INTERMOLECULAR FORCES PART 1: Evaporation and Intermolecular Attractions

Purpose: Investigate the relationship of dispersion forces and hydrogen bonding forces in intermolecular attractions.

IntroductionIn this experiment, temperature probes covered with filter paper are placed in various liquids.

Evaporation occurs when the probe is removed from the liquid’s container (see figure 1). 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. The temperature change is greater if more evaporation occurs (weaker intermolecular forces). 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 in this experiment — alkanes and alcohols. The two alkanes 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 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. Dispersion forces exist between any two molecules and generally increase as the molecular weight of the molecule increases. A hydrogen bond can occur when, in one molecule, a hydrogen atom is bonded directly to an N, O, or F atom (the donor) and that hydrogen is attracted to a lone pair on an N, O, or F atom in another molecule (the acceptor). Both a donor and an acceptor must be present for a hydrogen bond to occur.

Materials

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EQUIPMENT (Per Group)ComputerSerial Box InterfaceLoggerPro(2) Vernier Temperature Probes(6) Pieces of filter paper (2.5 cm x 2.5 cm)(2) Small rubber bandsMasking Tape(6) Pyrex test tubesTest tube racks

CHEMICALS (Per Group)10 mL Methanol (CH3OH)10 mL Ethanol (C2H5OH)10 mL 1-Propanol (C3H7OH)10 mL 1-Butanol (C4H9OH)10 mL Pentane (C5H12)10 mL Hexane (C6H14)

Procedures

CAUTION: In all experiments today keep liquids away from the computers and keyboards. Be very careful when stirring with the temperature probes. Clean them gently with water and wipe gently or blot them with Kim wipes.

1. Prepare the computer for data collection by opening LoggerPro. 2. You will need to then find the appropriate experiment using the following pathway:

FileOpen ExperimentsChemistry with Computers 09 Evaporation. 3. You may need to confirm the identity of the temperature probes that you are using in the Sensor

Confirmation dialog box IF it pops up when you open the program. If it does not, then the probes have been identified already.

4. You should see a temperature scale on the y-axis and time on the x-axis.You will need to set the time scale for data collection.

5. Under the Experiment menu, select Data Collection. 6. Change the Length to 600 seconds and click “OK’. 7. Then rescale the horizontal axis to 0 to 600 seconds by double clicking on any number on the x-

axis and then clicking on Axes Option. On the bottom right corner for the left and right values, set the Left to 0 and the Right to 600.

8. The vertical axis of the graph will have temperature scaled from 5 to 30°C, change this to read 0 to 30°C by clicking once on the minimum and changing it to 0.

9. Wrap Probe I and Probe 2 with square pieces of filter paper secured by small rubber bands as shown in Figure 1. 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.

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10. Add 10 mL of ethanol to a pyrex test tube and label it “Test tube 1.” Add 10 mL of 1-propanol to a second pyrex test tube and label it “Test tube 2.”

11. Stand Probe I in the test tube containing the 10 mL of ethanol and Probe 2 in the test tube containing 10 mL of 1-propanol. Make sure the containers do not tip over.

12. Prepare 2 pieces of masking tape, each about 10-cm long, to be used to tape the probes in position during step 13.

13. After the probes have been in the liquids for at least 30 seconds, begin data collection by clicking the green arrow that says “Collect.” Leave each probe in its respective test tube and monitor the temperature for 15 seconds to establish the initial temperature of each liquid.

14. Without hitting stop, simultaneously remove the probes from the liquids and tape them to the bench so the probe tips extend 5 cm over the edge of the table top as shown in Figure 1. Make sure that the filter paper goes to the tip of the probe, but not past the tip.

15. When both temperatures have reached minimums and have begun to increase, click STOP to end data collection.

16. Click the Statistics button on the top bar menu, then click OK to display a box for both probes. Record the maximum (T1) and minimum (T2) values for Temperature 1 (ethanol) and Temperature 2 (1-propanol) in the table.

17. For each liquid, subtract the minimum temperature from the maximum temperature to determine ∆T, the temperature change during evaporation.

18. Based on the ∆T values you obtained for these two substances, plus information in the Pre -Lab exercise, predict the relative size of the ∆T value for 1 -butanol. Compare its hydrogen-bonding capability and molecular weight to those of ethanol and 1-propanol.

