Last rev. 022007a

Last rev. 022007a

Pisgah High School Chemistry Pisgah High School Chemistry

Mr. Jones

Heat, Temperature

And

Phase Changes

Heat, Temperature

And

Phase Changes

Part OneHeat and

Temperature

Part OneHeat 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.

Ice melts at 0 C, so the air cools to that temperature.

The snow melts and absorbs heat from the air,

causing the air to cool.

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 is temperature?

What does it mean to have a temperature of 0 C?

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 mv2

Temperature is therefore proportional to the speed of the molecules of a substance.

T ½ mv2

Velocity or speed

T v

The higher the temperature, the greater the average speed of the molecules.

T ½ mv2

Velocity or speed

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 = mcT

q = mcT

Quantity of heat

mass

specific heat capacity

temperature change

q = mcT

Quantity of heat

specific heat capacity

The specific heat capacity of water

is 4.18 J/gC

q = m c T

How much heat is needed to raise the temperature of 25.6 grams of water from 20.0 C to 50.0 C?

q = (25.6g)(4.18J/gC)(30.0C)

q = 3210 J

q = m c T

What is the final temperature of 27.0 grams of liquid water, initially at 0C, after it absorbs 700.0 J of energy?

Hint: start by solving for T.

=qT m c

Answer: 6.20 C

Part TwoCalorimetry and

Specific Heat Capacity

Part TwoCalorimetry 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?

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:

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?

You could use a balance, a thermometer, a coffee cup calorimeter, and a hot plate.

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 do we need to record in a data table?

Investigate:

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 _________

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.

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

And since q = mcT then

mhcTh = -mccTc

Heat lost by the hot water =

Heat gained by the cold water

Investigate:The convention for T is final temperature minus initial temperature or Tfinal – Tinitial

mhc(Tf -Th) = -mcc(Tf -Tc)Use your algebra skills, to solve for Tf , the final temperature.

mhcTh = -mccTc

becomes

How did our predicted Tf compare to the one we

observed?

In the next investigation you will …develop a method to

find the specific heat capacity of a metal.

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.

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.

hotplate

Get the initial temperature of the metal.

The temperature of boiling water.

metal

Get initial temp of water in calorimeter cup.

Transfer the metal to the calorimeter.

Continue stirring until thermal equilibrium is reached.

Data: Mass of metalInitial temp of metal

Mass of waterInitial temp of water

Final temp of water and metal

qlost = -qgained

mmcmTm = -mwcwTw

-mwcwTw

mmTm

cm =

Mass of metal 40.0 gInitial T of metal 98.0 CMass of water in calorimeter 60.0 gInitial T of water 20.0 CFinal T of water and metal 22.9 C

Calculate the specific heat capacity of the

metal.

Aluminum 0.900Bismuth 0.123Copper 0.386Brass 0.380Gold 0.126Lead 0.128Silver 0.233Tungsten 0.134Zinc 0.387Mercury 0.140Ethanol 2.400Water 4.186Ice 2.050

Substancec in

J/g KTable of selected specific heats.

What is the unknown

metal?

Part ThreeCalorimetry and Phase Changes

Part ThreeCalorimetry 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?

2. Does the temperature of the water change?

Consider ice melting in water.

No

3. Is the water absorbing or releasing heat? Releasing heat

1. What is the temperature of a mixture of ice and water?0 C


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


The word fusion means “melting”.

How could you design an experiment to measure

the heat of fusion of ice?


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


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.


When the system reaches equilibrium at 0C, 15.95 grams of the ice has melted.

Ice


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


…we should be able to compute the heat of fusion of ice, Hf.

qlost by water= -qgained by ice

mwatercT = -miceHf

Ice


Go ahead and calculate the value of Hf.

qlost by water= -qgained by ice

mwatercT = -miceHf

Ice

We now know that heat is either absorbed or released during a phase change.

Heat is absorbed as solids melt, or liquids vaporize.

Heat is released as liquids freeze, or vapors condense.

We now know that heat is either absorbed or released during a phase change.

Ice And melts.

Heat is absorbed by the ice.

Heat is absorbed by the ice.

… making liquid water

One gram of ice at 0C absorbs 334 J as it melts to form water at 0C.

water

Heat is released by the water as it freezes.

334 joules is released when one gram of water freezes at 0C.

Ice

Ice

Ice absorbs 334 J per gram as it melts at 0C

Water releases 334 J per gram as it freezes at 0C

Heat is absorbed by the water as it vaporizes.

Hotplate

2260 joules is absorbed by one gram of water as it boils at 100C.

Hotplate

Heat is absorbed by the water as it vaporizes.

Hotplate

Water absorbs

2260 J/g as it boils at 100 C

Steam releases 2260 J/g as it condenses at 100 C

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.

40,000 + feet

Thunderhead

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.

If phase changes occur at a constant temperature, then what happens to the heat when water boils?

