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

2. 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. Water freezes and ice melts.
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. That’s 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
This allows more snow to lay. It doesn’t 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.
Velocity or speed
T µ KE
KE = ½ mv2
T µ v

17. The higher the temperature, the greater the average speed of the molecules.
Velocity or speed
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 mass specific heat capacity Quantity of heat
temperature change

22. The specific heat capacity of water is 4.18 J/gC
Quantity of heat
q = mcDT
The specific heat capacity of water is 4.18 J/gC

23. q = m c DT q = (25.6g)(4.18J/gC)(30.0C) q = 3210 J
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. q = m c DT = q DT m c Answer: 6.20 C
What is the final temperature of 27.0 grams of liquid water, initially at 0C, after it absorbs J of energy?
q = m c DT
Hint: start by solving for DT. = q DT
m c
Answer:
6.20 C

25. Calorimetry and Specific Heat Capacity
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 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?

30. 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.

31. 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.

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.
Note: 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. qlost = -qgained = mhcDTh = -mccDTc 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. mhc(Tf -Th) = -mcc(Tf -Tc)
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.

37. 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 grams of water at 10.0 C in an insulated container.

38. 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 grams of water at 10.0 C in an insulated container.

39. In the next investigation you will …
develop a method to find the specific heat capacity of a metal.

40. 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

41. 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

42. Specific heat capacity lab suggestions:
Heat a metal to a known temp
Transfer the metal to a known quantity of water at a known temperature
Measure the equilibrium temperature
Use qlost = qgained to compute the specific heat of the metal.

43. Get the initial temperature of the metal.
The temperature of boiling water.
metal
hotplate

44. Get initial temp of water in calorimeter cup.

45. Transfer the metal to the calorimeter.

46. Continue stirring until thermal equilibrium is reached.

47. Data: Mass of metal Initial temp of metal Mass of water
Initial temp of water
Final temp of water and metal

48. qlost = -qgained
mmcmDTm = -(mwcwDTw)
-(mwcwDTw)
cm =
mmDTm

49. Data: Suppose 104.58 g of a metal is heated to 100. C.
It is placed into 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.

50. Data: Suppose 104.58 g of a metal is heated to 100. C.
It is placed into 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.

51. Table of selected specific heats.

52. Calorimetry and Phase Changes
Part Three
Calorimetry and Phase Changes

53. Is heat is absorbed or released during a phase change?
How could you measure the heat absorbed or released as substances change phase?

54. Consider ice melting in water.
Does the temperature of the water change?
Is the water absorbing or releasing heat?
Does ice absorb heat or release heat as it melts?

55. Consider ice melting in water.
Does the temperature of the water change?
Is the water absorbing or releasing heat?
Does ice absorb heat or release heat as it melts? No
Releasing heat
Absorb heat

56. Consider ice melting in water.
The word fusion means “melting”.
How could you design an experiment to measure the heat of fusion of ice?

57. 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.

58. Consider ice melting in water.
Suppose some ice at 0C is placed into g of water at 25.5 C.
Ice
Copy down this information and the data that follows.

59. Consider ice melting in water.
When the system reaches equilibrium at 0C, grams of the ice has melted.
Ice

60. 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

61. 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

62. Consider ice melting in water.
Go ahead and calculate the value of Hf.
Ice
qlost by water= qgained by ice
mwatercDT = miceHf

63. We now know that heat is absorbed or released during a phase change.
Heat is absorbed as solids melt, or liquids vaporize.

64. We now know that heat is absorbed or released during a phase change.
Heat is released as liquids freeze, or vapors condense.

65. Heat is absorbed by the ice.
And melts.

66. 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

67. Heat is released by the water as it freezes.
334 joules is released when one gram of water freezes at 0C.
Ice
water

68. Ice Water releases 334 J per gram as it 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

69. Heat is absorbed by the water as it vaporizes.
Hotplate

70. Heat is absorbed by the water as it vaporizes.
2260 joules is absorbed by one gram of water as it boils at 100C.
Hotplate

71. Steam releases 2260 J/g as it condenses at 100 C
Water absorbs 2260 J/g as it boils at 100 C
Hotplate

72. Heat is released by water vapor as it condenses.

73. 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.

74. Thunderhead
40,000 + feet
The heat released by condensing water vapor causes convection and updrafts in thunderstorms.

75. Phase changes occur at a constant temperature as heat is absorbed or released.

76. Question for discussion:
If phase changes occur at a constant temperature, then what happens to the heat when water boils?

77. Heat energy is converted to matter (E=mc2) and it stays in the water.
Question for discussion – possible answers:
Heat energy is converted to matter (E=mc2) and it stays in the water.
The heat increases the speed of the water molecules.
The heat energy breaks the intermolecular bonds which keep the water in the liquid phase.
The temperature really does change, you just missed it.

78. Heat energy is converted to matter (E=mc2) and it stays in the water.
Question for discussion – possible answers:
Heat energy is converted to matter (E=mc2) and it stays in the water.
The heat increases the speed of the water molecules.
The heat energy breaks the intermolecular bonds which keep the water in the liquid phase.
The temperature really does change, you just missed it.

