1. Enthalpy Changes in Chemistry

2. Heat • 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.

6. 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 goes up).

7. 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 goes down).

8. 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.

9. Which of the following statements is correct? 3 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?

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

11. Question

12. Answer