Energy

1
Chapter 10

• Energy

2
Energy

• There are things that do not have mass and
  volume.
• These things fall into a category we call energy.
• Energy is anything that has the capacity to do
  work.
• Work if force times distance
• Work units are Joules kgm2/s2
• Force is a push and has units of Newtons
  (kgm/s2)
• Although chemistry is the study of matter, matter
  is effected by energy.
• It can cause physical and/or chemical changes in
  matter.

3
Law of Conservation of Energy

• Energy can neither be created nor destroyed.
• The total amount of energy in the universe is
  constant. There is no process that can increase
  or decrease that amount.
• However, we can transfer energy from one place in
  the universe to another, and we can change its
  form.

4
Matter Possesses Energy

• When a piece of matter possesses energy, it can
  give some or all of it to another object.
• It can do work on the other object.
• All chemical and physical changes result in the
  matter changing energy.

5
Kinetic and Potential Energy

• Potential energy is energy that is stored.
• Water flows because gravity pulls it downstream.
• However, the dam wont allow it to move, so it
  has to store that energy.
• Kinetic energy is energy of motion, or energy
  that is being transferred from one object to
  another.
• When the water flows over the dam, some of its
  potential energy is converted to kinetic energy
  of motion.

6
Forms of Energy

• Electrical
• Kinetic energy associated with the flow of
  electrical charge.
• Heat or Thermal Energy
• Kinetic energy associated with molecular motion.
• Light or Radiant Energy
• Kinetic energy associated with energy transitions
  in an atom.
• Nuclear
• Potential energy in the nucleus of atoms.
• Chemical
• Potential energy in the attachment of atoms or
  because of their position.

7
Converting Forms of Energy

• When water flows over the dam, some of its
  potential energy is converted to kinetic energy.
• Some of the energy is stored in the water because
  it is at a higher elevation than the
  surroundings.
• The movement of the water is kinetic energy.
• Along the way, some of that energy can be used to
  push a turbine to generate electricity.
• Electricity is one form of kinetic energy.
• The electricity can then be used in your home.
  For example, you can use it to heat cake batter
  you mixed, causing it to change chemically and
  storing some of the energy in the new molecules
  that are made.

8
Using Energy

• We use energy to accomplish all kinds of
  processes, but according to the Law of
  Conservation of Energy we dont really use it up!
• When we use energy we are changing it from one
  form to another.
• For example, converting the chemical energy in
  gasoline into mechanical energy to make your car
  move.

9
Losing Energy

• If a process was 100 efficient, we could
  theoretically get all the energy transformed into
  a useful form.
• Unfortunately we cannot get a 100 efficient
  process.
• The energy lost in the process is energy
  transformed into a form we cannot use.

10
Theres No Such Thing as a Free Ride

• When you drive your car, some of the chemical
  potential energy stored in the gasoline is
  released.
• Most of the energy released in the combustion of
  gasoline is transformed into sound or heat energy
  that adds energy to the air rather than move your
  car down the road.

11
Units of Energy

• Calorie (cal) is the amount of energy needed to
  raise one gram of water by 1 C.
• kcal energy needed to raise 1000 g of water 1
  C.
• food calories kcals.

Energy Conversion Factors
1 calorie (cal) 4.184 joules (J)
1 Calorie (Cal) 1000 calories (cal)
1 kilowatt-hour (kWh) 3.60 x 106 joules (J)
12
Energy Use
Unit Energy Required to Raise Temperature of 1 g of Water by 1C Energy Required to Light 100-W Bulb for 1 Hour Energy Used by Average U.S. Citizen in 1 Day
joule (J) 4.18 3.6 x 105 9.0 x 108
calorie (cal) 1.00 8.60 x 104 2.2 x 108
Calorie (Cal) 1.00 x 10-3 86.0 2.2 x 105
kWh 1.1 x 10-6 0.100 2.50 x 102
13
Chemical Potential Energy

• The amount of energy stored in a material is its
  chemical potential energy.
• The stored energy arises mainly from the
  attachments between atoms in the molecules and
  the attractive forces between molecules.
• When materials undergo a physical change, the
  attractions between molecules change as their
  position changes, resulting in a change in the
  amount of chemical potential energy.
• When materials undergo a chemical change, the
  structures of the molecules change, resulting in
  a change in the amount of chemical potential
  energy.

