Vapor Pressure

In this experiment, you will investigate the relationship between the vapor pressure of a liquid and its temperature. When a liquid is added to a test tube, it will evaporate into the air above it in the tube. Eventually, equilibrium is reached between the rate of evaporation and the rate of condensation. At this point, the vapor pressure of the liquid is equal to the partial pressure of its vapor in the flask. Pressure and temperature data will be collected using a pressure sensor and a temperature probe connected to a CBL System. The test tube will be placed in water baths of different temperatures to determine the effect of temperature on vapor pressure. You will also compare the vapor pressure of two different liquids, ethanol and methanol, at the same temperature.

MATERIALS

CBL System TI-8X Graphing Calculator Vernier Pressure Sensor Vernier Temperature Probe 2 Vernier adapter cables TI-Graph Link four 1-liter beakers

2-hole stopper with glass tube and Beral pipet two 125-mL Erlenmeyer flasks Parafilm methanol ethanol heavy-wall plastic tubing ice

PROCEDURE

1. Obtain and wear goggles.

2. Use 400 mL beakers to prepare four water baths, one in each of the following temperature ranges: 0 to 5°C, 10 to 15°C, 20 to 25°C (use room temperature water), and 30 to 35°C. For each water bath, mix varying amounts of warm water, cool water, and ice to obtain a volume of 300 mL in a 400 mL beaker. To save time and beakers, two lab groups can use the same set of water baths.

3. Obtain a rubber-stopper assembly, made up of a plastic hose, glass tube, and Beral-type pipet. Obtain the methanol container. Fill the Beral pipet about half full of methanol, then stretch and wrap a small piece of plastc wrap over the tip of the pipet. The plastic wrap must be tight enough so that no methanol escapes from the pipet in Step 9. Carry the stopper assembly back to your station with the tip of the pipet pointing upward. Keep the stopper assembly at your station, with the pipet tip up, until Step 9.

4. Prepare the temperature probe and pressure sensor for data collection.

• Plug the temperature probe into the adapter cable in Channel 1 of the CBL.
• Plug the pressure sensor into the adapter cable in Channel 2 of the CBL.
• Attach the heavy-wall plastic tubing of the rubber-stopper assembly to the end opening of the 3-way valve of the pressure sensor. If your pressure sensor has a metal 3-way valve (see Figure 1), firmly press the plastic tubing onto the end valve opening. If your pressure sensor has a plastic 3-way valve, connect the threaded adapter on the end of the plastic tubing to the end valve stem with a full clockwise turn (see Figure 4).

5. Turn on the CBL unit and the TI-8X calculator. Press PRGM and select CHEMBIO. Press ENTER, then press ENTER again to go to the CHEM MAIN MENU.

6. Set up the calculator and CBL for a temperature probe and a pressure sensor calibration (in mm Hg).

• Select SET UP PROBES from the CHEM MAIN MENU.
• Enter "2" as the number of probes.
• Select TEMPERATURE from the SELECT PROBE menu.
• Enter "1" as the channel number.
• Select PRESSURE from the SELECT PROBE menu.
• Enter "2" as the channel number.
• Select USE STORED from the CALIBRATION menu.
• Select MM HG from the PRESSURE UNITS menu.

7. Set up the calculator and CBL for data collection.

• Select COLLECT DATA from the CHEM MAIN MENU.
• Select TRIGGER from the DATA COLLECTION menu.
• You will collect "4" samples.

8. To take a reading of atmospheric pressure, first open the side valve stem of the 3-way valve. If your pressure sensor has a metal valve (see Figure 1), open its side valve by turning the knob one full turn counterclockwise. If your pressure sensor has a plastic valve, open its side valve by aligning the blue handle with the arm that leads to the rubber-stopper assembly (see Figure 2).

Read the atmospheric pressure from the CBL display. To do this, press [CH VIEW] on the CBL repeatedly until "CH2" blinks, indicating that pressure (in mm Hg) is being displayed. Record this value in your data table. Leave the side valve of the pressure sensor open until Step 10. If your pressure sensor has a plastic valve, align the blue handle with the arm that leads to the pressure sensor (see Figure 3).

