Author: J. M. McCormick
Last Update: October 15, 2008
Introduction
In this exercise you will determine the enthalpy of vaporization (ΔvapH) of water, and if time permits study the effect of a solute on water's ΔvapH. Please see the handouts from Garland1 and Atkins and dePaula2 for more background on the theory.
Procedure
You will use the boiling point method (procedure 1 in Garland)1 to determine ΔvapH for water using carbonate-free deionized water. While the procedure is similar to that of Garland,1 the apparatus that you will use has been modified significantly, as shown schematically in Fig. 1. Instead of a mercury manometer, a calibrated Vernier gas pressure sensor will be used (click here to review operation of the LabPro and the LoggerPro software that controls it), and a manifold will be used to control the pressure in the system. A McLeod gauge and the laboratory's mercury barometer will be used to calibrate the pressure sensor.
The kit for this exercise includes the items listed in Table 1. Please check that everything is present and in good condition before beginning.
| Number |
Item |
| 1 | 100 ml 19/22 One-neck round bottom flask |
| 1 | 19/22 Condenser |
| 1 | 19/22 Gas inlet adapter |
| 1 | 19/22 Claisen adapter |
| 1 | 19/22 Thermometer adapter (with o-ring) |
| 4 | Blue (19/22) Keck clips |
| 1 | Vernier pressure probe |
| 1 | Vernier temperature probe |
| 1 | Vial of boiling chips |
Table 1. Equipment needed for this exercise.
Figure 1. Schematic diagram of the apparatus for the determination of the vapor pressure of a liquid sample by the boiling point method.
Upon coming to laboratory check that the manifold, McLeod gauge and ballast bulb have already been assembled and make sure all valves on the manifold are closed. Do not over tighten the valves! Start the mechanical pump. Under no circumstances is the valve to the pump be left open while the system is open to the atmosphere and the pump is running.
Plug the fitting for the pressure sensor with a serum stopper being careful to completely cover the fitting. Slowly open the valve to the pump (the pump should make a gurgling noise) and then open it to approximately two-thirds full. Within a minute or two the pump should grow quiet as the air is removed from the system; if it does not then there is a leak somewhere that must be sealed. Once the air has been removed from the system, check that the McLeod gauge is in a horizontal position with the arm with the attached plastic scale is on top. Carefully open the manifold valve leading to the McLeod gauge; a little at first, and then fully open. The pump may gurgle at first, but should soon stop. Pump out the McLeod gauge for at least 10 to 15 minutes, during which time you can assemble the distillation apparatus. Once the McLeod gauge has been evacuated, close its valve on the manifold. The McLeod gauge must be left under vacuum at all times for it to function properly, so do not open the valve again when the system is not under vacuum. Close the pump's manifold valve and open the vent valve. When the system has returned to ambient pressure, remove the stopper and install the pressure sensor.
The LoggerPro software should automatically recognize the sensor and display the ambient pressure. If necessary, change the units to mm Hg. You will be performing a two-point calibration of the pressure sensor, one at ambient pressure and one at the pump's maximum pumping capability (~3 mm Hg). Record the ambient pressure in mm Hg from the laboratory barometer and correct the pressure using the equation posted by the barometer.3 Start the calibration routine in LoggerPro and enter the ambient pressure as the first calibration point (note that the vent valve must be open). Close the vent valve and open the pump valve (slowly at first and then fully open). The pump should gurgle at first and then become quiet while the pressure/voltage reading on LoggerPro should drop. If this does not happen, check for leaks. Once the pump has become quiet and the reading on LoggerPro has stabilized, carefully open the valve to the McLeod gauge. Wait a few minutes and slowly and carefully turn the gauge in a counterclockwise direction until it is in a vertical position. CAUTION! All movements of the McLeod gauge must be done slowly in the direction indicated using two hands to stabilize the gauge as it is turned. Rapid movement can break the gauge leading to a mercury spill. Be sure that the top black line on the scale is even with top of the left tube and read the pressure from the height of the mercury column. If this isn't about 3 mm Hg, or less, carefully turn the gauge in a clockwise direction back to the horizontal position and continue pumping. If the pressure is in the correct range, enter the pressure reading from the McLeod gauge as the second calibration point to finish the calibration. Return the gauge to the horizontal position by turning it clockwise and close the valves to the McLeod gauge and the vacuum pump. Vent the system. The pressure reading displayed by LoggerPro should return to ambient pressure. If it does not, you may repeat the calibration procedure until it does.
