TEMPERATURE S RELATIONSHIP TO GAS & VAPOR PRESSURE Adapted from "Chemistry with Computers" Vernier Software, Portland OR, 1997 ELECTRONIC LABORATORY NOTEBOOK (ELN) INSTRUCTIONS Read the directions and watch the podcast under Week 0 on how to use the ELN. Also, read the step-by-step directions for the ELN use with the first experiment. All work for this experiment must be recorded, attached, or answered in the ELN. Create a pre & inlab page in the Experiment #1 s folder containing the following sections: 1) Equipment and chemical tables. Pictures of typical equipment, instrumentation, special reaction set-ups should be attached. Chemicals with constants needed in the experiment and NFPA Fire Diamonds should be listed in the table. (A special Widget is available for making the Fire Diamonds.) In order to have a complete table, you must look ahead at the calculations and think about constants that will be needed. 2) Procedures and Observations. PROCEDURAL STEPS MUST BE IN YOUR OWN WORDS DO NOT COPY THE LAB MANUAL. All quantitative data should be recorded with the correct number of significant figures and units. All qualitative observations should indicate the color, depth of color, and either the crystallinity of a solid or the clarity of a liquid. Any cues that indicate a reaction has occurred (such as heat evolution, color change, gas evolution, et cetera) should be noted. It is acceptable to write the procedures and observations together in one section as you perform the experiment. All Logger/Pro data files created should be recorded or attached to this ELN page. Postlab questions at the end of this document should be answered on a new page in Experiment #1 s folder. 1
SAFETY Students must wear safety goggles and lab aprons at all times in the lab. Be careful when handling heated glassware. Use paper towels, toweling, or tongs to avoid burns. Ethanol is flammable; make sure there are no sparks or flames in lab. Ethanol fumes can irritate eyes and lungs; avoid contact with fumes. INTRODUCTION Molecules in the gas phase are in constant motion. Gas pressure is a measurement of the number of collisions of molecules with the walls of the container. The velocity and the number of collisions of these molecules changes when the temperature of the gas increases or decreases. In this experiment, the relationship between the temperature of a gas sample and the pressure it exerts will be investigated. Using the apparatus shown in Figure 1, an Erlenmeyer flask containing an air sample will be placed in water baths of varying temperature. Figure 1 Pressure will be monitored with a pressure sensor and temperature will be monitored using a temperature probe. The volume of the gas sample and the number of molecules it contains must be kept constant. Pressure and temperature data pairs will be collected during the experiment and then analyzed. From the data and graph, you will determine what kind of mathematical relationship exists between the pressure and absolute temperature of a confined gas. Finally, the value for absolute zero will be determined. In the next part of this experiment you will investigate the relationship between vapor pressure and temperature and find the heat of vaporization (ΔH vap ) of ethanol. When a liquid is added by syringe to the same sealed Erlenmeyer flask used for the first part of the experiment (Figure 2), it will begin to evaporate into the air above in the flask. 2
Figure 2 Evaporation will continue until equilibrium is established 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. The pressure sensor will be used to measure changes in the total pressure in the flask (the pressure resulting from air and gaseous ethanol molecules together). First, the pressure in the flask will be measured at room temperature and then the flask will be placed in a water bath so the temperature of the bath can be increased by small increments to determine the effect of temperature on vapor pressure. By plotting the natural log of the vapor pressure (ln P) versus inverse Kelvin temperature (1/T), the heat of vaporization (ΔH vap ) can be determined from the slope of the best fit line using the Clausius Clapeyron equation: (1) ln P = ( ΔH vap / R) (1/T) + C (where R = gas constant, 8.3145 J/mol-K and C is a constant.) Before starting the experiment, the TA will ask you to do a quick demonstration or talk-through one of the following: 1) Assemble the glassware setup for this experiment (without the parafilm) 2) How to use parafilm. Parafilm the glassware setup assembled by the previous student. 3) Find a stir plate in the room and show how to use it. What does the stir bar look like? 4) Where is the ice machine found for making the cold water bath? (student can volunteer for this one) 3
