1. (See Chapter 10.4) Chemical Reactions and Energy

2. What happens when you light a match? What happened when you mixed baking soda and strontium chloride in the lab? Chemical reactions gain or lose heat energy.

3. Enthalpy Enthalpy – amount of heat lost or gained during a chemical reaction Symbol: Δ H Units: kiloJoules per mol kJ/mol

4. Enthalpy For any chemical reaction Δ H = Δ H products – Δ H reactants

5. Exothermic Reactions What other words start with exo-? Exothermic reaction – “heat out”, reaction in which heat leaves (is lost) -Reaction feels hot Δ H < 0 Example – Lighting a match

6. Endothermic Reactions What’s the opposite of leaving? Endothermic reaction – “heat in”, reaction in which heat comes in (is gained) -Reaction feels cold Δ H > 0 Example – Chewing gum that makes your mouth cold

7. Practice What type of reaction is: A reaction with Δ H = +20 kJ/mol 1) Exothermic 2) Endothermic

8. Practice What type of reaction is: A reaction with Δ H = -45 kJ/mol 1) Exothermic 2) Endothermic

9. Activation Energy Diagram (Energy Graph) Terms Reactant(s) – The starting substance(s) in a reaction Product(s) – The substance(s) formed in a reaction Activation energy – The minimum amount of energy that particles must have in order to react = Peak ΔH – ΔH(reactants) Products Reactants Activation energy Energy  Reaction Progress 

10. Activation Energy Diagram (Energy Graph) Label the Activation Energy Activation Energy

11. Activation Energy Diagram (Energy Graph) What is the initial energy of the reactants? 100J

12. Activation Energy Diagram (Energy Graph) What is the activation energy of this reaction? It goes from 100J to 400 J, so 300J

13. Activation Energy Diagram (Energy Graph) What is the energy of the products? 300J

14. Activation Energy Diagram (Energy Graph) The products have more/less energy than the reactants. MORE

15. Activation Energy Diagram (Energy Graph) Do you think this reaction released or absorbed heat? Absorbed Δ H = Δ H P – Δ H R = 300 – 100 = +200J

16. Activation Energy Diagram (Energy Graph) Would you expect this reaction to feel hot or cold? Cold (absorbing heat from surroundings)

17. Activation Energy Diagram (Energy Graph) Is this process exothermic or endothermic? Endothermic

18. Activation Energy Diagram (Energy Graph) Label the Activation Energy 50 100 150 200 250 Activation Energy

19. 50 100 150 200 250 Activation Energy Diagram (Energy Graph) What is the initial energy of the reactants? 150J

20. Activation Energy Diagram (Energy Graph) What is the activation energy of this reaction? 50 100 150 200 250 It goes from 150J to 250 J, so 100J

21. Activation Energy Diagram (Energy Graph) What is the energy of the products? 50J 50 100 150 200 250

22. Activation Energy Diagram (Energy Graph) The products have more/less energy than the reactants. LESS 50 100 150 200 250

23. Activation Energy Diagram (Energy Graph) Do you think this reaction released or absorbed heat? Released Δ H = Δ H P – Δ H R = 50 – 150 = -100J 50 100 150 200 250

24. Activation Energy Diagram (Energy Graph) Would you expect this reaction to feel hot or cold? Hot (Releasing heat into surroundings) 50 100 150 200 250

25. Activation Energy Diagram (Energy Graph) Is this process exothermic or endothermic? Exothermic 50 100 150 200 250

26. What is the total heat gain or loss of the room? Window loses 1J of energy because it isn’t sealed well. Loses 1J

27. What is the total heat gain or loss of the room? Lose 1 J Heater puts in 5 J of hot air into the room Gains 4J

28. What is the total heat gain or loss of the room? Lose 1 J Gain 5J Lose 1 J Gains 2J

29. What did we need to know to figure out the TOTAL heat of the room? Gain 5JLose 1 J 1) Amount of heat gained or lost for each item 2) Total number of each item

30. Calculating Enthalpy StepExample 1) Look up the Enthalpies of Formation ( Δ H f ) of each compound in your chemical reaction. They will be in a table. 2H 2 (g) + O 2 (g)  2H 2 O(g) Δ H f (H 2 ) = 0 kJ/mol Δ H f (O 2 ) = 0 kJ/mol Δ H f (H 2 O) = -286 kJ/mol

31. Calculating Enthalpy StepExample 2) Multiply each Enthalpy of Formation ( Δ H f ) by the number of moles in the equation (which is the coefficient on that compound). 2H 2 (g) + O 2 (g)  2H 2 O(g) Δ H f (H 2 ) x 2 = 0 kJ Δ H f (O 2 ) x 1 = 0 kJ Δ H f (H 2 O) x 2 = -572 kJ

32. Calculating Enthalpy StepExample 3) Add the enthalpies of formation ( Δ H f ) of the reactants. Then add the enthalpies of formation ( Δ H f ) of the products. 2H 2 (g) + O 2 (g)  2H 2 O(g) Δ H f (reactants) = 0 kJ + 0 kJ = 0 kJ Δ H f (products) = -572 kJ

33. Calculating Enthalpy StepExample 4) Subtract the enthalpy of formation of the reactants from the enthalpy of formation of the products. 2H 2 (g) + O 2 (g)  2H 2 O(g) Δ H = -572 kJ – 0 kJ = -572kJ Easier to Write: Δ H = Δ H f (products) – Δ H f (reactants)

34. Calculating Enthalpy Step Example 2 1) 2Al(s) + 3H 2 O(l)  Al 2 O 3 (aq) + 3H 2 (g) Δ H f (Al) = 0 kJ/mol Δ H f (H 2 O) = -286 kJ/mol Δ H f (Al 2 O 3 ) = -1676 kJ/mol Δ H f (H 2 ) = 0 kJ/mol

35. Calculating Enthalpy Step Example 2 2) 2Al(s) + 3H 2 O(l)  Al 2 O 3 (aq) + 3H 2 (g) Δ H f (Al) = 0 kJ/mol x 2 = 0kJ Δ H f (H 2 O) = -286 kJ/mol x 3 = -858kJ Δ H f (Al 2 O 3 ) = -1676 kJ/mol x 1 = -1676kJ Δ H f (H 2 ) = 0 kJ/mol x 3 = 0kJ

36. Calculating Enthalpy Step Example 2 3) 2Al(s) + 3H 2 O(l)  Al 2 O 3 (aq) + 3H 2 (g) Δ H f (Al) + Δ H f (H 2 O) = -858 kJ Δ H f (Al 2 O 3 ) + Δ H f (H 2 ) = -1676 kJ

37. Calculating Enthalpy Step Example 2 4) 2Al(s) + 3H 2 O(l)  Al 2 O 3 (aq) + 3H 2 (g) Δ H f (products) - Δ H f (reactants) = -1676kJ – (-858 kJ) Δ H = -818kJ