Question for discussion:

a. Heat energy is converted to matter (E=mc2) making the water heavier.

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) making the water heavier.

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:

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 FourSublimation

and Phase Diagrams

Part FourSublimation

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.

Solid iodine, I2, undergoes sublimation when heated.

Hotplate

Watchglass

Beaker with iodine

Iodine vapor fills the beaker.

Solid iodine crystallizes on the bottom of the watchglass.

Hotplate

Watchglass

Beaker with iodine

The color of the vapor fades as the iodine deposits on the watchglass

A solid forming directly from the vapor is called deposition.

Hotplate

Watchglass

Beaker with iodine

The color of the vapor fades as the iodine deposits on the watchglass

Here we see the sublimation and

deposition of iodine.

Heat is absorbed as the iodine undergoes sublimation.

Heat is released as the iodine undergoes

deposition.

Dry Ice

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.

CO2 vapor

Dry Ice

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.

CO2 vapor

CO2 vapor

CO2 vapor

CO2 vapor

CO2 vapor

A phase diagram can help explain why dry ice undergoes sublimation.

Temperature

Pre

ssur

e

The phase diagram has three distinct regions.

2

31

Which phase is in each region?

Temperature

Pre

ssur

e

The phase diagram has three distinct regions.

2

31


Temperature

Pre

ssur

e

1= ???

1

2

3

2= ???

3= ???

Hint: What happens to ice as temperature increases?


Temperature

Pre

ssur

e

1= Solid

1

2

3

2= ???

3= ???



Temperature

Pre

ssur

e

1= Solid

1

2

3

2= Liquid

3= ???



Temperature

Pre

ssur

e

1= Solid

1

2

3

2= Liquid

3= Gas


The point where all three phases exist in equilibrium is called the

Temperature

S L

G

triple point. triple point.

Pre

ssur

e

At a pressure of 1 atm, most substances go through all three phases, as the temperature increases,

Temp.

S L

G1 atm

Solids melt to form liquids, which vaporize to form gases.

Temp.

S L

G1 atm

MP BP

Notice the melting point and boiling point.

At a pressure of 1 atm, most substances go through all three phases, as the temperature increases,

But the phase diagram for CO2 is a little different.

Temperature

S L

G1 atm

Notice that the triple point is above 1 atm.

5 atm

At 1 atm CO2 goes directly from solid to vapor as the temperature increases.

Temperature

S L

G1 atm

At 1 atm CO2 goes directly from solid to vapor as the temperature increases.

Temperature

S L

G1 atm

The sublimation point is –78.5 C

-78.5

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

Temperature

Pre

ssur

e

The line for one atmosphere of

pressure tells us that all three

phases can exist.

For more common substances we see a phase diagram like this.

1 atm

Temperature

Pre

ssur

e

Tell what phase change the

arrow indicates.

An arrow will appear in the following phase diagrams.

What phase change is occurring?

Melting (fusion)

Temperature

Pre

ssur

e

S L

G


Temperature

Pre

ssur

e

VaporizationS L

G


Temperature

Pre

ssur

e

CondensationS L

G


Temperature

Pre

ssur

e

SublimationS L

G


Temperature

Pre

ssur

e

Liquefying a gas by increasing the pressure.

S L

G

What do the lines between the different regions represent?

Each line represents an

equilibrium between two phases.

Equilibria occur at the boundaries between the regions.

Temperature

Pre

ssur

e

The equilibrium between the solid and liquid phases.

S L

G


Temperature

Pre

ssur

e

The equilibrium between the liquid and gaseous phases.

S L

G


Temperature

Pre

ssur

e

The equilibrium between the solid and gaseous phases.

S L

G

Consider the equilibrium between two phases.

Temperature

Pre

ssur

e

S L

G

Ice and water are in an insulated container.

Ice and water are in an insulated container.

Acme Digital Thermometer0.0 C

Some ice melts and forms liquid water.


Some water freezes and forms ice.


When the rates at which the ice melts and the water freezes are equal …


equilibrium is

established.


… an

The amounts of ice and water will remain constant…


…and the mixture of ice and water will remain at a constant 0C.


A mixture of ice and water can be used to calibrate a thermometer at 0C.


That’s 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.

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.

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.

Part FiveHeating and Cooling

Curves

Part FiveHeating and Cooling

Curves

A process that gives off heat is called

exothermic.

A process that absorbs heat is called endothermic.

Exothermic:

Endothermic:

FreezingCondensationDeposition

Melting (fusion)

VaporizationSublimationHeat is absorbed.

Heat is released.

Heat the mixture of water and ice on a hotplate and record the temperature as a function of time.

Investigate:

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.

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.

Investigate:

Thermometer

Stirring hotplate

TimeTem

pera

ture

Graph paper

Thermometer

Stirring hotplate

Time

Tem

pera

ture

CBL, LabPro, or computer

0.0 C

Stirring hotplate

Time

Tem

pera

ture

Temperature probe

Consider the following heating curve for water.

