Name Period Date

12. Heat of Fusion

Driving QuestionsA pot of boiling water will not get any hotter than 100 ºC, no matter how long or how vigorously you heat it. The energy being added must be doing something other than increasing the temperature of the water. What is happening to this added energy?

BackgroundAs atoms are held together to form molecules, molecules are held together in the various physical states of matter: solid, liquid, and gas. The forces attracting molecules to one another are called intermolecular forces.

In solids, the molecules are held by the intermolecular forces in tight, rigid patterns. In liquids, the molecules are still in contact with each other, but they have enough energy to flow around one another. In gases, the molecules have enough energy to break free from all of their attractions and exist independently from one another with space between them.

By adding energy to a substance, the molecules gain enough energy to overcome the intermolecular forces. The energy required to overcome intermolecular forces is called latent heat. The amount of energy required to break the attractions depends on the chemical structure of the substance. Water, for example, contains strong hydrogen bonds, so it requires a lot of energy to overcome the intermolecular forces.

The energy required to give the molecules enough energy to overcome the attractions in a solid and melt to become a liquid is called the heat of fusion. The heat of vaporization is the energy required to vaporize a liquid into a gas. Specific heat is the energy required to increase the temperature of a substance. Temperature is a measure of the kinetic energy (motion) of the molecules in a substance.

An input of energy first causes a solid substance to increase its temperature to its melting point, according to its specific heat. Once at the melting point, additional energy (measured as the heat of fusion) is used to overcome the intermolecular forces, turning the solid into a liquid. The temperature remains constant during a phase change because the energy is being used to break the intermolecular forces and not to increase the kinetic energy of the molecules.

After a substance is completely melted, adding more energy increases the temperature of the substance until it reaches its boiling point. At the boiling point, additional energy (measured as the heat of vaporization) converts the liquid into gas. Reversing the process releases the same amount of energy used to break the attractions when they reform.

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Heat of Fusion

Materials and Equipment

For each student or group:

Data collection system Polystyrene cup (2)

Fast response temperature sensor Lid for polystyrene cup

Graduated cylinder, 100-mL Paper towels

Beaker (2), 250-mL Water, 200 mL

Hot plate Ice cubes (2)

SafetyAdd these important safety precautions to your normal laboratory procedures:

Avoid being burned from contact with the hot plate. Allow it to cool thoroughly before storing.

Sequencing ChallengeThe steps below are part of the Procedure for this lab activity. They are not in the right order. Determine the proper order and write numbers in the circles that put the steps in the correct sequence.

ProcedureAfter you complete a step (or answer a question), place a check mark in the box () next to that step.

Note: When you see the symbol "�" with a superscripted number following a step, refer to the numbered Tech Tips listed in the Tech Tips appendix that corresponds to your PASCO data collection system. There you will find detailed technical instructions for performing that step. Your teacher will provide you with a copy of the instructions for these operations.

Set Up

1. Start a new experiment on the data collection system. �(1.2)

2. Connect a fast response temperature sensor to the data collection system. �(2.1)

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Record the mass of the calorimeter and the 50 ºC water.

Place a temperature sensor in the 50 °C water and start recording the temperature of the calorimeter water.

Add an ice cube to the 50 °C water and then cover the calorimeter. Continue to record data until the ice melts.

Heat 150 mL of water to 50 ºC and pour it into a calorimeter.

Remove the lid and sensor then record the mass of the calorimeter, water, and melted ice.

Student Inquiry Worksheet3. Display a graph of Temperature (°C) versus Time (s). �(7.1.1)

4. Using a graduated cylinder, add 100 mL of water to a 250-mL beaker.

5. Use a hot plate to warm the water to approximately 50 ºC.

6. Place one polystyrene cup in the other one to create a simple calorimeter and then place the calorimeter inside an empty 250-mL beaker for stability.

7. What happens to the temperature of the warm water when the ice is added?

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Collect Data

8. Measure and the mass of the beaker plus the empty calorimeter to two decimal places. Record the mass below.

Mass of the beaker plus the empty calorimeter (g):

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9. Pour the heated water into the calorimeter.

10. Measure the mass of the beaker plus the calorimeter with the water to two decimal places. Record the mass below.

Mass of the beaker plus calorimeter with water (g):

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11. How can the mass of water in the calorimeter be calculated?

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12. Insert the temperature sensor into the calorimeter making sure it is submerged in the water.

