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Last rev. 022007a Pisgah High School Chemistry Mr. Jones Heat, Temperature And Phase Changes Heat, Temperature And Phase Changes.

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Presentation on theme: "Last rev. 022007a Pisgah High School Chemistry Mr. Jones Heat, Temperature And Phase Changes Heat, Temperature And Phase Changes."— Presentation transcript:

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2 Last rev a Pisgah High School Chemistry Mr. Jones Heat, Temperature And Phase Changes Heat, Temperature And Phase Changes

3 Part One Heat and Temperature Part One Heat and Temperature

4 What temperature does the thermometer indicate?

5 What might be going on that would cause this temperature?

6 This is the view out the window, past the thermometer.

7 Yep. It is snowing.

8 Why would snow cause the air temperature to be at precisely 0C? What occurs at 0C? Water freezes and ice melts.

9 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.

10 The temperature hovers at zero Celsius as the snow melts.

11 So why is there snow on the ground if it is melting? Yep. That’s what allows the snow to accumulate.

12 As the snow melts, it absorbs heat and cools the ground, the car, and the grill.

13 This allows more snow to lay. It doesn’t melt because the ground is now at 0C.

14 What is temperature ? What does it mean to have a temperature of 0 C? Is temperature the same thing as heat ?

15 Temperature is a measure of how “hot” or “cold” something is. Temperature is measured in arbitrary units, like Fahrenheit or Celsius.

16 Temperature is proportional to the average kinetic energy of the molecules of the substance. T   ½ mv 2

17 Temperature is therefore proportional to the speed of the molecules of a substance. T   ½ mv 2 Velocity or speed T  v

18 The higher the temperature, the greater the average speed of the molecules. T   ½ mv 2 Velocity or speed T  v

19 Heat is the thermal energy transferred from a hot object to a cold object. Heat is measured in energy units -- Joules or calories.

20 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.

21 Heat has the symbol q and is calculated using … q = mc  T

22 Quantity of heat mass specific heat capacity temperature change

23 q = mc  T Quantity of heat specific heat capacity The specific heat capacity of water is 4.18 J/gC

24 q = m c T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

25 q = m c  T What is the final temperature of 27.0 grams of liquid water, initially at 0C, after it absorbs J of energy? Hint: start by solving for  T. = q TT m c Answer:6.20 C

26 Part Two Calorimetry and Specific Heat Capacity Part Two Calorimetry and Specific Heat Capacity

27 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.

28 Investigate: Suppose two different masses of water at different temperatures are mixed. Can you predict the final temperature?

29 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:

30 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?

31 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.

32 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 _________

33 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.

34 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.

35 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.

36 Investigate: To put it mathematically: q lost = -q gained And since q = mc  T then m h c  T h = -m c c  T c Heat lost by the hot water = Heat gained by the cold water

37 Investigate: The convention for  T is final temperature minus initial temperature or T final – T initial m h c(T f -T h ) = -m c c(T f -T c ) Use your algebra skills, to solve for T f, the final temperature. m h c  T h = -m c c  T c becomes

38 How did our predicted T f compare to the one we observed?

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 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 q lost = -q gained to compute the specific heat of the metal.

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

43 Get initial temp of water in calorimeter cup.

44 Transfer the metal to the calorimeter.

45 Continue stirring until thermal equilibrium is reached.

46 Data: Mass of metal Initial temp of metal Mass of water Initial temp of water Final temp of water and metal

47 q lost = -q gained m m c m  T m = -m w c w  T w -m w c w  T w mmTmmmTm c m =

48 Mass of metal40.0 g Initial T of metal98.0 C Mass of water in calorimeter60.0 g Initial T of water20.0 C Final T of water and metal22.9 C Calculate the specific heat capacity of the metal.

49 Table of selected specific heats. What is the unknown metal?

50 Part Three Calorimetry and Phase Changes Part Three Calorimetry and Phase Changes

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

52 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

53 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

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

55 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

56 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.