0

100

Time

Temp

0

100

Time

Temp

Ice at –30C absorbs heat. Temperature rises to 0C.


0

100

Time

Temp

Ice at 0C absorbs heat and melts at constant 0C to make water at 0C.


0

100

Time

TempWhen all ice melts, water at 0C absorbs heat and temperature rises to 100C.


0

100

Time

Temp

Water absorbs heat and boils at a constant temperature of 100C.


0

100

Time

Temp

Temperature of steam rises as it

absorbs heat after all of the water boils.


What is happening at each segment of the heating curve?

0

100

Time

Temp

Look at the different regions of the heating curve for water.

0

100

Time

Temp

Ice

Ice andwater

Water

Steam

Phase changes?

Water andsteam

Water andsteam

The temperature is constant during a phase change.

0

100

Time

Temp

Ice

Ice andwater

Water

Steam

Phase changes

Calculating heat at each segment of the heating curve.

0

100

Time

Temp

q1=mciT

The temperature of the ice is

increasing. The specific heat for ice

is 2.05 J/gC.


0

100

Time

Temp

q2=mHf

q1=mciT

A phase change occurs at a constant

temperature. Use the heat of fusion

since ice is melting.


0

100

Time

Temp

q2=mHf

q1=mciT

q3=mcwT The temperature of the water is

increasing. The specific heat of water

is 4.18 J/gC.


0

100

Time

Temp

q2=mHf

q1=mciT

q3=mcwTq4=mHv

A phase change occurs at a constant

temperature. Use the heat of vaporization

since water is boiling.


0

100

Time

Temp

q2=mHf

q1=mciT

q3=mcwTq4=mHv

q5=mcsT

The temperature of the steam is

increasing. The specific heat of steam

is 2.02 J/gC.


0

100

Time

Temp

q2=mHf

q1=mciT

q3=mcwTq4=mHv

q5=mcsT

Use q=mcT when there is a

temperature change.


0

100

Time

Temp

q2=mHf

q1=mciT

q3=mcwTq4=mHv

q5=mcsT

Use q=mHf or q=mHv when there is a phase change.


0

100

Time

Temp

q2=mHf

q1=mciT

q3=mcwTq4=mHv

q5=mcsT

The total amount of heat absorbed is the sum:

qtot= q1+q2+q3+q4+q5

What would the cooling curve of steam look like?

0

100

Time

Temp


IceIce andwater

Water

Water andsteam

Steam0

100

Time

Temp


IceIce andwater

Water

Water andsteam

Steam

Heat energy is released at each step.

0

100

Time

Temp

Why is a steam burn worse than one from boiling water, even if

both are at 100C?

IceIce andwater

Water

Water andsteam

Steam0

100

Time

Temp

Part SixFractionalDistillation

Part SixFractionalDistillation

Initial ObservationsA 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?

How might you separate a mixture of two clear, colorless liquids?

What if the liquids have different boiling points?

Heating curve for two liquids, A and B

time

tem

pera

ture

Boiling point of liquid A

Boiling point of liquid B

Fractional distillation can be used to separate the mixture into its various “fractions”.

Isolate each fraction at each of the different boiling points.

Collecting the first fraction

time

tem

pera

ture

12

3

A

B

Collect in the first test tubewhat comes off in region 1.

Collecting the second fraction

time

tem

pera

ture

12

3

A

B

Collect in the second test tubewhat comes off in region 2.

Collecting the third fraction

time

tem

pera

ture

12

3

A

B

Collect in the third test tubewhat comes off in region 3.

Predict what is in each tube

time

tem

pera

ture

A

B

12

3

Test tube 1May contain only A

Test tube 2

Test tube 3

May contain both A and B

May contain only B

Equipment setup for doing fractional distillation.

beaker or test tube

thermometer or temperature probe

boiling flask

hot plate

sidearm

Jones condenser

Ring stand and finger clamp

rubber tubing

Change the beaker or test tube to collect each fraction.

beaker or test tube

thermometer or temperature probe

boiling flask

hot plate

sidearm

Jones condenser

Ring stand and finger clamp

rubber tubing

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.

Record your observations

TT#Start

Temp

End

TempOdor Flammability

1

2

3

Questions?Questions?

Questions

2. A substance freezes at -80.0C. At what temperature does it melt?

1. Ice and water are placed in an insulated container. What will be the equilibrium temperature?

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 black ice to form on the road.

Questions

9. Explain why dry ice doesn’t form a puddle of liquid CO2.

10. What phase change is occuring?

Temperature

Pre

ssur

e

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.

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.

Heat

Temperature Phase change

Melting

BoilingFreezing

CondensationSublimation

Equilibrium Joule

Calorie

Celsius

Deposition

VaporizationHeating curve

Phase diagram

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