13. Start recording data. �(6.2)

14. Adjust the scale of the graph. �(7.1.2)

15. Allow the temperature to stabilize.

16. Use a paper towel to dry an ice cube.

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Heat of Fusion17. Why is it necessary to dry the ice cube with a paper towel before placing it in the

calorimeter?

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18. Without splashing any water from the calorimeter, add the ice cube and then cover the calorimeter with the lid.

19. What happens to your results if some of the water in the calorimeter splashes out when you add the ice?

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20. Gently swirl the calorimeter without splashing any water.

21. As soon as the ice finishes melting, stop recording data. �(6.2)

22. Name the data run “ice trial 1”. �(8.2)

23. Remove the lid and the temperature sensor.

24. Measure the mass of the beaker plus the calorimeter with the water and melted ice to two decimal places. Record the mass below.

Mass of the beaker plus calorimeter withwater and melted ice (g): ________________________

25. Repeat the process. Record the values in Table 1 below.

Table 1: Collected masses for trial 2

Mass Ice Trial 2

Beaker plus empty calorimeter (g)

Beaker plus calorimeter with water (g)

Beaker plus calorimeter with water and melted ice (g)

26. Name the data run “ice trial 2”.

27. Save your data file and clean up according to the teacher's instructions. �(11.1)

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Student Inquiry Worksheet

Data Analysis

1. Calculate the amount of water in the calorimeter before adding the ice. Record it in Table 2 below.

Table 2: Mass of water in the colorimeter

Mass Ice Trial 1 Ice Trial 2

Beaker plus calorimeter with water (g)

Beaker plus empty calorimeter (g)

Water (calorimeter with water – calorimeter empty) (g)

2. Calculate the amount of ice added to the calorimeter. Record it in Table 3 below.

Table 3: Mass of ice in the colorimeter

Mass Ice Trial 1 Ice Trial 2

Beaker plus calorimeter with water and melted ice (g)

Beaker plus calorimeter with water (g)

Ice (calorimeter with melted ice – calorimeter with water) (g)

3. Determine the number of moles of ice added to the calorimeter. Use the following equation, and record your work in Table 4 below.

Table 4: Moles of ice

Ice Trial 1 Ice Trial 2

Moles of ice (show your work)

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Heat of Fusion4. Use the graph of Temperature (°C) versus Time (s) to determine the final

temperature, initial temperature, and change in temperature for the calorimeter water. Follow the steps below to complete this on your data collection system:a. Display the run of data you want to analyze. �(7.1.7)

b. Measure the difference between the final temperature and the initial temperature. When this step is complete, the final temperature, initial temperature, and change in temperature will be displayed on the screen. �(9.2)

c. Record the values for each trial in Table 5 below.

Table 5: Collected and calculated temperature

Temperature Ice Trial 1 Ice Trial 2

Final temperature (˚C)

Initial temperature (˚C)

Change in temperature, ΔT = Tfinal – Tinitial (˚C)

5. Create a graph with both runs of data displayed on your data collection system. �(7.1.3)

6. Sketch or print a graph of Temperature (°C) versus Time (s). Make sure that each trial is labeled as well as the overall graph, the x-axis, the y-axis, and include units on the axes. �(11.2)

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Student Inquiry Worksheet7. Calculate the energy transferred from the water in the calorimeter to the ice qtrans

and the energy gained by the ice from the water –qtrans. Use the following equation, and show your work in Table 6 below.

Table 6: Energy transferred from water to ice

Ice Trial 1 Ice Trial 2

Energy transferred

from the water (J)

qtrans

Energy absorbed by the ice (J)

–qtrans

8. The energy gained by the ice –qtrans is used to melt the ice qmelt and to bring the temperature of the melted water up to the final temperature .

Determine the energy absorbed by the freshly melted ice to warm from 0 ºC to the final temperature. Use the following equation, and record your work in Table 7 below.

Table 7: Energy used to warm the melted ice water

Ice Trial 1 Ice Trial 2

Show your work here

Energy used to warm the ice water

(J)

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Heat of Fusion9. Determine the energy required to melt the ice (solid at 0 ºC to a liquid at 0 ºC).

Use the following equation, and record it in Table 8 below.

Table 8: Energy required to melt the ice

Ice Trial 1 Ice Trial 2

Show your work here

Energy used to melt the ice qmelt (J)

10. Calculate the heat of fusion for ice (in kJ/mol) using the amount of energy absorbed by the ice and the number of moles of ice in the calorimeter. Use the following equation and record it in Table 9 below.

