Energy Part I:
Introduction to Energy
Chapter 6 Sec 1-3
of Jespersen 7th ed)
Dr. C. Yau
Spring 2015
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Thermochemistry
Thermochemistry is a study of the heat
flow in chemical reactions.
Kinetic versus Potential Energy
Kinetic Energy (KE) is the energy of motion.
KE = mv2 m=mass v=velocity
Potential Energy (PE) is "stored" energy
an object has that has the potential to be
changed to other forms of energy.
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Forms of Energy
• kinetic energy
• potential energy
• thermal E (heat)
• light
PE 1
• electrical
• nuclear
PE 2
PE 1 > PE 2
3
Chemical Energy
• Chemical Energy is a form of potential
energy, due to the physical & chemical
bonds within a substance
• A book has chemical energy. How can we
get energy out of it?
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Potential Energy
Potential energy depends on position:
e.g. Position of an object about to fall down,
pulled by gravity.
e.g. Position of an object at a distance from
an object to which it is attracted.
e.g. Position of a molecule/atom/ion/electron
at a high energy level about to fall down to
a lower energy level
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Potential Energy
Consider a spring connecting two balls:
Natural position
It takes E to pull on the ball and stretch the spring out.
From its extended position, the system has a higher
potential energy.
When the ball is released, the spring returns to its natural
position, and the potential energy is released.
What happens if you PUSH the balls together?
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Potential Energy
The potential energy of a spring depends on its length.
Either stretching or squeezing the spring raises the P E.
PE is at its lowest when the spring is at its natural length. 7
Factors Affecting Potential Energy
Increase Potential Energy
• Pull apart objects that attract
each other
– Book/gravity
– N and S poles of magnets
– Positive and negative charges
• Push together objects that
repel each other
– Spring compressed
– N poles on two magnets
– 2 like charges (+) (+) or (-) (-)
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Factors Affecting Potential Energy
Decrease Potential Energy
• Objects that attract each other come together
– Book falls
– N and S poles of 2 magnets
S
N
N
S
– Positive and negative charges
• Objects that repel each
other move apart
– Spring released
S
S
N
N
N
N
S
S
– N poles on 2 magnets
– 2 like charges moving apart
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Your Turn!
Which of the following represents a
decrease in the potential energy of the
system?
A. A book is raised 6 feet above the floor.
B. A ball rolls downhill.
C. Two electrons come close together.
D. A spring is stretched completely.
E. Two atomic nuclei approach each other.
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1st Law of Thermodynamics
The Conservation of Energy
Energy cannot be created nor destroyed.
It may be transformed into a different form of
energy, but the total remains the same.
When you burn a book, what kind of
transformations are there in terms of
energy? How is energy conserved?
Etotal before reaction = E total after reaction
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Internal Energy
Internal energy (E) is the sum of energies for all of
the individual particles in a sample of matter.
It includes the "molecular kinetic energy": the
kinetic energy of the particles in constant motion
as they collide with each other and with the sides
of the container.
ΔE is the change in internal energy during a
process.
ΔE = Efinal – Einitial
REMEMBER THIS!... Final minus Initial
In a reaction,
ΔE = Eproducts– Ereactants
We cannot measure E, but we can measure ΔE.12
Temperature and Average Kinetic Energy
Large collection of molecules (gas)
 Wide distribution of kinetic energy (KE)
 Small number with KE = 0
o Collisions momentarily stopped molecule’s
motion
 Very small number with very high KE
o Unbalanced collisions give high velocity
 Most molecules intermediate KEs
 Result = distribution of energies
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Kinetic Energy
Atoms and molecules are in constant random
motion.
Within the sample each particle has different
kinetic energies, energies that are ever
changing as they collide with each other and
with the sides of the container.
HOWEVER, at a given temperature, the total
energy & the AVERAGE KE of all the
particles are constant.
The distribution of energy of the particles
within the same follows the Boltzmann
distribution.
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KE Distribution Curve
The KE distribution curve shows how the
fraction of particles with a given KE varies
with KE.
The area under the curve corresponds to the
sum of all the fractions = 1.
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The Bell Curve
If the KE were a Bell curve, the most probable KE
would also be the average KE. However, the
Boltzmann Distribution Curve is skewed.
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Fig. 6.4 p.257
Boltzmann Distribution
Most probable KE
Average KE
Most probable KE at the higher temperature
Average KE
At a higher temperature
Any given sample would have a "most probable"
KE. The AVERAGE KE is slightly to its right.
When temperature is increased, the graph
flattens and shifts to the right.
Learn to draw this!
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Fig. 6.4 p.257
Boltzmann Distribution
At a higher temperature
Of particular significance, the graph shows us that
beyond the maxima, at the higher temperature,
there are more molecules for a given KE.
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KE versus Temperature
The average KE correlates
directly to temperature.
When T increases, heat is added
to the sample and converted
into the KE of the molecules.
The higher the T, the faster the
gas particles are moving.
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“State” of an Object
• is a complete list of properties that specify
object’s current condition.
In Chemistry, “state” is defined by...
• Chemical composition
(Number of moles of all substances present)
• Pressure
• Temperature
• Volume
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“State Function”
"State function" is any property of a substance
that depends only on its current state and
NOT on how it got there.
It is not dependent on the path taken to
establish it.
