INNGANGUR
Gufuþrýstingur vökva (P) eykst með hitastigi (P) með þeim hætti að logarithmi gufuþrýstingsins vex línulega með 1/T:
-þar sem
 H er gufunarvarmi vökvans (enthalpybreyting fyrir uppgufun), z er leiðrettingarstuðull (lítilsháttar frábrugðinn gildinu 1 (?)), R er gas fastinn og C er fasti.
Því er unnt að ákvarða gufunarvarma vökva með því að ákvarða gufuþrýsting (og þá lnP) fyrir mismunandi hitastig og finna
 H út frá hallatölu bestu beinu línu fyrir lnP vs 1/T:

The vapor pressure of water is measured using an isoteniscope as the temperature is raised from room temperature to 75 degrees centigrade. Read carefully the discussion of this experiment in GNS7 / GNS8*, but use the following procedure whenever it differs from the one described in GNS7 / GNS8*.
1. Check the equipment: It is important to check how well the system holds vacuum. Turn on the water aspirator (pump) and pump out the air from the system as much as possible. Let the pump run for additional 10 minutes after minimum pressure has been achieved. Then close the system off from the pump and note whether the pressure rises. The pressure should not increase by more than a few mm of Hg in 10 minutes. If the pressure increases more than that, the equipment needs to be cleaned, relubricated, hoses replaced, etc.
2. Clean dissolved air out of the water sample: Fill the isoteniscope 2/3 full with the water sample and then fasten the stopper tightly. Part of the water is then distilled to clean out dissolved gas and air which is initially above the liquid in the isoteniscope. As the pressure is dropped, most of the air gets pumped out, but about 5% remains. In order to get rid of that, the water is boiled and the vapor then chases out the nitrogen and oxygen gas. Water vapor should be the only gas in the isoteniscope. It is good to tild the isoteniscope during the distillation in order to reduce the surface area of the liquid. Usually it is enough to pump out for about 20 minutes. Make sure the isoteniscope is still at least half full. Water is then poured into the U-shaped tube (the water-lock) and the isoteniscope placed in the water bath. The system should then be isolated from the pump by closing the valve. The system is then allowed to reach equilibrium. It may be necessary to adjust the pressure in order to make the pressure nearly equal on both sides of the U-tube. After equilibrium has been reached, the first reading is taken from all measuring devices.
3. Continuation with the measurements: The temperature is increased by 2-3 degrees at a time.
The vapor pressure of the liquid then increases and the water column in the U-tube gets lower on the side of the isoteniscope. Every once in a while, air needs to be let into the system on the other side to make the pressure on both sides nearly equal. It is extremely important that air not make it through the water-lock and into the isoteniscope. If that happens, the experiment needs to be repeated from the beginning. After increasing the temperature of the heat bath, make sure the system has enough time to equilibrate before taking a reading.
Follow the description for calculations in GNS7 / GNS8. Do the least squares fit in Matlab, including the fit of ln P as a quadratic polynomial in 1/T. Include a printout of your Matlab program with your report. Make the temperature correction to the manometer reading as described in GNS7 / GNS8. Carry out all the calculations described in NGS.
Test the accuracy of the assumption of ideal gas behaviour in the Clausius-Claperon equation.
How large an error could non-ideality of the water vapor be making in this case? You can use the van der Waals approximation to the equation of state for this test (the coefficients are

given in the text book by Silbey et al.). Also, test how good the approximation made on the change in molar volume in the derivation of the Clausius-Claperon equation is in this case.
Can you detect any variation of the enthalpy change with temperature? Estimate the expected variation from the heat capacity difference between water vapor and liquid water (see the theoretical discussion in GNS7 / GNS8).
The discussion about entropy in GNS7 / GNS8 will be postponed until the end of the semester and does not need to be addressed in the report for this exercise.
Full report is required (see also HERE ).
*
GNS7: "Experiments in Physical Chemistry" eftir C.W. Garland, J.W. Nibler og D.P.
Shoemaker, 7. útg., 2003, Experiment 13, page 199.
GNS8: "Experiments in Physical Chemistry" eftir C.W. Garland, J.W. Nibler og D.P.
Shoemaker, 8. útg., 2008.
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VEE-VP-09.pdf
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