Transcript Document

Fire Unit
Investigation III:
Energy for Change
Lesson 1: No Going Back
Lesson 2: Fire Starter
Lesson 3: Formations
Lesson 4: Ashes to Ashes
Fire Unit – Investigation III
Lesson 1:
No Going Back
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.
© 2004 Key Curriculum Press.
Unit V • Investigation III
The Big Question
• How do we keep track of the energy
changes in a chemical reaction?
© 2004 Key Curriculum Press.
Unit V • Investigation III
You will be able to:
• Describe the direction of energy changes in
a combustion reaction
© 2004 Key Curriculum Press.
Unit V • Investigation III
Notes
2 H 2 + O2
2 H2 + O2
2 H 2O
Reaction 1:
Combustion of hydrogen
2 H2 + O2
2 H 2O
2 H2 O
Reaction 2:
Decomposition of water
2 H 2O
2 H2 + O2
• Energy diagrams show the difference in
energy from the beginning of a reaction
to the end of the reaction.
© 2004 Key Curriculum Press.
Unit V • Investigation III
Activity
Purpose: In this lesson you will use
energy diagrams to examine the
energies from the beginning of a reaction
to the end.
(cont.)
© 2004 Key Curriculum Press.
Unit V • Investigation III
(cont.)
2 H2 + O2
2 H2 + O2
2 H 2O
Reaction 1:
Combustion of hydrogen
2 H2 + O2
2 H2O
2 H2 O
Reaction 2:
Decomposition of water
2 H2O
2 H2 + O2
(cont.)
© 2004 Key Curriculum Press.
Unit V • Investigation III
(cont.)
Reaction 1:
Combustion of methane
CH4 + 2 O2
CO2 + 2 H2O
Reaction 2:
Formation of methane
CO2 + 2 H2O
CH4 + 2 O2
© 2004 Key Curriculum Press.
Unit V • Investigation III
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.
© 2004 Key Curriculum Press.
Unit V • Investigation III
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.
© 2004 Key Curriculum Press.
Unit V • Investigation III
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?
© 2004 Key Curriculum Press.
Unit V • Investigation III
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.
© 2004 Key Curriculum Press.
Unit V • Investigation III
Fire Unit – Investigation III
Lesson 2:
Fire Starter
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.
© 2004 Key Curriculum Press.
Unit V • Investigation III
(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
–286 kJ/mol H2
2 H 2O
© 2004 Key Curriculum Press.
Unit V • Investigation III
The Big Question
• Why do some chemical reactions need
a “spark” or some other kind of energy
input to get them started?
© 2004 Key Curriculum Press.
Unit V • Investigation III
You will be able to:
• Explain the role of the “activation energy” for
a chemical reaction.
© 2004 Key Curriculum Press.
Unit V • Investigation III
Notes
Ea
reactants
products
• Energy of activation (activation
energy): The energy that is required to
get a reaction started.
© 2004 Key Curriculum Press.
Unit V • Investigation III
Activity
Purpose: In this lesson you will have
practice interpreting energy diagrams
and activation energies.
(cont.)
© 2004 Key Curriculum Press.
Unit V • Investigation III
(cont.)
Energy
change 200
in kJ/mol
Reaction 1
Reaction 2
100
0
-100
-200
(cont.)
© 2004 Key Curriculum Press.
Unit V • Investigation III
(cont.)
transition
state
bond
breaking
Ea
bond making
reactants
products
(cont.)
© 2004 Key Curriculum Press.
Unit V • Investigation III
(cont.)
2 HBr
H2 + Cl2
H2 + Br2
2 HCl
Reaction 1:
H2 + Cl2
2 HCl
Reaction 2:
H2 + Br2
2 HBr
(cont.)
© 2004 Key Curriculum Press.
Unit V • Investigation III
(cont.)
paper + KNO3
paper + O2
6 CO2 + 6 H2O
6 CO2 + 6 H2O
+ 6 KNO2
© 2004 Key Curriculum Press.
Unit V • Investigation III
Making Sense
• Explain the energy of activation and
the heat of reaction in terms of bond
breaking and bond making.
© 2004 Key Curriculum Press.
Unit V • Investigation III
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.)
© 2004 Key Curriculum Press.
Unit V • Investigation III
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
E
N
(cont.)
© 2004 Key Curriculum Press.
Unit V • Investigation III
(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
© 2004 Key Curriculum Press.
Unit V • Investigation III
Check-In
Use the energy diagram to answer the
questions.
a.
b.
c.