For example, the mp of a substance is a state
function.
The mp of ice is 0oC, regardless of how we get the
ice, and what we did with the H2O before we
changed it to ice.
Internal Energy is a "state function."
Be sure you understand the meaning of “state
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function.”
State Function
New
York
Los
Angeles
• Location is a state function: both train and
car travel to the same locations although
their paths vary.
• The actual distance traveled does vary with
path. Distance traveled is NOT a state
function.
The time it took to get there...is that a state function? 22
State Functions
• Some State functions in science:
Internal energy E = Ef – Ei
Pressure
P = Pf – Pi
Temperature
T = Tf – Ti
Volume
V = Vf – Vi
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Quick Review of Proportionality
Mathematically, if we say x is proportional to
y, how do we express it in an equation?
Note: It is not accurate to say that if x
increases and y increases, then x and y
are directly proportional.
Why isn’t it? What is a better way to put it?
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Heat vs. Temperature
Do not confuse "heat" with "temperature."
It is easy to confuse the two because when
we say it's "hot" we are actually referring
to the temperature.
Heat is not temperature.
We can measure temperature (with a
thermometer)
but we cannot measure heat directly.
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Heat
• Pour hot coffee into cold cup
– Heat flows from hot coffee to cold cup
– Faster coffee molecules bump into wall of cup
– Transfer kinetic energy
– Eventually coffee & cup reach same temperature
Thermal Equilibrium
• When both cup and coffee reach same
average Kinetic Energy and same temperature
– Energy transferred through heat comes from
object’s internal energy
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Heat versus T
• Remember that we cannot determine E
(including thermal E, heat), but we can
determine the CHANGE in E, ΔE.
ΔE for heat = q = amount of heat transferred
It is directly proportional to the change in T.
q = CΔT
y=k x
where C = heat capacity
and ΔT = change in T
T final – T initial
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Heat Capacity vs. Specific Heat
Heat capacity = specific heat x mass
C=
s
x m
m = mass of the sample
s = specific heat,
a characteristic of the
sample.
Remember that q = C ΔT
Here we have q = s m ΔT LEARN THIS!
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Specific Heat
• It is the amt of heat
needed to raise one gram
of substance by one
degree (either oC or K)
• Ones with * are all solids.
What do their specific
heats have in common?
• Substances with high
specific heats resist
temperature changes
• Note that water has a
very high specific heat
– (This is why coastal
temperatures are different
from inland temperatures.)
Table 6.1, p.260
Substance
Specific
Heat,
J/ g °C
(25 °C)
Carbon
(graphite)
0.711 *
Copper
0.387 *
Ethyl
alcohol
2.45
Gold
0.129 *
Granite
0.803 *
Iron
0.4498 *
Lead
0.128 *
Olive oil
2.0
Silver
0.235 *
Water
(liquid)
4.18
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Units of Heat
The SI unit of heat is the joule (J).
It is equivalent to the amount of KE
possessed by 2 kg of object moving at a
speed of 1 m/s:
KE = mv2
= (2 kg) (1 m/s)2
=1 kg m2 s-2 = 1 J (joule)
1 kJ = 1000 J
1 cal = 4.184 J
1kcal = 4.184 kJ
1 kcal = 1 dietary Cal (called the Big Cal)
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Calculate the specific heat of a metal if it
takes 235 J to raise the temperature of a
32.91 g sample by 2.53°C.
q  s m t
q
235 J
J
s


2
.
82
m t 32.91g  2.53  C
gC
Note the units of specific heat:
J per gram per deg Celsius, or J g-1 oC-1.
It tells us how much heat is needed to raise 1 g
of sample by 1 oC.
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Specific Heat of Water
This is how the unit, calorie, was defined.
One calorie is the amount of heat needed to
warm one gram (1 cc) of water by one
degree (Celsius or K, kelvin)….
for example, to raise from 25oC to 26oC.
If we were to use K, it would be from 298 to
299K. Note that ΔT is the same
regardless of whether it is in oC or K.
1 cal ≡ 4.184 J (We will stick to J and kJ.)
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The Sign of q
q = amount of heat transferred
Heat can be either transferred INTO a system
or OUT OF a system.
q is positive if heat is going INTO a system.
q is negative if heat is going OUT OF a system.
Consider a hot cup of coffee. As it sits in the room,
heat is going OUT OF the cup and INTO the
surroundings.
qcup is negative
and
qsurr is positive
The value of q must be the same (amount of heat that
goes into the surrounding must equal to the amount
of heat that left the cup), but must have opposite
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qsurr = - qcup
signs.
The Sign of q
qsurr = - qcup
Note that the negative sign does NOT mean that qsurr
or qcup is negative.
It only means qsurr has an opposite sign to qcup.
e.g. If the cup loses 4 J as it cools down, the
surroundings have gained 4 J.
qcup = – 4 J (Cup lost 4 J)
qsurr = + 4 J (same quantity but – (– 4J)
(same amt of heat, but sign is changed)
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The Sign of q
Now, let’s consider a can of cold soda warming
up to room temperature,
absorbing 5 J.
This equation still holds true…
qsurr = – qcan
qcan = + 5 J
(The can gained 5 J)
qsurr = – (+5 J) (same amt of heat, but sign is
changed)
(Surrounding is giving away 5 J.)
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