Transcript Heat, Temperature Phase Changes And

Heat,
Temperature
And
Phase Changes
Pisgah High School Chemistry
Mr. Jones
Last rev. 5/12/04
Part One
Heat and
Temperature
What temperature
does the thermometer
indicate?
What might be going
on that would cause
this temperature?
This is the view out
the window, past
the thermometer.
Yep.
It is
snowing.
Why would snow cause
the air temperature to be at
precisely 0C?
What occurs at 0C?
Water freezes and ice melts.
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.
The temperature
hovers at zero Celsius
as the snow melts.
So why is
there snow on
the ground if
it is melting?
Yep. That’s
what allows
the snow to
accumulate.
As the snow
melts, it
absorbs heat
and cools the
ground, the
car, and the
grill.
This allows
more snow to
lay. It doesn’t
melt because
the ground is
now at 0C.
What does it mean to have a
temperature of 0 C?
What is
temperature?
Is temperature the same
thing as heat?
Temperature is a measure
of how “hot” or “cold”
something is.
Temperature is measured in
arbitrary units, like
Fahrenheit or Celsius.
Temperature is
proportional to the average
kinetic energy of the
molecules of the substance.
T 
= ½
2
mv
Temperature is therefore
proportional to the speed of
the molecules of a substance.
Velocity or speed
T 
2
= ½ mv
Tv
The higher the temperature,
the greater the average speed
of the molecules.
Velocity or speed
T 
2
= ½ mv
Tv
Heat is the thermal energy
transferred from a hot object
to a cold object.
Heat is measured in energy
units -- Joules or calories.
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.
Heat has the symbol q and
is calculated using …
q = mcDT
Quantity
of heat
specific heat
capacity
q = mcDT
mass
temperature
change
Quantity
of heat
specific heat
capacity
q = mcDT
The specific heat
capacity of water
is 4.18 J/gC
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
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.
q
DT = m c
Answer: 6.20 C
Part Two
Calorimetry and
Specific Heat
Capacity
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.
Investigate:
Suppose two different masses of
water at different temperatures
are mixed. Can you predict the
final temperature?
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?
Investigate:
Develop a procedure where you could
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 would you need?
Investigate:
Develop a procedure where you could
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.
Investigate:
What do you need in a data table?
Mass of calorimeter cup
Mass of cool water
Initial temperature of cool water
Mass of warm water
Initial temperature of hot water
Final temperature after mixing
Feel free to make additions.
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.
Note: Hot plates and boiling water
can cause severe burns.
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.
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.
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
Investigate:
Finally, since DT includes the final
temperature (DT = Tfinal – Tinitial )
then replace DT:
mhc(Tf -Th) = -mcc(Tf -Tc)
Write an equation that solves
for the final temperature.
Investigate:
Use your equation to solve the
following problem:
Calculate the final temperature
when 20.0 grams of water at 85 C
is added to 35.0 grams of water at
10.0 C in an insulated container.
Investigate:
The answer to the following
problem is 37.3 C.
Calculate the final temperature
when 20.0 grams of water at 85 C
is added to 35.0 grams of water at
10.0 C in an insulated container.
In the next investigation you
will … develop a method to
find the specific heat
capacity of a metal.
Specific heat capacity …
1. …varies from one substance to
another
2. …a measure of how much heat
something can “hold”
3. …the amount of heat needed to
raise one gram of a substance
by one Celsius degree
Specific heat capacity …
1. …varies from one substance to
another
2. …a measure of how much heat
something can “hold”
3. …the amount of heat needed to
raise one gram of a substance
by one Celsius degree
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.
Get the initial
temperature of
the metal.
metal
The temperature
of boiling water.
hotplate
Get initial
temp of
water in
calorimeter
cup.
Transfer the metal to
the calorimeter.
Continue stirring until thermal
equilibrium is reached.
Data: Mass of metal
Initial temp of metal
Mass of water
Initial temp of water
Final temp
of water
and metal
qlost = -qgained
mmcmDTm = -(mwcwDTw)
-(mwcwDTw)
cm =
mmDTm
Data: Suppose 104.58 g of a
metal is heated to 100. C.
It is placed into 54.21 g
of water at 17.5 C.
What would be
the specific
heat capacity of
the metal?
The final temp
of water and
metal is 22.0 C.
Data: Suppose 104.58 g of a
metal is heated to 100. C.
It is placed into 54.21 g
of water at 17.5 C.
What could be
the identity of
the metal?
The final temp
of water and
metal is 22.0 C.
Table of selected
specific heats.
