Last rev. 022007a

1
Heat, Temperature And Phase Changes
Pisgah High School Chemistry
Mr. Jones
Last rev. 022007a
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Part One Heat and Temperature
3
What temperature does the thermometer indicate?
4
What might be going on that would cause this
temperature?
5
This is the view out the window, past the
thermometer.
6
Yep.
It is snowing.
7
Why would snow cause the air temperature
to be at precisely 0C?
What occurs at 0C?
Water freezes and ice melts.
8
Ice, in the form of snow, falls through the
slightly warmer air.
The snow melts and absorbs heat from the air,
causing the air to cool.
Ice melts at 0 C, so the air cools to that
temperature.
9
The temperature hovers at zero Celsius as the
snow melts.
10
So why is there snow on the ground if it is
melting?
Yep. Thats what allows the snow to accumulate.
11
As the snow melts, it absorbs heat and cools the
ground, the car, and the grill.
12
This allows more snow to lay. It doesnt melt
because the ground is now at 0C.
13
What does it mean to have a temperature of 0 C?
What is temperature?
Is temperature the same thing as heat?
14
Temperature is a measure of how hot or cold
something is.
Temperature is measured in arbitrary units, like
Fahrenheit or Celsius.
15
Temperature is proportional to the average
kinetic energy of the molecules of the substance.
T µ KE
KE ½ mv2
16
Temperature is therefore proportional to the
speed of the molecules of a substance.
T µ KE
KE ½ mv2
T µ v
17
The higher the temperature, the greater the
average speed of the molecules.
T µ KE
KE ½ mv2
T µ v
18
Heat is the thermal energy transferred from a hot
object to a cold object.
Heat is measured in energy units -- Joules or
calories.
19
The heat transferred is proportional to the mass
of the object, the specific heat capacity of the
object and the temperature change the object
undergoes.
20
Heat has the symbol q and is calculated using
q mcDT
21
q mcDT
22
q mcDT
The specific heat capacity of water is 4.18 J/gC
23
How much heat is needed to raise the temperature
of 25.6 grams of water from 20.0 C to 50.0 C?
q m c DT
q (25.6g)(4.18J/gC)(30.0C)
q 3210 J
24
What is the final temperature of 27.0 grams of
liquid water, initially at 0C, after it absorbs
700.0 J of energy?
q m c DT
Hint start by solving for DT.
Answer
6.20 C
25
Part Two Calorimetry and Specific Heat Capacity
26
Calorimetry is a collection of laboratory
procedures used to investigate the transfer of
heat.
In calorimetry experiments, one might be looking
for a final temperature or a specific heat
capacity.
27
Investigate
Suppose two different masses of water at
different temperatures are mixed. Can you
predict the final temperature?
28
Investigate
Will the final temperature be cooler than the
cool water, or will it be warmer than the warm
water?
Or will the final temperature be somewhere in
between?
29
Investigate
Develop a procedure where you mix a known mass of
cool water with a different mass of water at an
elevated temperature and measure the final
(equilibrium) temperature.
What equipment will you need?
30
Investigate
Develop a procedure where you mix a known mass of
cool water with a different mass of water at an
elevated temperature and measure the final
(equilibrium) temperature.
You could use a balance, a thermometer, a coffee
cup calorimeter, and a hot plate.
31
Investigate
What do we need to record in a data table?
Mass of calorimeter cup _________ Mass of cool
water and cup _________ Mass of cool water
_________ Initial temperature of cool water
_________ Initial temperature of hot water
_________ Final temperature after mixing
_________ Mass of mixed water and cup
_________ Mass of hot water _________
32
Investigate
Whenever we design an experiment we make some
assumptions. Here are a couple, can you add any
more?
The calorimeter cup is a perfect insulator and no
heat is exchanged with the surroundings.
Warning Hot plates and boiling water can cause
severe burns.
33
Investigate
You might need a hint about how to calculate the
results.
What is the law of conservation of energy?
Energy is neither created nor destroyed, only
changed in form.
34
Investigate
You might need a hint about how to calculate the
results.
The law of conservation of energy suggests that
the heat lost by the hot water as it cools is
equal to the heat gained by the cool water as it
warms up.
35
Investigate
To put it mathematically
qlost -qgained
Heat lost by the hot water
Heat gained by the cold water

And since q mcDT then
mhcDTh -mccDTc
36
Investigate
The convention for DT is final temperature minus
initial temperature or Tfinal Tinitial
mhcDTh -mccDTc
becomes
mhc(Tf -Th) -mcc(Tf -Tc)
Use your algebra skills, to solve for Tf , the
final temperature.
37
How did our predicted Tf compare to the one we
observed?
38
In the next investigation you will
develop a method to find the specific heat
capacity of a metal.
39
Specific heat capacity

• varies from one substance to another.
• a measure of how much heat something can hold.
• the amount of heat needed to raise one gram of a
  substance by one Celsius degree.

