Forces and Bonding: The Temperature Dependence of Vapor Pressure

• To use the ideal gas law.
• To use Dalton's law of partial pressures.
• To explore the relationship between P_vap of a liquid and T.
• To calculate DH^o_vap, DS^o_vap, and the normal boiling point of a liquid from experimental P_vap, T data.

In this experiment you will measure the equilibrium vapor pressure, P_vap, of a liquid at various temperatures using a cylinder/piston apparatus (syringe). The resulting P_vap, T data will allow calculation of the enthalpy and entropy of vaporization, and the normal boiling point of the liquid.

Vapor Pressure vs. T. The variation of vapor pressure with temperature is given by the Clausius-Clapeyron equation, (9), where P_vap is in atm, T is in K, DH^o_vap the standard enthalpy of vaporization in J/mole, DS^o_vap the standard entropy of vaporization in J/K-mole, and R the gas constant.

A plot of ln P_vap vs. l/T should be linear with slope related to DH^o_vap and intercept to DS^o_vap.

Equation (9) can be used to calculate the normal boiling point of the liquid.

1. The ideal gas law suggests that gas pressure should increase linearly with temperature, yet vapor pressure increases more rapidly than this. Explain why.
2. A very small volume of liquid is used in this experiment. Why is it important to use such a small volume?
3. If the temperature of the liquid were raised to its boiling point, what would you see happen in the system?

• Thermometer
• Iron ring
• Wire gauze
• Bunsen burner with rubber tubing
• Extension clamps
• Papertowels and Kimwipes
• Matches
• Wash bottles of acetone
• 10-mL glass syringe
• 30-mL sidearm test tube
• New septum cap to fit test tube
• 150-mL beaker
• 1-mL plastic syringe with attached needle
• 1 mL of a liquid (ethanol, methanol, chloroform, carbon tetrachloride, water)

Safety goggles are required at all times in the laboratory. SOME OF THE LIQUIDS ARE HIGHLY FLAMMABLE. KEEP YOUR SUPPLY OF LIQUID FAR AWAY FROM THE BUNSEN BURNER AT ALL TIMES.

Record all data in your laboratory notebook. The instructor will provide the barometric pressure.

PLEASE DO NOT LET THE SYRINGE PLUNGER ROLL OFF THE BENCH TOP OR SLIDE OUT OF THE SYRINGE BARREL; the syringes are very expensive.

Clean the syringe. Rinse the inside wall of the barrel and the surface of the plunger thoroughly with acetone. Wipe the entire surface of the plunger and dry the inner wall of the barrel with a Kimwipe. Insert the plunger and move it in and out several times. Withdraw it and let acetone evaporate from its surface. Repeat the insertion and withdrawal until the acetone is gone. At this point, the plunger should slide and rotate freely, with almost no friction. Get it to this point before proceeding.

Set up an iron ring and wire gauze on a ring stand. Fill a l50-mL beaker half full of ice and add some water to make a slush. Set up the apparatus in Figure l; make sure that the syringe plunger is all the way into the barrel. Submerge the bottom 1 inch or so of the sidearm test tube in the ice bath. Attach a Bunsen burner to a gas jet and place the unlit burner under the beaker.

Obtain about 1-2 mL of your first assigned liquid in a clean, dry 1-dram vial. Obtain a l-mL plastic syringe with needle attached. Draw about 0.1-0.2 mL of the liquid into the plastic syringe. Holding the syringe vertically with the needle up, push the plunger into the syringe to expel all air. Catch liquid that exits the needle using a tissue. Insert the needle of the plastic syringe through the septum cap until you see it enter the upper part of the sidearm test tube. Inject the liquid, and retract the needle. Remove the plunger from the barrel of the plastic syringe and set the pieces on a Kimwipe to dry. Stir the water in the ice bath with a thermometer and measure the ice bath temperature. Record the temperature and the position of the plunger of the glass syringe.

Fire up the Bunsen burner and heat the ice bath until the ice melts. Stop heating when the temperature of the bath is 3 or 4 degrees, and stir until the temperature stabilizes. The syringe plunger may tend to stick in the barrel, so frequently rotate the syringe plunger without moving it in or out of the barrel to keep it moving freely. Record the stable temperature and the position of the syringe plunger. Briefly heat the bath again to obtain a stable temperature about 10 degrees higher than the previous one. Record temperature and plunger position. Continue at 10-degree intervals to a maximum temperature of 50 ^oC. When finished, turn off the burner and disassemble the sidearm test tube-syringe apparatus. Aspirate all traces of liquid and vapor from the interior of the test tube and syringe.

Repeat the entire procedure for your 2 other assigned liquids.

After disassembling the syringe-test tube apparatus, fill the sidearm test tube with water. Insert the septum cap and allow all air bubbles to escape through the sidearm. Then fill the sidearm with water. Empty the test tube into a 50-mL graduated cylinder and record the water level. This gives you a pretty good measure of the volume capacity of the sidearm test tube.

For each liquid, use your data to calculate the vapor pressure of the liquid at each temperature using the following equation:

Make the appropriate plots of the data to determine the enthalpy and entropy of vaporization and the normal boiling point for each liquid.

Dispose of all leftover liquids in the organic waste bottles in the fume hoods.