Chemistry: Evaporation and Intermolecular Attractions

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Name: ______

Honors 227

Laboratory No. 5

Chemistry: Evaporation and Intermolecular Attractions

Introduction

When a liquid is exposed to the atmosphere, some of the molecules change state from a liquid to a gas. The rate at which this vaporization happens is not the same for all liquids and very much depends on chemical properties of the liquid. As molecules of a liquid change state to a gas, the temperature of the liquid changes, which is also a function of chemical properties of the chemical. Thus, by simply measuring temperature changes of a liquid as it evaporates, one may investigate some of the chemical properties of molecules.

The purpose of this experiment 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.

Background

Evaporation is an endothermic process that results in a temperature decrease. Evaporating molecules carry away thermal energy when they leave a liquid. The amount of temperature decrease is related to the strength of intermolecular forces of attraction between molecules of the liquid. Intermolecular forces include hydrogen bonding, polarity, and van der Vaal forces.

Alkanes and alcohols are two types of hydrocarbons (i.e., compounds composed mostly of carbon atoms linked together via covalent bonds and also bonded to hydrogen atoms). Alkanes have carbon and hydrogen atoms only. The two alkanes used in this activity are pentane, C5H12, and hexane, C6H14. In addition to carbon and hydrogen atoms, alcohols also contain the –OH functional group (hydroxyl). Two of the alcohols used in this activity are methanol, CH3OH, and ethanol, C2H5OH. You should assume that the differences in chemistry of these molecules translate into differences in evaporation, which in turn should be evident by different temperature profiles.

Pre-Lab Analysis

Complete Table 1 before beginning the laboratory activity. In order to diagram the structural formula, you need to refer to the text and lecture notes. An example of the structural formula for methanol is provided. The molecular mass is measured in units of g/mole and is calculated as the sum of the mass for each atom comprising the structural formula for the compound (refer to the Periodic Table of Elements in your text for this information). For example, the molecular mass of oxygen gas (O2) is 16 + 16 = 32 g/mole. 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. In Table 1, indicate whether or not each hydrocarbon molecule has hydrogen bonding and in parentheses indicate the number of hydrogen bonds.

Before you begin the laboratory and after completing Table 1, answer Question No. 1.

Laboratory Exercise

General Scheme

The general scheme is to use the Data Studio Temperature Sensors to measure changes in temperature of pieces of filter paper that have been soaked in various organic liquids. After measuring the change in temperature for Ethanol and Propanol, you will analyze the temperature changes in light of the data in Table 1.

Based on this analysis, make a prediction as to the temperature of vaporization for Butanol and Pentane. Once you have made that prediction, use Temperature Sensors to test your prediction. Then make a prediction for the change in temperature for Methanol and Hexane. Use the sensors to test your prediction. Use DataStudio to record, display, and analyze your data.

Safety

The following safety concerns must be adhered to:

1. Absolutely no open flames;
2. The individual handling the test tubes and sensors must wear goggles and rubber gloves (provided);
3. The used filter paper, gloves and any towels must be disposed of in the plastic bag on the instructor’s desk; and
4. At the end of the exercise, the test tubes need to be re-capped and the desk top left clean.

Part I: Computer Setup

1. Connect the ScienceWorkshop interface to the computer, turn on the interface, and turn on the computer (this should already be done).
2. Connect the DIN plug of one Temperature Sensor into Analog Channel A and the DIN plug of the other sensor to Analog Channel B on the interface (this should already be done).
3. Open the file titled as shown: C05: Evaporation and Intermolecular Attraction (on the C Drive, Program Files, Chemistry)

• The DataStudio file has a Workbook display. Read the brief instructions in the Workbook.

Part II: Sensor and Equipment Setup

1. Wrap the ends of Sensor A and Sensor B with square pieces of filter paper secured by small rubber bands. Roll the filter paper around the sensor tip in the shape of a cylinder.
2. The test tube holder on the workbench has six test tubes each marked as having one of six organic liquids. Arrange the test tubes in the following sequence
3. Ethanol
4. Propanol
5. Butanol
6. Pentane
7. Methanol
8. Hexane

Each test tube has been filled with sufficient volume of each hydrocarbon.

1. Stand Sensor A in the Ethanol test tube and Sensor B in the Propanol test tube. Make sure you know which sensor is A and which is B (look at the Data Interface Black Box and trace the lines). Let the sensors remain in the liquid for ~2 minutes.
2. Cut 2 pieces of masking tape, ~10-cm long, to be used to tape the sensors in position during Data Recording.