Substance
c in
J/g K
Aluminum
Bismuth
Copper
Brass
Gold
Lead
Silver
Tungsten
Zinc
Mercury
Ethanol
Water
Ice
0.900
0.123
0.386
0.380
0.126
0.128
0.233
0.134
0.387
0.140
2.400
4.186
2.050
Part Three
Calorimetry and
Phase Changes
Is heat is absorbed or
released during a phase
change?
How could you measure the
heat absorbed or released as
substances change phase?
Consider ice melting in water.
1. Does the temperature of the
water change?
2. Is the water absorbing or
releasing heat?
3. Does ice absorb heat or release
heat as it melts?
Consider ice melting in water.
1. Does the temperature of the
water change? No
2. Is the water absorbing or
releasing heat? Releasing heat
3. Does ice absorb heat or release
heat as it melts? Absorb heat
Consider ice melting in water.
The word fusion means “melting”.
How could you design an
experiment to measure
the heat of fusion of ice?
Consider ice melting in water.
You could measure
the heat lost by some
water as it cools.
That should equal
the heat gained by
the ice as it melts.
Ice
Consider ice melting in water.
Suppose some ice at
0C is placed into
50.00 g of water at
25.5 C.
Copy down this information
and the data that follows.
Ice
Consider ice melting in water.
When the system
reaches equilibrium
at 0C, 15.95 grams
of the ice has
melted.
Ice
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
Consider ice melting in water.
…we should be able
to compute the heat
of fusion of ice, Hf.
qlost by water= qgained by ice
mwatercDT = miceHf
Ice
Consider ice melting in water.
Go ahead and
calculate the value
of Hf.
qlost by water= qgained by ice
mwatercDT = miceHf
Ice
We now know that heat is
absorbed or released during
a phase change.
Heat is absorbed as solids
melt, or liquids vaporize.
We now know that heat is
absorbed or released during
a phase change.
Heat is released as liquids
freeze, or vapors condense.
Heat is absorbed by the ice.
Ice
And melts.
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
Heat is
released by
the water as
it freezes.
Ice
water
334 joules is released
when one gram of
water freezes at 0C.
Ice absorbs 334 J per gram
as it melts at 0C
Ice
Water releases 334 J per gram
as it freezes at 0C
Heat is absorbed by the
water as it vaporizes.
Hotplate
Heat is absorbed by the
water as it vaporizes.
2260 joules is
absorbed by one
gram of water as
it boils at 100C.
Hotplate
Steam
releases
2260 J/g
as it
condenses
at 100 C
Water
absorbs
2260 J/g
as it boils
at 100 C
Hotplate
Heat is released by water
vapor as it condenses.
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.
Thunderhead
40,000 + feet
The heat released
by condensing
water vapor causes
convection and
updrafts in
thunderstorms.
Phase changes
occur at a constant
temperature as heat
is absorbed or
released.
Question for discussion:
If phase changes occur at a
constant temperature, then
what happens to the heat
when water boils?
Question for discussion – possible answers:
a. Heat energy is converted to matter
(E=mc2) and it stays in the water.
b.The heat increases the speed of the
water molecules.
c. The heat energy breaks the
intermolecular bonds which keep
the water in the liquid phase.
d.The temperature really does
change, you just missed it.
Question for discussion – possible answers:
a. Heat energy is converted to matter
(E=mc2) and it stays in the water.
b.The heat increases the speed of the
water molecules.
c. The heat energy breaks the
intermolecular bonds which keep
the water in the liquid phase.
d.The temperature really does
change, you just missed it.
Question for discussion – possible answers:
a. Heat energy is converted to matter
(E=mc2) and it stays in the water.
b.The heat increases the speed of the
water molecules.
c. The heat energy breaks the
intermolecular bonds which keep
the water in the liquid phase.
d.The temperature really does
change, you just missed it.
The heat gained or lost in
phase changes can be
calculated using …
q = mHf
q = mHv
Heat of
fusion
(melting)
Heat of
vaporization
The values for water are …
Hf = 334 J/g
Hv=2260 J/g
Heat of
fusion
(melting)
Heat of
vaporization
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
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
Part Four
Sublimation
and
Phase Diagrams
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.
Dry ice is solid carbon dioxide, CO2.
At room temperature and
normal atmospheric pressures
dry ice undergoes sublimation.
CO2 vapor
Dry Ice
It goes directly from
the solid state to the
vapor state.
CO2 vapor
Dry ice is solid carbon dioxide, CO2.
CO2 vapor
At room temperature and
CO2 vapor
normal atmospheric pressures
dry ice undergoes sublimation.