79. Heat energy is converted to matter (E=mc2) and it stays in the water.
Question for discussion – possible answers:
Heat energy is converted to matter (E=mc2) and it stays in the water.
The heat increases the speed of the water molecules.
The heat energy breaks the intermolecular bonds which keep the water in the liquid phase.
The temperature really does change, you just missed it.

80. The heat gained or lost in phase changes can be calculated using …
Heat of fusion (melting)
q = mHf
q = mHv
Heat of vaporization

81. Hf = 334 J/g Hv=2260 J/g The values for water are …
Heat of fusion (melting)
Hf = 334 J/g
Hv=2260 J/g
Heat of vaporization

82. q = m Hf q = (150.0 g)(334 J/g) q = 50,100 J or 50.1 kJ
How much heat is absorbed by 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

83. q = m Hv q = (20.0 g)(2260 J/g) q = 45,200 J or 45.2 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

84. Part Four
Sublimation
and
Phase Diagrams

85. 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.

86. 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

87. Dry ice is solid carbon dioxide, CO2.
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

88. Solid iodine, I2, also undergoes sublimation when heated.
Iodine vapor fills the beaker.
Watchglass
Beaker with iodine
Hotplate

89. 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

90. 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

91. Heat is absorbed as the iodine undergoes sublimation.

92. Heat is released as the iodine undergoes deposition.

93. A phase diagram can help explain why dry ice undergoes sublimation.
The phase diagram has three distinct regions.
Temperature
Pressure 2 1 3

94. 2 1 3 Which phase is in each region?
The phase diagram has three distinct regions.
Temperature
Pressure 2 1 3

95. 2 1 3 Which phase is in each region? 1= ??? 2= ??? 3= ???
Hint: What happens to ice as temperature increases?
1= ???
Temperature
Pressure 2
2= ??? 1
3= ??? 3

96. 2 1 3 Which phase is in each region? 1= Solid 2= ??? 3= ???
Hint: What happens to ice as temperature increases?
1= Solid
Temperature
Pressure 2
2= ??? 1
3= ??? 3

97. 2 1 3 Which phase is in each region? 1= Solid 2= Liquid 3= ???
Hint: What happens to ice as temperature increases?
1= Solid
Temperature
Pressure 2
2= Liquid 1
3= ??? 3

98. 2 1 3 Which phase is in each region? 1= Solid 2= Liquid 3= Gas
Hint: What happens to ice as temperature increases?
1= Solid
Temperature
Pressure 2
2= Liquid 1
3= Gas 3

99. The point where all three phases exist in equilibrium is called the
triple point.
triple point. L S
Pressure G
Temperature

100. At a pressure of 1 atm, most substances go through all three phases, as the temperature increases,
Solids melt to form liquids, which vaporize to form gases.
1 atm G
Temp.

101. At a pressure of 1 atm, most substances go through all three phases, as the temperature increases,
Notice the melting point and boiling point.
1 atm G MP BP
Temp.

102. L S G But the phase diagram for CO2 is a little different.
Notice that the triple point is above 1 atm. L S
5 atm
1 atm G
Temperature

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

104. At 1 atm CO2 goes directly from solid to vapor as the temperature increases.
The sublimation point is –78.5 C
1 atm
-78.5 G
Temperature

105. An arrow will appear in the following phase diagrams.
Tell what phase change the arrow indicates.
Pressure
Temperature

106. L S G What phase change is occurring? Melting (fusion) Pressure
Temperature
Pressure S L G
Melting (fusion)

107. L S G What phase change is occurring? Vaporization Pressure
Temperature

108. L S G What phase change is occurring? Condensation Pressure
Temperature

109. What phase change is occurring?L G
Sublimation
Pressure
Temperature

110. L S G What phase change is occurring?
Liquefying a gas by increasing the pressure. S L G
Pressure
Temperature

111. Heating and Cooling Curves
Part Five
Heating and Cooling Curves

112. A process that absorbs heat is called endothermic.
A process that gives off heat is called exothermic.

113. Endothermic: Exothermic: Melting (fusion) Vaporization Sublimation
Heat is absorbed.
Freezing
Condensation
Deposition
Exothermic:
Heat is released.

114. Investigate:
Either recall an earlier experiment, or design an experiment to look at the temperature of water as phase changes take place.

115. 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.

116. Investigate:
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.)
Allow the temperature to equilibrate.
Turn on the hot plate and continue to record temperature at regular intervals until some of the water boils away.
Plot temperature as a function of time.

