Last rev. 5/12/04 Pisgah High School Chemistry Mr. Jones Heat, Temperature And Phase Changes

Last rev. 5/12/04

Pisgah High School Chemistry

Mr. Jones

Heat, Temperature

And

Phase Changes

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

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 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?

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

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 do you need in a data table?

Investigate:

Mass of calorimeter cupMass of cool waterInitial temperature of cool waterMass of warm waterInitial temperature of hot waterFinal 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

And since q = mcT then

mhcTh = -mccTc

Heat lost by the hot water =

Heat gained by the cold water

Investigate:

Finally, since T includes the final temperature (T = Tfinal – Tinitial ) then replace T:

mhc(Tf -Th) = -mcc(Tf -Tc)

Write an equation that solves for the final temperature.

Investigate:

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.

Use your equation to solve the following problem:

Investigate:

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.

The answer to the following problem is 37.3 C.

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.

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 =

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.

The final temp of water and metal is 22.0 C.

What would be the specific heat capacity of the metal?

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.

The final temp of water and metal is 22.0 C.

What could be the identity 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.

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?

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?


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?

No

Releasing heat

Absorb heat


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 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 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) 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:











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

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

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

Solid iodine, I2, also 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

Heat is absorbed as the iodine undergoes sublimation.

Heat is released as the iodine undergoes

deposition.

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

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

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.

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.

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

TimeTem

pera

ture

Graph paper

You could use a thermometer and

plot the temperature on graph paper or

on a computer.

Thermometer

Stirring hotplate

Time

Tem

pera

ture

CBL, LabPro, or computer

0.0 C

Stirring hotplate

Time

Tem

pera

ture

Temperature probe Or you could use a

thermometer probe connected to an interface and a

computer to plot the temperature.

CBL, LabPro, or computer

0.0 C

Stirring hotplate

Time

Tem

pera

ture

Temperature probe

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.

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

Water andsteam

Steam

Phase changes?

The temperature is constant during a phase change.

0

100

Time

Temp

Ice

Ice andwater

Water

Water andsteam

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


0

100

Time

Temp

IceIce andwater

Water

Water andsteam

Steam


0

100

Time

Temp

IceIce andwater

Water

Water andsteam

Steam

Heat energy is released at each step.

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.

Ice and water 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.


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 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.

Heat

Temperature Phase change

Melting

BoilingFreezing

CondensationSublimation

Equilibrium Joule

Calorie

Celsius

Deposition

VaporizationHeating curve

Phase diagram

Documents

Last rev. 5/12/04 Pisgah High School Chemistry Mr. Jones Heat, Temperature And Phase Changes