40
Specific heat capacity lab suggestions

1. Heat a metal to a known temp.
2. Transfer the metal to a known quantity of water
   at a known temperature.
3. Measure the equilibrium temperature.
4. Use qlost -qgained to compute the specific heat
   of the metal.

41
Get the initial temperature of the metal.
The temperature of boiling water.
metal
hotplate
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Get initial temp of water in calorimeter cup.
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Transfer the metal to the calorimeter.
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Continue stirring until thermal equilibrium is
reached.
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Data
Mass of metal Initial temp of metal
Mass of water Initial temp of water
Final temp of water and metal
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qlost -qgained
mmcmDTm -mwcwDTw
-mwcwDTw
cm
mmDTm
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Mass of metal 40.0 g Initial T of metal 98.0
C Mass of water in calorimeter 60.0 g Initial T
of water 20.0 C Final T of water and metal 22.9 C
Calculate the specific heat capacity of the metal.
48
Table of selected specific heats.
What is the unknown metal?
49
Part Three Calorimetry and Phase Changes
50
Is heat is absorbed or released during a phase
change?
How could you measure the heat absorbed or
released as substances change phase?
51
Consider ice melting in water.
1. What is the temperature of a mixture of ice
and water?
0 C
2. Does the temperature of the water change?
No
3. Is the water absorbing or releasing heat?
Releasing heat
52
Consider ice melting in water.
4. Does ice absorb heat or release heat as it
melts?
Absorbs heat
5. What is the temperature of the water from the
melting ice?
0 C
6. When will the temperature of the water change?
When all ice melts
53
Consider ice melting in water.
The word fusion means melting.
How could you design an experiment to measure the
heat of fusion of ice?
54
Consider ice melting in water.
You could measure the heat lost by some water as
it cools.
Ice
That should equal the heat gained by the ice as
it melts.
55
Consider ice melting in water.
Suppose some ice at 0C is placed into 50.00 g of
water at 25.5 C.
Ice
Copy down this information and the data that
follows.
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Consider ice melting in water.
When the system reaches equilibrium at 0C, 15.95
grams of the ice has melted.
Ice
57
Consider ice melting in water.
Knowing that the heat lost by the water as it
cools to 0C is equal to the heat gained by the
ice as it melts at 0C
Ice
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Consider ice melting in water.
we should be able to compute the heat of fusion
of ice, Hf.
Ice
qlost by water -qgained by ice
mwatercDT -miceHf
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Consider ice melting in water.
Go ahead and calculate the value of Hf.
Ice
qlost by water -qgained by ice
mwatercDT -miceHf
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We now know that heat is either absorbed or
released during a phase change.
Heat is absorbed as solids melt, or liquids
vaporize.
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We now know that heat is either absorbed or
released during a phase change.
Heat is released as liquids freeze, or vapors
condense.
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Heat is absorbed by the ice.
Ice
And melts.
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Heat is absorbed by the ice.
One gram of ice at 0C absorbs 334 J as it melts
to form water at 0C.
making liquid water
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Heat is released by the water as it freezes.
334 joules is released when one gram of water
freezes at 0C.
Ice
water
65
Ice absorbs 334 J per gram as it melts at 0C
Ice
Water releases 334 J per gram as it freezes at 0C
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Heat is absorbed by the water as it vaporizes.
Hotplate
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Heat is absorbed by the water as it vaporizes.
2260 joules is absorbed by one gram of water as
it boils at 100C.
Hotplate
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Steam releases 2260 J/g as it condenses at 100 C
Water absorbs 2260 J/g as it boils at 100 C
Hotplate
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Heat is released by water vapor as it condenses.
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Heat is released by water vapor as it condenses.
The heat released by condensing water vapor is a
major factor in weather phenomena like
thunderstorms and hurricanes.
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Thunderhead
40,000 feet
The heat released by condensing water vapor
causes convection and updrafts in thunderstorms.
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Phase changes occur at a constant temperature as
heat is absorbed or released.
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Question for discussion
If phase changes occur at a constant temperature,
then what happens to the heat when water boils?
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Question for discussion possible answers

1. Heat energy is converted to matter (Emc2) making
   the water heavier.
2. The heat increases the speed of the water
   molecules.
3. The heat energy breaks the intermolecular bonds
   which keep the water in the liquid phase.
4. The temperature really does change, you just
   missed it.