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

58 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

59 Consider ice melting in water. …we should be able to compute the heat of fusion of ice, H f. q lost by water = -q gained by ice m water c  T = -m ice H f Ice

60 Consider ice melting in water. Go ahead and calculate the value of H f. q lost by water = -q gained by ice m water c  T = -m ice H f Ice

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

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

63 Ice And melts. Heat is absorbed by the ice.

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

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

66 Ice absorbs 334 J per gram as it melts at 0C Water releases 334 J per gram as it freezes at 0C

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

68 2260 joules is absorbed by one gram of water as it boils at 100C. Hotplate Heat is absorbed by the water as it vaporizes.

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

70 Heat is released by water vapor as it condenses.

71 The heat released by condensing water vapor is a major factor in weather phenomena like thunderstorms and hurricanes.

72 40,000 + feet Thunderhead The heat released by condensing water vapor causes convection and updrafts in thunderstorms.

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

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

75 a.Heat energy is converted to matter (E=mc 2 ) 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:

76 a.Heat energy is converted to matter (E=mc 2 ) 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:

77 The heat gained or lost in phase changes can be calculated using … q = mH f q = mH v Heat of fusion (melting) Heat of vaporization

78 The values for water are … H f = 334 J/g H v =2260 J/g Heat of fusion (melting) Heat of vaporization

79 How much heat is absorbed by g of ice as it melts at 0C? q = m H f q = (150.0 g)(334 J/g) q = 50,100 Jor 50.1 kJ

80 How much heat is released by 20.0 grams of steam as it condenses at 100C? q = m H v q = (20.0 g)(2260 J/g) q = 45,200 Jor 45.2 kJ

81 Part Four Sublimation and Phase Diagrams Part Four Sublimation and Phase Diagrams

82 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.

83 Solid iodine, I 2, undergoes sublimation when heated. Hotplate Watchglass Beaker with iodine Iodine vapor fills the beaker.

84 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

85 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

86 Here we see the sublimation and deposition of iodine.

87 Heat is absorbed as the iodine undergoes sublimation.

88 Heat is released as the iodine undergoes deposition.

89 Dry Ice Dry ice is solid carbon dioxide, CO 2. At room temperature and normal atmospheric pressures dry ice undergoes sublimation. It goes directly from the solid state to the vapor state. CO 2 vapor

90 Dry Ice Dry ice is solid carbon dioxide, CO 2. At room temperature and normal atmospheric pressures dry ice undergoes sublimation. It goes directly from the solid state to the vapor state. CO 2 vapor

91 A phase diagram can help explain why dry ice undergoes sublimation. Temperature Pressure The phase diagram has three distinct regions

92 Which phase is in each region? Temperature Pressure The phase diagram has three distinct regions

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

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

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

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

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

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

99 Temp. S L G 1 atm MPBP Notice the melting point and boiling point. At a pressure of 1 atm, most substances go through all three phases, as the temperature increases,

100 But the phase diagram for CO 2 is a little different. Temperature S L G 1 atm Notice that the triple point is above 1 atm. 5 atm

101 At 1 atm CO 2 goes directly from solid to vapor as the temperature increases. Temperature S L G 1 atm

102 At 1 atm CO 2 goes directly from solid to vapor as the temperature increases. Temperature S L G 1 atm The sublimation point is –78.5 C -78.5

103 Carbon dioxide is a liquid at the bottom of the ocean where the pressure is well above 5 atmospheres.

104 Temperature Pressure 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

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

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

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

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

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

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

111 What do the lines between the different regions represent? Each line represents an equilibrium between two phases.

112 Equilibria occur at the boundaries between the regions. Temperature Pressure The equilibrium between the solid and liquid phases. S L G

113 Equilibria occur at the boundaries between the regions. Temperature Pressure The equilibrium between the liquid and gaseous phases. S L G

114 Equilibria occur at the boundaries between the regions. Temperature Pressure The equilibrium between the solid and gaseous phases. S L G

115 Consider the equilibrium between two phases. Temperature Pressure S L G Ice and water are in an insulated container.

116 Acme Digital Thermometer 0.0 C

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

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

119 When the rates at which the ice melts and the water freezes are equal … Acme Digital Thermometer 0.0 C

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

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

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

123 A mixture of ice and water can be used to calibrate a thermometer at 0C. Acme Digital Thermometer 0.0 C

124 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.

