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Zumdahl • Zumdahl • DeCoste
World of
CHEMISTRY
Chapter 10
Energy
10.1 The Nature of Energy
• Objective: To understand the general
properties of energy
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10.1 The Nature of Energy
• Energy
• The ability to do work or produce heat
• Potential Energy
• Energy due to position or composition
• Movement of water from high to low to do work
• Attractive and repulsive forces used to drive a
chemical reaction
• Example – combustion of gasoline
• Kinetic Energy
• Energy do to the motion of the object
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Law of Conservation of Energy
• Energy can be converted from one form
to another but can neither be created or
destroyed.
• Energy on the universe is constant
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Work
• Work
• Force acting over a distance
• Example: a ball rolling down hill.
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What happens if we change the surface of
our hill?
1. Does ball A lose more energy?
2. What changes?
1. The amount of heat released
2. The amount of force (work) applied
3. Regardless of the amount of heat or
work the energy change is constant.
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.
• The change in Energy is independent of
it pathway.
• Work and Heat are dependent of its
pathway.
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State Function
• A State Function is a property of the
system that changes independently of
its pathways (heat and work are not
state functions, they are dependent
upon the pathway in which the reaction
proceeds).
• Example of State Function
• Displacement; Energy change
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10.2 Temperature and Heat
Objective: To understand the concept of
Temperature and Heat
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What does temperature of a substance tell
us about that substance?
• Temperature is
• a measure of the random motion of the
components of a substance.
• Example: Ice →liquid H2O →gaseous H2O
• Heat can be defined as
• a measure of the random motion of the
components of a substance.
• .
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Figure 10.2: Equal masses of hot and cold water.
Suppose we place 1.00 Kg
hot water next to 1.00 Kg of
cold water in an insulated
box. What do you expect to
happen?
Assuming no energy is
lost to the air how can we
determine the final temp.
of the two samples?
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Figure 10.3: H2O molecules in hot and cold water.
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• Remembering that heat is the measure of
energy flow do to temperature difference:
Tfinal = Thot initial + Tcold initial = 90° C + 10°C = 50
2
2
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Figure 10.4: H2O molecules in same temperature
water.
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Review
• Temperature
• the measure of the random motions of the
components
• Heat
• the flow of energy due to temperature
differences
• Thermal Energy of an object
• The random motions of the components of the
object
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10.3 Exothermic and Endothermic
Objective: To consider the direction of
energy flow as heat
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What are the energy changed that
accompany chemical reactions?
• When a process results in the evolution
of heat, it is said to be
• Exothermic: energy flows OUT of a system
• When a process results absorbs energy
from its surrounding it is said to be
• Endothermic
• When heat flows into a system it is said to
be endothermic
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Key Terms
• System
• The portion of the universe that we single
out to study
• Surroundings
• The surroundings include everything else
in the universe.
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Endothermic Reactions
• Example: Boiling water to form steam
• Where does the energy (absorbed as
heat) come from in an endothermic
reaction?
• The energy is the difference between the
potential energy between the reactants and
the products
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If energy can neither be created or destroyed,
where does the change in energy come from?
• In an Endothermic Reactions:
• The energy gained by a system must equal
the energy lost by the surroundings.
• In an Exothermic Reaction:
• the energy lost by a system must equal
the energy gained by the surroundings
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Examples: Endothermic or Exothermic?
1. Your hands get cold when you touch ice?
Exothermic
2. Ice melts when you touch it.
Endothermic
3. Ice cream melts
Endothermic
4. Propane is burning in a propane torch
Exothermic
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5. Water drops on your skin evaporates
Endothermic
6. Two chemicals mixed in a beaker give
off heat.
Exothermic
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Figure 10.5: The energy changes
accompanying the burning of a match.
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Focus Questions
1. Explain why energy is a state function, but
heat and work are not.
2. What is probably the most important
characteristic of energy?
3. What is the difference between temperature
and heat?
1. Heat is the flow of energy due to temp.
difference.
2. Temperature is the measurement of random
motion
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10.4 Thermodynamics
Objective: To understand how energy flow
affects internal energy
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First Law of Thermodynamics
• The energy of the universe is constant
• The internal energy, E, of a system can
be defined as the sum of the kinetic and
potential energies of all particles in the
system.