14
Energy Changes in Reactions

• Chemical reactions happen most readily when
  energy is released during the reaction.
• Molecules with lots of chemical potential energy
  are less stable than ones with less chemical
  potential energy.
• Energy will be released when the reactants have
  more chemical potential energy than the products.

15
Exothermic Processes

• When a change results in the release of energy it
  is called an exothermic process.
• An exothermic chemical reaction occurs when the
  reactants have more chemical potential energy
  than the products.
• The excess energy is released into the
  surrounding materials, adding energy to them.
• Often the surrounding materials get hotter from
  the energy released by the reaction.

16
An Exothermic Reaction
17
Endothermic Processes

• When a change requires the absorption of energy
  it is called an endothermic process.
• An endothermic chemical reaction occurs when the
  products have more chemical potential energy than
  the reactants.
• The required energy is absorbed from the
  surrounding materials, taking energy from them.
• Often the surrounding materials get colder due to
  the energy being removed by the reaction.

18
An Endothermic Reaction
19
Thermochemical Equations

• When a chemical or physical change takes place
  energy is either lost of gained. A
  Thermochemical equation describes this change.
  Equations gaining energy are called endothermic
  and equations losing energy are called
  exothermic.

20
Thermochemical Equations

• When a chemical or physical change takes place
  energy is either lost of gained. A
  Thermochemical equation describes this change.
  Equations gaining energy are called endothermic
  and equations losing energy are called
  exothermic.

Examples
C3H6O (l ) 4O2 (g) 3CO2(g)
3 H2O (g)
?H -1790 kj
Exothermic
H2O (l) H2O (g)
?H 44.01 kj
Endothermic
21
Thermochemical Conversions

• How many kj of heat are released when 709 g of
  C3H6O are burned?

22
Thermochemical Conversions

• How many kj of heat are released when 709 g of
  C3H6O are burned?

C3H6O (l ) 4O2 (g) 3CO2(g)
3 H2O (g)
?H -1790 kj
709 g C3H6O
mole C3H6O
58.1 g C3H6O
23
Thermochemical Conversions

• How many kj of heat are released when 709 g of
  C3H6O are burned?

C3H6O (l ) 4O2 (g) 3CO2(g)
3 H2O (g)
?H -1790 kj
709 g C3H6O
mole C3H6O
58.1 g C3H6O
24
Thermochemical Conversions

• How many kj of heat are released when 709 g of
  C3H6O are burned?

C3H6O (l ) 4O2 (g) 3CO2(g)
3 H2O (g)
?H -1790 kj
709 g C3H6O
mole C3H6O
1790 kj
21800 kj
mole C3H6O
58.1 g C3H6O
25
Energy and the Temperature of Matter

• The amount the temperature of an object increases
  depends on the amount of heat energy added (q).
• If you double the added heat energy the
  temperature will increase twice as much.
• The amount the temperature of an object increases
  depending on its mass.
• If you double the mass, it will take twice as
  much heat energy to raise the temperature the
  same amount.

26
Heat Capacity

• Heat capacity is the amount of heat a substance
  must absorb to raise its temperature by 1 C.
• cal/C or J/C.
• Metals have low heat capacities insulators have
  high heat capacities.
• Specific heat heat capacity of 1 gram of the
  substance.
• cal/gC or J/gC.
• Waters specific heat 4.184 J/gC for liquid.
• Or 1.000 cal/gC.
• It is less for ice and steam.

27
Specific Heat Capacity

• Specific heat is the amount of energy required to
  raise the temperature of one gram of a substance
  by 1 C.
• The larger a materials specific heat is, the
  more energy it takes to raise its temperature a
  given amount.
• Like density, specific heat is a property of the
  type of matter.
• It doesnt matter how much material you have.
• It can be used to identify the type of matter.
• Waters high specific heat is the reason it is
  such a good cooling agent.
• It absorbs a lot of heat for a relatively small
  mass.

28
Specific Heat Capacities
29
Heat Gain or Loss by an Object

• The amount of heat energy gained or lost by an
  object depends on 3 factors how much material
  there is, what the material is, and how much the
  temperature changed.

