Fire Unit

1. Fire Unit Investigation III: Energy for Change Lesson 1: No Going Back Lesson 2: Fire Starter Lesson 3: Formations Lesson 4: Ashes to Ashes

2. Fire Unit – Investigation III Lesson 1: No Going Back

3. ChemCatalyst • Humans generate energy from burning fuels, such as coal, oil, natural gas, and hydrogen. For example, the combustion of coal can be written as • C(s) + O2(g) CO2(g) • Do you think you can reverse the reaction to form coal, C(s), and oxygen, O2, from CO2? Explain your thinking. Unit V • Investigation III

4. The Big Question • How do we keep track of the energy changes in a chemical reaction? Unit V • Investigation III

5. You will be able to: • Describe the direction of energy changes in a combustion reaction Unit V • Investigation III

6. 2 H2 + O2 2 H2 + O2 2 H2O 2 H2O Reaction 1: Combustion of hydrogen Reaction 2: Decomposition of water 2 H2 + O2 2 H2O 2 H2O 2 H2 + O2 Notes • Energy diagrams show the difference in energy from the beginning of a reaction to the end of the reaction. Unit V • Investigation III

7. Activity • Purpose: In this lesson you will use energy diagrams to examine the energies from the beginning of a reaction to the end. (cont.) Unit V • Investigation III

8. 2 H2 + O2 2 H2 + O2 2 H2O 2 H2O Reaction 1: Combustion of hydrogen Reaction 2: Decomposition of water 2 H2 + O2 2 H2O 2 H2O 2 H2 + O2 (cont.) (cont.) Unit V • Investigation III

9. Reaction 1: Combustion of methane Reaction 2: Formation of methane CH4 + 2 O2 CO2 + 2 H2O CO2 + 2 H2O CH4 + 2 O2 (cont.) Unit V • Investigation III

10. Making Sense • Humans generate energy from burning fuels we dig out of the earth, such as coal, oil, and natural gas. Do you think it will be easy to replenish these fuels? Explain your thinking. Unit V • Investigation III

11. Notes • Heat of reaction is the amount of energy gained or lost during a chemical reaction. If the sign for the heat of reaction is negative, the reaction is exothermic. If the sign is positive, the reaction is endothermic. • Conservation of energy is a law that states that energy is neither created nor destroyed. Thus, if a chemical process releases energy, then the reverse process must require an input of the exact same amount of energy. Unit V • Investigation III

12. Check-In • Sketch an energy diagram for the combustion of carbon (coal) to form carbon dioxide. The heat of reaction is –394 kJ/mol. • What energy is required to form coal from carbon dioxide? Unit V • Investigation III

13. Wrap-Up • The heat of reaction is the energy change in going from reactants to products. • The heat of reaction is positive for an endothermic reaction. It is negative for an exothermic reaction. • Energy is conserved in a chemical reaction. The reverse reaction requires an equal amount of energy transferred in the opposite direction. Unit V • Investigation III

14. Fire Unit – Investigation III Lesson 2: Fire Starter

15. ChemCatalyst • In the previous lesson we showed you an energy diagram for the combustion of hydrogen. In actuality, that diagram was simplified. This new energy diagram is more accurate. Unit V • Investigation III

16. 2 H2 + O2 –286 kJ/mol H2 2 H2O (cont.) • What is different about this diagram? Explain what you think is going on, and why you think the diagram has the shape it has. Unit V • Investigation III

17. The Big Question • Why do some chemical reactions need a “spark” or some other kind of energy input to get them started? Unit V • Investigation III

18. You will be able to: • Explain the role of the “activation energy” for a chemical reaction. Unit V • Investigation III

19. Notes Ea • Energy of activation (activation energy): The energy that is required to get a reaction started. reactants products Unit V • Investigation III

20. Activity • Purpose: In this lesson you will have practice interpreting energy diagrams and activation energies. (cont.) Unit V • Investigation III

21. Reaction 1 Energy change in kJ/mol Reaction 2 200 100 0 -100 -200 (cont.) (cont.) Unit V • Investigation III

22. transition state bond breaking Ea bond making reactants products (cont.) (cont.) Unit V • Investigation III

23. 2 HBr H2 + Cl2 H2 + Br2 2 HCl Reaction 1: H2 + Cl2 2 HCl Reaction 2: H2 + Br2 2 HBr (cont.) (cont.) Unit V • Investigation III

24. (cont.) paper + KNO3 paper + O2 6 CO2 + 6 H2O + 6 KNO2 6 CO2 + 6 H2O Unit V • Investigation III

25. Making Sense • Explain the energy of activation and the heat of reaction in terms of bond breaking and bond making. Unit V • Investigation III

26. Notes • Most chemical reactions (not just combustion reactions) require some sort of energy input to get them started. This is called the activation energy. (cont.) Unit V • Investigation III

27. N E E N Notes (cont.) • Bond breaking requires an input of energy into a system. • Bond making, on the other hand, releases a certain amount of energy. • Bond energy: The energy required to break a bond. Bond breaking is endothermic. Bond making is exothermic. (cont.) Unit V • Investigation III