Make sure you watch the videos on the course website and read the documents to prepare. These demonstrations will be done every week. Everyone will have presented at least one topic by the end of the quarter. The demonstrations should be short (>1 min) and will be graded. PROCEDURE Part A. Temperature & Gas Pressure Work in pairs. Wear safety goggles and lab apron in lab at all times. 1. Prepare 4 water baths containing approx 800-mL of water in 1-L beakers. Place a stir bar in each beaker and place the beaker on a stir plate. Stir on the stirrer (not the heat) to make sure that the temperature throughout each bath is even. Boiling-water bath. Use hot tap water; heat to boiling on a hot plate. Hot water bath. Use hot tap water. Room temperature bath. Use tap water and let stand at RT. Ice-water bath. Use cold tap water and ice mixture. 2. Prepare the glassware setup for the experiment. Obtain a rubber-stopper assembly with a piece of heavy-wall plastic tubing connected to one of its two valves (Figure 3). Figure 3 Insert the rubber-stopper assembly into a 125-mL Erlenmeyer flask. Important: Twist the stopper into the neck of the flask to insure a tight fit. Attach the connector at the free end of the plastic tubing to the gas pressure sensor valve with a clockwise turn. Wrap parafilm around the stopper and top of the flask to provide extra insurance against gas leaks. (Parafilm should be stretched when doing this or it will not provide a good seal.) 4
Close the 2-way valve above the rubber stopper as shown in Figure 4. Do this by turning the white valve handle so it is perpendicular with the valve step itself. closed Figure 4 3. Plug the temperature probe into CH 1 and the pressure sensor into CH 2 of the LabQuest2 interface box. Make the following adjustments: Go to the screen, click on graph from the menu and then graph options change X- axis column to temperature and under graph 2! Y-axis, unselect CH1:Temperature. Click OK. Go to the screen, click on mode and change to selected events. Click OK. Change the units on the pressure sensor to mmhg. Do this by clicking on the box labeled CH2:Pressure and in the drop down menu select Change units! mmhg. 4. Take a reading of the pressure of air trapped in the Erlenmeyer flask. Read the air pressure in mm Hg from the screen. The pressure reading of air in the flask should be the same as atmospheric pressure (which is around 760 mm Hg on a mild sunny day). Suspend the temp probe in the air (away from the hot plate!) and read the room air temperature in C from the live display below the graph (which is around 20 C in the lab). If your pressure and temperature values deviate significantly from the values given in parentheses, replace the temperature or pressure probes. Record these values in a data table. 5. Click in the screen to begin data collection of pressure vs. temperature data for the gas sample: Place temperature probe into the ice-water bath (Figure 1). Clamp the flask so that most of it is immersed. (The water level should be just below the parafilm used to seal in the stopper.) When the pressure and temperature readings displayed in the Meter window stabilize, click Keep. The first pressure-temperature data pair has now been saved. 5
6. Repeat the above step three more times using a room-temperature bath, a hot-water bath, and a boiling water bath. For the boiling water bath place the temperature probe in first. Once the temperature has stabilized use tongs to hold the flask in the boiling water bath. Clamp up tubing and probe wires to prevent melting from the hotplate. CAUTION: Do not burn yourself of the probe wires on the hot glassware or hot plate. When you have finished collecting data. Turn off the hot plate. 7. Email the resulting file to your ELN. Part B. Temperature & Vapor Pressure Note: Calculate air and vapor pressure as you collect each pressure, temperature data pair (see Part B Calculations #1) many students get erroneous results for this experiment, so calculating as you go will enable you to repeat measurements. Do not inhale fumes and wash hands thoroughly before leaving lab. Ethanol is very flammable, extinguish any open flames in lab. 8. You will need the boiling water and room temperature water baths from Part A. Make sure the room temperature bath is truly room temperature. (This is important, if the water is colder than room temperature, it will cause erroneous results for the first few pressure measurements.) Use the experimental set up from Part A. Make sure the 2-way valve shown in Figure 3 is closed. 9. Obtain 2-3 ml ethanol (EtOH) container and the syringe. (Note: 1 cc = 1 ml). With the two-way valve still closed, screw the syringe onto the two-way valve as shown in Figure 3, but do not place the flask into the water bath until directed to do so. Repeat the set up of the LabQuest2 in Part A. 10. Take a reading of the pressure of air trapped in the Erlenmeyer flask. The air pressure should be around 760 mm Hg and the room air temperature should be around 20 C. 11. Inject the ethanol into the flask: 6