30
PracticeCalculate the Amount of Heat Released
When 7.40 g of Water Cools from 49 to 29 C
31
PracticeCalculate the Amount of Heat Released
When 7.40 g of Water Cools from 49 to 29 C
First use the specific heat of water 4.184 j/g-C
and cross out all units except the heat unit, the
joule (j), using our four step process
4.184 j
g- C
32
PracticeCalculate the Amount of Heat Released
When 7.40 g of Water Cools from 49 to 29 C
First use the specific heat of water 4.184 j/g-C
and cross out all units except the heat unit, the
joule (j), using our four step process
4.184 j
7.40 g
g- C
33
PracticeCalculate the Amount of Heat Released
When 7.40 g of Water Cools from 49 to 29 C
First use the specific heat of water 4.184 j/g-C
and cross out all units except the heat unit, the
joule (j), using our four step process
4.184 j
7.40 g
20 C
g- C
34
PracticeCalculate the Amount of Heat Released
When 7.40 g of Water Cools from 49 to 29 C
First use the specific heat of water 4.184 j/g-C
and cross out all units except the heat unit, the
joule (j), using our four step process
4.184 j
7.40 g
20 C
620 j
g- C
35
Thermodynamics
First Law of thermodynamics Energy cannot be
created nor destroyed. Mathematical Statement
?E q w ?E is the change in internal energy
of matter. q is the amount of heat into the
system w is the amount of work on the
system The system is the test tube, beaker or
flask The Second Law of Thermodynamics Entropy
of the universe is always increasing.
36
Entropy
Entropy is a word used to describe the spreading
of matter. For example consider the Universe, is
it expanding?
37
Entropy
Entropy is a word used to describe the spreading
of matter. For example consider the Universe, is
it expanding? Yes How about where you live, does
matter spread there?
38
Entropy
Entropy is a word used to describe the spreading
of matter. For example consider the Universe, is
it expanding? Yes How about where you live, does
matter spread there? Yes How about our natural
resources, are they being spread about? Yes
39
Entropy
Entropy is a word used to describe the spreading
of matter. For example consider the Universe, is
it expanding? Yes How about where you live, does
matter spread there? Yes How about our natural
resources, are they being spread about? The
symbol for entropy is S and the change in entropy
is ?S. ?Sgt0, means the spreading of
matter. House cleaning would have ?Slt0 (more
order)
40
Spontaneous Processes
Some process proceed without constant outside
intervention.
41
Spontaneous Processes
Some process proceed with constant outside
intervention. For example air escaping out of a
tire with a hole in it. Have you ever observed
air flowing into a tire with a hole in it? How
about aging? Have you ever observed someone
getting younger?
42
Spontaneous Processes
Some process proceed with constant outside
intervention. For example air escaping out of a
tire with a hole in it. Have you ever observed
air flowing into a tire with a hole in it? How
about aging? Have you ever observed someone
getting younger? How about shuffling a deck of
cards? Does it ever become organized like it
came from the factory?
43
Spontaneous Processes
Some process proceed with constant outside
intervention. For example air escaping out of a
tire with a hole in it. Have you ever observed
air flowing into a tire with a hole in it? How
about aging? Have you ever observed someone
getting younger? How about shuffling a deck of
cards? Does it ever become organized like it
came from the factory? Theoretically, it is
possible to shuffle a deck of cards until it has
the same order as a new deck of cards.
44
Spontaneous Processes
Some process proceed with constant outside
intervention. For example air escaping out of a
tire with a hole in it. Have you ever observed
air flowing into a tire with a hole in it? How
about aging? Have you ever observed someone
getting younger? How about shuffling a deck of
cards? Does it ever become organized like it
came from the factory? Theoretically, it is
possible to shuffle a deck of cards until it has
the same order as a new deck of cards. How many
shuffles until a deck has a new order?
45
Spontaneous Processes
Some process proceed with constant outside
intervention. For example air escaping out of a
tire with a hole in it. Have you ever observed
air flowing into a tire with a hole in it? How
about aging? Have you ever observed someone
getting younger? How about shuffling a deck of
cards? Does it ever become organized like it
came from the factory? Theoretically, it is
possible to shuffle a deck of cards until it has
the same order as a new deck of cards. How many
shuffles until a deck has a new order? 1064
46
Predicting Spontaneity
Spontaneous process are always accompanied with
spreading of energy or matter one or the other
or both. Enthalpy is a term used to describe
spreading of energy. When energy is being spread
?Hlt0
47
Predicting Spontaneity
Spontaneous process are always accompanied with
spreading of energy or matter one or the other
or both. Enthalpy is a term used to describe
spreading of energy. When energy is being spread
?Hlt0 Is melting of ice spontaneous?
48
Predicting Spontaneity
Spontaneous process are always accompanied with
spreading of energy or matter one or the other
or both. Enthalpy is a term used to describe
spreading of energy. When energy is being spread
?Hlt0 Is melting of ice spontaneous? Yes What is
being spread, energy or matter?
49
Predicting Spontaneity
Spontaneous process are always accompanied with
spreading of energy or matter one or the other
or both. Enthalpy is a term used to describe
spreading of energy. When energy is being spread
?Hlt0 Is melting of ice spontaneous? Yes What is
being spread, energy or matter? Matter, right?
50
Predicting Spontaneity
Spontaneous process are always accompanied with
spreading of energy or matter one or the other
or both. Enthalpy is a term used to describe
spreading of energy. When energy is being spread
?Hlt0 Is melting of ice spontaneous? Yes What is
being spread, energy or matter? Matter, right?
Is burning of gasoline spontaneous?
51
Predicting Spontaneity
Spontaneous process are always accompanied with
spreading of energy or matter one or the other
or both. Enthalpy is a term used to describe
spreading of energy. When energy is being spread
?Hlt0 Is melting of ice spontaneous? Yes What is
being spread, energy or matter? Matter, right?
Is burning of gasoline spontaneous? Yes, after
it starts it does not stop. What is spread here?

