The thermodynamic basis of this experiment can be found in your text under its presentation of the Clausius-Clapeyron equation. Extensive experimentation has shown that the pressure of a pure liquid-vapor equilibrium two phase system in a closed container is determined by the temperature alone. Thermodynamic analysis shows that the variation of pressure with temperature in such a system is described by the Clausius-Clapeyron equation
where p is the equilibrium vapor pressure measured at temperature T, R is the gas constant, and DHvap is the molar heat of vaporization of the pure liquid. To the extent that the heat of vaporization is independent of temperature, this relation states that a plot of ln(p) vs 1/T will be a straight line whose slope is proportional to the enthalpy of vaporization. In this laboratory procedure we will measure the boiling temperature of a pure liquid at several different pressures; the data obtained will enable us to test the relations given above for their applicability to our system (in this case, water).
The vapor pressure apparatus is set up in the walk-in hood in the back of the laboratory. The procedure is a modification of the Ramsay-Young method described in "Physical Chemistry Laboratory: Principles and Experiments", by Salzberg, Morrow, Cohen and Green (McMillan, 1978). Consult this text for a discussion of the Cottrell pump and a general outline of experimental procedures for vapor pressure measurement. We will utilize a thermocouple for temperature measurement and a transducer to monitor pressure.
The equilibrium vapor temperature of the liquid (in this case, water) is read with a thermocouple circuit, the measuring junction of which is placed in the vapor of the boiling liquid at the top of a Cottrell pump. The Agilent 34970A may be configured to read a thermocouple voltage relative to an internal reference, eliminating the need to hold a reference junction at a known temperature. A type-T thermocouple will be in place in the apparatus; connect the +/- leads to the multiplexer in the 34970A, and configure to make a temperature measurement, type T thermocouple, internal reference, 0.1oC resolution. Be sure that the TC TEST feature is enabled on the 34970A (accessed under the Advanced button on the front panel.) Configure the thermocouple channel to scan 25 times.
The voltage output from the pressure transducer will correspond to the pressure difference between the system and atmospheric pressure, so it will be necessary to record both the offset voltage from the transducer before the experiment begins, and the barometric pressure from the laboratory barometer. Don't forget to monitor the power supply voltage to the transducer. Connect the tranducer leads to the multiplexer and configure to make a DC volt measurement on this channel. Configure the transucer channel to scan 25 times.
The Ramsay-Young apparatus is connected to a vacuum aspirator, which limits the pressure ranges which can be used, but which otherwise will suffice. Please note the instructions in the hood near the apparatus: the stopcock must not be opened to the aspirator unless the water is running at full tilt. Turn the water valve on full, and connect the system to the vacuum by rotating the stopcock clockwise. Watch the manometer in the hood, and the voltage output from the transducer; after a short time, the voltage (and the mercury level in the manometer) will remain constant. This is the lowest pressure that we can obtain using a water aspirator as a vacuum source. At this point, you may disconnect the apparatus from the aspirator by turning the stopcock a half-turn. You may also shut the water off. Begin heating the water gently to boiling with the Bunsen burner; try to avoid 'bumping' the liquid. Do not heat the liquid so much that it enters the condensers above the boiling apparatus.
Once the water has begun to boil, be sure that the pressure and temperature remain relatively constant (for a period of about 5 minutes to ensure thermal equilibrium) and collect a pair of data which are a voltage corresponding to the pressure, and the celcius temperature. Then, increase system pressure by about 5 cm Hg by carefully opening the stopcock to the 'vent' (this is accomplished by turning the stopcock counterclockwise) and noting the voltage output by the pressure transducer. When the pressure on the system has been increased, heat the liquid to boiling, collect another pair of pressure-temperature data, and continue at 5 cm increments until atmospheric pressure is reached.
Note that chilled water is pumped through the condensers on the Ramsay-Young apparatus, and a trap is in the line between the Cottrell pump/condenser and the ballast tank. We would typically have a cryogen such as liquid nitrogen in the Dewar surrounding the trap. The purpose of the trap is to prevent vapors from entering the vacuum source (typically a vacuum pump). However, in this experiment we are not much concerned about water vapor entering the aspirator. Nonetheless, the operator must exercise care to prevent too vigorous boiling, which can flood the condenser and trap. The least troublesome sequence is that suggested above: start with the lowest pressure, achieve steady boiling, record the equilibrium pressure and temperature, raise the pressure (boiling immediately stops), and increase the heating rate with the Bunsen burner. When your measurements are completed, turn off the gas and vent the ballast tank to the atmosphere.
You will need to think about how to calculate the actual pressure on the boiling water for this experiment. It is this quantity which you will need to plot.
The report format should follow the usual guidelines. In particular:
| Back to labs | Schedule | Links |
| SOU Chemistry Page | SOU Home Page |