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∆ E=q+w
∆ (delta) means a change in the function
that follows
q represents heat
w represents work
E represent the internal energy
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Thermodynamic Quantities Consist of 2 Parts
1. A number
giving the magnitude of the change
2. Sign
indicating the direction of the flow of
energy
The sign represents the systems point of
view
+ indicated energy flows into a system
- indicates energy flows out of a system
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Common Units of Energy
• Calorie (metric unit)
• The amount of energy (heat) required to
raise the temperature of one gram of water
by one Celsius degree
• Joule (an SI unit)
• 1 calorie = 4.184 joules
• 1 cal = 4.184 J
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• If 1 cal/4.184 J is required to change
1.0g of water by 1° C we can assume
that it will take twice as much energy to
change 2 g of water by 1 ° C
How much energy (heat) in joules is
required to raise the temperature of 7.40
gram of water from 29 ° C to 46 ° C ?
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10.5 Measuring Energy Changes
Objective: To understand how heat is
measured
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Conversion between calories and joules
• Express 60.1 cal of energy in units of joules
60.1 cal X 4.184 J = 251 J
1cal
How many calories of energy correspond to 28.4 J?
28.4J X 1 cal = 6.67 cal
4.184 J
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Specific Heat Capacity
• The amount of energy required to
change the temperature of one gram of
a substance by one Celsius degree.
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The Energy (heat) Required to change the
temperature of a substance depends on:
1. The amount of substance being heated
(number of grams).
2. The temperature change (number of
degrees)
*Note: different substance behave differently to
being heated. Some substances require large
amounts of energy to change temperature , whereas
others require less.
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Calculations Involving Specific Heat Capacity
pg. 298
1. What quantity in joules is required to
heat a piece of iron weighing 1.3 g
from 25°C to 46°C?
1. What is the answer in calories?
2. A 5.63 sample of solid gold is heated
from 21°C to 32°C. How much energy
in joules is required?
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Q=sXmX∆T
•
•
•
•
Q = Energy required
S = specific heat of capacity
M = mass in grams
∆ T = change in temperature
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Quiz 10.1 – 10.3
1. The portion of the universe that we single out to
study is known as:__________
2. A measure of the random motion of the
components of a substance is known as_____
3. Explain why energy is a state function, but heat
and work are not.
4. The flow of energy due to temperature differences
is known as___________.
5. __________is the ability to do work or produce
heat
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10.6 Thermochemistry (Enthalpy)
Objective: To consider the heat (enthalpy)
of chemical reactions
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Enthalpy
• At constant pressure, the change in
enthalpy (∆ H) is equal to the energy
that flows as heat.
• ∆ Hp= heat
• Where “p” indicates that the process
occurred under conditions of constant
pressure.
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Practice problem pg 302
• When 1 mol of methane is burned at
constant pressure, 890 kJ of energy is
released as heat. Calculate the ∆ H for
a process in which 5.8 grams sample of
methane is burned at constant
pressure.
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Calorimeter
• A calorimeter is a device used to
determine the heat associated with a
chemical reaction.
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Figure 10.6: A coffee-cup calorimeter.
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10.7 Hess’s Law
Objective: To understand Hess’s Law
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Enthalpy is a state function
• The change in enthalpy for a given
process is independent of the pathway
for the process
• In going from a particular set of reactants
to products the change in enthalpy is the
same whether the reaction takes place in
one step or in a series of steps.
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Characteristics of Enthalpy Changes
1. If a reaction is reversed, the sign of ∆ H is
also reversed.
2. The magnitude of ∆ H is proportional to the
quantities of reactants and products in the
reaction.
If the coefficients in a balanced reaction
are multiplied by an integer, then the value
of ∆ H must also be multiplied by the same
inetger.
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1. If enthalpy is the heat for a reaction, it must
have a sign as well as a magnitude. What
sign for an exothermic reaction have? Why?
2. Suppose you ran a chemical reaction in a
calorimeter. If the temperature of the solution
goes from 27 °C to 36°C for a 5.0g sample,
how would you determine the energy
produced by the reaction?
3.What is Hess’s Law and why is it useful?