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Question for discussion possible answers

1. Heat energy is converted to matter (Emc2) making
   the water heavier.
2. The heat increases the speed of the water
   molecules.
3. The heat energy breaks the intermolecular bonds
   which keep the water in the liquid phase.
4. The temperature really does change, you just
   missed it.

76
The heat gained or lost in phase changes can be
calculated using
q mHf
q mHv
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The values for water are
Heat of fusion (melting)
Hf 334 J/g
Hv2260 J/g
Heat of vaporization
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How much heat is absorbed by 150.0 g of ice as it
melts at 0C?
q m Hf
q (150.0 g)(334 J/g)
q 50,100 J
or 50.1 kJ
79
How much heat is released by 20.0 grams of steam
as it condenses at 100C?
q m Hv
q (20.0 g)(2260 J/g)
q 45,200 J
or 45.2 kJ
80
Part Four Sublimation and Phase Diagrams
81
Sublimation is an unusual phase change.
Sublimation occurs when a solid changes directly
into a gas without going through the liquid phase.
Heat is absorbed when sublimation occurs.
82
Solid iodine, I2, undergoes sublimation when
heated.
Iodine vapor fills the beaker.
Watchglass
Beaker with iodine
Hotplate
83
Solid iodine crystallizes on the bottom of the
watchglass.
The color of the vapor fades as the iodine
deposits on the watchglass
Watchglass
Beaker with iodine
Hotplate
84
A solid forming directly from the vapor is called
deposition.
The color of the vapor fades as the iodine
deposits on the watchglass
Watchglass
Beaker with iodine
Hotplate
85
Here we see the sublimation and deposition of
iodine.
86
Heat is absorbed as the iodine undergoes
sublimation.
87
Heat is released as the iodine undergoes
deposition.
88
Dry ice is solid carbon dioxide, CO2.
At room temperature and normal atmospheric
pressures dry ice undergoes sublimation.
It goes directly from the solid state to the
vapor state.
Dry Ice
CO2 vapor
89
CO2 vapor
Dry ice is solid carbon dioxide, CO2.
CO2 vapor
At room temperature and normal atmospheric
pressures dry ice undergoes sublimation.
CO2 vapor
CO2 vapor
It goes directly from the solid state to the
vapor state.
Dry Ice
CO2 vapor
90
A phase diagram can help explain why dry ice
undergoes sublimation.
The phase diagram has three distinct regions.
2
1
3
91
Which phase is in each region?
The phase diagram has three distinct regions.
2
1
3
92
Which phase is in each region?
Hint What happens to ice as temperature
increases?
1 ???
2
2 ???
1
3 ???
3
93
Which phase is in each region?
Hint What happens to ice as temperature
increases?
1 Solid
2
2 ???
1
3 ???
3
94
Which phase is in each region?
Hint What happens to ice as temperature
increases?
1 Solid
2
2 Liquid
1
3 ???
3
95
Which phase is in each region?
Hint What happens to ice as temperature
increases?
1 Solid
2
2 Liquid
1
3 Gas
3
96
The point where all three phases exist in
equilibrium is called the
triple point.
triple point.
L
S
Pressure
G
Temperature
97
At a pressure of 1 atm, most substances go
through all three phases, as the temperature
increases,
L
S
Solids melt to form liquids, which vaporize to
form gases.
G
Temp.
98
At a pressure of 1 atm, most substances go
through all three phases, as the temperature
increases,
L
S
Notice the melting point and boiling point.
G
MP
BP
Temp.
99
But the phase diagram for CO2 is a little
different.
Notice that the triple point is above 1 atm.
L
S
G
Temperature
100
At 1 atm CO2 goes directly from solid to vapor as
the temperature increases.
L
S
G
Temperature
101
At 1 atm CO2 goes directly from solid to vapor as
the temperature increases.
L
S
The sublimation point is 78.5 C
G
Temperature
102
Carbon dioxide is a liquid at the bottom of the
ocean where the pressure is well above 5
atmospheres.
http//www.mbari.org/ghgases/deep/release.htm
103
For more common substances we see a phase diagram
like this.
The line for one atmosphere of pressure tells us
that all three phases can exist.
Pressure
Temperature
104
An arrow will appear in the following phase
diagrams.
Tell what phase change the arrow indicates.
Pressure
Temperature
105
What phase change is occurring?
Melting (fusion)
Pressure
Temperature
106
What phase change is occurring?
Vaporization
Pressure
Temperature
107
What phase change is occurring?
Condensation
Pressure
Temperature
108
What phase change is occurring?
Sublimation
Pressure
Temperature
109
What phase change is occurring?
Liquefying a gas by increasing the pressure.
Pressure
Temperature
110
What do the lines between the different regions
represent?
Each line represents an equilibrium between two
phases.
111
Equilibria occur at the boundaries between the
regions.
The equilibrium between the solid and liquid
phases.
Pressure
Temperature
112
Equilibria occur at the boundaries between the
regions.
The equilibrium between the liquid and gaseous
phases.
Pressure
Temperature
113
Equilibria occur at the boundaries between the
regions.
The equilibrium between the solid and gaseous
phases.
Pressure
Temperature
114
Consider the equilibrium between two phases.
Ice and water are in an insulated container.
Pressure
Temperature
115
Ice and water are in an insulated container.
116
Some ice melts and forms liquid water.
117
Some water freezes and forms ice.
118
When the rates at which the ice melts and the
water freezes are equal
119
an
equilibrium is established.
120
The amounts of ice and water will remain constant
121
and the mixture of ice and water will remain at
a constant 0C.
122
A mixture of ice and water can be used to
calibrate a thermometer at 0C.
123
Thats because phase changes occur at a single
temperature.
Water freezes and ice melts at 0C.
At sea level, water boils and steam condenses at
100C.
124
Therefore, it can be seen that when ice and water
are placed into a perfectly insulated container

