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In our first activity, we investigated structure and functional groups as we modeled several molecular compounds. In this activity, you will now investigate some of the components of crude oil to make sense of how they are separated through distillation.

In this experiment, temperature probes are placed in various liquids. Evaporation occurs when the probe is removed from the liquid’s container. This evaporation is an endothermic process that results in a temperature decrease. The magnitude of a temperature decrease is, like viscosity and boiling temperature, related to the strength of intermolecular forces of attraction. In this experiment, you will study temperature changes caused by the evaporation of several liquids and relate the temperature changes to the strength of intermolecular forces of attraction. You will use the results to predict, and then measure, the temperature change for several other liquids.

You will encounter two types of organic compounds (that you were introduced to in AC1 – Molecular Geometries) in this experiment—alkanes and alcohols. The three alkanes are n-pentane, C5H12, n-hexane, C6H14, and n-heptane. 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 we will use in this experiment. You will examine the molecular structure of alkanes and alcohols for the presence and relative strength of two intermolecular forces—hydrogen bonding and dispersion forces.

Prior to doing the experiment, copy and complete Table 2.1 on the top of the next page into your laboratory notebook. The name or formula is given for each compound. Draw a structural formula for a molecule of each compound. Then determine the molecular weight of each of the molecules. Next, examine each structural formula for the presence of hydrogen bonding. Tell whether or not each molecule has a polar bond in it.

As always, state the objectives for this activity.

P REVIEWING STRUCTURES • LEARNING OBJECTIVES

P R E P A R I N G

AE

Activity 2: Petroleum Mixture Evaporation & Intermolecular Forces

Fig 2.1

Fig 2.2

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Table 2.1: A lkanes and Alcohols

SUBSTANCE FORMULA STRUCTURAL FORMULAS MOLECULAR WEIGHT

POLAR BOND? (YES OR NO)

CH3OH

C2H5OH

1-propanol

1-butanol

n-pentane C5H12

C6H14

n-heptane

P a r t A : C o l l e c t i n g I n i t i a l D a t a 1. Obtain and wear goggles!

CAUTION: The compounds used in this experiment are flammable and poisonous. Avoid inhaling their vapors. Avoid contact with your skin or clothing. Be sure there are no open flames in the lab during this experiment. Notify your instructor immediately if an accident occurs.

2. Connect three temperature probes to the LabQuest and turn it on. You should see three temperature readings. To see the graph, use the blue-green stylus in the back of the unit to click on the small graph picture on the top of the screen.

3. Wrap Probe 1, Probe 2, and Probe 3 with square pieces of filter paper secured by small rubber bands as shown in Figure 2.3. You will need to cut the strip of filter paper into the small squares. Roll the filter paper around the probe tip in the shape of a cylinder. Hint: First slip the rubber band up on the probe, wrap the paper around the probe, and then finally slip the rubber band over the wrapped paper. The paper should be even with the probe end.

4. Stand Probe 1 in the ethanol container, Probe 2 in the 1-propanol container, and probe three in the methanol container. Make sure the containers do not tip over.

E COLLECTING DATA & PREDICTING

E X P E R I M E N T I N G

Fig 2.3

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5. Prepare 3 pieces of masking tape, each about 10 cm long, to be used to tape the probes in position during Step 6.

6. After the probes have been in the liquids for at least 30 seconds, begin data collection by clicking the green collect arrow in the lower left-hand corner of the screen. Monitor the temperature for 15 seconds to establish the initial temperature of each liquid. Then simultaneously remove the probes from the liquids and tape them so the probe tips extend 5 cm over the edge of the table top as shown in Figure 2.3.

7. When all three temperatures have reached minimums and have begun to increase, click the red stop button to end data collection. Click “Analyze” and “Statistics.” Select the three runs. On the right-hand side of the screen you can see the statistics for each line (you’ll need to scroll down to see it all). Record the maximum (t1) and minimum (t2) values for Temperature 1 (ethanol), Temperature 2 (1-propanol), and Temperature 3 (methanol).

8. For each liquid, subtract the minimum temperature from the maximum temperature to determine Δt, the temperature change during evaporation.

9. Roll the rubber band up the probe shaft and dispose of the filter paper as directed by your instructor (in trash). Reset the LabQuest for the next data run by selecting “File,” “New,” and click on “Discard.”

P a r t B : P r e d i c t i n g B a s e d o n D a t a In Part B you will make and test predictions based on your data from Part A. Call your instructor over once you have your first prediction.

10. Based on the Δt values you obtained for these three substances, plus information in the preparing section, predict the size of the Δt value for 1-butanol in your observations before you collect the data (hence “predict”). Record your predicted Δt and explain how you arrived at this answer in your observations section. Do the same for n-pentane. It is not important that you predict the exact Δt value; simply estimate a logical value that is higher, lower, or between the previous Δt values. Call over your instructor to check out your prediction.

11. Test your prediction in Step 10 by repeating Steps 3-9 using 1-butanol for Probe 1, and n-pentane for Probe 2. You can disconnect Probe 3.

12. Based on the data collected in steps 10 and 11, predict the size of the Δt value for n-hexane and n-heptane in your observations. Record your predicted Δt and explain how you arrived at this answer in your observations section.

13. Test your prediction in Step 12 by repeating Steps 3-9 using n-hexane for Probe 1, and n-heptane for Probe 2.

R e a d i n g : P e t r o l e u m M i x t u r e

Read Zumdahl 14.1 - 14.4 on energy requirements and phase changes

A EVAPORATION & INTERMOLECULAR FORCES

A N A L Y Z I N G

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Q u e s t i o n s : P e t r o l e u m M i x t u r e

1. Two of the liquids, n-pentane and 1-butanol, had nearly the same molecular weights, but significantly

different Δt values. Explain the difference in Δt values of these substances.

2. Plot a graph of Δt values of the four alcohols v. their respective molecular weights. Plot molecular weight on the horizontal axis (what you changed) and Δt on the vertical axis.

3. Plot a second graph of Δt values of the three alkanes v. molecular weights.

W h a t d o e s t h e a c t i v i t y m e a n ? Chemistry explains the macroscopic phenomenon (what you observe) with an explanation of what happens at the nanoscopic level (atoms and molecules) using symbolic structures as a way to communicate. Explain the meaning of this activity by completing the MNS table.

M A C R O N A N O S Y M B O L I C

Focus on the phase change of ethanol for this m/n/s.

H o w d o I k n o w ? Which of the alkanes studied has the stronger intermolecular forces of attraction? The weakest

intermolecular forces? Explain using the results of this experiment. Which of the alcohols (alkanes with a hydroxyl functional group) studied has the strongest intermolecular forces of attraction? The weakest intermolecular forces? Explain using the results of this experiment.

W h y d o I b e l i e v e ? How does this relate to what you learned in the first activity? How might it help explain the illustration below (Fig 2.4)?

W h y d o I c a r e ? The premise of this lab is that molecules with few intermolecular attractions will evaporate fastest, making a large temperature change quickly. Why is this? How do you think this might relate to fuels?

CT

MEAN? • KNOW? • BELIEVE? • CARE?

C R I T I C A L L Y T H I N K I N G

Fig 2.4