1. Investigation III: Energy for Change
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.
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4. The Big Question
How do we keep track of the energy changes in a chemical reaction?
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5. You will be able to:
Describe the direction of energy changes in a combustion reaction
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6. Notes
2 H2 + O2
2 H2O
Reaction 1:
Combustion of hydrogen
2 H2 + O H2O
Reaction 2:
Decomposition of water
2 H2O H2 + O2
Energy diagrams show the difference in energy from the beginning of a reaction to the end of the reaction.
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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.)
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8. (cont.) (cont.) 2 H2 + O2 2 H2O 2 H2O 2 H2 + O2 2 H2 + O2 2 H2O
Reaction 1:
Combustion of hydrogen
2 H2 + O H2O
Reaction 2:
Decomposition of water
2 H2O H2 + O2
(cont.)
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9. (cont.) Reaction 1: Combustion of methane CH4 + 2 O2 CO2 + 2 H2O
Formation of methane
CO2 + 2 H2O CH4 + 2 O2
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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.
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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.
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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?
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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.
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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.
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16. (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.
2 H2 + O2
2 H2O
–286 kJ/mol H2
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17. The Big Question
Why do some chemical reactions need a “spark” or some other kind of energy input to get them started?
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18. You will be able to:
Explain the role of the “activation energy” for a chemical reaction.
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19. Notes Ea
reactants
products
Energy of activation (activation energy): The energy that is required to get a reaction started.
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20. Activity
Purpose: In this lesson you will have practice interpreting energy diagrams and activation energies.
(cont.)
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21. Energy change in kJ/mol
(cont.)
Reaction 1
Reaction 2
Energy change in kJ/mol
200
100
-100
-200
(cont.)
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22. (cont.) (cont.) transition state bond breaking Ea bond making
reactants
products
transition state
bond breaking
bond making
(cont.)
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23. (cont.) (cont.) Reaction 1: Reaction 2: H2 + Cl2 H2 + Br2
H2 + Cl HCl
2 HCl
2 HBr
Reaction 2:
H2 + Br HBr
(cont.)
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24. (cont.) paper + KNO3 paper + O2 6 CO2 + 6 H2O 6 CO2 + 6 H2O + 6 KNO2
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25. Making Sense
Explain the energy of activation and the heat of reaction in terms of bond breaking and bond making.
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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.)
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27. Bond breaking requires an input of energy into a system.
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. E N
(cont.)
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28. (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.
effect of catalyst
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29. Check-In Use the energy diagram to answer the questions. a. b. c.
(cont.)
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30. Which arrow represents the activation energy—heat going into system?
(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.
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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.)
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32. (cont.)
The heat of reaction is the difference between the energy required to break bonds and the energy released in forming bonds.
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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?
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35. The Big Question
How can we calculate the energy of a reaction without measuring it experimentally?
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36. You will be able to:
Use the concept of “heat of formation” to calculate the energy changes for various chemical reactions.
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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.)
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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)
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39. Activity
Purpose: This lesson provides you with practice calculating heats of reaction using heats of formation values. Heats of formation:
(cont.)
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40. Substance Heat of formation ∆Hf° CO2 (g) –394 kJ/mol C2H6 (g)
C (s)
C6H12O2 (s)
– kJ/mol
H2O (l)
–286 kJ/mol
Fe (s)
O2 (g)
Fe (g)
416 kJ/mol
N2 (g)
FeO (s)
–272 kJ/mol
N (g)
473 kJ/mol
Fe2O3 (s)
–822 kJ/mol
NO (g)
90 kJ/mol
CaO (s)
–636 kJ/mol
NO2 (g)
34 kJ/mol
HCl (aq)
–167 kJ/mol
N2O4 (g)
9.7 kJ/mol
CaCO3 (s)
–1207 kJ/mol
CH4 (g)
–75 kJ/mol
MgO (s)
–602 kJ/mol
O (g)
248 kJ/mol
Mg (s)
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41. (cont.) ∆Hf° = 0 (elements) –∑∆Hf°(reactants)
= –[∆Hf° (CaO) + ∆Hf° (CO2)]
∑∆Hf°(products)
= ∆Hf° CaCO3
∆Hrxn = (∆Hf products) - (∆Hf reactants)
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42. Making Sense
Explain how you use heats of formation to determine the heat of a reaction.
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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.)
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44. Notes (cont.)
Calculate the heat of reaction for the reaction of NO2 with itself to form N2O4:
2 NO 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.)
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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.)
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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
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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) MgO(s)
Write out the formula for this calculation, using the compounds in the above reaction.
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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.)
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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.
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50. Fire Unit – Investigation III
Lesson 4:
Ashes to Ashes

51. ChemCatalyst
Many reactions are easily reversible. However, when a tree burns down, it is essentially impossible to recover the tree by reversing the combustion reaction. Examine the two chemical equations and explain why only one is easily reversible.
2 NO N2O ∆H = 9.7 kJ/mol
2 C8H O CO H2O ∆H = –5439 kJ/mol
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52. The Big Question
How are the concepts in the Fire unit useful in describing the energy related to chemical changes?
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53. You will be able to:
Identify the essential concepts of the Fire unit and explain how they can be used to describe energy changes in chemical reactions.
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54. Activity
Purpose: This lesson provides you with practice problems that will allow you to review the concepts you've learned in this unit.
(cont.)
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55. (cont.)
(cont.)
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56. (cont.) 2 H2 + O2 –58 kcal/mol 2 H2O (g) 2 H2O (l) –10 kcal/mol
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57. (cont.)
+ 68 kcal/mol
2 H2O (l)
2 H2 + O2
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58. Making Sense
What information would you need to tell if a chemical reaction might result in a fire?
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59. Notes
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60. Check-In
No Check-In.
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61. Wrap-Up
No Wrap-Up.
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