VAPOR PRESSURE AND ENTHALPY OF VAPORIZATION OF A PURE LIQUID: THE ISOTENISCOPE METHOD

Transcription

1 Physical Chemistry Laboratory Experiment I-3b VAPOR PRESSURE AND ENTHALPY OF VAPORIZATION OF A PURE LIQUID: THE ISOTENISCOPE METHOD References: See relevant sections of undergraduate Physical Chemistry text Background: Study the references carefully. Know the following: Objectives: a) Definitions of equilibrium vapor pressure of a pure substance and enthalpy of vaporization; b) Condition for thermodynamic equilibrium between two phases; c) Thermodynamic relations leading to the derivation of the Clausius-Clapeyron equation, and all valid assumptions. 1) Measurement of the equilibrium vapor pressure of a pure liquid as a function of temperature. 2) Determination of the Clausius-Clapeyron equationparameters, the normal boiling point, the enthalpy and entropy of vaporization. Principle of the Isoteniscope method: The isoteniscope method is based on the principle of the null- manometer. The isoteniscope, shown in the schematic diagram, consists of a U-tube (the null manometer), one end of which is sealed, and a side arm test tube. The sample placed in the closed-end of the U-tube and an additional portion in the test tube comprise the manometer liquid. The vapor pressure of the sample is determined by adjusting the level of the liquid in the open end of the U-tube, to that in the test tube. When these two levels are the same the pressure in the U-tube, which is the vapor pressure of the sample, is equal to the pressure in the test tube. The pressure in the test tube is determined from the difference between the barometric pressure in the laboratory, and the Hg-manometer reading. Procedure:

2 Experiment I-3b Physical Chemistry Laboratory Read the barometric pressure in the laboratory at the beginning and at least 3 more times during each laboratory period. 1) Check the assembled apparatus, as indicated in the schematic diagram. 2) Before placing the sample in the isoteniscope evacuate the system and check for leaks. The three-way stopcock should be in position (1) and the needle valve (N) should be closed. Although small leaks are not harmful they may be annoying and are worth eliminating. (Before proceeding have your instructor check out your apparatus) Add sufficient liquid to the U-tube with a medicine dropper to fill approximately 3/4 of the closed end. 3) Place enough liquid in the side-arm test tube to cover about 3/4 of the U-tube, then place the U-tube in the test tube. 4) Place the isoteniscope in the water bath, which should be at room temperature. 5) Reduce the pressure in the system, with the 3-way stopcock in position 1 and the needle valve (N) closed. Reducing the pressure sweeps the air out of the U- tube, producing bubbles in the test tube liquid. Two to three minutes of bubbling, at a reasonable rate, are sufficient to sweep the air out of the U-tube. Avoid evaporating too much of the liquid in the U- tube. 6) Carefully admit air through the needle valve (N) until the liquid levels in the open end of the U-tube and the test tube are equal. 7) Read and record the temperature of the bath and the pressure on the Hg-manometer, at the instant when the null manometer indicates no difference in the liquid

3 Physical Chemistry Laboratory Experiment I-3b levels. 8) To insure removal of all the air in the U-tube repeat the procedure above (steps 4, 5, 6) until successive readings are in very good agreement. Compare to a literature value if available. 9) Heat the water bath to a new temperature about 4 C above room temperature. As the temperature is varied, keep the liquid levels in the isoteniscope approximately equal at all times. This is necessary to avoid evaporation of the sample or displacement of the sample by the liquid in the test tube. Pressure adjustments in the system are made by means of the 3-way stopcock and valve (N), whereby air is admitted or removed to increase or decrease the pressure as desired. 10) When the bath temperature is steady at its new value, adjust the pressure until the liquid levels are equal. Read and record both temperature and pressure. Repeat to obtain three readings in very good agreement. 11) Take readings at approximately 4 C intervals up to a temperature of 65 to 70 C, or at least 12 readings. 12) After the last data point reading, turn the pump off and bring the system to atmospheric pressure (stopcock in position 2 and needle valve (N) opened). Clean isoteniscope, remove heater and stirrer from the bath. Calculations and treatment of data: 1) Correct the barometric pressure readings for temperature according to the relationship posted in the lab. 2) Tabulate the experimental data with the following information: T(K) T -1 (K -1 ) Δh(mm) P(mm) ln P/P ø

4 Experiment I-3b Physical Chemistry Laboratory 3) Plot the experimental and literature data on two graphs: a) vapor pressure vs. temperature b) ln P/P ø vs. T -1 4) By linear least squares analysis of ln P/P ø vs T -1 evaluate the Clausius-Clapeyron equation parameters (slope and intercept) and report them in the results section. Also apply linear least squares method to literature data. 5) Calculate!H vap for the liquid sample from the Clausius-Clapyron parameters. 6) Using the standard deviation of the slope, calculate the standard deviation in!h vap. 7) Compare the experimental!h vap with a literature value; comment on any deviation found (remember your value is an average over the temperature range). 8) Comment on the physical significance of the intercept of the Clausius-Clapeyron equation. 9) Extrapolate the data analytically to a pressure of 1 atm to obtain the normal boiling point. Compare with the literature value. 10) Calculate ΔS vap at 400 torr and at one atm.

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