1. Learning objective: To explain why there are energy changes during a reaction. Must: Be able to identify exothermic and endothermic reactions. Must: Be able to interpret energy level diagrams. Should: Explain why some reactions are exothermic and some are endothermic. Could: Calculate the energy change given the bond enthalpy.

2. Table of results

3. Discover – Hot or cold investigation. Follow the instructions for each reaction and record your results. Wear goggles at all times. Key skills Read and understand texts and take appropriate action Use appropriate mathematical procedures. Team worker – collaborate with others to work towards common goals

4. Exothermic & Endothermic

5. 1.An exothermic reaction is one which releases heat energy to the surroundings 2.The temperature of the surroundings increases 3.The energy released from forming new bonds is greater than the energy needed to break old bonds Exothermic reactions

6. During a chemical reaction there is an energy transfer. Combustion of methane in a Bunsen burner 6 Explain Two things happen to bonds. reactantsproducts methane oxygen carbon dioxide water

7. The atoms in the reactants are held together by chemical bonds. Chemical energy 7 Explain Low energy High energy

8. 8 Explain Bonds break Heat energy from a match breaks the bonds and splits the atoms apart A small amount of energy from a match. Chemical energy Low energy High energy 1

9. Atoms join to make products. As new bonds form, energy is released. 9 Bonds are made Explain A lot of energy is released. Chemical energy Low energy High energy 2

10. Exothermic reactions

11. 11 Explain An energy level diagram for an exothermic reaction. SS3 energy products reactants exothermic

12. 12 Explain In an exothermic reaction, the reactants contain more energy than the products. I get it. HEAT OUT! The left-over energy is released as heat.

13. Exothermic reactions

14. 1.An endothermic reaction is on which takes in heat energy from the surroundings 2.The temperature of the surroundings decreases 3.The energy needed to break old bonds is greater than the energy released from forming new bonds Endothermic reactions

17. 17 Explain An energy level diagram for an endothermic reaction. The products contain more energy than the reactants. This extra energy comes from the surroundings. energy reactants products endothermic

18. In an exothermic reaction more energy is released making new bonds than is taken in breaking bonds. There is heating. It’s all ‘ Make and Break ’! In an endothermic reaction less energy is released making new bonds than is taken in breaking bonds. There is cooling.

19. Bond energies The energy needed to break a chemical bond Different chemical bonds have different bond energies Chemical bondBond energy, kJ/mole H―H436 O=O498 O―H464

20. Working out ∆H Draw an energy level diagram with all the reactants and products on it energy course of reaction 2H 2 + O 2 2H 2 O

21. Working out ∆H Show all the bonds in the reactants energy course of reaction 2H 2 O H―H O=O+

22. Working out ∆H Show all the bonds in the products energy course of reaction H―H O=O+ HH O HH O

23. Working out ∆H Show the bond energies for all the bonds energy course of reaction 436 O=O+ HH O HH O

24. Working out ∆H Show the bond energies for all the bonds energy course of reaction 436 498+ HH O HH O

25. Working out ∆H Show the bond energies for all the bonds energy course of reaction 436 498+ HH O 464 +

26. Working out ∆H Show the bond energies for all the bonds energy course of reaction 436 498+ 464 + +

27. Working out ∆H Add the reactants’ bond energies together energy course of reaction 464 + + 1370

28. Working out ∆H Add the products’ bond energies together energy course of reaction 1370 1856

29. Working out ∆H ∆H = energy in ― energy out energy course of reaction 1370 1856 1370 1856 ―

30. Working out ∆H ∆H = energy in ― energy out energy course of reaction 1370 1856 1370 1856 -486 ―

31. Working out ∆H ∆H = energy in ― energy out energy course of reaction 1370 1856 ∆H = -486

32. Working out ∆H Summary The energy values have units of kJ/mole In the exam, you will be given the energy values and all the bonds to make or break ∆H is energy in – energy out Energy goes in to break bonds Energy goes out when bonds are made