125 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.

126 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.

127 Part Five Heating and Cooling Curves Part Five Heating and Cooling Curves

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

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

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

131 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.

132 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 :

133 Thermometer Stirring hotplate Time Temperature Graph paper

134 Thermometer Stirring hotplate Time Temperature

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

136 Consider the following heating curve for water Time Temp

137 0 100 Time Temp Ice at –30C absorbs heat. Temperature rises to 0C. Consider the following heating curve for water.

138 0 100 Time Temp Ice at 0C absorbs heat and melts at constant 0C to make water at 0C. Consider the following heating curve for water.

139 0 100 Time Temp When all ice melts, water at 0C absorbs heat and temperature rises to 100C. Consider the following heating curve for water.

140 0 100 Time Temp Water absorbs heat and boils at a constant temperature of 100C. Consider the following heating curve for water.

141 0 100 Time Temp Temperature of steam rises as it absorbs heat after all of the water boils. Consider the following heating curve for water.

142 What is happening at each segment of the heating curve? Time Temp

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

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

145 Calculating heat at each segment of the heating curve Time Temp q 1 =mc i  T The temperature of the ice is increasing. The specific heat for ice is 2.05 J/gC.

146 Calculating heat at each segment of the heating curve Time Temp q 2 =mH f q 1 =mc i  T A phase change occurs at a constant temperature. Use the heat of fusion since ice is melting.

147 Calculating heat at each segment of the heating curve Time Temp q 2 =mH f q 1 =mc i  T q 3 =mc w  T The temperature of the water is increasing. The specific heat of water is 4.18 J/gC.

148 Calculating heat at each segment of the heating curve Time Temp q 2 =mH f q 1 =mc i  T q 3 =mc w  T q 4 =mH v A phase change occurs at a constant temperature. Use the heat of vaporization since water is boiling.

149 Calculating heat at each segment of the heating curve Time Temp q 2 =mH f q 1 =mc i  T q 3 =mc w  T q 4 =mH v q 5 =mc s  T The temperature of the steam is increasing. The specific heat of steam is 2.02 J/gC.

150 Calculating heat at each segment of the heating curve Time Temp q 2 =mH f q 1 =mc i  T q 3 =mc w  T q 4 =mH v q 5 =mc s  T Use q=mc  T when there is a temperature change.

151 Calculating heat at each segment of the heating curve Time Temp q 2 =mH f q 1 =mc i  T q 3 =mc w  T q 4 =mH v q 5 =mc s  T Use q=mH f or q=mH v when there is a phase change.

152 Calculating heat at each segment of the heating curve Time Temp q 2 =mH f q 1 =mc i  T q 3 =mc w  T q 4 =mH v q 5 =mc s  T The total amount of heat absorbed is the sum: q tot = q 1 +q 2 +q 3 +q 4 +q 5

153 What would the cooling curve of steam look like? Time Temp

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

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

156 Why is a steam burn worse than one from boiling water, even if both are at 100C? Ice Ice and water Water Water and steam Steam Time Temp

157 Part Six Fractional Distillation Part Six Fractional Distillation

158 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?

159 How might you separate a mixture of two clear, colorless liquids? What if the liquids have different boiling points?

160 Heating curve for two liquids, A and B time temperature Boiling point of liquid ABoiling point of liquid B

161 Fractional distillation can be used to separate the mixture into its various “fractions”. Isolate each fraction at each of the different boiling points.

162 Collecting the first fraction time temperature A B Collect in the first test tube what comes off in region 1.

163 Collecting the second fraction time temperature A B Collect in the second test tube what comes off in region 2.

164 Collecting the third fraction time temperature A B Collect in the third test tube what comes off in region 3.

165 Predict what is in each tube time temperature A B Test tube 1 May contain only A Test tube 2 Test tube 3 May contain both A and B May contain only B

166 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

167 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

168 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.

169 Record your observations TT# Start Temp End Temp OdorFlammability 1 2 3

170 Questions?

171 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?

172 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?

173 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?

174 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.

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

176 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?

177 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 10 6 J of energy? Hint: there are three changes that take place.

178 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.

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


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