Cooling Curve/Heat of Crystallization



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Cooling & Heating Curves for Sodium Thiosulfate Pentahydrate

PARTS A, B, AND C MUST BE COMPLETED PRIOR TO COMING TO CLASS THE DAY OF THE LAB – THEY ARE PRE-LAB COMPONENTS!

A. QUESTION: Read the lab thoroughly, highlighting key non-calculator parts of the procedure. Scientists always begin exploration with a question. In this case we are guiding you to the question, and we want you to learn to articulate the question. Propose a scientific question that can be answered using the procedures and chemicals provided.

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B. VARIABLES: In the box below, write down the variables for this experiment. There should be ONE independent variable (the manipulated variable), ONE dependent variable (the measured variable), and as many control variables as needed (those variables that could also effect the dependent variable.)

|INDEPENDENT | |

|DEPENDENT | |

|CONTROL(S) | |

C. SAFETY: In the box below indicate the general safety concerns involved in this lab and the specific health concerns for sodium thiosulfate. The MSDS sheet for sodium thiosulfate can be found on the Flinn MSDS site (search) or go to

|GENERAL SAFETY |SPECIFIC SODIUM THIOSULFATE SAFETY |

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Define the term “supercooled”: _______________________________________________

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D. PROCEDURE:

Part One: Calculator Set-Up

✓ Plug the Temperature Probe into Channel 1 of the LabPro or CBL 2 interface. Use the link cable to connect the TI Graphing Calculator to the interface. Firmly press in the cable ends.

✓ Use an AC adapter to plug the LabPro in and check the calculator for batteries.

✓ Turn on the calculator and select the purple “APPS” key.

✓ Select the DATAMATE program (#7).

✓ Press [pic] to reset the program.

✓ Select “SETUP” (#1 on bottom menu)

✓ If the calculator displays “Stainless Temp (°C)” in CH 1, proceed directly to the next section on setting up the data collection. If it does not, continue with this step to set up your sensor manually.

✓ Press [pic] to select CH 1.

✓ Select TEMPERATURE from the SELECT SENSOR menu.

✓ Select the Stainless Temp (°C) from the TEMPERATURE menu.

Set up the data-collection mode:

• To select MODE, use arrows to highlight “Mode” and press [pic].

• Select TIME GRAPH (#2) from the SELECT MODE menu.

• Select CHANGE TIME SETTINGS (#2) from the TIME GRAPH SETTINGS menu.

• Enter “10” as the time between samples in seconds. Hit “Enter”.

• Enter “90” as the number of samples. The length of the data collection will be 15 minutes. Hit “Enter”.

• Select OK (#1) to return to the setup screen. Select OK (#1) again to return to the main screen.

Part Two: Cooling Curve Data

1. Adjust the ring on a ringstand so that it is 10 cm above the top of the Bunsen burner. Place the wire gauze on the ring.

2. Fill a 400 mL beaker approximately ¾ full of tap water an place on the gauze. Adjust a test-tube clamp on the ringstand above the beaker.

3. Heat the water to approximately 85oC. Use the thermometer to determine the temperature of the water.

4. Obtain a test-tube containing approximately 15 g of sodium thiosulfate. Clamp the test-tube into the test tube clamp on the ring stand. Place the temperature probe into the test tube so that the end is in the solid. (This means the stopper should be removed!!!)

5. Lower the test-tube into the hot water bath.

6. Allow the solid to melt and then heat the liquid to approximately the same temperature as the hot water bath. Use the CBL to determine the temperature of the sample (the temperature will be in the upper right-hand corner of the screen) and use the thermometer for the temperature of the water.

7. While the liquid is heating, prepare a cold-water bath and a room temperature bath. Fill a beaker approximately ½ full of tap water. Add enough ice to bring the temperature of the water to approximately 10oC. For the room temperature bath, simply fill a beaker approximately ¾ full.

8. Remove the test tube from the hot water bath and place it in the room temperature bath.

PRESS START (#2) on the calculator to begin collecting data.

9. Stir the liquid GENTLY with the CBL temperature probe during this time.

10. When the temperature of the liquid sodium thiosulfate pentahydrate has dropped below 40oC, remove the test tube from the room-temperature bath and immediately put it into the cold-water bath. NOTE: The calculator should still be collecting data!

11. Using forceps, drop a seed crystal of sodium thiosulfate into the test tube. This will help prevent supercooling, as it will get crystallization to begin.

12. Continue to stir and take temperature readings of the sodium thiosulfate pentahydrate until the liquid solidifies and a constant temperature is maintained. At this point, you may not be able to stir anymore – do not force the stirring, or you could break the probe. Continue to take readings until the temperature has dropped an additional 5oC. Keep the test tube submerged until you are ready to begin the next part.