Open the 2-way valve above the rubber stopper: turn the white valve handle so it is aligned with the valve stem itself as shown in Figure 5. open Figure 5 Squirt the EtOH into the flask by pushing in the plunger of the syringe. Quickly close the 2-way valve by turning the white valve handle so it is perpendicular with the valve stem. Remove the syringe from the 2-way valve with a counter-clockwise turn taking care not to loosen the stopper assembly from the flask. Leaks are disastrous to this experiment! 12. To monitor and collect pressure and temperature data: a. Click in the screen. b. After a few moments equilibrium will be established between EtOH liquid and vapor at a given temperature. (This is indicated when the P and T readings displayed on the computer monitor stabilize.) Once this happens, click Keep. The first P-T data measurement (at room temperature) is now stored. c. Calculate the vapor pressure at room temperature. 13. Place the sealed flask in the water bath with the entire flask covered as shown in Figure 2. Place the temperature probe in the water bath. Next, without disturbing the flask, increase the temperature of the water bath about 3 C by adding a small amount of the boiling water prepared on the hot plate. (Use a pipet to measure out hot water or pour the hot water from the beaker protecting your hands with paper towels.) Stir the water bath with the temperature probe. Make sure the flask is still covered with water, wait until P and T readings on the computer monitor stabilize and then click Keep. Your second PT data pair is now stored. Calculate the air and vapor pressure at this temperature. 14. Next, without disturbing the flask, increase the temperature of the water bath about 3 C by adding a small amount of the boiling water prepared on the hot plate. (You may need to remove water from the flask first. Use a dipper to measure out hot water or toweling to protect your hands while you pour hot water from the beaker.) Stir the water bath with the 7
temperature probe. Make sure the flask is still covered with water, wait until P and T readings on the computer monitor stabilize and then click Keep. Your third PT data pair is now stored. Calculate the air and vapor pressure at this temperature. 15. Repeat until you have at least 5 measurements between room temperature and 40 C. Calculate the air and vapor pressure at each temperature as you go. (You may have to pour off some water from the water bath before adding more hot water at each step. EtOH boils at 78 C and above 40 C the stopper begins to pop out of the flask.) 16. When you have completed your measurements for EtOH, open the 2-way valve above the rubber stopper to release the pressure inside the flask. Remove the stopper assembly and fill the flask to the top with water and pour the solution in the sink. (If an aqueous solution is 40% or less ethanol it poses no environmental threat and can be poured down the drain.) 17. Email the resulting plot to your ELN. Make sure to clear your email address and password of the LabQuest2 so others can t access your email account. Shutdown the LabQuest2 and not simply put it to sleep. To shutdown the LabQuest2: press the home key, select System! Shut Down! OK. 8
CALCULATIONS & DISCUSSION Part A. Temperature & Gas Pressure 1. For this experiment, what variable is (are) a. constant? b. independent? c. dependent? 2. Is the relationship between pressure and temperature is either direct (P = kt) or inverse (P = k/t)? Calculate the average numerical value of k from this experiment. (Note: Kelvin units must be used.) In terms of the ideal gas law, what variables are represented by k? 3. Using Excel, create a plot of temperature (K) versus pressure (atm). Note: Typically, the dependent variable is plotted on the y-axis and the independent variable on the x-axis. You are being asked to do the exact opposite here. All work below should be shown on the plot printout. a. Graphically determine absolute zero using this plot (Hint: Changing the minimums for the axes may be helpful i.e., the experimental data line, for once, does not have to fill the plot area). b. Mathematically determine absolute zero using this plot (Hint: Equations generated by Excel should always be on plots generated for general chemistry lab.) 4. Using Excel, create a plot of temperature ( C) versus pressure (atm). Repeat the procedure given in #3. 5. Calculate the percent error for the value you found in #4. (Look up the true value in your textbook.) These calculations and values should be shown on the plot. 9
Part B. Temperature & Vapor Pressure (1) Find the vapor pressure at each temperature. The total pressure in the flask at temperature, T, is equal to sum of the pressure of air in the flask and the vapor pressure of the ethanol: (P total) T = (P air) T + (VP ethanol) T. Show one sample calculation and then report all values in a Results Table. Remember, for trials at temperatures other than room temperature, even if no ethanol was present, the air pressure would increase due to a higher temperature, or decrease due to a lower temperature (remember those pesky gas laws?). Therefore, the air pressure at room temperature must be corrected to the air pressure at the temperature of the water bath. Use the gas-law equation: P 2 / T 2 = P 1 / T 1 (T should have Kelvin units). (2) Construct a Clausius-Clapeyron plot: a) In Excel, construct a graph of vapor pressure of EtOH vs. Celsius temperature. What is the general relationship between vapor pressure and temperature? b) Convert vapor pressure of ethanol to ln VP and Celsius temperature to inverse Kelvin temperature (1/T). Construct a graph of ln VP vs. 1/T. Calculate ΔH vap from the slope. c) The true value for ΔH vap of ethanol is 42.3 kj/mol. Calculate your percent error: % error = [(true expt)/ true] x 100 10