the mixture will stay at a constant zero
degrees Celsius by establishing an equilibrium.
125
An ice/water equilibrium occurs when the rate at
which water freezes is equal to the rate at which
ice melts.
The amount of ice and water will never change.
If the container is completely insulated.
126
Part Five Heating and Cooling Curves
127
A process that absorbs heat is called endothermic.
A process that gives off heat is called
exothermic.
128
Melting (fusion) Vaporization Sublimation
Endothermic
Heat is absorbed.
Freezing Condensation Deposition
Exothermic
Heat is released.
129
Investigate
Heat the mixture of water and ice on a hotplate
and record the temperature as a function of time.
130
Investigate
The following are suggested procedures you could
use to record the temperature of water at regular
intervals.
Note Hot plates and boiling water can cause
severe burns.
131
Investigate

1. Clamp a thermometer with the bulb in a mixture of
   ice and water in a beaker on a hot plate. (The
   hot plate is off.)
2. Allow the temperature to equilibrate.
3. Turn on the hot plate and continue to record
   temperature at regular intervals until some of
   the water boils away.
4. Plot temperature as a function of time.

132
Thermometer
Temperature
Time
Graph paper
Stirring hotplate
133
Thermometer
Stirring hotplate
134
Temperature probe
0.0 C
Temperature
Time
CBL, LabPro, or computer
Stirring hotplate
135
Consider the following heating curve for water.
136
Consider the following heating curve for water.
Ice at 30C absorbs heat. Temperature rises to 0C.
137
Consider the following heating curve for water.
Ice at 0C absorbs heat and melts at constant 0C
to make water at 0C.
138
Consider the following heating curve for water.
When all ice melts, water at 0C absorbs heat and
temperature rises to 100C.
139
Consider the following heating curve for water.
Water absorbs heat and boils at a constant
temperature of 100C.
140
Consider the following heating curve for water.
Temperature of steam rises as it absorbs heat
after all of the water boils.
141
What is happening at each segment of the heating
curve?
142
Look at the different regions of the heating
curve for water.
Water and steam
Steam
Water
Ice and water
Phase changes?
Ice
143
The temperature is constant during a phase change.
Water and steam
Steam
Water
Ice and water
Phase changes
Ice
144
Calculating heat at each segment of the heating
curve.
The temperature of the ice is increasing.
The specific heat for ice is 2.05 J/gC.
q1mciDT
145
Calculating heat at each segment of the heating
curve.
A phase change occurs at a constant temperature.
Use the heat of fusion since ice is melting.
q2mHf
q1mciDT
146
Calculating heat at each segment of the heating
curve.
q3mcwDT
The temperature of the water is increasing.
The specific heat of water is 4.18 J/gC.
q2mHf
q1mciDT
147
Calculating heat at each segment of the heating
curve.
q4mHv
q3mcwDT
A phase change occurs at a constant
temperature. Use the heat of vaporization since
water is boiling.
q2mHf
q1mciDT
148
Calculating heat at each segment of the heating
curve.
q5mcsDT
q4mHv
q3mcwDT
The temperature of the steam is
increasing. The specific heat of steam is 2.02
J/gC.
q2mHf
q1mciDT
149
Calculating heat at each segment of the heating
curve.
q5mcsDT
q4mHv
q3mcwDT
Use qmcDT when there is a temperature change.
q2mHf
q1mciDT
150
Calculating heat at each segment of the heating
curve.
q5mcsDT
q4mHv
q3mcwDT
Use qmHf or qmHv when there is a phase
change.
q2mHf
q1mciDT
151
Calculating heat at each segment of the heating
curve.
q5mcsDT
q4mHv
q3mcwDT
The total amount of heat absorbed is the sum
q2mHf
qtot q1q2q3q4q5
q1mciDT
152
What would the cooling curve of steam look like?
153
What would the cooling curve of steam look like?
Water and steam
Water
Steam
Ice and water
Ice
154
What would the cooling curve of steam look like?
Heat energy is released at each step.
Water and steam
Water
Steam
Ice and water
Ice
155
Why is a steam burn worse than one from boiling
water, even if both are at 100C?
Water and steam
Water
Steam
Ice and water
Ice
156
Part Six Fractional Distillation
157
Initial Observations