52
Predicting Spontaneity
Spontaneous process are always accompanied with
spreading of energy or matter one or the other
or both. Enthalpy is a term used to describe
spreading of energy. When energy is being spread
?Hlt0 Is melting of ice spontaneous? Yes What is
being spread, energy or matter? Matter, right?
Is burning of gasoline spontaneous? Yes, after
it starts it does not stop. What is spread here?
Both matter and energy!
53
Practice

• During a strenuous workout, a student
  generates 2000 kJ of heat energy. What mass of
  water would have to evaporate from the students
  skin to dissipate this much heat?

54
Practice

• During a strenuous workout, a student
  generates 2000 kJ of heat energy. What mass of
  water would have to evaporate from the students
  skin to dissipate this much heat?

g
2257 j
55
Practice

• During a strenuous workout, a student
  generates 2000 kJ of heat energy. What mass of
  water would have to evaporate from the students
  skin to dissipate this much heat?

g
10 3 j
2257 j
kj
56
Practice

• During a strenuous workout, a student
  generates 2000 kJ of heat energy. What mass of
  water would have to evaporate from the students
  skin to dissipate this much heat?

g
10 3 j
2000 kj
2257 j
kj
57
Practice

• During a strenuous workout, a student
  generates 2000 kJ of heat energy. What mass of
  water would have to evaporate from the students
  skin to dissipate this much heat?

g
10 3 j
2000 kj
886 g water
2257 j
kj
58
Practice5.53 From Text

• Exactly 10 mL of water at 25oC was added to a
  hot iron skillet. All of the water was converted
  into steam at 100oC. If the mass of the pan was
  1.20 kg and the molar heat capacity of iron is
  25.19 J/moloC, what was the temperature change
  of the skillet?