19. Record your predicted ∆T and explain how you arrived at this answer in the space provided. Do the same for pentane. 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 (> 4°C or < 8°C will suffice).

20. Test your predictions in Step 18 by repeating Steps 9-17 using 1-butanol for Probe 1 and pentane for Probe 2.

21. Based on the ∆T values you have obtained for all four substances, plus information in the Pre - Lab exercise, predict the ∆T values for methanol and hexane. Compare the hydrogen-bonding capability and molecular weight of methanol and hexane to those of the previous four liquids. Record your predicted ∆T, then explain how you arrived at this answer in the space provided.

22. Test your prediction in Step 21 by repeating Steps 9-17 using methanol with Probe 1 and hexane with Probe 2.

Data and Results

Table 1: Temperature change during evaporation

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Substance Tmax ( C)⁰ Tmin (⁰C) ∆T ( C)⁰(Tmax – Tmin)

Ethanol

1-Propanol

Table 2: Comparing and predicting temperature changes during evaporation

Substance Explanation Predicted∆T ( C)⁰

Tmax

( C)⁰Tmin

(⁰C)∆T ( C)⁰

(Tmax – Tmin)

1-Butanol

Pentane

Methanol

Hexane

Using Excel, plot a graph of ∆T ( C)⁰ values of the four alcohols versus their respective molecular weights. Plot molecular weight on the x-axis and ∆T on the y-axis. Label each point with the identity of the alcohol. Can you draw any conclusions regarding intermolecular forces and molecular weight from this graph?

Discussion The following list was designed to help you construct a good discussion as well as to illustrate how you should go about analyzing the theory behind a particular experiment and relating it to the results and observations. For this experiment, consider and include in your discussion (but not limit it to) the following:- Correlate how readily a sample evaporated to the strength of the intermolecular forces in the

molecule- Compare substances with nearly the same molecular weights but significantly different ∆T values

and explain this difference based on or in terms of their intermolecular forces. Be specific!- Compare the two alkanes in terms of structure, molecular weight and thus intermolecular forces- Compare the four alcohols in terms of structure, molecular weight and thus intermolecular forces- Analyze the graph!Pre-Lab Questions (To be included and answered in your Notebooks)

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1. Complete the following table:

Substance Formula Structural Formula(condensed formula is ok)

Molecular Weight(g/mol)

Hydrogen Bond

(Yes or No)

Ethanol C2H5OH

1-Propanol C3H7OH

1-Butanol C4H9OH

Pentane C5H12

Methanol CH3OH

Hexane C6H14

2. Which will evaporate faster, 1-Butanol or Pentane? Explain your choice.

3. A thermometer is placed in a test tube of chipped ice at -5.0 ⁰C. The temperature is recorded at the time intervals shown below until room temperature is reached. Plot the data given below on graph paper and explain all flat, horizontal portions of the curve. Plot time on the X-axix!

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Time (min) Temperature (⁰C)0 -5.02 -2.54 -1.06 0.010 0.015 0.020 0.025 0.030 1.535 4.040 8.045 11.550 15.055 17.560 19.065 20.070 20.075 20.080 20.0

4. Define the following thermic processes and state whether they are ENDO or EXOthermic: Melting _________________________i. Definition:

Freezing ________________________i. Definition:

Vaporization _____________________i. Definition:

Condensation _____________________i. Definition:

Sublimation________________________i. Definition:

Deposition ___________________________i. Definition:

Post-Lab Questions (To be answered in your Lab Report)

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1. In the space below, organize your samples from smallest ∆T to largest ∆T for your alkanes, and again for your alcohols:

Alkanes: _____________________ _____________________

Alcohols: _______________<________________< _________________< _________________

2. Discover what the ∆T “tells” you about how readily a sample evaporates. Complete following statements by adding in the blank the correct response (DOES or DOES NOT):

Large ∆T values mean that the sample ___________________________ evaporate readily.

Small ∆T values mean that the sample ___________________________ evaporate readily.

3. Looking at the formulas for your alkanes, what is the only difference between formulas?

4. What IMF(s) are present in the alkanes (list all present)?

5. Looking at the formulas for your alcohols, what is the only difference between the formulas?

6. What IMF(s) are present in the alcohols (list all present)?

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