definitions
In your notes write down a definition of an endothermic reaction;
AND
Describe what happened in the two experiments you have just seen.

9. Endothermic Reactions
Correct definition: An endothermic chemical reaction is where the products have more energy than the reactants. Energy is taken in from the surroundings (temperature of the surroundings goes down, because the surroundings are losing heat to the reaction).

10. Which of the following statements is correct?
The products of a chemical reaction have more energy than the reactants so an exothermic reaction must have taken place.
The products of a chemical reaction have less energy than the reactants so an endothermic reaction must have taken place.
The products of a chemical reaction have less energy than the reactants so an exothermic reaction must have taken place.

11. Which of the following statements is correct?
The products of a chemical reaction have less energy than the reactants so an exothermic reaction must have taken place. Q - How can we illustrate this?

12. Energy level diagrams – copy into your notes
Endothermic Exothermic
Heat taken in
Heat given out
Energy level of products is higher than reactants so heat taken in.
Energy level of products is lower than reactants so heat given out.

13. Question

14. Answer

15. Wrap up – Can you recall?
Draw an energy profile for an exothermic reaction marking clearly the activation energy.