Thermodynamics

In this experiment you will learn about two ways to study phase transitions by analysis of iodine and of binary solutions.

Experimental procedure

Part I: Binary Solutions

  1. Measure the air temperature in the lab (think why).
  2. Measure the vapor pressure of all the pure substances (methanol, ethanol, 1-propanol, 2-propanol (isopropanol) ,n-butanol, 2-butanol, chloroform, acetone, and toluene) according to the instructor's direction. Before you begin, determine the order in which you should make these measurements. Why? What are your expectations from each pure substance? The procedure for measuring the vapor pressure of any liquid can be found in the Experimental Setup section. Make sure the chamber is properly pumped before the beginning of each measurement (you will know it is properly pumped if the pressure reading remains close to 0 even when the valve to the vacuum pump is closed).
  3. Choose two pure substances and prepare five or six binary solutions of these components, and calculate their molar fraction (make sure that the molar fractions you select are spread in a uniform distribution).
    4. Solution # Moles of pure substances 1 [mol] Moles of pure substances 2 [mol] Molar fraction of substances 1 (or 2)
    ... ... ... ...
    ... ... ... ...
  4. Measure the vapor pressure of each solution.
  5. Draw a graph of the vapor pressure vs. the molar fraction of the solutions. Are the Solutions ideal? If not, how do they deviate from ideal?

Part II: Iodine Sublimation
Measure the Lamp Temperature

  1. Turn on the software, enter your group number.
  2. Turn on the light source. Adjust the integration period, average period, and boxcar smoothing parameters so that you get a clean spectrum. The peak should be near the red line.
  3. Turn off the lamp (or insert a blind between the lamp and the fiber) and take a measurement of the "dark" intensity.
  4. Turn the lamp back on (or remove the blind) and take a measurement of the "blank" intensity. The measured intensity is I(λ) = IBlank(λ) − IDark(λ).
  5. Calculate the lamp color temperature according to equation (5) and compare to the expected value. Remember to include the efficiencies of the grating and the CCD!
  6. Compare your result to the expected value. What is the source of any difference?

Measuring the Enthalpy of Iodine Sublimation

  1. Put the sample holder in place, and insert the thermocouple into its hole. Readjust the integration period, average period, and boxcar smoothing parameters so that you get a clean spectrum. From this stage forward the sample should not be moved.
  2. Connect the heater to the power source. Make sure the voltage is set to 0 V before you turn it on.
  3. Connect the thermocouple to the voltmeter and measure the initial conditions. Take a reading of the thermocouple voltage, and, using a thermometer, determine the temperature of the room.
  4. For the blank measurement:
    1. Take a reading of the thermocouple voltage and convert it to degrees. This is the difference between the room temperature and the temperature in the cuvette.
    2. Save the spectrum to a file and give it an appropriate name.
  5. Continue to increase the heating voltage slowly (notice that the heating process is one-way only), wait for the temperature reading to stabilize after each increase, and measure the temperature and spectrum as you did before. You should reach 70-80 °C.
  6. Result analysis:
    1. Find the maximum absorption wavelength from the first spectra you saved.
    2. Calculate the absorption at this wavelength for each temperature.
    3. Calculate the enthalpy according to equation (10). Compare your result to the literature.