CO2 vapor
CO2 vapor
Dry Ice
It goes directly from
the solid state to the
vapor state.
Solid iodine, I2, also undergoes
sublimation when heated.
Watchglass
Beaker
with iodine
Hotplate
Iodine
vapor
fills the
beaker.
Solid iodine crystallizes on the
bottom of the watchglass.
Watchglass
Beaker
with iodine
Hotplate
The color of
the vapor
fades as the
iodine
deposits on
the watchglass
A solid forming directly from
the vapor is called deposition.
Watchglass
Beaker
with iodine
Hotplate
The color of
the vapor
fades as the
iodine
deposits on
the watchglass
Heat is
absorbed as
the iodine
undergoes
sublimation.
Heat is
released as
the iodine
undergoes
deposition.
Pressure
A phase diagram can help explain
why dry ice undergoes sublimation.
2
1
3
Temperature
The phase
diagram
has three
distinct
regions.
Pressure
Which phase is in each region?
2
1
3
Temperature
The phase
diagram
has three
distinct
regions.
Which phase is in each region?
Pressure
Hint: What happens to ice as temperature increases?
2
1= ???
2= ???
1
3
Temperature
3= ???
Which phase is in each region?
Pressure
Hint: What happens to ice as temperature increases?
2
1= Solid
2= ???
1
3
Temperature
3= ???
Which phase is in each region?
Pressure
Hint: What happens to ice as temperature increases?
2
1= Solid
2= Liquid
1
3
Temperature
3= ???
Which phase is in each region?
Pressure
Hint: What happens to ice as temperature increases?
2
1= Solid
2= Liquid
1
3
Temperature
3= Gas
Pressure
The point where all three phases
exist in equilibrium is called the
triple point.
S
L
G
Temperature
At a pressure of 1 atm, most
substances go through all three
phases, as the temperature increases,
S
L
1 atm
G
Temp.
Solids melt to
form liquids,
which
vaporize to
form gases.
At a pressure of 1 atm, most
substances go through all three
phases, as the temperature increases,
S
L
1 atm
G
MP BP
Temp.
Notice the
melting point
and boiling
point.
But the phase diagram for CO2 is a
little different.
Notice that the
triple point is
L
above 1 atm.
5 atm
S
1 atm
G
Temperature
At 1 atm CO2 goes directly from
solid to vapor as the temperature
increases.
S
L
1 atm
G
Temperature
At 1 atm CO2 goes directly from
solid to vapor as the temperature
increases.
S
L
The sublimation
point is –78.5 C
1 atm
G
-78.5
Temperature
Pressure
An arrow will appear in the
following phase diagrams.
Tell what phase
change the
arrow indicates.
Temperature
Pressure
What phase change is occurring?
S
L
Melting (fusion)
G
Temperature
Pressure
What phase change is occurring?
S
L
Vaporization
G
Temperature
Pressure
What phase change is occurring?
S
L
Condensation
G
Temperature
Pressure
What phase change is occurring?
S
L
Sublimation
G
Temperature
Pressure
What phase change is occurring?
S
L
G
Temperature
Liquefying a
gas by
increasing
the pressure.
Part Five
Heating and Cooling
Curves
A process that
absorbs heat is called
endothermic.
A process that gives off
heat is called
exothermic.
Melting (fusion)
Endothermic: Vaporization
Heat is absorbed. Sublimation
Freezing
Exothermic: Condensation
Heat is released. Deposition
Investigate:
Either recall an earlier
experiment, or design an
experiment to look at the
temperature of water as
phase changes take place.
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.
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.
Thermometer
Time
Graph paper
Stirring hotplate
Thermometer
You could use a
thermometer and
plot the temperature
on graph paper or
Time
on a computer.
Graph paper
Stirring hotplate
Thermometer
Time
Stirring hotplate
Temperature
probe
OrCyou
0.0
could use a
thermometer probe
connected to an
Time a
interface
and
CBL,
LabPro,
or
computer
to plot the
computer
temperature.
Stirring hotplate
Temperature
probe
0.0 C
CBL,
LabPro, or
computer
Stirring hotplate
Time
Phase changes occur at a
single temperature.
Water freezes and
ice melts at 0C.
At sea level, water boils and
steam condenses at 100C.
Consider the following
heating curve for water.
Temp
100
0
Time
Consider the following
heating curve for water.
Temp
100
0
Ice at –30C absorbs heat.
Temperature rises to 0C.
Time
Consider the following
heating curve for water.
Temp
100
0
Ice at 0C absorbs
heat and melts at
constant 0C to
make water at 0C.