59
Sample Problem Solution
mole-C
25.19 j
60
Sample Problem Solution
mole-C
55.85 g
25.19 j
mole
61
Sample Problem Solution
mole-C
55.85 g
kg
25.19 j
mole
103 g
62
Sample Problem Solution
mole-C
55.85 g
kg
25.19 j
mole
103 g
1.20 kg
63
Sample Problem Solution
mole-C
55.85 g
kg
25.19 j
mole
103 g
1.20 kg
64
Sample Problem Solution
mole-C
55.85 g
kg
25.19 j
mole
103 g
1.20 kg
Now the energy required to heat 10mL of water
from 25C to 100C and then to vaporize the water
is outlined below.
65
Sample Problem Solution
mole-C
55.85 g
kg
25.19 j
mole
103 g
1.20 kg
Now the energy required to heat 10mL of water
from 25C to 100C and then to vaporize the water
is outlined below.
Heating from 25C to 100C
4.184 j
g-C
66
Sample Problem Solution
mole-C
55.85 g
kg
25.19 j
mole
103 g
1.20 kg
Now the energy required to heat 10mL of water
from 25C to 100C and then to vaporize the water
is outlined below.
Heating from 25C to 100C
10.0g
4.184 j
g-C
67
Sample Problem Solution
mole-C
55.85 g
kg
25.19 j
mole
103 g
1.20 kg
Now the energy required to heat 10mL of water
from 25C to 100C and then to vaporize the water
is outlined below.
Heating from 25C to 100C
10.0g
75 C
4.184 j
g-C
68
Sample Problem Solution
mole-C
55.85 g
kg
25.19 j
mole
103 g
1.20 kg
Now the energy required to heat 10mL of water
from 25C to 100C and then to vaporize the water
is outlined below.
Heating from 25C to 100C
10.0g
75 C
4.184 j
3138 j
g-C
Evaporating 10.0 mL of water
69
Sample Problem Solution
mole-C
55.85 g
kg
25.19 j
mole
103 g
1.20 kg
Now the energy required to heat 10mL of water
from 25C to 100C and then to vaporize the water
is outlined below.
Heating from 25C to 100C
10.0g
75 C
4.184 j
3138 j
g-C
Evaporating 10.0 mL of water
2257 j
g
70
Sample Problem Solution
mole-C
55.85 g
kg
25.19 j
mole
103 g
1.20 kg
Now the energy required to heat 10mL of water
from 25C to 100C and then to vaporize the water
is outlined below.
Heating from 25C to 100C
10.0g
75 C
4.184 j
3138 j
g-C
Evaporating 10.0 mL of water
10.0 g
2257 j
g
71
Sample Problem Solution
mole-C
55.85 g
kg
25.19 j
mole
103 g
1.20 kg
Now the energy required to heat 10mL of water
from 25C to 100C and then to vaporize the water
is outlined below.
Heating from 25C to 100C
10.0g
75 C
4.184 j
3138 j
g-C
Evaporating 10.0 mL of water
10.0 g
2257 j
22570 j
g
72
Sample Problem Solution
mole-C
55.85 g
kg
25.19 j
mole
103 g
1.20 kg
Now the energy required to heat 10mL of water
from 25C to 100C and then to vaporize the water
is outlined below.
Heating from 25C to 100C
10.0g
75 C
4.184 j
3138 j
g-C
Now Combine
3138 j 22570 j 25708j
Evaporating 10.0 mL of water
10.0 g
2257 j
22570 j
g
73
Sample Problem Solution
mole-C
55.85 g
kg
25708 j
47.5 C
25.19 j
mole
103 g
1.20 kg

74
Hesss Law

• Hesss law states that the enthalpy change of a
  reaction that is the sum of two or more reactions
  is equal to the sum of the enthalpy changes of
  the constituent reactions.

75
Calculations via Hesss Law

• If a reaction is reversed, ?H sign changes.
  N2(g) O2(g) ? 2NO(g) ?H 180 kJ
• 2NO(g) ? N2(g) O2(g) ?H ?180 kJ
• 2. If the coefficients of a reaction are
  multiplied by an integer, ?H is multiplied by
  that same integer.
• 6NO(g) ? 3N2(g) 3O2(g) ?H ?540 kJ

76
Example
Calculate the enthalpy change for C2H4(g)
H2(g) C2H6(g) using the following data.

• H2(g) 1/2O2(g) H2O(l)
  -285.8 kJ
• C2H4(g) 3O2(g) 2H2O(l) 2CO2(g)
  -1411 kJ
• C2H6(g) 7/2O2(g) 3H2O(l) 2CO2(g)
  -1560 kJ

77
Example
Calculate the enthalpy change for C2H4(g)
H2(g) C2H6(g) using the following data.

• H2(g) 1/2O2(g) H2O(l)
  -285.8 kJ
• C2H4(g) 3O2(g) 2H2O(l) 2CO2(g)
  -1411 kJ

3H2O(l) 2CO2(g) C2H6(g) 7/2O2(g)
1560 kJ
H2(g) 1/2O2(g)C2H4(g)3O2(g)3H2O(l)2CO2(g)
H2O(l) 2H2O(l) 2CO2(g) C2H6(g) 7/2O2(g)
-136.8
simplify
C2H4(g) H2(g) C2H6(g) -136.8 kj
78

• The End