Data collection will stop after 15 minutes.

Data for graph: Use the cursor to move along the graph to collect data points. You will need about 20 data points from the beginning to the end. Make sure you take enough points to define the melting point region.

Analyze the flat part of the graph to determine melting temperature of your solid. To do this:

• Press [pic] to return to the main screen, then select ANALYZE.

• Select STATISTICS from the ANALYZE OPTIONS menu (#4).

• Use [pic] to move the cursor to the beginning (left side) of the flat section of the curve. Press [pic] to select the left boundary of the flat section.

• Move the cursor to the end (right side) of the flat section of the graph, and press [pic] to select the right boundary of the flat section. The program will now calculate and display the statistics for the data between the two boundaries.

• Record the MEAN value as the freezing temperature in your data table (round to the nearest 0.1°C).

• Press [pic] to return to the ANALYZE OPTIONS menu, then select RETURN TO MAIN SCREEN (#1).

Enter your data from the calculator into the grid below. You must put the correct units into the headers on each column.

|Time ( ) |Temperature ( ) |Time ( ) |Temperature ( ) |Time ( ) |Temperature ( ) |

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AVERAGE FREEZING POINT: ________

Part Three: Heating Curve Data

1. Reheat the hot water bath to 75oC. (Again, measure the temperature with a thermometer, not the CBL.) Try not to allow the temperature of the water to exceed 75oC. Turn the flame down as low as possible and move the burner so only the outer edge of the bottom of the beaker is heated. Adjust the position and size of the flame so the temperature of the hot water bath remains 75oC. Make sure this remains constant for a few minutes

2. Immerse the test tube with the solid in the hot-water bath.

PRESS START (#2) on the calculator to begin collecting data.

3. GENTLY use the thermometer as a stirring rod when it becomes free of solid. Don’t try to force it!

Data collection will stop after 15 minutes.

Data for graph: Use the cursor to move along the graph to collect data points. You will need about 20 data points from the beginning to the end. Make sure you take points to define the melting point region.

Analyze the flat part of the graph to determine melting temperature of your solid. To do this:

• Press [pic] to return to the main screen, then select ANALYZE.

• Select STATISTICS from the ANALYZE OPTIONS menu (#4).

• Use [pic] to move the cursor to the beginning (left side) of the flat section of the curve. Press [pic] to select the left boundary of the flat section.

• Move the cursor to the end (right side) of the flat section of the graph, and press [pic] to select the right boundary of the flat section. The program will now calculate and display the statistics for the data between the two boundaries.

• Record the MEAN value as the freezing temperature in your data table (round to the nearest 0.1°C).

• Press [pic] to return to the ANALYZE OPTIONS menu, then select RETURN TO MAIN SCREEN (#1).

4. Allow the sample to cool and return the test tube with the sodium thiosulfate to the test tube holder in the front of the room.

5. Enter your data from the calculator into the grid below. You must put the correct units into the headers on each column.

|Time ( ) |Temperature ( ) |Time ( ) |Temperature ( ) |Time ( ) |Temperature ( ) |

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AVERAGE FREEZING POINT: ________

E. PROCESSED DATA:

Graph: Prepare one graph showing both the heating and cooling curves. Graph paper has been provided on the next page of this packet. Time and Temperature will be on the two axes… use DRY MIX to figure out which one goes on each axis! Make sure the lines are distinct by using different symbols or colors. FOLLOW ALL GRAPHING GUIDELINES FROM YOUR HANDOUT!

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F. READING & REFLECTION: Answer the following questions in complete sentences.

1. How does this relate to what we are learning in this unit? Give a specific reference to the notes!

2. Propose an experimental question for a potential next step of experimentation. Think like a scientist! You need new independent &/or dependent variables or a new chemical. EX: “What would be the difference in the heating/cooling curves for anhydrous sodium thiosulfate (without the “pentahydrate”?”

3. How do the concepts and/or chemicals studied in this experiment impact the world in which we live? (Real life application!)

G. QUESTIONS: Answer the following questions in complete sentences.

1. As the liquid cools, what is happening to the kinetic energy of the system?

2. Indicate whether the following values increase (I), decrease (D), or remain constant (C) during a phase transition from solid to liquid.

Temperature ________ Kinetic energy _______ Potential energy _______

3. Does your graph show a dip before it levels off at the melting point? What could a dip indicate?

4. Why was it necessary to add a seed crystal to the liquid?

5. Honey is a supercooled liquid. How could you crystallize a jar of honey?

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Execution [pic]

Report [pic]

TOTAL [pic]

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Teacher Initials

Pre-Lab Completed Before Class:

RAW DATA

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PROCESSED DATA:

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