28. effect of catalyst (cont.) • Reaction rate: The speed at which a reaction proceeds. The reaction rate is effected by temperature, mixing, and surface area. Reactions with high activation energies proceed slowly. • Catalyst: A substance that lowers the activation energy for a reaction. A catalyst is not consumed by the reaction. Unit V • Investigation III

29. a. b. c. Check-In • Use the energy diagram to answer the questions. (cont.) Unit V • Investigation III

30. (cont.) • Which arrow represents the activation energy—heat going into system? • Which arrow represents the heat of reaction—net energy released by the reaction? • For the reaction described by the energy diagram, is the energy required to break bonds greater than the energy released upon forming bonds? Explain. Unit V • Investigation III

31. Wrap-Up • The energy of activation for a chemical reaction is the energy that is required to get a reaction started. • Breaking bonds requires energy. Making bonds releases energy. • Energy is required to start a reaction because bonds need to be broken as a first step. (cont.) Unit V • Investigation III

32. (cont.) • The heat of reaction is the difference between the energy required to break bonds and the energy released in forming bonds. Unit V • Investigation III

33. Fire Unit – Investigation III Lesson 3: Formations

34. ChemCatalyst • H2 (g) + 1/2 O2 (g) H2O (l) + 68 kcal • H2 (g) + 1/2 O2 (g) H2O (l)∆H = –68 kcal/mol H2O • These two equations seem to contradict each other, but they both refer to the exact same chemical reaction. What does each equation mean? Unit V • Investigation III

35. The Big Question • How can we calculate the energy of a reaction without measuring it experimentally? Unit V • Investigation III

36. You will be able to: • Use the concept of “heat of formation” to calculate the energy changes for various chemical reactions. Unit V • Investigation III

37. Notes • You could say that the focus of the first equation is the combustion of hydrogen as a fuel. • You could say that the focus of the second equation is the formation of liquid water. (cont.) Unit V • Investigation III

38. Notes (cont.) • Sometimes it takes heat to form a certain product and sometimes heat is released in the formation of a certain product. • Whether the heat is positive or negative, it is referred to as the heat of formation. • Its symbol is ∆Hf°. ∆Hrxn = (the sum of ∆Hf products) – (the sum of ∆Hf reactants) Unit V • Investigation III

39. Activity • Purpose: This lesson provides you with practice calculating heats of reaction using heats of formation values. Heats of formation: (cont.) Unit V • Investigation III

40. Unit V • Investigation III

41. (cont.) ∆Hf° = 0 (elements) –∑∆Hf°(reactants) = –[∆Hf° (CaO) + ∆Hf° (CO2)] ∑∆Hf°(products) = ∆Hf° CaCO3 ∆Hrxn = (∆Hf products) - (∆Hf reactants) Unit V • Investigation III

42. Making Sense • Explain how you use heats of formation to determine the heat of a reaction. Unit V • Investigation III

43. Notes • Hess's Law, also known as the Law of Heat Summation, states that the sum of the heats of formation of the various steps of a reaction will be equal to the heat of the overall reaction. (cont.) Unit V • Investigation III

44. Notes (cont.) • Calculate the heat of reaction for the reaction of NO2 with itself to form N2O4: • 2 NO2 N2O4 • ∆Hrxn = (∆Hf° products) – (∆Hf° reactants) • ∆Hrxn = (∆Hf° N2O4) – 2∆Hf° (NO2) • Now solve for ∆Hf°rxn: • heat of reaction= (+9.7 kJ/mol) – 2(34 kJ/mol) • = (9.7 kJ/mol) – (68 kJ/mol) • = –58 kJ/mol (cont.) Unit V • Investigation III

45. Notes (cont.) • Enthalpy of reaction: Enthalpy is simply the energy of the reaction adjusted to take into account atmospheric pressure. • ∆Hrxn = ∑ ∆H(products) – ∑ ∆H(reactants) (cont.) Unit V • Investigation III

46. Notes (cont.) • Heat of reaction - energy input or output of a reaction • Molar heat of reaction - energy input or output of a reaction per mole of reactant (or product) used • Enthalpy - the heat (or energy) content of a system at constant pressure • Heat of formation - the heat released or required (the change in enthalpy) during the formation of a pure substance from its elements Unit V • Investigation III

47. Check-In • Explain how you can you calculate the heat of reaction (or the enthalpy of reaction) for the following reaction, from the heats of formation of the reactants and products. • 2Mg (s) + O2 (g) 2 MgO(s) • Write out the formula for this calculation, using the compounds in the above reaction. Unit V • Investigation III

48. Wrap-Up • The heat of formation of a substance is the energy required to create a mole of the substance from its constituent elements in their standard states. • We can calculate the "energy" of a reaction by measuring the difference in energy between the reactants and products. ∆H = ∆H(products) – ∆H(reactants). (cont.) Unit V • Investigation III

49. (cont.) • Enthalpy is a more accurate value to use when talking about the energy content of a reaction. • Enthalpy is similar to heat of reaction except that it takes into account atmospheric pressure and the work that gases do when they are produced or removed by a reaction. Unit V • Investigation III

50. Fire Unit – Investigation III Lesson 4: Ashes to Ashes