Phase Diagram of tert-butyl Alcohol

Transcription

1 Phase Diagram of tert-butyl Alcohol Bill Ponder Department of Chemistry Collin College Phase diagrams are plots illustrating the relationship of temperature and pressure relative to the phase (or state of matter) of a substance. For this experiment, we are only interested in the three phases most commonly encountered in everyday life: solid, liquid and vapor. It should be noted that there are other phases, such as plasma and Bose-Einstein condensates. However, they are outside the scope of this investigation. In this experiment you will construct a phase diagram of tert-butyl alcohol. To construct the phase diagram, you will need to determine the ambient boiling point and freezing point of tert-butyl alcohol. In addition to these data, you will also determine the boiling-point temperature of tert-butyl alcohol at different pressures, and record the temperature and pressure of the triple point. The triple point will be discussed in further detail below. To more completely understand the data associated with a phase diagram, you must first understand equilibrium vapor pressure. Imagine a closed container of water at room temperature, and the space above the water in the container is a perfect vacuum. Furthermore, imagine that time has been turned off to reach this hypothetical situation. Once time is resumed, what will occur in the closed container? Immediately, molecules of water will begin rapidly leaving the liquid phase (boiling) and enter the vapor phase. As more and more water molecules enter the vapor phase, the pressure over the liquid phase will begin to increase beyond that of a vacuum. As the number of water vapor molecules in the container increases, the chances that a water vapor molecule will reenter the water phase also increases (i.e. the rate of water vapor molecules reentering the liquid phase increases as the pressure increases). Eventually, equilibrium is reached (e.g. the rate that water molecules are entering the vapor phase equals the rate the vapor molecules are returning to the liquid phase) and boiling stops. The pressure at which boiling ceases, in the case just described, is the equilibrium vapor pressure of water at room temperature. Suppose the above experiment is repeated at different temperatures, and the resulting equilibrium vapor pressures are plotted against temperature. A curve similar to that of Figure 1 is obtained.

2 Pressure Pressure Temperature Figure 1. Boiling point curve The data above yield several important pieces of information. As temperature is increased, the equilibrium vapor pressure for water increases. The curve above is called the liquid-vapor coexistence boundary for water. At each temperature along the curve there exists a specific pressure where the liquid and vapor phases of water are in equilibrium. Perhaps more descriptive, the data also show that combinations of temperature and pressure above the resulting curve will result in liquid conditions of water. Conversely, combinations of pressure and temperature below the curve will result in vapor conditions of water. Does this make intuitive sense? At a given temperature, is liquid water more likely to exist at lower or higher pressures? Now imagine a similar experiment where the melting temperature of ice is measured at different pressures (all substances melt at different temperatures depending on pressure). The resulting plotted data will look similar to Figure 2. Temperature Figure 2. Melting point curve

3 Pressure Using similar reasoning as applied to Figure 1, would combinations of temperature and pressure to the left of the line result in ice or liquid water? What about combinations of temperature and pressure to the right of the line? Combining the data from Figures 1 and 2 gives the resulting curve in Figure 3. Notice that the two curves intersect. The point of intersection is of special significance and is a key intensive characteristic of pure substances. It is called the triple point. Following the liquid-vapor coexistence curve through lower temperatures, two phases are in equilibrium: liquid and vapor. Temperature Figure 3. The intersection of the boiling point and melting point curves Following the solid-liquid coexistence curve through lower pressures, two phases are in equilibrium: solid and liquid. Therefore, at the triple point, all three phases are in equilibrium. In other words, water (as well as any other pure substance) both boils and freezes at a specific combination of temperature and pressure! Questions to answer prior to performing the lab: Following are some questions to consider before starting the lab. At a given temperature, is liquid water more likely to exist at lower or higher pressures? Would combinations of temperature and pressure to the left of the melting point curve result in ice or liquid water? What about combinations of temperature and pressure to the right of the curve? If you were to draw the curve that extends below the triple point, what curve would this be? What states would be at equilibrium at this curve?