Time
Consider the following
heating curve for water.
Temp
100
0
When all ice
melts, water at
0C absorbs heat
and temperature
rises to 100C.
Time
Consider the following
heating curve for water.
Temp
100
0
Water absorbs heat
and boils at a constant
temperature of 100C.
Time
Consider the following
heating curve for water.
Temp
100
0
Temperature of
steam rises as it
absorbs heat after all
of the water boils.
Time
What is happening at each
segment of the heating curve?
Temp
100
0
Time
Look at the different regions of
the heating curve for water.
Temp
100
Water
Ice and
water
0
Water and
steam
Steam
Phase changes?
Ice
Time
The temperature is constant
during a phase change.
Temp
100
Water
Ice and
water
0
Water and
steam
Steam
Phase changes
Ice
Time
Calculating heat at each segment
of the heating curve.
Temp
100
0
q1=mciDT
The temperature
of the ice is
increasing. The
specific heat for ice
is 2.05 J/gC.
Time
Calculating heat at each segment
of the heating curve.
Temp
100
0
q2=mHf
q1=mciDT
A phase change
occurs at a constant
temperature. Use
the heat of fusion
since ice is melting.
Time
Calculating heat at each segment
of the heating curve.
Temp
100
0
q3=mcwDT
q2=mHf
q1=mciDT
The temperature
of the water is
increasing. The
specific heat of water
is 4.18 J/gC.
Time
Calculating heat at each segment
of the heating curve.
Temp
100
0
q3=mcwDT
q2=mHf
q1=mciDT
q4=mHv
A phase change
occurs at a constant
temperature. Use the
heat of vaporization
since water is boiling.
Time
Calculating heat at each segment
of the heating curve.
q5=mcsDT
Temp
100
0
q3=mcwDT
q2=mHf
q1=mciDT
q4=mHv
The temperature
of the steam is
increasing. The
specific heat of steam
is 2.02 J/gC.
Time
Calculating heat at each segment
of the heating curve.
q5=mcsDT
Temp
100
0
q3=mcwDT
q2=mHf
q1=mciDT
q4=mHv
Use q=mcDT
when there is a
temperature change.
Time
Calculating heat at each segment
of the heating curve.
q5=mcsDT
Temp
100
0
q3=mcwDT
q2=mHf
q1=mciDT
q4=mHv
Use q=mHf or
q=mHv when there
is a phase change.
Time
Calculating heat at each segment
of the heating curve.
q5=mcsDT
Temp
100
0
q3=mcwDT
q2=mHf
q1=mciDT
q4=mHv
The total amount of
heat absorbed is the sum:
qtot= q1+q2+q3+q4+q5
Time
What would the cooling curve of
steam look like?
Temp
100
0
Time
What would the cooling curve of
steam look like?
Temp
100
Water and
steam
Steam
0
Water
Ice and
water
Time
Ice
What would the cooling curve of
steam look like?
Heat energy
Temp
100
Water and
steam
Steam
0
Water
is released
at each step.
Ice and
water
Time
Ice
There is something else special
about a mixture of ice and water.
Suppose ice and water were
placed into a perfectly insulated
container.
The mixture would stay at a
constant zero degrees Celsius by
establishing an equilibrium.
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.
0.0 C
Acme Digital Thermometer
Ice and
water in an
insulated
container.
0.0 C
Acme Digital Thermometer
Some ice
melts and
forms liquid
water.
0.0 C
Acme Digital Thermometer
Some water
freezes and
forms ice.
0.0 C
Acme Digital Thermometer
When the
rates at
which the ice
melts and the
water freezes
are equal …
0.0 C
Acme Digital Thermometer
… an
equilibrium
is
established.
0.0 C
Acme Digital Thermometer
The amounts
of ice and
water will
remain
constant…
0.0 C
Acme Digital Thermometer
…and the
mixture of ice
and water will
remain at a
constant 0C.
0.0 C
Acme Digital Thermometer
A mixture of
ice and water
can be used to
calibrate a
thermometer
at 0C.
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?
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?
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?
Questions
7. Explain how sweating cools
your body.
8. Explain how liquid water
evaporating from a roadway can
cause ice to form on the road.
Questions
10. What phase
change is
occuring?
Pressure
9. Explain why dry ice doesn’t
form a puddle of liquid CO2.
Temperature
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?
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.
Heat
Deposition
Melting
Temperature
Phase change
Equilibrium
Joule
Phase diagram
Vaporization
Heating curve
Calorie
Condensation
Sublimation
Freezing
Celsius
Boiling