(cont.)
© 2004 Key Curriculum Press.
Unit V • Investigation III
(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.
© 2004 Key Curriculum Press.
Unit V • Investigation III
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.)
© 2004 Key Curriculum Press.
Unit V • Investigation III
(cont.)
• The heat of reaction is the difference
between the energy required to break
bonds and the energy released in
forming bonds.
© 2004 Key Curriculum Press.
Unit V • Investigation III
Fire Unit – Investigation III
Lesson 3:
Formations
ChemCatalyst
• H2 (g) + 1/2 O2 (g)
• H2 (g) + 1/2 O2 (g)
H2O (l) + 68 kcal
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?
© 2004 Key Curriculum Press.
Unit V • Investigation III
The Big Question
• How can we calculate the energy of a
reaction without measuring it
experimentally?
© 2004 Key Curriculum Press.
Unit V • Investigation III
You will be able to:
• Use the concept of “heat of formation” to
calculate the energy changes for various
chemical reactions.
© 2004 Key Curriculum Press.
Unit V • Investigation III
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.)
© 2004 Key Curriculum Press.
Unit V • Investigation III
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)
© 2004 Key Curriculum Press.
Unit V • Investigation III
Activity
Purpose: This lesson provides you with
practice calculating heats of reaction
using heats of formation values. Heats of
formation:
(cont.)
© 2004 Key Curriculum Press.
Unit V • Investigation III
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
© 2004 Key Curriculum Press.
Unit V • Investigation III
(cont.)
∆Hf° = 0 (elements)
–∑∆Hf°(reactants)
= –[∆Hf° (CaO) + ∆Hf° (CO2)]
∑∆Hf°(products)
= ∆Hf° CaCO3
∆Hrxn = (∆Hf products) - (∆Hf reactants)
© 2004 Key Curriculum Press.
Unit V • Investigation III
Making Sense
• Explain how you use heats of
formation to determine the heat of a
reaction.
© 2004 Key Curriculum Press.
Unit V • Investigation III
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.)
© 2004 Key Curriculum Press.
Unit V • Investigation III
Notes (cont.)
• Calculate the heat of reaction for the
reaction of NO2 with itself to form N2O4:
2 NO2
N 2O 4
∆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)
(cont.)
= –58 kJ/mol
© 2004 Key Curriculum Press.
Unit V • Investigation III
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.)
© 2004 Key Curriculum Press.
Unit V • Investigation III
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
© 2004 Key Curriculum Press.
Unit V • Investigation III
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.
© 2004 Key Curriculum Press.
Unit V • Investigation III
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.)
© 2004 Key Curriculum Press.
Unit V • Investigation III
(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.
© 2004 Key Curriculum Press.
Unit V • Investigation III
Fire Unit – Investigation III
Lesson 4:
Ashes to Ashes
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
N 2O 4
∆H = 9.7 kJ/mol
• 2 C8H18 + 25 O2
16 CO2 + 18 H2O
∆H = –5439 kJ/mol
© 2004 Key Curriculum Press.
Unit V • Investigation III
The Big Question
• How are the concepts in the Fire unit
useful in describing the energy related
to chemical changes?
© 2004 Key Curriculum Press.
Unit V • Investigation III
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.
© 2004 Key Curriculum Press.
Unit V • Investigation III
Activity
Purpose: This lesson provides you with
practice problems that will allow you to
review the concepts you've learned in
this unit.
(cont.)
© 2004 Key Curriculum Press.
Unit V • Investigation III
(cont.)
(cont.)
© 2004 Key Curriculum Press.
Unit V • Investigation III
(cont.)
2 H2 + O2
–58 kcal/mol
2 H2O (g)
–10 kcal/mol
2 H2O (l)
(cont.)
© 2004 Key Curriculum Press.
Unit V • Investigation III
(cont.)
2 H2 + O2
+ 68 kcal/mol
2 H2O (l)
© 2004 Key Curriculum Press.
Unit V • Investigation III
Making Sense
• What information would you need to
tell if a chemical reaction might result
in a fire?
© 2004 Key Curriculum Press.
Unit V • Investigation III
Notes
© 2004 Key Curriculum Press.
Unit V • Investigation III
Check-In
• No Check-In.
© 2004 Key Curriculum Press.
Unit V • Investigation III
Wrap-Up
• No Wrap-Up.
© 2004 Key Curriculum Press.
Unit V • Investigation III