# Physical Chemistry Laboratory I CHEM 445 Experiment 6 Vapor Pressure of a Pure Liquid (Revised, 01/09/06)

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1 1 Physical Chemistry Laboratory I CHEM 445 Experiment 6 Vapor Pressure of a Pure Liquid (Revised, 01/09/06) The vapor pressure of a pure liquid is an intensive property of the compound. That is, the vapor pressure of a liquid is independent of the amounts of the two phases as long as both phases are present. This phenomenon is shown in many physical chemistry textbooks in plots of PV isotherms at temperatures below the critical temperature. The horizontal part of the P vs. V isotherm corresponds to the constant vapor pressure of the liquid. The vapor pressure of every liquid is a strongly increasing function of temperature and a very slightly increasing function of applied pressure. The equation for the variation of vapor pressure of a liquid with temperature is derived in standard texts and often called the Clausius - Clapeyron equation.. Reasonable approximations have been made to achieve this equation. 1 dp H Vap = (1) 2 P dt RT In Eq. (1), P is the vapor pressure of the liquid, T is the absolute temperature in K, and HVap the molar heat of vaporization of the liquid. The vapor pressure of a pure liquid is often represented by Eq. (2), the integrated form of Eq. (1) with the additional assumption that the heat of vaporization is independent of temperature. HVap ln{ P} = A (2) RT Although the heat of vaporization of a liquid is actually not independent of temperature, Eq. (2) will give an accurate fit to vapor pressure data over a wide range of temperatures, as long as the temperature is well below the critical temperature. There are compensating errors in the approximations. If very accurate data are obtained or if vapor pressures are obtained over a very wide range of temperatures, then the variation of heat of vaporization with temperature should be considered. The following equation is often used, H Vap = a b T (3) Substituting Eq. (3) into equation Eq. (1) and integrating will give a more elaborate equation for vapor pressure as a function of temperature, in which the constants (either positive or negative) are obtained from a fit to the experimental data. This equation (Kirchoff equation) was proposed empirically many years ago to fit accurate vapor pressure data, a ln{p} = c + + bln{t} (4) T Other equations have been proposed for the variation of vapor pressure of a liquid with temperature. For the analysis of your data, Eq. (2) is sufficient and H is an average molar heat of vaporization over the temperature range. Vap is

2 2 H From standard thermodynamics, Vap = the molar entropy of vaporization. It was T observed experimentally and predicted theoretically that this quantity at the normal boiling point would be relatively constant for many non-associated liquids at ~ 21 cal/mol o or ~ 88 J/mol o, Trouton s Rule. [1] Consequently, one may estimate the temperature variation of vapor pressure of a liquid reasonably well near its normal boiling point, simply from the boiling point. Experimental procedure You will measure the vapor pressure of methanol. In this experiment you will use an isoteniscope in a stirred water bath whose temperature you can change relatively easily. The system {see picture} consists of a vacuum pump to reduce pressure, an ice/water cold trap to capture sample vapors, a 3-way stopcock, an isoteniscope (see picture) to contain your sample, a large empty jar (ballast tank) to reduce the pressure fluctuations, and an MKS Baratron to measure the system pressure. A Lauda water bath is provided to control the temperature of the isoteniscope and cyclohexane. The 3-way stopcock controls pressure changes in the system. If the 3-way stopcock is open to the pump, the pressure is reduced (performed initially to pump down the system). If the 3-way stopcock is opened to the atmosphere, the pressure is increased in the system (performed during heating) and to shut down the system. IMPORTANT: Before you begin the experiment, check the bath temperature. It should be approximately the same as room temperature. If colder, fine; if much warmer, remove some water and use ice to reduce bath to ~ room temperature. Remove an isoteniscope from the storage canister and remove the cap. Place the cap in the canister for safe keeping. Pack some ice around the glass cold trap in the Styrofoam container. Fill the metal bulb portion ~ ¾-full with methanol using a Pasteur pipette or a plastic squeeze bottle. The exact amount of liquid is not critical, but the liquid should be clearly visible. When you remove the air, some (much) of the liquid will be lost. No liquid should be in the U- tube portion, although a little is OK as long as the tube is not blocked, because it will evaporate as you reduce the pressure in the system. Slightly tricky manipulation of the isoteniscope is needed to get the liquid into the bulb. Place the isoteniscope in the constant temperature bath at room temperature and attach the connecting hose to the vacuum system. Turn on the vacuum pump with the 3-way stopcock closed (perpendicular to the gas flow guarantees that the stopcock is closed). Slowly turn the 3-way stopcock to the vacuum line (color coded) to reduce the pressure in the isoteniscope and the ballast tank. The liquid may boil (bubble) as the pressure is reduced, but it vaporizes even if visible boiling does not occur. You can tell from the sound level of the pump and from the changes in pressure that the vacuum pump is evacuating the system. Pump on the system for a minute or two. Close the stopcock and let the pressure equilibrate, and note the pressure on the MKS Baratron, which gives a direct reading of the pressure in the ballast tank and the isoteniscope. Repeat (pump, equilibrate, read) a few times. Remember that when you evaporate a liquid by reducing the pressure, the temperature of the liquid drops and time is needed for the temperature of the liquid in the isoteniscope to equilibrate with the surrounding water bath. When the pressure no longer changes on evacuating the chamber, the air has been evacuated from the system. Then remove isoteniscope from the bath and tilt the bulb to let

5 5 Analyze the following set of data {Boublik, Fried, Hala, The Vapor Pressures of Pure Substances, Elsevier, 1984} as you analyzed your data: plots of P vs. T, ln{p} or log{p} vs. 1/T, {P(Exp) P(Calc)}/P(Exp) } vs. T, and calculate nbp, H Vap from Eq. (2). These are data for the vapor pressure for another compound and are clearly not the same as your data. t, o C P, mm Hg t, o C P, mm Hg References 1. S. Glasstone, Textbook of Physical Chemistry, 2 nd Ed., D. van Nostrand Co., New York, 1946

6 6

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