Part III: Data Recording and Analysis– Ethanol and Propanol

1. After the sensors have been in the liquids for ~ 2 minutes, start recording data. The following procedure is important to follow:
2. Click on Temperature Graph on Display Menu (lower left menu bar)
3. Click on the Start Icon (upper horizontal menu bar).
4. Allow the graph to run for ~20 seconds, and then remove each sensor from the liquid but still “housed” within the test tube. Allow the liquid to drip for ~ 2 seconds, and then remove the sensor from the test tube and place the senor on the counter top so the end of the sensor extends over the counter top. Tape the sensor in position with masking tape.
5. Observe the change in temperature of the two sensors for ~230 seconds. On the Menu Bar for the Temperature Graph, you can change dimensions of the graphic display by clicking on several of the icons (start with the far left and experiment with the next two icons which are in the shape of magnifying glasses); while the data are being generated, click on the icons to understand how then individually change the graphic display.
6. After 230 seconds, click Stop.
7. Graph the entire 230 second response in Figure 1; mark the coordinates at 5-10 second intervals and draw a line illustrating the response. Mark the line for each hydrocarbon.
8. As shown by the instructor, use the graphical display icons to determine the following (using the sixth icon from the left and the cursor arrays):
9. Initial/maximum temperature (Tmaximum) in oC
10. Minimum temperature (Tminimum) in oC
11. Temperature Difference or T (Tmaximum – Tminimum) in oC (include the direction of change, positive or negative)
12. Record these data for each chemical in Table 2.
13. Recognizing the safety precautions, dispose of the filter paper as directed by your teacher and wipe off the sensor tip with a paper towel. Clean up any hydrocarbon residue that has drip onto the countertop or the floor.

Part IV: Predictions for Butanol and Pentane

1. Based on the T values you obtained for Ethanol and Propanol, plus information in the Pre-Lab exercise, predict the size of the T value for Butanol. (Hint: compare its hydrogen-bonding capability, polarity and molecular mass to those of Ethanol and Propanol. 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.)
2. Record your predicted T, and explain how you arrived at this answer in the space below:

HydrocarbonT valueExplanation

1. Make a prediction for Pentane and record (above) your predicted T and explanation.

Part V: Data Recording – Butanol and Pentane

1. Test your prediction by repeating the data recording procedure using Butanol for Sensor A and Pentane for Sensor B.
2. Start recording data. Record 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 tabletop.
3. Stop data recording at ~230 seconds.
4. Graph the entire 230 second response in Figure 1; mark the coordinates at 5-10 second intervals and draw a line illustrating the response. Mark the line for each hydrocarbon.
5. Analyze the data using the cursor and record the data in Table 2.
6. Dispose of the filter paper as directed above.

Part VI: Make a Prediction for Methanol and Hexane

1. Using your measured T values, predict the T values for Methanol and Hexane. Compare the hydrogen-bonding capability and molecular mass of Methanol and Hexane to those of the previous hydrocarbons.
2. Record your predicted T, and explain how you arrived at this answer in the space below:

HydrocarbonT valueExplanation

Part VII: Data Recording – Methanol and Hexane

1. Test your prediction by repeating the data recording procedure using Methanol for Sensor A and Hexane for Sensor B.
2. Start recording data. Record 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 tabletop.
3. Stop data recording at ~230 seconds.
4. Graph the entire 230 second response in Figure 1; mark the coordinates at 5-10 second intervals and draw a line illustrating the response. Mark the line for each hydrocarbon.
5. Analyze the data using the cursor and record the data in Table 2.
6. Dispose of the filter paper as directed above.
7. Wipe each sensor with a wet (water) towel and place them on a clean towel on the countertop.

Part VIII: Analyzing the Data

Complete Table 2 and answer the questions in the Lab Report section.

1 Alcohol

2 Alkane

Questions

Question No. 1. What do you think the relationship will be between the strength of intermolecular attraction for a liquid and the amount of temperature change caused by evaporation of the liquid? Explain your answer.

Question No. 2. Pentane and Butanol have almost the same molecular mass, but significantly different T values. Explain the difference in T values of these substances, based on their intermolecular forces.

Question No. 3. Which of the alcohols studied has the strongest intermolecular forces of attraction? Which have the weakest intermolecular forces? Explain using the results of this experiment.

Question No. 4. Which of the alkanes studied has the strongest intermolecular forces of attraction? Which have the weakest intermolecular forces? Explain using the results of this experiment.

Question No. 5. In Figure 2, plot a graph of T values (vertical axis) for the four alcohols versus their respective molecular masses (horizontal axis) and draw a line that best represents the relationship. In words, explain how molecular mass relates to T?

Question No. 6. If you were given an alcohol with a molecular weight of 101 g/mole, how volatile would it be? Explain your answer using the relationship in Figure 2.

Figure 2. Relationship between molecular mass and T