Vapor pressure and heat of vaporization lab report answers

Experiment 4: Vapor Pressure of Liquids Background Like solids, liquids are referred to as a condensed state of matter because the molecules are in contact with each other. Unlike solids, liquid substances have molecules that are able to move around each other. As heat is added to a liquid, the kinetic energy of the particles, and thus the temperature of the substance, increases. When liquid particles gain enough kinetic energy to overcome the attractive forces between them they escape into the gas phase generating a gas vapor. This lab activity will demonstrate how intermolecular forces affect the pressure generated by a gas vapor. To explain your observations on a molecular level it will be important for you to describe: 1. Why do certain molecules have weaker intermolecular forces than other? 2. What is happening to the molecules in a substance when heat is added? 3. Why is a gas vapor generated at temperatures below the boiling point of the substance? 4. What is the kinetic energy distribution of different molecules in a liquid substance? 5. How does the rate of vaporization compare to the rate of condensation in an open container? In a closed container? The total pressure in the sealed flask is due to the vaporized liquid plus air molecules present in the flask: Ptotal = Pvapor + Pair (1) In this experiment, you will investigate the relationship between the vapor pressure of a liquid and its temperature. Pressure and temperature data will be collected using a gas pressure sensor and a temperature (Figure 1). Vapor pressures will be determined by subtracting atmospheric pressure from the total pressure. The flask will be placed in water baths of different temperatures to determine the effect of temperature on vapor pressure. You will measure the vapor pressure of methanol and ethanol and determine the enthalpy (heat) of vaporization for each liquid. Objectives Figure 1 In this experiment, you will • Investigate the relationship between the vapor pressure of a liquid and its temperature. • Compare the vapor pressure of two different liquids at the same temperature. • Use pressure-temperature data and the Clausius-Clapeyron equation to determine the heat of vaporization for each liquid. Caution! The alcohols used in this experiment are flammable and poisonous. Avoid inhaling their vapors. Avoid contacting them with your skin or clothing. Be sure there are no open flames in the lab during this Procedure 1. Wear goggles! You will work in pairs for this lab, but you may share water baths with your table. 2. Prepare four water baths: 20 to 25°C (use room temperature water), 30 to 35°C, 40 to 45°C, and 50 to 55°C. You should also have some hot water on a hot plate on reserve. 3. Obtain a temperature probe and gas pressure sensor. The sensor comes with a rubber-stopper assembly (Figure 2). The stopper has three holes, one of which is closed. Make sure your tubing and valve are not inserted in the closed hole. Figure 2 Insert the rubber-stopper assembly into a 125 mL Erlenmeyer flask. Important: Twist the stopper into the neck of the flask to ensure a tight fit. 4. Plug the temperature probe and pressure sensor into the interface box. Prepare the computer for data collection by opening the file “10 Vapor Pressure” from the Chemistry with Vernier folder of Logger Pro. The temperature and pressure readings should now be displayed. Figure 3 5. Turn the two-way valve above the rubber stopper to the open position (see Figure 3). Record the value for the atmospheric pressure (round to the nearest 0.1 kPa) and the temperature in the table in the DATA section. (All subsequent trials should be performed at temperatures higher than this temperature. Adjust the temperatures of the water baths as necessary.) open closed 6. Perform the following to finish setting up the apparatus as shown in Figure 1: a. Place the Temperature Probe in the room-temperature (20–25°C) water bath. b. Hold the flask in the water bath, with the entire flask covered as shown in Figure 1. c. After 30 seconds, close the two-way valve above the rubber stopper as shown in Figure 3—do this by turning the white valve handle so it is perpendicular with the valve stem itself. 7. Obtain about 10 mL of the liquid (methanol or ethanol) in a small beaker. Draw about 2-3 mL of the liquid up into the syringe (note the exact position of the syringe plunger). With the two-way valve still closed, screw the syringe onto the two-way valve, as shown in Figure 1. 8. Perform the following steps to introduce the liquid into the Erlenmeyer flask: a. Open the 2-way valve above the rubber stopper—do this by turning the white valve handle so it is aligned with the valve stem (see Figure 3). b. Squirt the liquid into the flask by pushing in the plunger of the syringe. c. (Important step!!) Quickly return the plunger of the syringe back to where it was before your injection (essentially, you have replaced a volume of air with liquid), then close the 2-way valve by turning the white valve handle so it is perpendicular with the valve stem. d. Remove the syringe from the 2-way valve. 9. Click to monitor and collect pressure and temperature data.