117. Thermometer
Temperature
Time
Graph paper
Stirring hotplate

118. Thermometer
You could use a thermometer and plot the temperature on graph paper or on a computer.
Temperature
Time
Graph paper
Stirring hotplate

119. Thermometer
Time
Temperature
Stirring hotplate

120. Temperature probe
Or you could use a thermometer probe connected to an interface and a computer to plot the temperature.
0.0 C
Temperature
Time
CBL, LabPro, or computer
Stirring hotplate

121. 0.0 C Temperature probe CBL, LabPro, or computer Stirring hotplate
Time
CBL, LabPro, or computer
Stirring hotplate

122. Phase changes occur at a single temperature.
Water freezes and ice melts at 0C.
At sea level, water boils and steam condenses at 100C.

123. Consider the following heating curve for water.
100
Time
Temp

124. Consider the following heating curve for water.
100
Time
Temp
Ice at –30C absorbs heat. Temperature rises to 0C.

125. Consider the following heating curve for water.
100
Time
Temp
Ice at 0C absorbs heat and melts at constant 0C to make water at 0C.

126. Consider the following heating curve for water.
When all ice melts, water at 0C absorbs heat and temperature rises to 100C.
100
Time
Temp

127. Consider the following heating curve for water.
100
Time
Temp
Water absorbs heat and boils at a constant temperature of 100C.

128. Consider the following heating curve for water.
100
Time
Temp
Temperature of steam rises as it absorbs heat after all of the water boils.

129. What is happening at each segment of the heating curve?
100
Time
Temp

130. Look at the different regions of the heating curve for water.
100
Time
Temp
Water and
steam
Steam
Water
Ice and
water
Phase changes?
Ice

131. The temperature is constant during a phase change.
100
Time
Temp
Water and
steam
Steam
Water
Ice and
water
Phase changes
Ice

132. Calculating heat at each segment of the heating curve.
100
Time
Temp
The temperature of the ice is increasing. The specific heat for ice is 2.05 J/gC.
q1=mciDT

133. Calculating heat at each segment of the heating curve.
100
Time
Temp
A phase change occurs at a constant temperature. Use the heat of fusion since ice is melting.
q2=mHf
q1=mciDT

134. Calculating heat at each segment of the heating curve.
100
Time
Temp
q3=mcwDT
The temperature of the water is increasing. The specific heat of water is 4.18 J/gC.
q2=mHf
q1=mciDT

135. Calculating heat at each segment of the heating curve.
100
Time
Temp
q4=mHv
q3=mcwDT
A phase change occurs at a constant temperature. Use the heat of vaporization since water is boiling.
q2=mHf
q1=mciDT

136. Calculating heat at each segment of the heating curve.
q5=mcsDT
100
Time
Temp
q4=mHv
q3=mcwDT
The temperature of the steam is increasing. The specific heat of steam is 2.02 J/gC.
q2=mHf
q1=mciDT

137. Calculating heat at each segment of the heating curve.
q5=mcsDT
100
Time
Temp
q4=mHv
q3=mcwDT
Use q=mcDT when there is a temperature change.
q2=mHf
q1=mciDT

138. Calculating heat at each segment of the heating curve.
q5=mcsDT
100
Time
Temp
q4=mHv
q3=mcwDT
Use q=mHf or q=mHv when there is a phase change.
q2=mHf
q1=mciDT

139. Calculating heat at each segment of the heating curve.
q5=mcsDT
100
Time
Temp
q4=mHv
q3=mcwDT
The total amount of heat absorbed is the sum:
q2=mHf
qtot= q1+q2+q3+q4+q5
q1=mciDT

140. What would the cooling curve of steam look like?
100
Time
Temp

141. What would the cooling curve of steam look like?
100
Time
Temp
Water and
steam
Water
Steam
Ice and
water
Ice

142. What would the cooling curve of steam look like?
Heat energy is released at each step.
100
Time
Temp
Water and
steam
Water
Steam
Ice and
water
Ice

143. 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.

144. The amount of ice and water will never change.
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.

145. Ice and water in an insulated container.
Acme Digital Thermometer
0.0 C
Ice and water in an insulated container.

146. Some ice melts and forms liquid water.
Acme Digital Thermometer
0.0 C
Some ice melts and forms liquid water.

147. Some water freezes and forms ice.
Acme Digital Thermometer
0.0 C
Some water freezes and forms ice.

148. Acme Digital Thermometer
When the rates at which the ice melts and the water freezes are equal …

149. equilibrium is established.
Acme Digital Thermometer
0.0 C
… an
equilibrium is established.

150. The amounts of ice and water will remain constant…
Acme Digital Thermometer
0.0 C
The amounts of ice and water will remain constant…

151. …and the mixture of ice and water will remain at a constant 0C.
Acme Digital Thermometer
0.0 C
…and the mixture of ice and water will remain at a constant 0C.

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

153. 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?

154. 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?

155. 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?

156. 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.

157. Questions 9. Explain why dry ice doesn’t form a puddle of liquid CO2.
10. What phase change is occuring?
Temperature
Pressure

158. 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?

159. 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.

160. Heat
Deposition
Melting
Temperature
Phase change
Equilibrium
Joule
Phase diagram
Vaporization
Heating curve
Calorie
Condensation
Sublimation
Freezing
Boiling
Celsius