Concept: phase change
Time: 30 m
SW Interface: 300, 500 & 700
Macintosh® file: C05 Evaporation
Windows® file: C05_EVAP.SWS
Adapted by Terri Case, J.I. Case High School, from Chemistry with Computers, by Dan D. Holmquist and Donald L. Volz, distributed by Vernier Software, 8565 S.W. Beaverton Hillsdale Hwy., Portland, OR 97225-2429, (503) 297-5317.
EQUIPMENT NEEDED
ð temperature sensors (2)
ð six test tubes
ð apron and safety goggles
ð 1-propanol
ð ethanol (ethyl alcohol)
ð methanol (methyl alcohol)
ð n-hexane
ð n-pentane
ð masking tape
ð six pieces of filter paper (2.5cm x 2.5cm)
ð two small rubber bands
The purpose of this laboratory activity is to study temperature changes caused by evaporation of several liquids and relate the temperature changes to the strength of the forces of attraction between molecules of the liquid.
THEORY
When temperature sensors are removed from a liquid, evaporation occurs. Evaporation is an endothermic process that results in a temperature decrease. The magnitude of a temperature decrease is related to the strength of intermolecular forces of attraction.
You will encounter two types of organic compounds called "hydrocarbons" ÷ alkanes and alcohols. The two alkanes used in this experiment are pentane, C5H12, and hexane, C6H14. In addition to carbon and hydrogen atoms, alcohols also contain the -OH functional group. Methanol, CH3OH, and ethanol, C2H5OH, are two of the alcohols that will be used in this experiment. The molecular structure of alkanes and alcohols will be examined for the presence and relative strength of two intermolecular forces ÷ hydrogen bonding and dispersion forces.
PRE-LAB EXERCISE
Complete the PRE-LAB table before beginning the experiment. The name and formula are given for each compound. Draw a structural formula for a molecule of each compound and determine the molecular weight of each molecule. Dispersion forces exist between any two molecules, and generally increase as the molecular weight of the molecule increases. Next, examine each molecule for the presence of hydrogen bonding. Before hydrogen bonding can occur, a hydrogen atom must be bonded directly to an N (nitrogen), O (oxygen), or F (fluorine) atom within the molecule. Indicate whether or not each molecule has hydrogen-bonding capability in the pre-lab table.
PRE-LAB
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2. Follow your teacherâs directions for the handling and disposal of all chemicals.
PART I: Computer Setup
2. Connect the DIN plug of one temperature sensor to Analog Channel A and the DIN plug of the other to Analog Channel B on the interface.
3. Open the Science Workshop file titled as shown;
Windows file: C05_EVAP.SWS
ð Note: For quick reference, see the Experiment Notes window. To bring a display to the top, click on its window or select the name of the display from the list at the end of the Display menu. Change the Experiment Setup window by clicking on the "Zoom" box or the Restore button in the upper right hand corner of that window.
4. The "Sampling Options·" for this experiment are: Periodic Samples = Slow at one measurement per 10 seconds and the Stop Condition = Time at "200" seconds.
1. Wrap the ends of Sensor A and Sensor B with square pieces of filter paper secured by small rubber bands as shown in the diagram. Roll the filter paper around the sensor tip in the shape of a cylinder. Hint: First slip the rubber band up on the sensor, wrap the paper around the sensor, and then slip the rubber band over the wrapped paper. The lower edge of the paper should be even with the sensor end.
2. Stand Sensor A in the ethanol container and Sensor B in the 1-propanol container. Make sure the containers do not tip over.
3. Cut 2 pieces of masking tape, about 10-cm long, to be used to tape the sensors in position during Data Recording.
2. Observe the Digits display until the temperature stabilizes.
3. Record this temperature as the initial temperature of each liquid. Then simultaneously remove the sensors from the liquids and tape them so the sensor tips extend 5 cm over the edge of the table top as shown in the diagram.
4. Click the "STOP" button to end data recording when both temperatures have reached minimums and have begun to increase.
2. Record the minimum and maximum temperature for Temp 1 (ethanol) and Temp 2 (1-propanol). Subtract the minimum temperature from the maximum temperature to determine _T, the temperature change during evaporation.
3. Based on the _T values you obtained for ethanol and 1-propanol, plus information in the PRE-LAB exercise, predict the size of the _T value for 1-butanol.
4. Compare its hydrogen-bonding capability and molecular weight to those of ethanol and 1-propanol. Record your predicted _T, then explain how you arrived at this answer in the space provided. It is not important that you predict the exact _T value; simply estimate a logical value that is higher, lower, or between the previous _T values.
5. Dispose of the filter paper as directed by your teacher.
6. Test your prediction by repeating the data recording procedure using 1-butanol for Sensor A and n-pentane for Sensor B.
7. Using your measured _T values, predict the _T values for methanol and n-hexane. Compare the hydrogen-bonding capability and molecular weight of methanol and n-hexane to those of the previous four liquids. Record your predicted _T, then explain how you arrived at your answer.
8. Test your prediction by repeating the data recording procedure using methanol for Sensor A and n-hexane for Sensor B.
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QUESTIONS
2. Which of the alcohols studied has the strongest intermolecular forces of attraction? The weakest intermolecular forces? Explain using the results of this experiment.
3. Which of the alkanes studied has the strongest intermolecular forces of attraction? The weakest intermolecular forces? Explain using the results of this experiment.