• A clear, colorless liquid has a strong, odor.
  When placed on a watch glass and ignited, it
  burns, but not completely. Some nonflammable
  liquid remains.

Is the liquid a pure substance? Is the liquid a
mixture? Heterogeneous or homogeneous?
158
How might you separate a mixture of two clear,
colorless liquids?

• What if the liquids have different boiling points?

159
Heating curve for two liquids, A and B
160
Fractional distillation can be used to separate
the mixture into its various fractions.

• Isolate each fraction at each of the different
  boiling points.

161
Collecting the first fraction
B
3
A
Collect in the first test tube what comes off in
region 1.
2
1
162
Collecting the second fraction
B
3
A
Collect in the second test tube what comes off in
region 2.
2
1
163
Collecting the third fraction
B
3
A
Collect in the third test tube what comes off in
region 3.
2
1
164
Predict what is in each tube
Test tube 1
May contain only A
Test tube 2
May contain both A and B
Test tube 3
May contain only B
165
Equipment setup for doing fractional
distillation.
Ring stand and finger clamp
thermometer or temperature probe
sidearm
rubber tubing
Jones condenser
boiling flask
beaker or test tube
hot plate
166
Change the beaker or test tube to collect each
fraction.
Ring stand and finger clamp
thermometer or temperature probe
sidearm
rubber tubing
Jones condenser
boiling flask
beaker or test tube
hot plate
167
Test the contents of each test tube for

1. Odor does it have an odor? Is it strong or
   weak?
2. Flammability does it burn? A lot or a little?

Place a small amount on a watch glass and ignite
it with a match.
168
Record your observations
TT Start Temp End Temp Odor Flammability
1
2
3
169
Questions?
170
Questions
1. Ice and water are placed in an insulated
container. What will be the equilibrium
temperature?
2. A substance freezes at -80.0C. At what
temperature does it melt?
171
Questions
3. A liquid gradually turns solid at a constant
temperature. Is heat being added, or removed?
4. How does melting snow affect the air
temperature?
172
Questions
5. When water vapor condenses to form liquid
water, is heat released or absorbed?
6. What is the connection between condensing
water vapor and updrafts in thunderstorms?
173
Questions
7. Explain how sweating cools your body.
8. Explain how liquid water evaporating from a
roadway can cause black ice to form on the road.
174
Questions
9. Explain why dry ice doesnt form a puddle of
liquid CO2.
175
Questions
11. How many joules of heat are released when
50.0 grams of water cools from 80.0C to 20.0C?
12. How many joules of heat are needed to melt
15.0 grams of ice at 0C?
176
Questions
13. How many joules of heat are needed to take
15.0 grams of ice at 20.0C to steam at 150.0C?
14. How many grams of steam must cool from
125.0C to 80.0C to release 2.00 x 106 J of energy?
Hint there are three changes that take place.
177
Questions
15. Based on your experience in doing fractional
distillation, how practical would it be to
separate a mixture of benzene and ethanol?
Benzene has a boiling point of 80.C. Be sure to
fully explain your reasoning.
178
Heat
Deposition
Melting
Temperature
Phase change
Equilibrium
Joule
Phase diagram
Vaporization
Heating curve
Calorie
Condensation
Sublimation
Freezing
Boiling
Celsius