9. Holding the stopper assembly with the pipet tip pointing up, insert the 2-hole stopper assembly up into an inverted large test tube. Note: This allows you to firmly twist the stopper for an airtight fit, without inadvertently causing methanol to drip from the pipet tip. It is important that no methanol be squeezed from the bulb until Step 10.

10. To find a pressure-temperature data pair using the 20-25°C (room-temperature) water bath:

• Turn the test tube apparatus right side up and place it into the water bath, with the entire test tube covered.
• Place the temperature probe in the water bath.
• After the test tube has been in the water bath for 30 seconds, close the side valve stem of the 3-way valve. If the valve is metal, firmly turn the knob clockwise to close it. If the valve is plastic, align the blue handle with the side stem, as shown in Figure 4.
• Squeeze the bulb of the Beral pipet to squirt the methanol into the test tube (the plastic wrap will give way).

11. To monitor and collect pressure and temperature data:

• Press the [CH VIEW] button on the CBL repeatedly to alternate between the temperature in Channel 1 ("CH1" in the upper-left corner of the CBL display blinks) and the pressure reading in Channel 2 ("CH2" blinks).
• When the temperature and pressure readings displayed on the CBL screen have both stabilized, equilibrium between methanol liquid and vapor has been established. Press TRIGGER on the CBL to store the first pressure-temperature data pair.

12. To collect another data pair using the 30-35°C water bath:

• Place the test tube assembly and the temperature probe into the 30-35°C water bath. Make sure the entire test tube is covered.
• When the temperature and pressure readings displayed on the CBL screen have both stabilized, press TRIGGER on the CBL to store the second data pair.

13. For Trial 3, repeat the Step-12 procedure, using the 10-15°C water bath. Then repeat the Step-12 procedure for Trial 4, using the 0-5°C water bath. Remove the test tube and the temperature probe after you have pressed TRIGGER and finished collecting data.

14. Use RIGHT ARROW to trace the data points along the displayed graph of pressure vs. temperature. As you move the cursor right or left, the temperature (X) and pressure (Y) values of each data point are displayed below the graph. Record the pressure and temperature data pairs in a data table. Round pressure to the nearest 1 mm Hg and temperature to the nearest 0.1°C.

15. Open the side valve of the pressure sensor so the test tube is open to the atmosphere. Remove the stopper assembly from the test tube and dispose of the methanol as directed by your teacher.

16. Obtain another clean, dry test tube. Draw air in and out of the Beral pipet enough times that you are certain that all of the methanol has evaporated from the inside of the bulb. Use the Step-3 procedure to fill the Beral pipet with ethanol.

17. Set up the calculator and CBL to repeat the data collection.

• Press ENTER to return to the CHEM MAIN MENU.
• Select COLLECT DATA from the CHEM MAIN MENU.
• Select MONITOR INPUT from the DATA COLLECTION menu.
• Press [CH VIEW] on the CBL repeatedly until "CH2" begins to blink, displaying the pressure reading.
• Repeat Steps 9 and 10 to do one trial only for ethanol in the room temperature water bath. When the pressure stabilizes, record its value in the data table.
• Press [+] to quit MONITOR INPUT.

18. Open the side valve of the pressure sensor. Remove the stopper assembly from the flask and dispose of the ethanol as directed by your teacher.

PROCESSING THE DATA

1. Convert each of the Celsius temperatures to Kelvin (K). Write the answer in the space provided.

2. To obtain the vapor pressure of methanol and ethanol, the air pressure must be subtracted from each of the measured pressure values. However, for Trials 2-4, even if no methanol was present, the pressure in the flask would have increased due to a higher temperature, or decreased due to a lower temperature (remember those gas laws?). Therefore, you must convert the atmospheric pressure at the temperature of the first water bath to a corrected air pressure at the temperature of the water bath in Trial 2, 3, or 4. To do this, use the gas-law equation (use the Kelvin temperatures):

where P₁ and T₁ are the atmospheric pressure and the temperature of the Trial 1 (room temperature) water bath. T₂ is the temperature of the water bath in Trial 2, 3, or 4. Solve for P₂, and record this value as the corrected air pressure for Trials 2, 3, and 4. For Trial 1 of methanol and Trial 1 of ethanol, it is not necessary to make a correction; for these two trials, simply record the atmospheric pressure value in the blank designated for air pressure.