Table 9: Heat of fusion

Ice Trial 1 Ice Trial 2

Show your work

Heat of fusion

(kJ/mol)

11. Calculate the average value for the heat of fusion of ice from the two trials.

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Student Inquiry Worksheet

Analysis Questions

1. The accepted value for the heat of fusion for ice is 6.01 kJ/mol. What is the percent error for the experimentally determined heat of fusion?

2. What are some possible sources of error and how might you correct or at least account for them?

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Synthesis QuestionsUse available resources to help you answer the following questions.

1. How much energy would be required to melt a 50.0 kg block of ice at 0 ºC? The heat of fusion for ice is 6.01 kJ/mol.

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Heat of Fusion2. How are heat of fusion and heat of vaporization similar? How are they different?

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Use the following information to answer Synthesis Questions 3 and 4 below:

An unknown substance that has a mass of 250.0 g reaches its melting point in a coffee cup calorimeter. As the substance changes from solid to liquid its temperature remains unchanged. The mass of the water in the calorimeter is 500.0 g. The water has an initial temperature of 90.0 ˚C and drops to 19.0 ˚C as the unknown solid melts.

3. How many kJ of energy the water release while the solid was melting:

4. If an unknown substance has a formula weight of 77.3 g/mol, what is the heat of fusion for the substance?

5. How is heat of fusion different from specific heat? For which is there no observed temperature change?

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Student Inquiry Worksheet

Multiple Choice QuestionsSelect the best answer or completion to each of the questions or incomplete statements below.

1. What happens to the average kinetic energy of the molecules as ice changes to liquid water at 0 ºC?

A. IncreasesB. DecreasesC. Remains the sameD. Unable to determine with this information

2. The heat of fusion for a substance is the energy measured during what?

A. Phase change from solid to liquidB. Phase change from a liquid to a gasC. Temperature changeD. Change in kinetic energy

3. When 20.0 g of a substance is completely melted at its melting point, 820 joules of energy are absorbed. What is the heat of fusion for this substance?

A. 0.024 J/gB. 16.4 J/gC. 24 J/gD. 41 J/g

4. The heat of fusion for ethanol is 5.02 kJ/mol. How much energy is required to melt 24.5 g of ethanol at its freezing point? The molar mass of ethanol is 46.07 g/mol.

A. 0.205 kJB. 2.67 kJC. 5.67 kJD. 123 kJ

Use the graph below to answer Multiple Choice Questions 5 and 6. The graph represents the relationship between temperature and energy added to a substance. At point “A”, the substance is a solid.

5. What is the freezing point of the substance depicted in the graph above?

A. 20 ºCB. 70 ºCC. 120 ºC

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Heat of FusionD. 160 ºC

6. During which interval does the heat absorbed by the substance represent the heat of vaporization?

A. B to C B. C to DC. D to ED. E to F

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Student Inquiry Worksheet

Key Term ChallengeFill in the blanks from the list of words in the Key Term Challenge Word Bank.

1. When a substance changes physical state, it requires ________________________. The molecules must overcome and break free from their ________________________ to each other. These forces are called ________________________. In water, they are hydrogen bonds. The energy necessary to overcome these forces is called the ________________________. Specifically, the energy required to change a substance from a solid to a liquid is the heat of ________________________, and the energy required to change from a liquid to a gas is the heat of ________________________. Fusion is another term for ________________________.

2. The stronger the intermolecular forces within a substance, the ________________________ the latent heat is. Water has particularly high latent heats because its hydrogen bonds are among the ________________________ types of intermolecular forces. The stronger, or more tightly, they “hang on” to one another, the ________________________ energy that is necessary to pull them apart. The ________________________ amount of energy that water absorbs when it melts is released when it freezes. An input of energy, however, can be used either to overcome the attractions or increase the kinetic energy of the molecules. Increasing the kinetic energy can be seen as an increase in ________________________. During an ________________________ phase change requiring an input of energy, such as fusion or vaporization, the temperatures ________________________. After the phase change is complete, further addition of energy causes the temperature to ________________________.

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Heat of Fusion

Key Term Challenge Word BankParagraph 1 Paragraph 2attraction

boiling

chemical bonds

condensing

energy

evaporating

freezing

fusion

intermolecular forces

intramolecular forces

latent heat

magnetic forces

melting

repulsion

space

specific heat

vaporization

decrease

endothermic

exothermic

greater

increase

less

lesser

mass

more

pressure

remain constant

same

strongest

temperature

volume

weakest

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