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4. The enthalpy of a combustion of solid
carbon to form carbon dioxide is -393.7
kJ/mol °C, and the enthalpy of
combustion of carbon monoxide to form
carbon dioxide is -283.3 kJ/mol °C.
Using this data, calculate the change in
enthalpy for the reaction:
2C(s) + O2 (g) → 2 CO (g)
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10.8 Quality Versus Quantity of Energy
Objective: To see how the quantity
changes as it is used
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If energy is conserved, why are we concerned
about having enough energy to use?
• Suppose we drive from Evansville to
Indianapolis – we put gas in our car to do
work.
• The energy in the bonds react with oxygen
(combust – bond are broken) and energy is
released to do work.
• During this process we release energy in the
form of heat (i.e. the quantity of heat is
transferred , thus conserved).
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What happens to the quality of the energy?
C8H18(l) + O2(g) → CO2 (g) + H2O (g) + energy
• Which energy is easier to use?
• When we utilize energy to do work we
degrade its usefulness (quality).
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“heat death” of the Universe
• Eventually all energy of the universe will
be spread out evenly throughout the
universe and everything will be the
same temperature – at this point the
energy will no longer be able to do
work.
• The Universe will be ‘dead’
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10.9 Energy and Our World
Objective: To consider the energy
resources of our world
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Where does our energy ultimately come from?
• By the process of photosynthesis plants
store energy which then can be
converted over millions of years to fossil
fuels.
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Figure 10.7: Energy sources used in the
United States.
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Petroleum and Natural Gas
• Petroleum and Natural gas is most likely
formed from the remains of marine
organism that lived more than 500
million years ago
• Petroleum is a thick dark liquid
composed of hydrocarbons.
• Natural Gas consist mostly of methane
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Coal
• Coal is formed from the remains of
plants that were buried and subjected to
high pressure and heat over long
periods of time.
• The energy available from the
combustion of coal depends upon the
grade of coal. The more carbon in the
coal the higher the energy yield.
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Coal supplies 20% of our energy in the US
What are the problems associated with coal?
1. expensive and dangerous to mine
2. Strip mining of fertile farmland
3. Burning of coal released Sulfur Dioxide.
4. Combustion of coal releases high levels of
carbon dioxide, which in turns effects the
earths temperature.
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Figure 10.8: The earth’s atmosphere.
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Effects of Carbon Dioxide on Climate
The Greenhouse Effect
• The temperature of the earth is
controlled to a significant extend by the
CO2 and H2O content of the
atmosphere.
Connection: What happened when the
production of heat when the moisture in
the earths atmosphere increases
(humidity)?
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Figure 10.9: The atmospheric CO2 concentration over
the past 1000 years.
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New Energy Sources
•
•
•
•
•
Solar
Nuclear
Wind
Biomass
Synthetic fuels
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10.10 Energy as a Driving Force
Objective: To understand energy as a
driving force for natural processes
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Why do thing occur?
Why do some reactions in nature proceed in
a particular direction?
Wood + O2(g) → CO2 (g) + H2) (g) + energy + ashes
Why doesn’t the reverse happen?
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2 Factors That are important to Driving Forces
• Energy Spread
• Matter Spread
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Energy Spread
• Energy Spread means that in a given
process, concentrated energy is
dispersed widely.
• This distribution always happens every
time an exothermic reaction occurs.
• The energy that flows into the surroundings
through heat increases the thermal
motions of the molecules in the
surroundings.
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Matter Spread
• Matter spread says that molecules of a
substance are spread out and occupy a
large volume.
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Entropy
• Entropy (S) is the natural tendency of
the world to become disordered.
• As randomness increases, S increases.
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Figure 10.10: Comparing the entropies of ice and
steam.
Which has
Greater
entropy?
Ice or steam?
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What happens to the disorder of the universe
as energy spread and matter spread occur
during a reaction?
• ENERGY SPREAD→ Faster random
motions of the molecules in surroundings
• MATTER SPREAD → Components of
matter are dispersed – they occupy a larger
volume.
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1. If energy is conserved, how can there be
an “energy crisis”
2. What is “cracking” of petroleum products?
How did it help increase production of
gasoline?
3. What is the greenhouse effect and what
are the key molecules that cause it?
4. What driving force must be predominant
for an endothermic reaction to occur?
Why?
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