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.

4
The Big Question

• How do we keep track of the energy changes in a
  chemical reaction?

5
You will be able to

• Describe the direction of energy changes in a
  combustion reaction

6
Notes

• Energy diagrams show the difference in energy
  from the beginning of a reaction to the end of
  the reaction.

7
Activity

• Purpose In this lesson you will use energy
  diagrams to examine the energies from the
  beginning of a reaction to the end.

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(cont.)
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(cont.)
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.

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.

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?

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.

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.

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.

17
The Big Question

• Why do some chemical reactions need a spark or
  some other kind of energy input to get them
  started?

18
You will be able to

• Explain the role of the activation energy for a
  chemical reaction.

19
Notes
Ea
reactants
products

• Energy of activation (activation energy) The
  energy that is required to get a reaction started.

20
Activity

• Purpose In this lesson you will have practice
  interpreting energy diagrams and activation
  energies.

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(cont.)
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(cont.)
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(cont.)
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(cont.)
paper O2
paper KNO3
6 CO2 6 H2O 6 KNO2
6 CO2 6 H2O
25
Making Sense

• Explain the energy of activation and the heat of
  reaction in terms of bond breaking and bond
  making.

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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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.)
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(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.

29
Check-In

• Use the energy diagram to answer the questions.

(cont.)
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(cont.)

• Which arrow represents the activation energyheat
  going into system?
• Which arrow represents the heat of reactionnet
  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.

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.)
32
(cont.)

• The heat of reaction is the difference between
  the energy required to break bonds and the energy
  released in forming bonds.

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?

35
The Big Question

• How can we calculate the energy of a reaction
  without measuring it experimentally?

36
You will be able to

• Use the concept of heat of formation to
  calculate the energy changes for various chemical
  reactions.

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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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)

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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Substance Heat of formation ?Hf Substance Heat of formation ?Hf
CO2 (g) 394 kJ/mol C2H6 (g) 85 kJ/mol
C (s) 0 C6H12O2 (s) 1273.0 kJ/mol
H2O (l) 286 kJ/mol Fe (s) 0
O2 (g) 0 Fe (g) 416 kJ/mol
N2 (g) 0 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) 0
41
(cont.)
?Hf 0 (elements)
??Hf(reactants) ?Hf (CaO) ?Hf (CO2)
??Hf(products) ?Hf CaCO3
?Hrxn (?Hf products) - (?Hf reactants)
42
Making Sense

• Explain how you use heats of formation to
  determine the heat of a reaction.

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.

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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 ?Hfrxn
• 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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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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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

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.

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).

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(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.

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 NO2 N2O4 ?H 9.7 kJ/mol
• 2 C8H18 25 O2 16 CO2 18 H2O ?H 5439
  kJ/mol

52
The Big Question

• How are the concepts in the Fire unit useful in
  describing the energy related to chemical changes?

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.

54
Activity

• Purpose This lesson provides you with practice
  problems that will allow you to review the
  concepts you've learned in this unit.

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(cont.)
2 H2 O2
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(cont.)
58
Making Sense

• What information would you need to tell if a
  chemical reaction might result in a fire?

59
Notes
60
Check-In

• No Check-In.

61
Wrap-Up

• No Wrap-Up.