4 What happens at the molecular level at very high temperature and very high pressure? What do you expect to observe for the melting point curve of water compared to that of an organic compound, say C6H6? Explain your prediction at the molecular level. Does density affect the melting or boiling point curves? Explain your answer. What is the structure of tert-butyl alcohol? What intermolecular forces would it exhibit? Do intermolecular forces play a role in the melting and boiling point curves? Use examples to explain your answer. Figure 4. Picture of apparatus Procedures Part A-Determining the triple point of tert-butyl alcohol 1. Construct the experimental apparatus as shown in your lab.

5 2. Add approximately 15 ml of tert-butyl alcohol to the 25 ml filter flask along with several boiling chips. 3. Stopper the 25 ml filter flask, ensuring that the tip of the temperature probe is immersed into the tert-butyl alcohol. 4. Turn on the aspirator. Click the Start button on the Pasco software control screen and note the temperature and pressure reported by the software. Pressure inside the Erlenmeyer flask containing the tert-butyl alcohol can be adjusted by opening or closing the valve of the Bunsen burner. Confirm this by slowly turning the knob of the Bunsen burner and noting that the pressure reading decreases. 5. Completely close the Bunsen burner valve and closely examine the tert-butyl alcohol contained in the flask. Also, closely monitor both temperature and pressure readings. Eventually the tert-butyl alcohol will both freeze and boil simultaneously. This is the triple point. Record the temperature and pressure. Click Stop on the Pasco data collection screen. 6. Slowly open the pressure release valve on the Bunsen burner. Once the pressure within the system has equalized with atmospheric pressure, turn off the aspirator. Part B-Determining multiple boiling points of tert-butyl alcohol. 1. Using tap water contained in a 250 ml beaker equipped with a stir bar as a water bath, suspend into the bath the 25 ml Erlenmeyer flask containing the tert-butyl alcohol. While stirring, slowly warm the water bath using a hotplate to approximately 40 ⁰C. Make sure the Bunsen burner valve is fully open. Use an alcohol thermometer to determine the temperature of the water bath. 2. Turn on the aspirator and click Start on the Pasco data collection screen to begin temperature and pressure data acquisition. Slowly, decrease the pressure within the system by partially closing the Bunsen burner valve. Continue decreasing pressure until the tert-butyl alcohol begins to boil. 3. Click Stop on the Pasco data collection screen. Record the pressure and temperature and then completely open the Bunsen burner valve to raise the pressure within the system. 4. Adjust the water bath temperature to 50 ⁰C, and repeat steps 2 and Collect boiling point data for temperatures of 60 ⁰C,, 70 ⁰C and 80 ⁰C by repeating step 4. Part C-Determining multiple freezing points of tert-butyl Alcohol

6 1. Starting with tap water contained in a 250 ml beaker equipped with a stir bar cool the water to approximately 15 ⁰C using ice. 2. Remove the rubber stopper on the 25 ml Erlenmeyer flask and add a small stir bar to the container. Replace the rubber stopper. Cool the tertbutyl alcohol to approximately 30 ⁰C using the cool water bath. 3. Turn on the aspirator and adjust the pressure to approximately half of atmospheric pressure. 4. While the 25 ml Erlenmeyer flask containing the tert-butyl alcohol is suspended in the 15 ⁰C water bath initiate stirring. Click Start to begin collecting temperature and pressure data. 5. Closely observe the tert-butyl alcohol. When the tert-butyl alcohol becomes turbid (cloudy), click Stop and record the temperature and pressure. 6. Remove the 25 ml Erlenmeyer flask containing the tert-butyl alcohol from the water bath and warm it to approximately 30 ⁰C using a warm water bath. Use hot tap water for the warm water bath. 7. Adjust the pressure to approximately 25% of atmospheric pressure. 8. Repeat steps 4 through Adjust the pressure to approximately 10% of atmospheric pressure. 10. Repeat steps 4 through When you have finished collecting all of the data, disassemble the apparatus. Clean all used glassware and return to their place of origin. Critical Data/Discussion to Include in Your Lab Report: Graph of data with title, properly scaled axes, and labeled axes Answer to all the posed questions Discussion of the data collected and calculated % error for the triple point Work cited