Assemble the distillation apparatus as shown in Garland1 (substitute a lab jack for the supporting ring). Be sure to make all hose connections to the distillation apparatus before clamping it in place and do not forget to grease the ground glass joints with a small amount of high-vacuum silicone grease. You are advised to use the Keck clips to hold the joints together. It is recommended that two clamps (one on the neck of the round bottom flask and one on the condenser) be used to hold the apparatus in place. To insert the temperature probe into the Claisen adapter, first slip the retaining ring on to the probe and then the o-ring. Carefully place the probe into the Claisen and turn the retaining ring until it just engages. Adjust the height of the temperature probe such that the bottom of the probe is level with the bottom of the Claisen's arm. Use a small clamp to hold the probe in a vertical position not touching the sides of the Claisen. Carefully tighten the retaining ring. Do NOT over tighten!
Don't forget to add boiling stones to the carbonate-free deionized water and to start the cooling water flow. As you will be using the LoggerPro pressure sensor to record the temperature, it will need to be calibrated at three points (0 °C, room temperature, and 100 °C are convenient). Helpful hint: the calibration of the temperature probe can be done while the apparatus is being assembled or when you are waiting for the system to be evacuated.
When you are ready to make a run, close the vent valve, and open the valve to the distillation apparatus. Slowly open the valve to the pump and evacuate the system to the desired pressure (take care that the sample does not start to boil) and then close the valve. You are advised to start at low pressures first and record the boiling point as a function of ascending pressure. With the heating mantle on a low setting, initiate boiling (a steady boiling, without bumping). You want the water to condense on the temperature probe and drip back into the round bottom, but not much higher in the apparatus. When the temperature and pressure have stabilized record both. Open the vent valve slightly to raise the pressure and then close it (during this operation boiling should cease). Continue heating until boiling starts again. Repeat this until ambient pressure is reached (pump valve closed, vent valve open). The procedure is similar to collect data at decreasing pressures, except that before the pressure is lowered you must drop the heating mantle away and cool the sample flask with an ice bath (why?).
When you are finished, close the pump valve and open the vent valve to return the system to ambient pressure (the valve to the distillation apparatus should be open at this point). Turn off the pump and open the pump valve on the manifold to vent the pump. Disassemble the distillation apparatus. Don't forget to remove the grease from the joints with a paper towel soaked in a little acetone and to dry all the glassware before returning it to the storage box. Remove the pressure and temperature sensors and place them in the storage box. Leave vent, pump and system valves on the manifold open (to the valve to the McLeod gauge must be closed).
To study the effect that solutes have on the vapor pressure (and ΔvapH), you need only repeat the measurement with a solution instead of pure water. You may choose either an electrolyte (e. g., NaCl, KNO3, etc.) or a non-electrolyte (e. g., sucrose). The only restrictions are that it is available in the laboratory, so please check with the instructor for the availability of the solute before coming to the laboratory.
Results and Discussion
You will not need to correct the manometer readings, as we are not using a mercury manometer. Although, you may wish to apply the correction to the two pressures used to calibrate the pressure sensor. The remainder of the calculations are to be done as outlined in Garland,1 with the exception of those related to statistical thermodynamics. Determine ΔvapH at 1 bar for pressure and 298.15 K. In your Results section be sure to include a graph of boiling point as a function of ambient pressure (two, if you were able to study the effect of a solute). Discuss your findings.
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