Online Photographic Pre-lab Demonstration for C_P/C_V Ratios for Gases

Note: the complete text of the experimental procedure from the "2004-2005 Physical Chemistry Lab Manual" is given here (in this type). If additional comments or directions are needed, they are added (in italics). So that each photo has its own block of text, extra numbers have been added as needed (i.e. 2a), 2b), etc.). The text is above the picture illustrating it in each case. For simple or known operations, no photo is included.

Using the Open Tube Manometer

1.      The open tube manometer is filled with dibutyl phthalate, which has a density of approximately 1.046 g/mL at 20 °C (see the bottle label for the manufacturer's density). If dibutyl phthalate is spilled and/or needs to be refilled, wear gloves for the clean up (paper towels and acetone work well) and refilling (a dispo pipette through the stopcock works well). See the instructor for proper disposal of spilled dibutyl phthalate. 

Figure One: Apparatus for determining the C_P/C_V ratio of gases using the adiabatic expansion method (set up for gas as dense or denser than air shown).

2.      The two columns (both sides of the U) should be at equal height when at atmospheric pressure. If the pressure inside the carboy is greater than atmospheric (which should be the case in every trial here), the column of dibutyl phthalate in the side with the tubing to the carboy will be lower than before (i.e. lower than it was at atmospheric pressure) while the column of dibutyl phthalate on the side with the stopc°Ck will be higher than it was before. The difference in height between these two columns of dibutyl phthalate is the pressure difference between the carboy and the atmosphere. 

3.      Attach pieces of paper to the board holding the manometer and record the heights of each column of dibutyl phthalate (each arm of the U-tube) on this paper. Mark each height with the corresponding identification  p₁, etc. Also mark the paper with the gas, trial, date, and group identity so that you can use the data later. 

4.      To convert the measured difference in height (mm dibutyl phthalate) to pressure in mm Hg, multiply the height of the dibutyl phthalate column in mm by the ratio of the density of dibutyl phthalate to the density of mercury (d_d.p./d_Hg). The density of Hg at 20 °C is 13.55 g/mL. This excess pressure must be added to the atmospheric pressure as measured with the barometer in lab (in mm Hg).  

5.      Measure the barometric pressure in mm Hg (torr) at least once during the experiment. This is p₂ for the calculations. Unless the weather conditions indicate a rapid change in the atmospheric pressure, it shouldnot be necessary to measure this more than once a lab period. For each trial, you should also measure the room temperature. 

The Adiabatic Expansion Experiment

6.      Before starting to work with the carboy, adjust the initial flow rate of the gas from the cylinder to about 6 L/min (100 mL/s). The best way to do this is take a bucket of water and invert a 1 L beaker in the water.

        Place the tube from the gas cylinder so that all of the gas will fill the beaker (which should be completely full of water initially). Use a stopwatch to measure the amount of time it takes to fill the beaker with the gas and adjust the flow rate accordingly. It should take about 10 seconds to fill a 1 L beaker.

7.      This procedure assumes the tubes are set up as shown in Figure One. If tubes a and b are switched (for gas much less dense than air), then switch them in the description that follows. Once the flow rate has been set, make sure the tubes are attached as described above, and place the stopper firmly in the neck of the carboy. The carboy and tubing with the open tube manometer (with paper for recording heights) in the background.

8.      Open tubes a and b (tank and vent) and close tube c (manometer). Do an initial purge of the carboy by letting gas flow through for 15 minutes. This serves to sweep the air or gas inside it out of the carboy by running through about 5 carboy volumes of the gas to be studied. Operating a screw clamp to open or close a piece of rubber tubing.

9.      After 15 minutes, slow the gas flow by partially closing tube a (from the gas cylinder). Open tube c (manometer) very carefully, then carefully close tube b (vent). It is very easy here to blow dibutyl phthalate out the top of the manometer! Watch the level carefully and open tube b (vent) if needed to vent excess gas pressure. Allow the pressure to rise to about 600 mm dibutyl phthalate (this is the difference between the two levels), then close tube a.  

10.  Allow the gas to stand 15 minutes to reach thermal equilibrium with the carboy and surroundings. The manometer pressure should be constant (especially after the first few minutes), if it rises steadily tube a is not closed tightly. If it decreases steadily there is a leak, so check that all the clamps are tightly closed and repeat steps 9 and 10. Record this pressure after 15 minutes (in mm dibutyl phthalate), convert it to mm Hg, and add it to the barometric pressure to obtain p₁ (mm Hg). 

11.  The next step should be practiced before the carboy is filled with the gas to be studied (and is the adiabatic expansion step which gives the method its name). This step is done as quickly as possible to prevent heat transfer, hence the "adiabatic" name. In the shortest time possible, remove the stopper completely from the carboy (a distance of 2 to 3 inches is good) and then put it securely back in the neck of the carboy ("pop it out and put it right back in"). The stopper as far out as it should go when "popping it out and popping it right back in".

12.  Let the gas equilibrate thermally for another 15 minutes - it should warm and the pressure will rise to a constant value. Determine the final pressure, which is p₃ (convert this from mm dibutyl phthalate to mm Hg). The barometric pressure in the laboratory is p₂ (mm Hg). 

13.  Do at least three such runs for each gas. After the initial trial's 15 minute flushing purge, you can shorten the time for subsequent purges to about 3 minutes (or 1 volume of the carboy). Three gases will be studied. He, N₂, and CO₂ are typical examples of a set of gases. For each new gas, adjust a or b as the tube attached to the tank, depending on the mass of the gas relative to air (which is about 30 g/mol).