3. Obtain the vapor pressure by subtracting the corrected air pressure from the measured pressure in Trials 2-4. Subtract the uncorrected air pressure in Trial 1 of methanol (and Trial 1 of ethanol) from the measured pressure.

4. Plot a graph of vapor pressure vs. temperature (°C) for the four data pairs you collected for methanol. Temperature is the independent variable and vapor pressure is the dependent variable. As directed by your teacher, plot the graph manually, or use the TI-8X calculator. To plot a graph of pressure vs. temperature (°C) using the TI-83:

• Select QUIT to exit the program.
• Press STAT, then select 1:Edit to view the lists.
• Clear L1 and L 2 by moving the cursor to the L1 or L 2 headings, then pressing CLEAR and ENTER.
• Enter the four temperature values in L1 and the four corresponding vapor pressure values in L2. Important: Enter the values in order of increasing temperature.
• Press [2nd][STAT PLOT], then select 1:Plot1. Use the arrow keys to position the cursor on each of the following Plot1 settings. Press ENTER to select any of the settings you change: Plot1 = On, Type = , Xlist = L1, Ylist = L 2, and Mark = .
• To plot a graph of vapor pressure vs. temperature (°C), press ZOOM, then select ZoomStat.

5. How would you describe the relationship between vapor pressure and temperature, as represented in the graph you made in the previous step? Explain this relationship using the concept of kinetic energy of molecules.

6. Which liquid, methanol or ethanol, had the larger vapor pressure value at room temperature? Explain your answer. Take into account various intermolecular forces in these two liquids.

EXTENSION

The Clausius-Clapeyron equation describes the relationship between vapor pressure and absolute temperature:

where ln P is the natural logarithm of the vapor pressure, Hvap is the heat of vaporization, T is the absolute temperature, and B is a positive constant. If this equation is rearranged in slope-intercept form (y = mx + b):

the slope, m, should be equal to -Hvap / R. If a plot of ln P vs. 1/T is made, the heat of vaporization can be determined from the slope of the curve. Follow these steps to plot the graph on the TI-83 calculator:

• If you have not already done so, enter the temperature (°C) and vapor pressure data pairs in L1 and L2, respectively (see Step 4 of Processing the Data). Enter the values in order of increasing temperature.
• Press [2nd][QUIT], then CLEAR.
• To create a list of reciprocal of Kelvin temperature values (in L 3), press ([2nd][L1] + 273)[x^-1][STO>][2nd][L 3] ENTER .
• To create a list of natural log (ln) of vapor pressure values (in L 4), press [ln][L 2][STO>][2nd][L 4]ENTER.
• Press STAT, RIGHT ARROW to display the CALC menu. Select 5:LinReg(ax+b).
• To identify the lists that correspond to the two variables, press [2nd][L 3][,][2nd][L 4] ENTER. The statistics are displayed for the equation in the form:

  y = ax + b

  where x is 1/ Kelvin temperature, y is ln vapor pressure, a is the slope, and b is the y-intercept. Record the value of the slope, a, to use in the calculation of heat of vaporization below.

• To display a best-fit regression line on a graph of pressure vs. Kelvin temperature, first press [Y=]. Press CLEAR to clear the Y1= equation, then press VARS. Select 5:Statistics, then press RIGHT ARROW, RIGHT ARROW to display the EQ menu. Select 7:RegEQ to copy the linear regression equation to Y1=.
• Press [2nd][STAT PLOT], then select 1:Plot1. Use the arrow keys to position the cursor on each of the following Plot1 settings. Press ENTER to select any of the settings you change: Plot1 = On, Type = , Xlist = L 3, Ylist = L 4, and Mark = .
• To plot a graph of ln vapor pressure vs. 1/Kelvin temperature, press ZOOM, then select ZoomStat.

Use the slope value to calculate the heat of vaporization for methanol (m = a = -Hvap / R).